Our equipment can be fitted with Constant Tension and a Passive and/or Active Heave Compensation System to compensate the relative motion between the vessel, its equipment and fixed objects such as platforms or the seabed.
The Huisman Heave Compensation Systems use one or a combination of the systems mentioned below.
Constant Tension (CT) is used to achieve a constant line-pull set by the Operator. The line-pull is measured by a load cell and monitored by the control system. If the actual value differs from the preset value the winch will pay in or pay out wire rope to maintain the preset value. Instant adjustment of the cable tension is possible by changing the tension setpoint value.
Constant Tension can for example be used to pretension a wire rope prior to lifting a load from a supply boat or after landing a load onto the seabed. CT prevents the loads to bounce on the deck or being picked up again. CT can also be used on tugger winches.
Passive Heave Compensation
Using Passive Heave Compensation (PHC) the line pull can be kept constant as well. It does not require an input signal and is therefore also called Passive Constant Tension. The PHC system acts as a gas loaded spring system. It consists of a hydraulic cylinder which is connected via a medium separator to an accumulator which is charged with gas.
Passive Heave Compensation is mostly used in situations where the load is standing or working on the seabed, for example as a drill string compensator. It can however also be used for motion compensation. The motions of a vessel cause drag forces on the load which will decrease or increase the line-pull. As a result, the hydraulic cylinder will move in or out and reduce the motions of the load.
Active Heave Compensation
To control the relative position of a load to a fixed object, Active Heave Compensation (AHC) is used. The position is determined by the control system using the real-time signal of a Motion Reference Unit (MRU) as an input signal. In response to this signal the AHC system will pay in or pay out to keep the load at a constant elevation. Huisman can deliver Active Heave Compensation on hydraulic or electric winches and on hydraulic cylinders.
For hydraulic winches Huisman uses the unique in-house developed AHC system with secondary controlled hydraulics. We have over 20 years of successful experience with this system. In secondary controlled hydraulic systems the pump unit controls a constant system pressure and works independently of the connected motors. The motor torque is controlled by varying the swash plate angle of the hydraulic motor. By changing the swash plate angle an acceleration or deceleration of the winch is achieved. This results in an extreme fast response enabling real-time compensation of the heave movement. With secondary controlled hydraulics it is possible to store energy in an accumulator during AHC. For smaller capacities it is also possible to deliver AHC on electric winches.
Especially for higher capacities hydraulic cylinders are used for Active Heave Compensation. In this case the load is balanced using a Passive Heave Compensation System. By adding active cylinders which are connected to a hydraulic power unit and a MRU, it is possible to control the position of the load. Since the main part of the load is compensated by the PHC system, the active cylinders only have to overcome accelerations, friction forces and a possible unbalance in the passive system and the actual load. Therefore limited power is required for AHC.