The Mechanics of Ship Explosions

The Mechanics of Ship Explosions

By Rik van Hemmen, President & Senior Partner, Martin & Ottaway

Picture 1

Explosions tend to be among the more spectacular events that involve salvors. While oftentimes the explosion has occurred before the salvor arrives, that is not always the case, and we will discuss that a little further in the article.

Fortunately, today, explosions are relatively rare events, but historically that has not always been the case. The invention of gunpowder and shipboard guns led to very impressive gunpowder explosions on armed merchant vessels and naval vessels all through the age of sail. In the early days of steam, boiler explosions were quite common and actually resulted in an extensive package of legislation that tried to reduce the risk of explosions. As a result of shipboard boiler explosions, many countries enacted pressure vessel legislation that required the approval of boilers and safety devices and the licensing of operating engineers, all of which were eventually adopted under the SOLAS (Safety of Life at Sea) Convention.

The development of more powerful explosives resulted in a number of ammunition carrier explosions, with the most spectacular mishap occurring in 1917 in Halifax, Nova Scotia, Canada, in which the SS Mont-Blanc and SS Imo collided, resulting in the ammunition cargo of the SS Mont-Blanc detonating and causing the largest manmade explosion up to that time.

In the 1960s there was a very disconcerting amount of crude oil tanker explosions, which resulted in the international requirement for Inert Gas Systems.

While larger chemical tankers also required inert gas systems, the intermixing of different cargoes in adjacent tanks due to nuisance fracturing, underway repairs, and poor general maintenance practices resulted in a spate of chemical carrier explosions in the 1980s. Improved maintenance approaches and systems maintenance services (SMS) appear to have reduced these types of explosions, but, in general, explosions still occur.

Today, in our experience, most explosions occur due to misunderstandings regarding explosion hazards in ship operations of all types. As such, it is too commonly assumed that heavy fuel oils (HFO) will not result in explosive atmospheres. While heavy fuel oils will not burn easily, the headspaces in heavy fuel oil tanks can often have explosive atmospheres. The actual mechanics are complicated and can be accidental or can be caused by careless blending by fuel oil blenders. Martin & Ottaway has dealt with a number of heavy fuel oil explosions where crew members assumed that HFO did not carry an explosion hazard and used open flame or welding gear around such tanks and deck equipment. (Picture 2)

Picture 2

This can result in a situation where at one moment a tug and tank barge are happily tootling along, but moments later the crew is dead and the barge is drifting around with loss of control and loss of most of the deck. (Picture 3)

Picture 3

When such explosions occur, too often with loss of life, the salvors and forensic engineers are called in.

As a salvor, the evaluation of a vessel after an explosion needs to focus on the vessel’s residual strength and the safety of the salvage personnel. That requires atmospheric evaluations and working practices that prevent open flames. The larger salvage companies are well equipped with spark and explosion proof equipment and are familiar with safe working practices in potentially hazardous atmospheres, and they will know how to start handling the casualty as long as there is no resulting fire.

When there is a resulting fire, follow-up explosions due to boiling liquid expanding vapor explosions (BLEVE’s) are a very serious risk. BLEVE’s are caused by heating of a contained flammable liquid, where the container gets pressurized from the surrounding heat and starts to develop a large amount of pressurized combustible vapors. The increasing pressure will cause the container to rupture and the hot vapor will be ignited by the surrounding fire, often resulting in a very severe explosion. When the risk of BLEVE’s is present, firefighting can be a very difficult task, and sometimes the only recourse is to wait for the BLEVE to actually occur before further salvage activities can take place.

Remarkably, there are many explosions where there is no resulting fire, and, even after a massive cargo tank or bunker tank explosion, the ship may actually still be afloat despite possibly losing her entire main deck. The reason that ships stay afloat after explosions is a function of the way ships are designed and the hydrostatic effect of the water in which a ship floats. If a cargo tank headspace experiences an explosion, the tank evenly pressurizes over the entire tank. However, a vessel’s bottom structure tends to be heavier than its deck structure, and the hydrostatic pressure on the bottom of the vessel will provide a cushion against the internal over-pressurization.

A rope breaks at its weakest point, and a ship also fails at its weakest point. On a tank barge with an over-pressurized tank, the weakest point is the deck. The deck will balloon up from overpressurization and, at a certain stage, will rip or pop the same way a balloon pops. This then allows the deck to fly upwards and fold away. After a cargo tank explosion, the vessel is often still afloat, but it now resembles an open shoe box and has about as much residual strength. For salvors, at that stage, the fun begins, and they get to figure out how to remove the cargo and to bring the ship to a safe berth without the vessel folding up.

While salvors have little interest in what caused an explosion (as long as it does not affect the safety of the salvage operation), the rest of the world wants to know what caused the explosion, and that is where the forensic engineers come into the operation.

Picking through the pieces post explosion

The forensic engineers pick over the debris and try to figure out where the explosion originated and hopefully can identify an ignition source. While explosions are violent events and appear to be instantaneous, they are actually more accurately compared to supersonic wave fronts. As such, the explosion starts somewhere and the resulting flame and pressure front travels through the explosive vapor. That means that a close study of the failure path will lead you back to the initial point of ignition.

This now provides you with enough information to figure out where the explosion started in the picture above.

With respect to the rapidly growing LNG fuel industry, while explosion concerns with LNG are real, the above discussion indicates that as long as natural gas (or even hydrogen) explosions occur in open spaces or are explosions of relatively small amounts of gas, the damage would be limited. However, that in no way means we should let our guard down at any time when handling any material that can emit explosive vapors.

Rik van Hemmen

Rik Van Hemmen is the President and Senior Partner at MARTIN, OTTAWAY, van HEMMEN & DOLAN, INC. Rik’s areas of specialization include forensic engineering, human factors, vehicle design and operations, ship appraisals, ship salvage, structural surveys, naval architecture and project management. (Photo by Madhavan Nayar)