Deep Water Ship Handling
WARNING: All rights reserved. Any unauthorised copying, hiring, lending, exhibition diffusion, sale, public performance or other exploitation of this video and accompanying workbook training package is strictly prohibited and may result in prosecution. © VIDEOTEL MMXIII This video and accompanying workbook training package is intended to reflect the best available techniques and practices at the time of production, it is intended purely as comment. No responsibility is accepted by Videotel, or by any firm, corporation or organisation who or which has been in any way concerned with the production or authorised translation, supply or sale of this video and accompanying workbook training package for accuracy of any information given hereon or for any omission herefrom.
DEEP WATER SHIP HANDLING A VIDEOTEL PRODUCTION
in association with Norwest Interaction Ltd
The producers would like to acknowledge the assistance of International Maritime Organization (IMO) Louis Dreyfus Armateurs S.A.S. MSC Shipmanagement Ltd The Nautical Institute North of England P&I Association Ltd Pacific International Lines (Pte) Ltd Safety at Sea Ltd, part of the Brookes Bell Group Southern Shipmanagement
PRINT AUTHOR: Anne Wilson WRITER/DIRECTOR: Julian Grant PRODUCER: Amanda Gross
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Deep Water Ship Handling
CONTENTS 1. INTRODUCTION
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2. SHIP STABILITY
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3. HEAD SEAS AND THEIR EFFECTS ON SHIP STABILITY
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4. BEAM SEAS AND THEIR EFFECTS ON SHIP STABILITY
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5 STERN SEAS AND THEIR EFFECTS ON SHIP STABILITY
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6. COMPLEX ROLLING MOTIONS IN BEAM AND STERN SEAS
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7. AWARENESS AND ACTION
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8. SUMMARY
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9. GLOSSARY
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10. FURTHER RESOURCES
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11. ASSESSMENT QUESTIONS
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12. ASSESSMENT ANSWERS
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1. INTRODUCTION ABOUT DEEP WATER SHIP HANDLING Many different variables determine the behaviour of a ship in deep water, including the weather, the state of the sea, the ship’s design, rudder and propeller type, its loading state, trim and speed. Knowing how to monitor, balance and manage these variables is largely a matter of experience. Nonetheless, every seafarer can play their part in keeping ship, cargo, crew and passengers safe if they know the basics of ship stability and ship handling in deep water. This training programme consists of a video, workbook or interactive CD-ROM, aimed at junior deck officers and above, as well as a non-seafaring audience who need to understand the issues, such as court or tribunal officials.
1.1 Learning outcomes By the end of the training, participants will: • • • • • •
Understand why good ship handling is so important Know the basics of ship stability (static and dynamic) Understand the main effects of head seas (in terms of motion, stress and risks) Understand the effects of beam seas and stern seas Be aware of the effects of complex rolling motions in beam and stern seas Understand the importance of knowing, balancing and managing the variables which determine a ship’s behaviour in deep water • Be aware of a range of actions to take in order to deal with a developing situation • Be aware of commercial pressures not to act – and the risks of taking no action • Understand the cost of accidents in human, environmental and commercial terms
1.2 How to use the training package The programme can be used either for individual training, or for group sessions facilitated by a trainer. In the first session it is recommended that the video be played from beginning to end. In subsequent sessions, the sections can be shown (or viewed) individually to remind participants of the key points of each of the topics. The workbook looks in much greater depth at the topics covered in the video, adding technical detail and giving examples (case studies). It also provides a glossary of
Introduction
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Deep Water Ship Handling terms and gives further reference sources on the subject of deep water ship handling. After watching the video, participants should read and study the workbook and, if necessary, look for further explanations and examples among the references listed at the end. Deep water ship handling is a complex subject and this workbook is necessarily limited in scope. It is highly recommended that participants build on what they have learned by reading more widely on the subject and drawing on the knowledge of more experienced colleagues. Additional video programmes are available that look into the practical and technical aspects of handling ships in heavy weather situations.
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Introduction
2. SHIP STABILITY A ship suffers damage in heavy weather because of inadequate preparation – lives could have been lost… A ship in force 4 to 5 winds encountered worsening weather conditions, with rough seas and winds to force 8 or 9. They reduced speed, but when the bow thruster room bilge alarm sounded, there were found to be a number of holes in the port side of the bow thruster room shell plating, through which water was pouring. What caused the damage? The port anchor chain lashing had released and the anchor had fallen against the windlass brake tension into the water. As the ship continued to pitch in the heavy seas the anchor impacted against the hull. It was later found that five adjacent compartments had also flooded. Despite the forecasted poor weather conditions no specific heavy weather checks had been carried out. By the time they were considered necessary it was too dangerous for personnel to go on to the deck, so the anchor could not be properly secured. The port anchor chain lashing arrangement failed because neither it, nor the windlass brake, was sufficiently tightened and the hawse pipe cover was not fitted. This was no ‘accident’. The damage was due to lack of good preparation for encountering heavy weather. The cost of mistakes in handling a ship in deep water and in adverse weather conditions can be very high indeed, in human, commercial and environmental terms. Although commercial vessels are always under pressure to move quickly, it is essential to think in terms of risk assessment and to balance meeting deadlines against the well-being of the ship, cargo and crew. Good ship handling is about balancing situational awareness and environmental sensitivity with commercial interests. Situational awareness means recognising and responding to what is going on around you by gathering information, interpreting it, and then thinking about what might happen next. There are many different variables to consider when handling a ship – the prevailing weather (wind speed and direction), the swell, length and height of the waves, the ship’s design and manoeuvring capability, its load, type of cargo, and so on. Handling a ship safely and efficiently means knowing, managing and balancing all these variables at the same time. It is the responsibility of every crew member to practice good situational awareness by observing and gathering information, interpreting what they observe, and anticipating what might happen. In heavy weather particularly, every crew member should be alert and use their experience to guide them in making decisions, or seek the advice of a more experienced crew member.
Ship Stability
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Deep Water Ship Handling
2.1 Understanding ship stability This section is a reminder of the basics of ship stability. Most seafarers will already be familiar with these, but may not have connected them with the principles of good ship handling in heavy weather (which are outlined in later sections of this workbook). First, we need to understand the principles of stability at static equilibrium – that is, knowing what a ship does – and why – when no forces other than gravity and buoyancy are acting upon it, such as when the ship is in port. Let’s take a look first at the different ways a ship can move.
2.2 The three axes of motion As a common convention, it is assumed that vessel motions take place along and around three reference axes: vertical, longitudinal and transverse. These are shown in Figure 1.
3 AXES OF MOTION
Vertical Longitudinal
Transverse / Lateral
Figure 1: The 3 axes of motion
The ship can move along an axis in either direction, or rotate around it. Thus six possible movements are possible (known as the six degrees of freedom) each with its own name: • up and down the vertical axis is heave (the ship goes up and down) • rotation around the vertical axis is yaw (the bow and stern swing to one side or the other) • along the longitudinal axis is surge (the ship moves forwards or back) • rotation around the longitudinal axis is roll (the deck tips sideways) • along the lateral axis is sway (the ship moves sideways) • rotation around the lateral axis is pitch (the bow and stern go up or down) 8
Ship Stability
Movement on or about any of these axes can lead to stresses on the ship’s hull, cargo and engines, and can also affect the ship’s stability. We will be looking in detail at these different types of movement later in this workbook. It is also important to note that for personnel on board the ship’s motions can cause physical, mental and emotional stress. Fatigue is now recognised to be a significant contributor in marine incidents, so signs of stress or fatigue in personnel when the ship is pitching or rolling heavily should always be taken seriously.
2.3 Gravity and buoyancy Imagine a model ship which you are trying to balance on just one finger. You need to get it into just the right place so that its weight is equal on all sides. When you find that perfect balance, your finger is directly under the centre of gravity (G). G is the point at which all the downward forces acting on a ship (i.e. the combined weights of every part of it), are perfectly balanced on every side. If you put a heavy weight in one side of the ship this will move the centre of gravity (G) directly towards the centre of gravity of the added weight – so that the ship will list (to the side) and/or trim (by the bow or stern) in that direction. Now imagine you put the model ship into water and it floats. If you push it down into the water at one end its buoyancy makes the other end rise up until it stops moving and balances again. However, if you push it down again, roughly in the middle, there will be one place where the ship goes down, but remains horizontal; your finger is directly above the centre of buoyancy (B). B is the point at which the buoyant upward forces on a ship are perfectly balanced on every side. For the ship to remain stable, and not move or rotate in any direction, these two forces – weight and buoyancy – must be equal and opposite. For a vessel floating on an even keel, or upright, G and B are at the same vertical line. This is true whether we consider the vessel lengthways (longitudinal stability) or across the vessel (transverse stability).
Ship Stability
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Deep Water Ship Handling
Figure 2: G and B – Equal and Opposite
The relative positions of the centre of gravity and the centre of buoyancy determine the stability of the vessel.
2.4 Stability and the metacentre (M) In calm waters if an external force acts on a ship to tip it from the upright, the force that acts to return it to upright appears to ‘pivot’ at the metacentre (‘M’). Imagine two vertical lines, one going through the centre of buoyancy when the ship is stable and the other through the new centre of buoyancy when the ship is inclined to one side. The metacentre is the point of intersection between these two lines.
