Hydraulic motion system tames rough seas

Image courtesy of Barge Master bv.

Parveen Gupta • Director – Sales, Production Machinery, Bosch Rexroth

Safely moving heavy loads from ship to sea floor is a basic requirement in the offshore industry. Undersea workof this type is common in the oil and gas industry. It typically includestasks such as pipeline construction and repair, placement of oil andgas well heads, installing subassemblies on drilling rigs, as well asmaintenance and repair activities. Similar operations entail building andmaintaining offshore wind turbines, underwater salvage, geotechnicalsurveys, and dredging.

These applications frequently involve large and expensive parts and require a high level of control, particularly when being placed with or linked to other components on the seabed. A major risk, in heavy seas, is the load acts as a destructive hammer to itself or nearby structures.

Such activities would be exceptionally hazardous to people, equipment and the environment if it were impossible to compensate for ship movements caused by wave action. Thus, there is a growing need for advanced heave compensation systems because of increasing activities on and below the sea.

Active heave compensation systems permit safe movement of personnel and equipment even when a ship is buffeted by large waves.

Active heave compensation

The systems compensate for the movement of a ship as a result of wave action. Rexroth has decades of experience in developing, commissioning and maintaining active heave compensation (AHC) systems. Engineering is based on a large portfolio of proven components, control systems and software. Designs range from a single 5 metric ton winch to some of the world’s largest working vessels that lift loads of nearly 50,000 metric tons.

AHC systems rely on actively controlled devices which can operate in various modes depending on the required characteristics. They maintain the constant vertical position of a free hanging load or a constant tension to a supported or fixed load.

Three variants have been developed: linear AHC (LAHC), rotating or secondary regulated AHC (RAHC) and primary regulated AHC (PAHC). All work by measuring vessel movement by means of Motion Reference Unit (MRU) acceleration sensors, and relaying that data to the main controller. Based on this data, the control software calculates the necessary counter motion and directs the hydraulic system to make adjustments in real time. Depending on the type of AHC, that can involve a hydraulic motor connected directly to a winch (RAHC), a variable pump (PAHC), a proportional valve (LAHC), or even an electric motor.

These components power the winch so the load hangs idle relative to the fixed world (such as a drilling rig or seabed). When the ship moves upwards, the winch pays out and when it comes down again, the winch hauls in. This makes it possible to position loads accurately and lets ship personnel work safely even in rough weather. By compensating for more than 90% of a ship’s movements, it’s possible to realize unparalleled precision operations at virtually any depth.

AHC systems rely on actively controlled hydraulics that sense motion and maintain a constant vertical position of a free hanging load or a constant tension to a supported or fixed load.

Primary AHC

The latest development in AHC systems is primary active heave compensation. PAHC systems are based on standard Rexroth hydraulic pumps and motors. This involves regulating speed and direction of the winch by varying the swivel angle of an axial-piston pump and thus regulating the volume flow rate. The MRU sends data to a digital controller, such as Bosch Rexroth’s type HNC 100, which calculates the set point and, in real time, controls the hydraulic variable pump that is connected to a fixed motor. To change the winch’s rotation direction, the hydraulic pump swivels over center and works as a motor. Thus, the PAHC is a closed-loop hydraulic system.

The main advantages of this primary system are lower investment costs and compact construction. Primary rotary active heave systems integrate into the winch drive system and require minimal space on deck. And Rexroth can adapt the software to meet customer-specific requirements.

Another characteristic feature of PAHC is that this type of heave compensation is suitable for recovering energy. Among other things, the energy released by easing the load (when the ship moves upwards) can be stored in a hydraulic accumulator. This energy becomes available again when the ship comes down and the load must be taken up. This energyefficient system uses a modular kit with safety components, a brake and the required accumulators.

Linear AHC measures heave motion where the winch rope leaves the vessel, then compensates for it with a special cylinder. A sheave assembly attached to the cylinder moves cable back and forth, counteracting ship movements and maintains nearly steady position at the lift point.

