Active heave compensation uses proven technologies and components for safe positioning during complex offshore activities.
Contributed by 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 work of this type is common in the oil and gas industry. It typically includes tasks such as pipeline construction and repair, placement of oil and gas wellheads, installing subassemblies on drilling rigs, as well as maintenance and repair activities. Similar operations entail building and maintaining offshore wind turbines, underwater salvage, geotechnical surveys, 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
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.
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 energy-efficient system uses a modular kit with safety components, a brake and the required accumulators.
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 ac-servomotor 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.
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 active-heave 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 spring-like 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.
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, self-sufficient system that is ready to be installed with its own controls, software and power supply.
Barge Master and Bosch Rexroth have teamed up to engineer and produce a next-generation gangway. The system is designed for the safe and efficient transfer of personnel and cargo from ships to drilling and offshore production platforms, wind turbines and other ships.
Barge Master specializes in the development and fabrication of motion-compensating systems for use on ships while offshore. The technique counteracts ships’ movements so that ship-mounted cranes and multipurpose platforms remain stable and cargo can be transferred and positioned without problems, even in rough seas.
For the safe transfer of personnel from a ship to wind turbines or production and drilling platforms at sea, the company developed the Barge Master Gangway in close collaboration with Bosch Rexroth. The design of the gangway has been assessed and approved by global quality assurance and risk management company DNV GL (DNVGL-ST-0358).
According to Ron van den Oetelaar, managing director of Bosch Rexroth’s Benelux unit, the development of the Gangway presented an engineering challenge. “Due to the simultaneous linear and rotary motions along several axes, which are moreover accompanied by large forces, the active motion-control technology applied in the system is extremely complex,” he said.
The gangway mounts on a small diameter pedestal with a hinge and a luffing cylinder to compensate the roll motions of the vessel. Hydraulic motors let the gangway translate and rotate (surge and yaw) in the horizontal plane around the pedestal. Telescopic sections with winches in the bridge ensure smooth translation in sway direction. Extremely fast sensors and controls for the Barge Master Gangway translate the ship’s movements into counter-movements for the gangway. The system can compensate for vessel motions despite wave heights of more than three meters. In addition to the gangway system, customers can add modules such as a height-adjustable pedestal with an integrated elevator. This option ensures stepless trans-shipment of pallet trolleys between vessel and platform.
The development, fabrication and timely delivery of the system involved close collaboration with engineers from Bosch Rexroth, says Martijn Koppert, CEO of Barge Master. Although some parts were custom built, most were standard components, he said. “The fact that Bosch Rexroth has an extremely wide product range helps enormously in this respect.” The result was lower costs and faster delivery in addition to a small footprint, light weight and modular design.
150 ton shipboard AHC
A 150-ton active heave compensation system from Bosch Rexroth enables an SBM diving-support vessel to position sensitive equipment at depths down to 3,000 m. The AHC system compensates for heave caused by ocean swell, and allows loads to be placed on the seabed gently and under full control. At the same time, it protects the hoisting cable from potential breakage due to internal resonance caused by wave motion. It handles wave amplitudes of up to ±3 m and compensates for load movement by more than 90%.
The linear active heave compensation system operates continuously based on measurements made by the Motion Reference Unit that track the vertical movements of the ship. An advanced controller processes this data and uses it to control a hydraulic cylinder. The cylinder extends or retracts to tighten or relax the hoisting cable, such that the load is suspended nearly motionless in relation to the seabed.
The system mounts below deck and includes a specially-designed compensation cylinder, which combines both a pneumatic and hydraulic cylinder. The pneumatic compartment bears and compensates for the static load, while the hydraulic compartment provides the dynamic forces to almost entirely compensate for movements of the load. Thanks to this unique combination, the required hydraulic capacity is only a fraction of the theoretical peak capacity that would otherwise be required from the winch if no heave compensation was installed. This results in considerable energy savings.
To ensure reliable operation under harsh conditions at sea, the hydraulic cylinders are treated with Enduroq 2200 surface technology, an extremely wear-resistant and corrosion-resistant cylinder rod finish. In addition, essential system components are dual or multiple-redundant. For example, the Bosch Rexroth CIMS measuring system integrated into the cylinder is equipped with three sensors.
From design to commissioning, Bosch Rexroth developed the entire system for SBM and supplied the required components. These include the drive section including the hydraulic power unit and the hydraulic cylinder, the air supply unit, all pipework, plus the controls and software, control cabinets and operator panels.
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