Figure 3: Metacentric height
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Ship Stability
The metacentric height (GM) is the distance between the centre of gravity (G) and its metacentre (M). The value of GM determines the initial static stability of a floating body – a larger metacentric height implies greater initial stability against overturning. If a rower stands up in a vessel it becomes more unstable, because when he is seated, the centre of gravity is low down, well below the metacentre. When he stands up (raising the centre of gravity), he is also raising G towards M, so reducing the metacentric height (GM). When M is above G, the ship is stable:
Figure 4: Stable ship
When M is below G, the ship is unstable:
Figure 5: Unstable ship
Ship Stability
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Deep Water Ship Handling When G and M coincide, the ship is in neutral equilibrium:
Figure 6: Ship in neutral equilibrium
So what is happening when a ship rights itself? As the vessel tips sideways, more of the hull on the lower side goes deeper into the water, whilst more of the hull on the higher side comes out of the water. The result is that the centre of buoyancy shifts to the lower side, where more water is being displaced. The centre of gravity, of course, remains in the same place, since the overall weight of the vessel has not changed. The upward force (through the centre of buoyancy) has thus shifted sideways, away from the centre of gravity, and therefore the two forces have become unbalanced, thus creating a torque, or turning moment, which opposes the tipping of the vessel and rights it again.
Figure 7: Damped rolling forces
When a vessel is heeled further by an external force, the centre of buoyancy and centre of gravity are not in the same line – a horizontal distance exists known as the
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Ship Stability
‘righting lever arm’, or GZ. The moment acting to turn the vessel back to the upright position is equivalent to the product of the vessel’s buoyancy, which is the same as the ship’s weight (W), given in tonnes (t), multiplied by the righting lever arm (GZ), given in metres, so that: The righting moment = W x GZ (tonnes-metres) This righting lever varies with the angle of heel and is expressed by what is known as the GZ curve.
2.5 Rolling – the ship as a pendulum Metacentre and metacentric height also help us understand the natural period of rolling of a hull, with very large metacentric heights being associated with shorter periods of roll. We can compare the ship’s roll to the pendulum of a clock:
Figure 8: The ship’s roll explained as a clock pendulum
Think of the pivot point of the pendulum as the metacentre (M). As the ‘pendulum’ deviates from the vertical, the weight (G) naturally acts to counter the motion, and bring it back to the central position. Also, just as the length of a pendulum determines how long it takes to swing to one side and return to centre, so the amount of time it takes for the ship to return to the upright position is determined by the metacentric height (GM). A ship with a small GM will be ‘tender’ – that is, it will have a long roll period. However, if the GM is too great, the vessel could be too ‘stiff’. The roll period of a ‘stiff’ ship is short and quick. An ideal ship strikes a balance between being ‘stiff’ and being ‘tender’.
Ship Stability
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Deep Water Ship Handling A stiff vessel responds quickly to the wave in attempting to right itself, rolling with a short period. Due to the large forces trying to bring the vessel upright, back into a state of equilibrium, the angular acceleration is likely to be high, resulting in a very fast roll. This increases the risk of damage to the ship as well as the risk of cargo breaking loose or shifting. It can also be uncomfortable for those trying to work or rest. A tender ship lags behind the motion of the waves and tends to roll to greater amplitudes. A passenger ship will typically have a longer rolling period for comfort, perhaps 15 to 18 seconds, while a tanker or freighter might have a rolling period of 9 to 12 seconds. Professional mariners usually develop a higher tolerance to shorter rolling periods than non-seafarers. Imagine now that you use your hand to push the clock’s pendulum back and forth, so its swing is wider and more forceful. This action would be like what happens on a ship when external forces act on it, such as waves striking the ship’s side. Tender ships are particularly vulnerable to the movement of fluid-like cargo (for example, fish, ice, grain, aggregates, brine, drilling mud or cement). As the ship rolls, the surface of the liquid, or ‘semi-fluid’, stays level, but it also rushes forcefully into the side which has tipped, tipping it over even further. As water moves to the low side, the centre of gravity (G) also moves to the low side. The fluid (or fluidlike) cargo becomes deeper, raising its volumetric centre of gravity slightly so that the ship’s centre of gravity (G) also rises slightly. The result is a slight reduction in the distance G to M (GM), and represents a small reduction in the vessel’s stability. This decrease in stability is known as the ‘free surface effect’. If this effect is large enough it can have a devastating impact on vessel stability, particularly on tender ships.
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A cruise ferry disaster in which free surface effect had a catastrophic impact… The ‘free surface effect’ was a major cause of a deadly cruise ferry disaster in the 1990s which cost 852 lives. The ferry was crossing the Baltic Sea in bad (but not extreme) weather, when the locks on the bow door failed from the pressure of the waves. The door separated from the rest of the vessel, pulling ajar the ramp and allowing water onto the vehicle deck. According to the accident report, this water ingress was the main cause of the vessel capsizing and sinking. Ro-Ro ferries, with their wide vehicle decks, are particularly vulnerable to capsizing if the car deck is even slightly flooded because of free surface effect. The fluid’s swilling motion across such a large area hampers the vessel’s ability to right itself after rolling with a wave. The report was critical of the crew’s actions, particularly for failing to reduce speed before investigating the noises coming from the bow, and for being unaware that the list was being caused by water entering the vehicle deck. There were also general criticisms of the delays in sounding the alarm, the passivity of the crew, and the lack of guidance from the bridge.
2.6 Newton’s laws of motion, applied to ships Newton’s laws of motion are scientific principles which explain why ships (and other objects) move in the way they do. Newton’s first, second, and third laws of motion help explain some of the things we have already seen, such as how the forces of gravity and buoyancy combine to make the ship tilt. Newton’s laws are also essential to understand the effects of waves and weather on a vessel’s structure, motion, and its stability. Newton’s first law of motion: ‘In the absence of any external (or internal) force to disturb it, a vessel at rest remains at rest, and a vessel in motion will continue with that motion.’ Perhaps the hardest of the three laws to demonstrate on Earth where friction and gravity are ever present (external forces), but in outer space far away from any planets or stars, an object given an initial push (force) will continue forever in the same direction, at a constant speed. Most importantly, what this law says is that all objects, including ships, act predictably. An essential part of good ship handling is being aware of your environment, including ‘sensing’ what your ship is doing. Ask yourself: is the ship’s motion normal for the situation? Are you comfortable with what you are ‘feeling’? Has anyone else commented on how the ship is ‘performing’? Ship Stability
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Deep Water Ship Handling If your ship begins to act in an unusual, unexpected or uncharacteristic manner then, according to Newton’s first law, something is happening to your ship. If you don’t know what that ‘something’ is, then your suspicions should be aroused and you should investigate it – but not before alerting someone to your suspicions and taking measures to protect yourself and the vessel while you undertake an investigation. Newton’s second law of motion: Newton’s second law of motion states that: ‘When a force (F) acts on an object, it will cause the object to accelerate (A). The larger the mass (m) of the object, the greater the force (F) will need to be to cause it to accelerate (A)’. (F = mA) So why is this important to someone handling a ship? Newton’s second law says that it takes more force to move a heavy object than it does to move a light object. But it also says that that the bigger your ship is, the more force (energy) it has when moving, so the more damage it is likely to do if it hits, or is hit, by something – including waves. There is a hidden element to this law that has resulted in quite severe ship damage that went virtually unnoticed until too late, as shown in the following excerpt from a ship’s accident investigation.
A ship suffers hull stress fracture due to pitching and slamming, causing an environmental and commercial disaster… A 62,000-ton container vessel faced increasingly heavy weather, causing the ship to pitch heavily. The vessel was making good a speed of 11 knots and the height of the waves was up to 9m. Suddenly, the hull of the vessel gave way and water flooded her engine room. The Master decided to abandon ship; the crew were rescued and there were no injuries. The ship was subsequently taken under tow but, as the disabled vessel approached the coast, it became evident there was a severe risk she might break up or sink, and so she was intentionally beached. More than 100 containers went overboard and 58 were washed ashore, leading to a scavenging and looting spree. About 1,900 seabirds along the coast were affected after 200 tonnes of oil leaked from the vessel soon after grounding.
In this situation this ship was large enough not to ‘feel’ the impact of large waves too badly. Newton’s second law predicted that a large ship could ‘take a hit’ from severe waves and not feel it too badly. Unfortunately, the fact that ‘the ship could take the waves’ did not mean that the individual elements of the ship could sustain a constant pounding – something which all ship-handlers should be made aware of.