Secondary rotary systems

Rexroth has also developed a rotating system that uses energy-efficient secondary control. It is based on use of a secondary regulated motor connected directly to the winch. In the RAHC system, the winch is fitted with adjustable hydraulic axial-piston motors, the size of which is determined by the required winch capacity. The speed and torque capacity of the motors can be adjusted to vary the speed and direction of the winch.

In this configuration, the primary load and wave-motion oscillations are managed and compensated by a secondary control system. During operation, the cable load and ship movements are constantly monitored with fast and accurate sensors, which serve as the basis for dynamic control of the secondary motors, and active control of the load in real time.

The design offers an additional advantage of using an integrated energy recovery system based on hydraulic accumulators. The principle is much like in the PAHC: during upward movement of the vessel the drive unwinds the winch cable and the hydraulic motor acts as a pump, converting motion energy into hydraulic pressure that is stored in the accumulators. In the subsequent downward movement, the drive works like a motor, reusing the stored energy from the accumulator.

Thus, the AHC system can operate with far less installed power capacity versus a system without energy regeneration. Energy savings of up to 65% are reported. Other advantages include smaller footprint for the hydraulic power unit and tanks, as well as lower ship fuel costs and reduced exhaust emissions. More than one hundred RAHC systems from Rexroth are in service in cranes and winches, safely positioning loads of hundreds of tons.

The company also offers electric rotary AHC systems that work similarly to RAHC, in that they also recover, store and reuse energy. The hardware controls are identical to the hydraulic variants, simplifying operation, diagnosis and spare parts management. The portfolio of acservomotor drives ranges from 11 kW to 4 MW of power, and the electromechanical drives are adapted to the special requirements of the maritime and offshore industries.

Bosch Rexroth designed an AHC system that lets SBM’s Normand Progress diving-support vessel position sensitive equipment at depths to 3,000 meters.

Linear compensation

LAHC systems are suited for standard winch-driven cranes and hoists. The system fits with nearly every existing winch system as an add-on, and it consumes considerably less energy than other activeheave systems.

In LAHC, the MRU measures heave motion where the winch rope leaves the vessel, then compensates for it with a special cylinder that combines both active and passive compensation capabilities. (A passive heave compensation unit acts as a springlike device and typically consists of a hydraulic cylinder and a gas accumulator.) The passive part accounts for the average load while the active part compensates for the wave-induced load variations.

The cylinder stroke determines the length of the winch cable and, thus, the load position. This cylinder pays out wire rope when the vessel lifts up, and retracts the wire when the vessel lowers. The control system uses the MRU output to calculate the required AHC cylinder movement and follow the desired movement as accurately as possible. A sheave assembly attached to the AHC cylinder moves the cable back and forth, counteracting the ship movements. This results in nearly steady position of the lift point (at the location of the overboard sheave) and thus maintains a near-steady load with respect to the fixed world.

Based on closed-loop force control, LAHC requires only a simple mechanical interface for the vessel structure. The LAHC is designed to be modular, making it easy to integrate into nearly every vessel arrangement. It consists of four main modules: the cylinder-based integrated cable-actuation system, hydraulic power supply, high-pressure air system, and an integrated control system, including the MRU.

The next-generation gangway developed by Barge Master and Bosch Rexroth relies on active motion-control technology to compensate for complex linear and rotary motions along several axes.

Mastering multiple degrees of freedom

In recent years Rexroth has been a pioneer in developing motion-compensation systems that handle six degrees of freedom. First used in simulators for aviator training, the company revised this concept to fit maritime and offshore applications. It has already been applied to helicopter decks on ships and to cranes on flat-top barges and supply vessels.

For example, one motion-compensating platform for crane applications was developed in close collaboration with marine company Barge Master, Rotterdam, The Netherlands. To prevent the crane-carrying platform from moving, the dominant three degrees of freedom — heave, roll and pitch — are compensated, while the other degrees of freedom — sway, surge and yaw — are restrained. Motion is compensated by a trio of hydraulic actuators.

The first platform was installed on a standard flat-top barge, carrying a standard crawler crane with a maximum lifting capacity of 400 to 600 tons. It’s an economical, selfsufficient system that is ready to be installed with its own controls, software and power supply.

Bosch Rexroth | boschrexroth.com