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Ship Stability
Consequently, the vessel failed to take adequate precautions when encountering heavy (potentially damaging) seas – but carried on, regardless. That decision to carry on regardless was made despite knowledge of Newton’s third law of motion. Newton’s third law of motion: ‘Action and Reaction are equal and opposite, in both magnitude and direction.’ What this law in effect says is: if your ship hits something, it will hit you back just as hard – and regardless of whether the ship, as a whole, will be able to absorb the energy being directed at it. This law describes why waves and other external influences will always have some effect on a ship. In short – everything that happens to a ship is dependent upon some other factor. For example, a ship floats at rest because the downward force of gravity (its weight) is opposed and cancelled out by its buoyancy, acting upwards. If you put more weight in the vessel, it sinks lower, displacing more water which in turn increases its buoyancy, so that it remains still (albeit lower in the water). Summarising the relevance of these three natural laws: • Smaller ships will feel big waves and swells more keenly – so are likely to take more account of them • Larger ships will receive an impact similar to smaller ships, but because of their size, are likely to feel the effect less keenly, so are unlikely to pay as much attention to the effect of severe weather • Regardless of their relative sizes, ships, large and small, will incur damage if they do nothing to mitigate the effects of encountering severe weather • It might take the officers of a larger ship longer to realise that something is wrong and investigate – because the overall weather effects will be less evident
2.7 The effects of weather on stability – anticipating and taking action Since weather can crucially affect a ship’s stability, it is essential to look ahead and be prepared. Master and crew should identify weather windows and use both common sense and a thorough risk assessment before deciding when, and how, to proceed. Every seafarer knows how quickly the weather at sea can deteriorate and change. This is why it is so important for the officer of the watch (and others) to notice what is happening, to monitor constantly the wind and waves and the motion of the ship.
Ship Stability
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Deep Water Ship Handling A good understanding of ship stability can help crew members identify what is happening to the ship and notice important changes. Many disasters could have been avoided by vigilant crew members alerting the Master in good time of unexpected developments, allowing them to take remedial action. A good rule to follow is that if you are the officer of the watch and are wondering whether or not to call the Master, you probably should have already done so.
Incidents which don’t make the headlines… • The Master who, when planning a voyage, chooses an optimum route based on forecasts of weather, sea conditions, and his ship’s individual characteristics for a particular transit. • The officer of the watch on a ship steaming into a head sea observes the ship make a significant roll when it is already pitching heavily. The wave length seems to be about the same length as the ship. The Master is informed and takes evasive action to avoid parametric rolling. • An officer of the watch who notices that the ship is pounding heavily. The Master is called and the speed reduced, with the result that the pounding (and hence the stress) on the ship’s hull is significantly reduced.
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3. HEAD SEAS AND THEIR EFFECTS ON SHIP STABILITY In any kind of sea, there are three factors to consider in terms of possible damage to the vessel: the motion of the vessel, the mechanical stress caused by that motion, and the risk of damage (to ship, cargo and personnel) that results. Encountering head seas (where the waves are running directly against the course of the ship); stern seas (where the waves run in the same direction as the ship is heading); and beam seas (where the waves run against the sides of the ship) all affect a ship’s motion, stresses and stability in different ways. This section considers the effects of head seas on these three elements.
3.1 Pitching and its effects Pitching is a rotational motion around the transverse axis of a ship. The sea lifts the ship at the bow and lowers it at the stern, and vice versa, with gravity pulling the ship down (sometimes at speed) every time it is disturbed in this way. The main stress on the hull as it comes back into the water is called ‘slamming’. Slamming is the ‘slap’ of the sea surface, or the waves, hitting the side, bow, or bottom of a ship in what is known as ‘hydrodynamic impact’. Slamming will generally accompany violent pitch and heave actions encountered in heavy seas. It is caused by the ship hitting the water (or the water hitting the ship structure) very fast as it drops under the force of gravity. When a solid surface (particularly a flat surface like the bottom of a ship’s hull) impacts the water like this, the water can behave almost like a solid. The forces experienced during slamming are sudden and violent. Strong acceleration/deceleration forces are exerted when the ship hits the water or drops back into it. This may manifest as shuddering and loud banging and can create powerful vibrations that may be felt throughout the whole ship. Slamming is known to cause metal fatigue resulting in significant damage to ships.
Head Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
Figure 9: Bottom slamming
Slamming also occurs when a wave hits the bow of the ship at force, referred to as ‘bow wave slamming’. Whether the wave hits the bottom of the ship or the bow, slamming can put considerable stress on the structure of a ship and so poses a risk to ship, cargo, and personnel.
Figure 10: Bow wave slamming
In head seas, the effects of the pitching motion and the slamming which results from it are intensified because of the added force of the waves coming from the opposite direction. Violent slamming loads in severe wave conditions can make the hull vibrate, either occasionally or even continuously. This is called ‘whipping’ (transient) or ‘springing’ (continuous) vibration. 20
Head Seas and Their Effects on Ship Stability
3.2 Hogging and sagging Head seas can put other types of stresses on a ship which create risk. If we think of a ship as being shaped like, say, a plank of wood, when a ship is at the crest of a wave, it can bend upwards in the middle and downwards at each end. This is called ‘hogging’. When the ship ends are lifted by the crests of two waves at once, it can sag in the middle and bend upwards at each end. This is called ‘sagging’. Both of these situations put stress on the hull. The effects of both hogging and sagging can be exaggerated by unevenly loaded cargo.
Figure 11: Hogging
Head Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
Figure 12: Sagging
A personal experience of hogging and sagging… “I was in a 264,000 VLCC approaching the Cape of Good Hope off the southern tip of South Africa. The weather was fair, but we were encountering very long ocean swells with a fairly high amplitude.” “As we were undertaking crew-change and storing by helicopter, the vessel had to maintain its course with the swell coming from approximately 50 to 60 degrees abaft the port beam. This meant that the ship would, at one moment, have a wave crest amidships and then shortly afterwards have wave crests on the port quarter and starboard bow.” “From the ship’s bridge, I could clearly see the bow rise and fall out of sequence with after end of the ship. The vessel also seemed to roll differently at the bow and stern.” “Very clearly, I was observing extreme hogging, sagging and torsional racking of the vessel. We altered course to change the encounter angle of the swell and reduce these effects.”
Hogging and sagging occur along the longitudinal axis of a ship, but a ship can also be distorted laterally, in a twisting motion called ‘torsion’. This can be caused by external forces (like waves), by internal loading stresses (such as unevenly loaded cargo) or by a combination of the two.
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Head Seas and Their Effects on Ship Stability
‘Racking’ is the distortion of the ship from the ideal oblong shape in the vertical plane. Racking, like hogging and sagging, can stress a ship’s hull causing physical damage. It is vital that officers of the watch monitor the motion of the ship, be aware of the different stresses that can arise, and be able to recognise the signs that something might be happening. Many disasters could have been anticipated, or their effects reduced, by a vigilant officer of the watch.
Figure 13: Racking – due to internal weight imbalance
Figure 14: Racking – due to environmental influences
Head Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
Figure 15: Torsion/Twisting
3.3 Shipping green seas In head seas, a combination of the movements of the ship and the force of the waves can cause large amounts of sea water to flow onto the bow (and/or deck) of a ship. This is known as ‘shipping green seas’, in order to distinguish it from less dangerous spray or foam. The weight and movement of a large amount of sea water on the deck or bow, especially in a heavily pitching ship, can seriously affect its stability. There can also be damage to small deck components (pipes, ventilator openings, hatches, etc.) allowing water into compartments so that it affects buoyancy and stability. Deck structure damage can allow water ingress to machinery spaces such as the bowthruster rooms, forecastle stores, emergency generator/fire-pump rooms, engine spaces and other compartments.
Figure 16: Water is taken into a ship’s cargo hold
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Head Seas and Their Effects on Ship Stability
A lot of water on deck (for example, in a cargo hold or large open deck area) can adversely affect stability. If water is shipped on board, the effect is three fold (Figure 17). Firstly a weight is added to the vessel so that the ship’s centre of gravity (G) moves towards the new added weight and changes the GM – so residual stability is altered. Secondly, as water moves freely, it gets deeper in places and its volumetric centre of gravity rises (Figure 18). This results in a slight raising of the ship’s centre of gravity (G) producing a slight reduction in GM – known as free surface effect. Because GM is reduced, the righting lever arm (GZ) – developed as the ship heels – is also reduced, making it harder for the vessel to return to an upright position. Thirdly, the added weight causes the vessel to sink deeper in the water, thereby reducing freeboard, making the shipping of green seas even more likely with consecutive waves.
Figure 17: Shipping water – sinkage due to added weight
1. Water on deck. Vessel sinks deeper. G rises to G1 and stability (GM) reduces.
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Figure 18: Shift of G due to water ‘sloshing’ around
2. Vessel rolls. Water on deck gets deeper on one side. G1 moves to G2 (rising and transferring laterally). GM (stability) again reduces as G rises – this component of stability reduction is known as ‘Free Surface Effect’. Shipping green seas causes a ship to suffer catastrophic hull failure… A bulk carrier facing a tropical storm sank with the loss of all 42 crew members on board, plus the wives of two crew members. Evidence from the underwater surveys showed that the covers for ventilator openings in the bow section of the ship were missing; it was concluded that this had allowed seawater to flood into the ship and cause it to trim down by the bow. This adverse forward trim enabled storm force waves to batter the foremost cargo hold hatch covers, causing them to collapse, and the forward cargo hold to then flood with sea water. The same process was repeated on the number two and number three cargo holds. The additional weight of seawater, coupled with the heavy seas, caused the main hull to suffer a catastrophic structural collapse and the vessel to founder. Shipping green seas caused structural damage that allowed water ingress so that the weight of the bow increased dramatically. This then set off a sequence of events which caused the ship to sink astonishingly quickly, with no time to react.
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Head Seas and Their Effects on Ship Stability
3.4 Engine labouring and over-speed effects The additional power needed to maintain way in heavy seas, or against a strong current, if unregulated, could result in damage to the main engine by forcing it to exceed its design limit. This might happen if the ship is pitching so heavily that the propeller lifts out of the water or rises close to the surface. If the propeller suddenly comes ‘off-load’, it causes the engine to race. In normal operating conditions, the engine governor is designed to cope with variations in propeller loading, but in extreme conditions, where the governor is excessively loaded, it may malfunction and cause an engine shutdown. To avoid this, it is advisable to reduce engine power demand and the speed of the ship in heavy seas. This action produces three direct benefits. Firstly, it reduces the risk to the engine and governor. Secondly, reducing speed lowers the incidence of shipping green seas and so reduces the risk of structural damage, particularly at fore end. Thirdly, anticipating risks to the engine and governor and pre-empting any problems, lessens risk of mechanical breakdown, so that the ship remains safely navigable. A cruise ship sustains engine damage in heavy seas… A cruise ship, with 88 passengers and 77 crew members aboard, suffered damage after being battered by brutal waves. The ship declared an emergency, reporting it had suffered engine damage amid heavy seas and 55mph winds when it was north east of the South Shetland Islands (New Zealand) on its way towards its destination in Argentina. The crew of the 2420-ton ship reduced speed and the ship limped back to its scheduled port at four knots. There were no injuries to passengers, although one member of the crew sustained minor injuries.
Head Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
4. BEAM SEAS AND THEIR EFFECTS ON SHIP STABILITY Every seafarer knows that ships roll in open water and that they roll more when the swell and the waves are hitting the beam of the ship. But behind these simple statements is rather complex physics. For the experienced ship’s officer, it is a simple matter to measure the rolling angle of a ship in particular conditions and then to calculate the amount of force needed for it to right itself.
4.1 Ordinary damped rolling As a swell approaches the side of the ship, the ship rides up onto the face of the swell and leans away from it. It’s a bit like driving a car round the face of a hill. The pressure of the swell hitting the side of the ship also tries to push the ship sideways. Because the air surrounding the part of the ship out of the water offers less resistance than the water in which the hull is riding, the top of the ship will move faster than the bottom, emphasising the motion. The combined effect of all the forces acting together is called a ‘roll’. The ship achieves a state of steady rolling once its damping tendency becomes equal to the force being input by each wave. If no other wave causes the ship to roll again immediately, the ship will continue to ‘rock’ to and fro, but with reduced amplitude each time. This sequence describes what is referred to as ‘ordinary damped rolling’. Like the pendulum referred to earlier, each vessel has a natural rolling period which is a function of its metacentric height – the shorter the metacentric height, the longer the period of roll.
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Beam Seas and Their Effects on Ship Stability
Figure 19: Example of a vessel rolling cycle (one of many sequences depending on the sea and natural period of the ship)
4.2 Deck edge immersion Most ships are designed to be able to take water on deck whilst at sea – it is a normal occurrence, especially in heavy weather. However, under certain circumstances, especially in situations where the stability is already very low, if the deck edge becomes immersed under the water and reduces the water plane area, the consequences for stability can be serious.
Figure 20: How deck edge immersion changes water plane width
Beam Seas and Their Effects on Ship Stability
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Deep Water Ship Handling By reducing the water plane width, deck edge immersion reduces the vessel’s ability to right itself. Knowing where deck edge immersion is likely to occur is also valuable when it comes to understanding the ship’s stability curves, as this is the point at which vessel stability starts to disappear rapidly.
4.3 The importance of ship stability curves The metacentric height (GM) is only indicative of stability for low angles of heel. For information about overall stability, we need to look at its stability curves. A ship’s stability curves give us important information about how it is likely to react in different conditions. Here is a brief refresher on the information we can get from them: 1. 2. 3. 4. 5.
The GZ at any angle and the moment of statical stability at that angle. The maximum GZ and the angle at which it occurs. The angle of vanishing stability. The range of positive stability (the range where the graph is above zero). The angle of deck edge immersion (the point of ‘contraflexion’ – where the curve direction changes). 6. The GM of the vessel. 7. The dynamical stability at any angle.
Figure 21: Example of a GZ curve for a ‘stable’ vessel having a positive GM
For an unstable vessel (curve 2) the initial GM is negative. The range of positive stability is considerably less than for a stable vessel. When at rest, the vessel will assume a natural inclination (the ‘angle of loll’) at the point where the stability curve crosses the x-axis.
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Beam Seas and Their Effects on Ship Stability
Figure 22: GZ curve for an ‘initially unstable’ vessel
The vessel’s dynamic stability is a measure of the total work done by an external force in inclining the vessel to a given angle. That is, the external force has to overcome the righting moments (W x GZ) produced by successive GZ levers and is therefore represented by the area beneath the GZ curve to the required angle of inclination. Thus, dynamic stability is equal to the area under the GZ curve multiplied by the vessel’s displacement and is given in ‘metre-radian-tonnes’ (or m.r.t).
Figure 23: Area under the GZ curve
The area under the curve will give an indication of the power/force (in metre-radiantonnes) that will act to right the ship. The introduction of free surface effect (FSE) has effectively raised the base-line for the dynamic stability curve so that the area under the curve is reduced. In essence, as can be seen from the curve below, by introducing free surface effect, the ship has less inherent capability for resisting an external inclining force.
Beam Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
Figure 24: Area under GZ curve reduced due to FSE
The conclusion from these graphs must be that: • Excessive amounts of water shipped aboard a vessel with a small GM can seriously compromise an inclined ship’s ability to right itself • Slack tanks and compartments should, so far as possible, be avoided if the vessel’s GM is low
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Beam Seas and Their Effects on Ship Stability
Deck edge immersion further reduced the stability of a ferry in heavy weather… Deck edge immersion, subsequent flooding, and the free surface effect of the water which got into the hold, were all important factors in the stability problems encountered by a 6,000 GRT Ro-Ro trailer ferry crossing the Irish Sea. The passage had been uneventful with the vessel’s movement being ‘comfortable’. As she approached port, the sea state deteriorated and the vessel’s rolling increased. A series of large rolls caused two trailers to slip from their trestles, loosening their lashings. The rolling then decreased for a short period and it was noted the vessel had developed a slight port list. The vessel then experienced more extreme rolling, with the vessel hanging to port after a particularly large roll. At this point cargo on trailers started to shift, and some trailers themselves shifted slightly. The Master then attempted to turn the vessel back into the westerly wind by turning rapidly to starboard in order to try to reduce the rolling. This manoeuvre exacerbated the port list still further (to 50 degrees) and the deck edge on the port side became immersed. Water flooded into the internal spaces, probably through vents to the lower hold. Although these were fitted with watertight flaps these were still open and the vents were seen to be submerged by waves breaking on the deck. During the incident water was sighted in the lower hold and heard in the vicinity of the main deck vehicle space, although it is not clear how it got there. The presence of this additional water and its free surface effect further reduced the vessel’s residual stability. The ferry ultimately foundered on a beach on the UK coast and had to be broken up. However, all of the19 crew and 4 passengers were successfully rescued by helicopter and no pollution incident occurred.
Beam Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
5 STERN SEAS AND THEIR EFFECTS ON SHIP STABILITY In stern or following seas, the main dangers to the dynamic stability of a ship are a sudden loss of control, large amplitude rolling and in extreme cases, capsizing. There have been many studies done on the behaviour of ships in stern seas in order to understand how wave action (and other factors) in these conditions can cause a ship to unexpectedly capsize. Good housekeeping practices, an awareness of your environment, and timely preparation for heavy weather can help prevent many problems – for example, making any necessary repairs before leaving port, battening down all doors, hatches, vents, deadlights, etc., and putting the engines on standby (if necessary).
5.1 Pooping of green seas ‘Pooping of green seas’ is when the vessel ships green seas over the stern. Like any shipping of green seas, this may cause a problem with stability because of the weight of the water and the effects of its movement. Wave height and dynamic pressure can also cause hatch covers/sounding pipes, etc. to fail, sometimes with catastrophic results. Of course, pooping green seas can also be a hazard for personnel working around the accommodation area, especially if this happens unexpectedly.
Sea damage to small parts on deck can have big consequences… A tanker, laden with 85,000 tonnes of crude oil lost engine power and was drifting in force 10-11 winds. The cause was found to be seawater which had entered the engine fuel-oil service tanks through a broken pipe on deck. The engine subsequently stopped so that the tanker lost navigational control and ran aground – causing shoreline pollution and considerable damage to marine life.
5.2 Propeller immersion and engine racing In stern seas, vessels can also experience a ‘surfing’ effect which causes the engine to race. Surfing can also result in the ship’s ability to steer being compromised. As the vessel surfs on each wave, the normal direction of water flow (forward to aft) around the propeller and rudder diminishes and may start to come from astern, so reducing the effectiveness of the rudder and decreasing the effect of thrust from the propeller. As the propeller load lessens, this causes the engine to race. Also, the water becomes aerated, so the engine is even less effective.
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Stern Seas and Their Effects on Ship Stability
5.3 Broaching-to, loss of stability and risk of capsizing Broaching-to is an unintentional change in heading angle of a vessel caused by wave action alone. Broaching tends to happen when waves come from behind and are moving in a direction which forms a small angle with the longitudinal axis of the ship. It is a sudden and marked yaw which peaks within a single wave length. Characteristically, control is lost when the middle of the ship lies somewhere on the wave down-slope – near to the trough. Broaching has been known to cause even large vessels to capsize suddenly. Such a situation can arise if the forces of buoyancy instantly reduce while the vessel is broaching-to at the same time. In such cases, the ship will heel significantly. In extreme situations, if the ship already has a low GM and all stability is lost, capsize may occur within a few seconds. Experiments have shown that the risk is greater for fine-lined vessels such as cruise ships, reefers and container vessels and less so with full bodied ships such as bulk carriers and tankers. Even so, all mariners should be aware of the potential risks. Capsizing can be sudden in stern seas‌ A Spanish fishing vessel was sailing in following seas with significant wave height of 2.7 metres, at a speed of 10.5 knots, when she suddenly capsized. There was no previous distress signal from the crew. The circumstances in which the ship was sailing (speed, heading, ship size) fell into the conditions which the IMO guidelines for Masters consider dangerous and should be avoided. The sudden capsizing of the vessel suggests that it was due entirely to sea conditions. All 10 crew were lost.
Stern Seas and Their Effects on Ship Stability
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Deep Water Ship Handling
6. COMPLEX ROLLING MOTIONS IN BEAM AND STERN SEAS Some combinations of wave length and wave height under certain operational conditions may lead to dangerous situations for virtually any ship. These hazardous wave configurations can generate a large rolling angle in vessels which can increase suddenly and unexpectedly. Generally speaking, these complex rolling motions are characterised by a specific relationship between the wave encounter interval and the natural roll period of the ship. Hull form, loading condition and speed are known to be important contributory variables. It is important to be aware of these types of roll in order to know how to react to reduce, or avoid, them. There are two main types of rolling to consider and both of them involve changes in every one of the ship’s axes of motion at the same time.
6.1 Synchronous rolling Synchronous rolling can occur when the interval between waves is approximately equal to that of the natural roll period of the vessel. What happens is that waves hit the vessel and cause it to roll. If the natural rolling period of the ship is equal to the wave encounter period and they come into phase with each other, the ship completes one rolling cycle and, as it comes back to the start position, it is struck by the next wave so that more energy is input and the next roll amplitude is greater. If this continues, the roll amplitude increases with each successive wave impact. The rolling motion may increase within just a few cycles, and can quickly reach 30 degrees or more. If no action is taken to break the synchronism, the vessel may heel to dangerous angles of inclination and, in extreme situations, to beyond the point from which it can return to an upright position. The risk of capsizing is increased if the GM is low. It is essential for all seafarers – but particularly the officer of the watch – to be aware of this kind of rolling motion and alert the Master if they have concerns. Changes to speed and course are the first measures to take in countering this type of rolling motion.
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Complex Rolling Motions in Beam and Stern Seas
Figure 25: Synchronous rolling
When a Master failed to take the right corrective action… An accident investigation found that the most plausible explanation for a container ship’s severe, and at times violent, rolling motions was synchronous rolling, as a result of the ship’s natural roll period matching that of the encounter period of the waves experienced. While the Master took action to avoid the rolling, in accordance with the guidance in the ship’s Safety Management System, this action was not sufficient. The option of altering the ship’s stability by adjusting the seawater ballast in its tanks, and therefore its natural roll period, was not considered. Poor lashings in one of the container bays combined with the ship’s rolling motions led to the holing of the hull and subsequent pollution of the coastline.
6.2 Parametric rolling The term ‘parametric roll’ is used to describe the phenomenon of a large instability induced rolling motion that can occur suddenly. In addition, the high amplitude (roll angles) associated with the onset of parametric rolling are a cause of real concern for the safety of vessels, crew and cargo. Parametric rolling has caused ships to lose containers; it can cause machinery to fail, structural damage, and even cause a vessel to capsize. The extreme movement can also cause deck or hold cargo to shift, so adding to the problem. Parametric rolling is the result of an adverse combination of environmental, operational and design factors. Characteristically, it occurs in conditions where the waves are Complex Rolling Motions in Beam and Stern Seas
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Deep Water Ship Handling high, the interval between waves is approximately twice that of the natural roll period of the vessel and, in addition, the pitch period is about twice that of the roll period. This particular combination of pitching and rolling can produce long, slow and very high amplitude rolls. In order to fully understand the phenomenon of ‘parametric rolling’, it is necessary to consider what happens during a wave encounter and how this affects the stability of the vessel. As suggested by the name, rolling is induced as a result of some factor that alters the vessels essential stability parameters. But how does this happen? Normally, a vessel is supported (buoyed up) along its length by the sea it floats in. This, in turn, results in the vessel having a specific water-plane area that will determine the stability (GM) of the vessel and give rise to typical dynamic stability-curve values. Consider how the water-plane area appears for two types of vessel: full-form, i.e. Tankers, Bulkers etc. and fine-form, i.e. Container ships, Passenger vessels, Reefers, Ro-Ros, etc.
Figures 26 & 27: Water-plane areas in flat sea conditions
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Complex Rolling Motions in Beam and Stern Seas
In each case, the stability curves will look similar and both vessels will be stable at all conditions of inclination as shown in the following stability curve.
Figure 28: Typical stability curve for a stable vessel
When high waves are introduced into the picture, the water-plane area of the two hull forms begins to look different at various stages of the wave encounter. Firstly, consider what happens when there is a deep trough amidships.
Figures 29 & 30: Water-planes for vessels having a trough amidships
In each case, there appears to be little change in the water-plane area from when the vessel was floating in flat seas (above). In this situation, the stability curve will change Complex Rolling Motions in Beam and Stern Seas
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Deep Water Ship Handling little, showing that the vessel continues to have adequate dynamic capability at all angles of inclination (roll). Now consider what happens when there is a high wave amidships on each hull form.
Figures 31 & 32: Water-planes for vessels having a wave crest amidships
While both vessels experience a reduction in their water-plane area, the fine-form (low block co-efficient) vessel experiences a very significant loss in water-plane area. Consequently, the full-form vessel will suffer only a slight reduction in its stability due to a small decrease in its water-plane area. The fine-form hull, by contrast, will experience a significant effect on its stability corresponding to the large reduction in water-plane area. In some instances, this can lead to the vessel having a temporary negative GM – as demonstrated in the following stability curve.
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Complex Rolling Motions in Beam and Stern Seas
Figure 33: Typical stability curve for an initially unstable vessel
Notice, in particular, three aspects of this curve: 1. The initial GM has become negative – the vessel will want to roll over to begin with. 2. The vessel does have a range of positive dynamic stability, but before that comes into effect the initial tendency is for the vessel to incline until it reaches the angle of loll. 3. The angle of ‘loll’ (the ‘angle of loll’ is the equilibrium state for a ship that is unstable when upright, i.e. has a negative metacentric height and so takes on angle of heel to either port or starboard). Once this situation arises, the vessel WILL begin to roll (either to port or starboard) and will continue to do so, accelerating and gathering momentum as it does so, until the angle of loll is reached, when the positive dynamic righting capability begins to counter the rolling action. Significant research by DNV has shown that parametric rolling may occur in headon seas. However, experience shows that it is more likely to be noticed in following seas where the wave encounter duration is much longer and the consequences of ‘acquiring’ a negative GM, even temporarily, have the time to take effect. Typically, when a vessel begins to experience the effect of ‘surfing’ on a following sea, the effect becomes more noticeable.
Complex Rolling Motions in Beam and Stern Seas
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Deep Water Ship Handling
Figure 34: Reduced water-plane area for a ‘fine-form’ ship pitching downward
The same effect can, to a lesser extent, also be experienced on the upward (bow-up) part of the pitching cycle, however this is much less probable as the vessel will be moving more slowly (not surfing) and will sink deeper into the water as it tries to ‘drive’ over the upward incline. Also, on the bow-up part of the cycle, mariners will note that the vessel has more steering functionality. Therefore, in terms of ‘parametric rolling’ we can begin to identify the most hazardous elements of the pitch-cycle.
Figure 35: Example of a pitching-cycle curve
From Figure 35 it should be noted that following each part of the bow-down ‘hazardous phase’ of the pitching-cycle, the vessel becomes level and that normal stability parameters are re-acquired so that, under most conditions, parametric rolling does not usually have the opportunity to build up into what is known as ‘synchronous parametric rolling’. This possibility is addressed in section 6.3. So far, we have shown that large container ships and vessels of similar hull-form are particularly vulnerable to parametric rolling because their hull shapes typically have a pronounced bow flare, flat transom stern and wall-sided mid-ship section. Thus, 42
Complex Rolling Motions in Beam and Stern Seas
when conditions allow, as the wave crest travels along the hull it produces a constantly changing underwater hull geometry which, in turn, causes a rapid changes in the effective waterplane area. This may reduce the GM, even making it slightly negative, so altering the ship’s ability to right itself after having been inclined. If the GM becomes negative the vessel will roll (either to port or starboard) and try to assume the angle of loll. So what are the effects of parametric rolling? As well as being an unpleasant sensation for the crew, it can result in heavy stresses on the ship’s structure. It can cause extreme stresses on the container securing system and even cause them to fail so that containers are lost. It can generate load variations on the ship’s engine(s). In the most extreme situations it may cause even huge container ships to capsize. If parametric rolling is encountered, it is very important to stay calm and take immediate action. Reducing speed and changing course to alter the wave encounter of a following sea can be effective remedial actions – these and other options are considered in section 7.
Complex Rolling Motions in Beam and Stern Seas
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Deep Water Ship Handling A personal experience of parametric rolling… “The vessel was a medium sized feeder vessel, fully loaded and trading between North Spain, the Bristol Ports and Northern Europe.” “These vessels are high sided so are allowed to load with very low GM – as they are not expected to ship green seas on deck. They tend to have fine lines, a long rolling period and are highly susceptible to the effect of wind.” “In the south-western approaches we encountered severe storm force winds coming from the quarter. As the wind rose, the ship was driven along and we regularly got caught on the down-slope of large swells. When this happened, the steering became sluggish, the wind would catch the quarter and begin to turn the vessel (against my wishes) and as the approaching swell caught up, the vessel would take a sudden ‘sheer’ and heel very heavily – holding it’s angle of heel until the wave had passed and we had a slight ‘lee’ in the trough of the swell.” “Once this had happened a few times, I decided to turn the vessel and ‘heave to’ as it was getting dark. I didn’t want the ship to be going through this during the night – when I couldn’t see what was happening with the sea.” “The turn to ‘heave to’ was quite an experience and had to be timed carefully so that the ship was clear of the swell down-slope, had full steering capability and reserve engine power to drive the ship round before the next swell hit. Even so, in stormy conditions, it was quite a frightening experience – although I wouldn’t admit that at the time, or to my crew… we survived!”
6.3 Synchronous parametric rolling Before leaving the subject of parametric rolling, one further complication must be considered – that of ‘synchronous parametric rolling’. When we considered ‘parametric rolling’, we noted that the most hazardous situation was when the vessel was proceeding on the downward (bow-down) part of the pitchcycle. We also noted that the effect would disappear when the vessel reached the bottom of the cycle – when it became level and was fully supported (buoyed) fore, aft and amidships by the sea. This proposition, however, assumes that the pitching cycle is relatively short and does not exceed more than one to one-and-a-half times the roll period of the vessel – so that the parametric rolling will always be quickly interrupted when the vessel becomes level and full stability is re-acquired. But why is that so? If we assume, for a moment, that the pitch and roll periods are similar, then the hazardous parametric roll element, at worst, could only occur during the bow-down
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Complex Rolling Motions in Beam and Stern Seas
part of the pitch-cycle, that is, a negative stability event might only arise over half of the pitch cycle, equivalent to half of the roll cycle. Firstly, consider a situation where the pitch and roll periods are similar, but they are out of phase:
Figure 36: Pitch and Roll period similar, but cycles out of phase
From Figure 36, where the roll amplitude is maximum (so that maximum dynamic stability is required to counter the rolling), in each case the vessel pitch attitude is zero. That is, the vessel is level in the water so that the dynamic stability is maximum and the full righting capability can be exerted. Also note that when the vessel’s pitch attitude is bow-down, with the greatest risk of dynamical stability reduction due to lost water-plane area, the roll attitude of the vessel is zero, or close to zero. Thus, the vessel risk from the effects of parametric rolling is significantly lessened. Now consider what happens with exactly the same pitch/roll values, but with them ‘in phase’ – that is, maximum stability loss due to reduced water-plane area during the pitching bow-down phase is experienced at the same time as maximum rolling effect.
Figure 37: Pitch and Roll period similar, but cycles in phase
In Figure 37, maximum roll (to port in this case, but it could equally be to starboard) occurs when the vessel is pitching bow-down and when it is most likely to be encountering a loss of stability due to a reduction in water-plane area. Complex Rolling Motions in Beam and Stern Seas
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Deep Water Ship Handling At the very moment the vessel needs all its dynamic righting capability, it suffers the greatest reduction and may even acquire a temporarily negative GM. This is a very hazardous situation – indicated by the black arrows. Note again, that when the vessel becomes level, maximum dynamic stability is reacquired and the parametric roll component will cease. Incidentally, the two graphs in Figures 36 and 37 explain why ‘breaking phase matching’ is critical when trying to avoid the dangers of parametric rolling or any other synchronous effect. Taking the parametric roll situation to its ultimate conclusion, we now consider what happens if the pitch-cycle duration lengthens so that the bow-down period increases and reaches a time interval equal to, or even exceeding, the normal roll-period of the ship (port to starboard and back again or vice-versa).
Figure 38 – Synchronous parametric rolling
For this situation to arise, the total pitch-cycle interval would need to be at least twice that of the vessels natural roll-cycle period, assuming that equal duration is spent on both the up and down phases of the pitch-cycle. Consequently, within the hazardous bow-down phase of the pitch-cycle, the vessel will be able to complete at least one full roll-cycle. Having lost its initial dynamic capability, there is nothing to counter the roll as it begins. In fact, the negative dynamic capability will actually make the situation worse as it implements an ‘inverting’ tendency until the ‘angle of loll’ is reached. Only then will the countering dynamic capability begin to take effect. However, the vessel will experience exactly the same situation as it rolls the other way and as a result, the amplitude of the next roll will be much greater. This tendency will continue until the pitching vessel becomes level and full dynamic capability is restored.
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Complex Rolling Motions in Beam and Stern Seas
7. AWARENESS AND ACTION Therefore, if the pitch-cycle exceeds twice the natural roll period of the vessel, the situation will become worse with each successive roll. In vessels with a small GM, the acceleration experienced on rolling may be so great, and the dynamic righting capability so small, that the rolling cannot be stopped. In such situations, the vessel may keep rolling and fully invert (capsize). In such extreme situations a ship can lose all its damping tendencies in an instant and, if also surfing, may not be able to counteract the roll, pitch and yaw, so that it becomes subject to the inverting action of the next waves. When this happens, the ship has lost navigational/directional control and may broach-to at the same time. This loss of stability, simultaneously with a loss of engine and rudder control, can cause the vessel to capsize with significant casualties.
Awareness and Action
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Deep Water Ship Handling Understanding and predicting the behaviour of a vessel in deep water is a complex matter, but a skill that can be mastered through time, and with experience. The weather, the sea state, loading condition, the ship’s speed, engine and rudder configurations, plus the planned course and vessel schedule, all have a bearing on how the ship should be handled. Every crew member can develop their skills in observing the sea state, understanding the motion of their vessel and anticipating how it might develop. Seafarers need to know how to manage and balance the variables which affect the ship’s motion and performance. By understanding what is happening to the ship, they learn to trust their own instincts and voice their concerns to the right person, at the appropriate time.
7.1 Collating information, anticipating problems and being prepared Good planning before the vessel sets sail can reduce at least some of the natural risks the vessel might encounter. The voyage should be planned to avoid, as far as possible, areas where the vessel is likely to encounter weather and sea conditions which are beyond what is acceptable in terms of its load, design and size. A prudent voyage plan, with the exercise of good navigation and seamanship, can help to reduce the risk of the ship’s motions becoming excessive or dangerous. Enroute, all possible information about the weather should be taken into account when making decisions about speed and course. There are also a number of measures which can either help avoid any problems, or help the crew react to them more quickly. It is advisable, for example, to put the engines on standby, so that the ship is ready for a change of speed or course if necessary. Battening down all doors, hatches, vents, deadlights and so on is a small but vital step that will help to maintain the watertight integrity of the vessel. Once this is done, appropriate attention can be given to navigating the vessel in a weather sensitive manner that will ultimately prove beneficial to the commercial interests of all. However, just because everything is secure to begin with, doesn’t mean that it will remain that way – so good seamanship demands that regular checks should also be carried out. Every member of the crew needs to know when heavy weather is expected and needs to be clear about their responsibilities in relation to handling the ship safely and effectively.
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A personal experience of the importance of good communication between Master and crew… “I was a 3rd officer of a VLCC (Supertanker) which was transiting the Atlantic in hurricane conditions. The main concerns were the stresses on the hull and possible damage to the ship.” “The Master and the bridge team were all briefed and the team was appraising the Master of the ongoing situation. A laden ship is not able to out run a hurricane, so we had to do everything we could to stay out of its possible path. We did 4 hourly positioning of the hurricane to plot its course and possible rebound, and adjusted the tanker’s speed and courses accordingly. By gathering information and acting on it in a timely fashion, we avoided putting the ship in a situation where it would encounter potentially damaging stresses on the hull.”
7.2 Indicators that something is happening Situational awareness is the ability to identify, process, and comprehend the critical information about what is happening on and around the vessel – in other words, noticing what is going on around you. A vigilant officer of the watch constantly observes and gathers information (“what can I see?”), interprets what they are seeing (“what does that mean?”) and thinks ahead to situations which might occur as a result (“what could happen next?”). It is the responsibility of every crew member to develop good situational awareness habits. As heavy weather develops, every crew member should be alert to any significant changes in the sound of the ship’s engine or engines or sluggish steering or manoeuvring. They should monitor the ship’s period of roll and its relationship to the pitching interval; they should note the wave length and height, how these relate to the ship’s current movement and how that movement is changing over time. They should learn about synchronous and parametric rolling effects and be sensitive to their possible onset. If not sure, they should be prudent in taking advice from more experienced shipmates, who may have practice in assessing all the factors and determining the risk of synchronous or parametric rolling. Situational awareness means listening for any sound or sensation of something impacting on the ship’s hull, noting if green seas are being shipped and the impact this has on the motion of the ship. It means noticing if the ship is reacting uncharacteristically to wind, waves and swell, or if there is unusual or excessive ‘flexing’ of the ship. They should be aware of factors which might affect a ship’s stability and be prepared to brief the Master on their observations. They should, at all times, put the safety of the ship, crew and cargo before all other considerations. On larger ships, it can be more difficult to detect the signs that something unusual is happening. For example, the nearer the vessel’s accommodation to its centre of Awareness and Action
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Deep Water Ship Handling rotation, the more difficult it is to detect any change in motion that might indicate the onset of change, particularly if those effects only affect the ship in small ways. The distance between the bridge and the bow and the amount of cargo on deck can also affect a crew member’s ability to spot developing situations. It is imperative to be extra vigilant in heavy weather. A prudent officer will go outside onto to the bridge wings or the deck to observe the situation regularly. This is particularly important if the bridge is fully enclosed – it is difficult to fully appreciate what is going on around the vessel if you never step outside. Additionally, in heavy weather it is wise to make the rounds of the ship after your watch period is over, provided it is safe to do so, and report to the bridge afterwards.
7.3 Possible actions to counter risks to stability If an officer of the watch detects something amiss, they should remain calm while they decide what action to take. If there is time to call the Master to the bridge, they should do so. If there is a company policy on this, they should follow it. The two major factors over which the ship has control are speed and heading – although heading is easier to change quickly, especially if the engines are ‘locked down’. These, then, are the first two tactical options to consider if action is needed. Some officers may be apprehensive about the personal consequences for them if they either change the vessel’s course or speed without first checking with the Master, or if they disturb him when he is not on duty. However, all seafarers should understand that failing to act when a problem arises could put both ship and crew at risk. The risk of upsetting the Master by calling him to the bridge is less important than risking the lives of everyone on board by doing nothing. It is also worth noting that more experienced Masters report that they have more confidence in officers who have demonstrated a willingness to call them to the bridge whenever they are uncertain or have concerns. Synchronous rolling and parametric rolling occur when the ship is encountering waves at fairly specific intervals, so changing the ship’s heading, reducing speed, or indeed both, will alter that interval, and minimise the risk. Acting decisively buys valuable thinking and planning time for Master and crew. Timing is critical – particularly on smaller ships, where everything happens much faster than on large ships. This is especially true of parametric rolling where a serious threat to the ship can develop within a few roll cycles. In this case, swift action is essential. If the ships heading is changed because of wave interval problems, it may then be possible to bring the ship back onto the original heading very slowly. However, in some circumstances, it may be better to reduce any roll by heading into the sea and avoiding beam or quartering seas.
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Awareness and Action
8. SUMMARY The first part of this workbook was a reminder to all seafarers of what they learned in college about ship stability. Although it is not necessary for most seafarers to understand the complex physics of a ship’s stability and movement in water, it is essential to remember the basic principles and apply them to observations made in real life. It is important to understand the three axes of motion and the movements associated with them; the force changes associated with loading and discharging of cargo or ballast, the effect of gravity (downward) and buoyancy (upward) acting in opposition to each other. It is also useful to remember that all vessels have a natural rolling period, determined by their metacentric height – a ship with a small GM being ‘tender’ and a ship with a large GM being ‘stiff’. An excessively small GM increases the risk of high amplitude rolling and the potential for capsizing in rough weather as the ‘righting lever’ may not be enough to return the vessel upright after a roll. But a large GM can make the roll period too sharp and quick, which is uncomfortable for passengers and crew and can dislodge cargo, causing it to break free and move. Good passage planning includes looking ahead at the weather before setting sail. This simple precaution can help vessels to avoid many problems. The safety of ship, crew, passengers and cargo must be treated as paramount, regardless of commercial pressures. Good seamanship means looking ahead, planning, anticipating and continually re-evaluating decisions about course and speed in the light of developments in the weather and observations of the behaviour of the ship. The section on head seas and their effect on ship motion, handling, structural stresses and stability looked at pitching (motion round the transverse axis) and slamming (the stress on the hull as it comes back into the water). Slamming, in its various forms, can put considerable stress on the structure of a ship and can cause significant damage. Violent slamming loads in severe wave conditions can make the hull vibrate either transiently (‘whipping’) or continuously (‘springing’). Effects such as ‘hogging’ (when the vessel bends upwards in the middle) and ‘sagging’ (bending downwards in the middle) can also put severe stress on a hull. Under the influence of long swells or heavy seas, it is possible for a vessel to hog and sag in a continuously repeating cycle. This may be particularly noticeable on larger vessels and may stress the hull longitudinally. Shipping green seas can also seriously affect the stability of a heavily pitching (or rolling) ship. As well as causing damage to small deck components (pipes, ventilator openings, etc.), shipping green seas adds weight to the ship, moving its centre of gravity (G) towards the added weight, and towards the metacentre (M), thus reducing
Summary
51
Deep Water Ship Handling stability. As the water on deck (or in a ship’s compartment) moves around, we have demonstrated how it may cause a further slight reduction in stability known as the ‘free surface effect’. Free surface effect reduces the ship’s effective dynamic stability, causing the rolling period to be slightly longer and making it harder for the ship to return to an upright position when heeled over. In heavy weather, a ship can pitch so violently that its propellers come close to the surface, and even completely out of the water. This can cause the engine to race or put it under extra stress. Where such risk exists, it is advisable to reduce engine speed in heavy seas to reduce the risk to the engine. In beam seas, the focus of concern is often the rolling motion of the ship. As long as its damping tendency is equal to the force generated by each wave, it can stabilise itself (described as ‘ordinary damped rolling’). We have explained what happens if the deck edge becomes immersed, how water on deck adds weight to the ship and reduces stability. We have considered what happens when this action effectively reduces the beam of the ship and how that influences the ship’s ability to right itself. In stern seas, we considered the combination of effects (including ineffective propeller thrust and loss of direction keeping ability) producing the hazard known as ‘broachingto’. Broaching-to tends to happen when waves come from behind and are moving in a direction which forms a small angle with the longitudinal axis of the ship. Typically, it is characterised by a sudden and marked yaw which peaks within a single wave length. This may be accompanied by large amplitude rolling and, in extreme cases, dramatic loss of stability – which has been known to result in capsizing. The next section looked at complex rolling motions – synchronous rolling (when the interval between waves is approximately equal to the natural roll period of the vessel and they operate ‘in phase’). Parametric rolling is a wave induced instability rolling action that arises from unfavourably combining pitch and roll motions. Parametric instability is caused when the vessel experiences variations in buoyancy along its length as a result of the waterplane area altering significantly in passing waves. During recorded incidences of synchronous parametric rolling, machinery has been noted to fail, cargo has been lost, structural damage has been recorded… and even large vessels have been known to capsize. Synchronous parametric rolling occurs when parametric rolling synchronises within an extended pitching cycle and the effect builds up. It results from a combination of environmental, operational and design factors. It tends to occur where: the waves are high; the interval between passing waves (usually from astern) is not less than 52
Summary
twice the natural roll period of the vessel; and the pitching period is about twice the rolling period. This produces long, slow and very high amplitude rolls resulting from a sudden loss of stability. The action can also cause deck or hold cargo to shift, which makes the rolling worse as this also affects the stability of the ship. We then looked at what actions seafarers can take to anticipate, avoid and reduce all these effects. Good situational awareness is crucial – being aware of what is happening, what it means to the ship, its personnel and its cargo, and what might happen next. It is also essential to take action quickly and decisively if a problem is seen to be developing, whether that is changing course, reducing speed or calling the Master. Collating information ahead of time, anticipating problems and being prepared can all help reduce the risk. In heavy weather, it is advisable to have the engine on standby, so that the ship is ready to change speed if necessary. Battening doors, hatches, vents, deadlights, etc. is important in order to keep the ship watertight. Every member of the crew needs to know if heavy weather is expected and be clear about their role and responsibilities.
Summary
53
Deep Water Ship Handling
9. GLOSSARY Broaching-to – Sudden, unintentional change of direction in heading of vessel caused by wave action. Centre of buoyancy (B) – B is the point at which the buoyant upward forces on a ship are perfectly balanced on every side. Centre of gravity (G) – G is the point at which the downward forces acting on a ship (i.e. the combined weights of every part of it), are perfectly balanced on every side. Deck edge immersion – When the ship heels to an angle that allows the deck edge to submerge under the water – causing an effect on stability. Free surface effect – The reduction in stability of a vessel caused by liquids moving about freely in a tank or hold, or on deck. Hogging – When the buoyancy force, in the middle of a ship, is greater than the force of its weight, bending it upwards in the middle. Metacentre (M) – If an external force acts on a ship to tip it from the upright, the force that acts to return it to upright appears to ‘pivot’ from the metacentre (‘M’). It can also be described as the intersection of two vertical lines, one going through the centre of buoyancy when the ship is stable and the other through the new centre of buoyancy when the ship is inclined to one side. Metacentric height (GM) – The distance between the centre of gravity (G) and its metacentre (M). Ordinary damped rolling – The effect of consecutively reducing rolling motions that arise after a ship is caused to roll and the cause is removed. Results from the way a ship stabilises itself once the righting tendency of the ship becomes equal to the force generated by a wave. Parametric rolling – Large instability induced rolling motion which usually occurs in conditions where the waves are high, the interval between waves is approximately twice that of the natural roll period of the vessel and, in addition, the pitch period is about twice that of the roll period. Pitching – Rotational motion around the transverse axis of a ship. The sea lifts the ship at the bow and lowers it at the stern, and vice versa, with gravity forcing the ship down (sometimes at speed) every time it is lifted. Pooping of green seas – When the vessel ships green seas over the stern. Racking – When the ship is distorted laterally in the vertical plane. 54
Glossary
Sagging – When the excess weight is around the mid-ships area, bending it upwards at each end. Shipping green seas – When sea water (as opposed to foam or spray) comes onto the deck or bow of a ship. Slamming – The stress on the hull as it comes back into the water under the force of gravitational acceleration. Stiff – A ship with a large metacentric height (GM) is said to be ‘stiff’ – that is, resistant to upsetting by external forces. The roll period of a ‘stiff’ ship is short and quick which can be uncomfortable for passengers and crew. Surfing effect – The effect of a vessel surfing on the leading edge of an advancing wave. This will affect the water flow around rudder and propeller, potentially reducing their effectiveness and may result in the engine racing. Synchronous rolling – Amplified rolling motion that occurs when the interval between waves is approximately equal to that of the natural roll period of the vessel and the two operate in phase to produce consecutively higher amplitude rolling. Tender – A ship with a small GM will be ‘tender’ – that is, it will have a long roll period.
Glossary
55
Deep Water Ship Handling
10. FURTHER RESOURCES Regulations STCW Ref: Table A-II/1, A-II/2 and A-II/3 Publications NP 100 – The Mariner’s Handbook MCA Stability Information Booklet ABS Pub133, Assessment of Parametric Roll Resonance in the Design of Carrier Containers ‘Parametric Roll Resonance of a Large Passenger Ship in Dead Ship Condition in All Heading Angles’ by Abdul Munif, Yoshiho Ikeda, Tomo Fujiwara and Toru Katayama (Contemporary Ideas on Ship Stability and Capsizing in Waves, Fluid Mechanics and Its Applications, Volume 97, 2011, pp 331-345) Ship Stability: Notes and Examples, Third Edition, Kemp & Young Ship Stability for Masters and Mates, Seventh Edition, Barrass and Derrett Basic Shiphandling for Masters, Mates and Pilots, P.F. Willerton Ship Stability Mates/Masters, Witherby Seamanship International Nautical Institute Publications Ship Dynamics for Mariners Numerical Weather Prediction The Evasion of Tropical Cyclones Relevant UK Marine Guidance Notices (MGNs) MGN 351 (M) Leadership Qualities for Effective Safety Management MGN 335 (M) Bulk Carriers – Guidelines on Early Assessment of Hull Damage and Need for Abandonment MGN 328 (M) Research Project 502: High-Speed Craft Dynamic Stability in Following & Quartering Seas – Operational Guidance MGN 315 (M) Keeping a Safe Navigational Watch on Merchant Vessels MGN 210 (M) Advice on the Dangers of Flooding of Forward Compartments MGN 108 (M) Hull Stress Monitoring Systems Related Videotel Programmes Ship Handling in Following Seas (636) Ship Handling in Head Seas (661) Ship Handling Series (95, 129, 321) Wind, Waves and Storms: Part 1- Understanding Weather Systems (738) 56
Further Resources
11. ASSESSMENT QUESTIONS 1. What is the metacentre? a) The intersection of two vertical lines, one going through the centre of buoyancy when the ship is stable and the other through the new centre of buoyancy when the ship is inclined to one side. b) The point at which the upward forces on the ship are balanced on every side. c) The point at which the downward forces on the ship are balanced on every side. d) The mid-point of the natural period of roll of any vessel. 2. What is the centre of buoyancy? a) The point of the ship which exerts the greatest force onto the surface area of the water. b) The point at which the upward forces on a ship are perfectly balanced on every side. c) The point at which the downward forces on a ship are perfectly balanced on every side. 3. A larger metacentric height implies greater initial stability against overturning than a smaller metacentric height. TRUE or FALSE 4. What is the ‘free surface effect’? a) The stress on the hull as the ship comes back into the water after pitching. b) When sea water (as opposed to spray or foam) comes onto the deck of a ship. c) The reduction in stability of a vessel caused by liquids moving about freely in a tank or hold, or on deck. d) The total stress on the combined surface area of a ship. 5. A ship which is ‘stiff’ is more comfortable for passengers and crew and safer for cargo. TRUE or FALSE 6. Good situational awareness from the officer of the watch can play a major part in preventing a ship from becoming unstable in heavy weather. TRUE or FALSE
Assessment Questions
57
Deep Water Ship Handling 7. What is ‘hogging’? a) When the motion of the ship is like that of a pig rolling in mud. b) The upward bend observed along the ship’s length that can occur when buoyancy forces amidships exceed those at either end. c) When the buoyancy forces at each end of the ship are greater than its weight. 8. Spray or foam on deck has an equal effect on stability to shipping green seas. TRUE or FALSE 9. If the ship is pitching so heavily that the propellers come out of the water, the engine or engines can… a) Show the effects of water damage. b) Use exactly three times the usual amount of fuel. c) Start racing, putting it/them under extra stress. 10. Reducing speed lowers the likelihood of shipping green seas and so reduces the risk of structural damage, particularly at the fore end. TRUE or FALSE 11. Ordinary damped rolling can be described as: a) The motion of the ship when the wave encounter interval is equal to the ship’s natural roll cycle and they are in phase. b) The way a ship naturally reduces its rolling amplitude over time as it tries to regain equilibrium after being disturbed by a wave. c) A rolling motion which frequently results in a vessel capsizing. 12. An ideal ship strikes a balance between being ‘stiff’ and being ‘tender’. A stiff vessel: d) Responds quickly to a wave in attempting to right itself. e) Is easier to overturn but has a longer roll period. f)
58
Has a righting lever which is twice the value of GM.
Assessment Questions
13. Broaching-to is an unintentional change in the heading angle of a vessel, caused by wave action alone. Broaching tends to happen when: a) In a head sea, a vessel pitches then slams forcefully after the wave has passed. b) Waves come from behind or the quarter and are moving in a direction which forms a small angle with the longitudinal axis of the ship. c) In a beam sea, two waves roll the vessel in quick succession. 14. Synchronous rolling can occur when the interval between waves is approximately equal to that of the natural roll period of the vessel. TRUE or FALSE 15. The term ‘parametric roll’ is used to describe: a) A roll on a ship’s longitudinal axis which results in the ship capsizing. b) The motion of a ship which occurs when the wave length is twice the length of the ship. c) The phenomenon of a large rolling motion occurring in stern seas and made worse by the instability that occurs when a ship spends an extended period on the crest of a wave. 16. Parametric rolling builds slowly, giving plenty of time for Master and crew to assess the risks. TRUE or FALSE 17. Parametric rolling and broaching-to are most likely to be encountered by small vessels – they rarely affect large container ships. TRUE or FALSE 18. An officer on watch in heavy weather notices that the ship is reacting uncharacteristically to the waves and swell. Then he detects in the combination of pitch and roll the start of a long rolling motion. The Master is off duty and the officer is reluctant to call him. What is the correct action to take? a) Wait until he is sure that something is wrong, then call the Master. b) Take corrective action and call the Master. c) Observe the ship carefully and think about reducing speed.
Assessment Questions
59
Deep Water Ship Handling
12. ASSESSMENT ANSWERS Question
Correct Answer
Found in Section
1
a
2.4, Glossary
2
b
2.3, Glossary
3
True
2.5
4
c
2.5, Glossary
5
False
2.5, Glossary
6
True
7.2
7
b
3.2, Summary, Glossary
8
False
3.3
9
c
3.4
10
True
3.4
11
b
4.1
12
a
2.5
13
b
5.3
14
True
6.1, Glossary
15
c
6.2, Glossary
16
False
6.2
17
False
6.2
18
b
7.3
60
Assessment Answers
NOTES
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Deep Water Ship Handling
NOTES
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