Active heave compensation has become a must-have for offshore lifting and landing. But is it the best solution?
Heave compensation systems were introduced to the offshore market in the early 1980s to enable the safe landing and lifting of loads. Since then, heave compensation systems have developed extensively and are considered essential tools for offshore lifting and landing.
However, selecting the right system for the right job is not as simple as it seems and opinions vary over the benefits, or otherwise, of different systems. Equipment builders, operators and clients often have a different perspectives and interests.
OE asked director of operations and offshore at Jumbo Shipping Patrick van Eerten and Huisman chief executive Joop Roodenburg to give their views on the benefits and drawbacks of passive and active heave compensation systems.
There has been a growing interest in AHC systems, as well as combined AHC and PHC systems, with many tenders today specifying AHC systems.
However, very few [people] actually understand these systems. While AHC systems have many advantages, we see several limitations to their use.
Jumbo prefers an engineered-solution, using PHC, based on experience of maritime heavy lifting and ship motion. Lessons learnt have led to the development of calculations to fine-tune the interaction between heavy lifting and ship motion.
The motions are carefully calculated during the engineering-phase of a project, leading to a tailored passive compensator [that is] able to accomplish a desired landing speed at the ideal pressure balance for different circumstances.
Once overboard, the hydrodynamic forces are relatively easy to overcome as drag and added mass can be fairly accurately predicted and total forces can be absorbed or minimized.
For example, landing speed can be achieved at below 0.5m/s with good resistance control, using a far less complex, more robust and safer system than AHC.
PHC is a very straightforward hydraulic cylinder and a gas accumulator. The piston’s load is balanced by nitrogen pressure and the stiffness depends on the nitrogen volume.
As the load is compensated at exactly the same frequency as the motion of the waves, there is hardly any risk of resonance. In an AHC system, resonance is hard to manage and, when it occurs, the damage can be substantial. When using PHC, the load also remains stationary after landing.
The hoist and heave systems are independent, making the system safer. PHC can also be used with a “normal” crane, simply by adding a heave compensation cylinder. PHC also requires no external power, making [possible] the design a failsafe system to reduce wave impact on subsea operations. There are some instances where you have to use AHC, no doubt. But, in general, you do not need it.
More advanced equipment, technology, and offshore lifting procedures can be the right answer, but there is no need for it to be too complex. Preparation of equipment, personnel, and procedures are what are important.
Active and passive heave compensation work with completely different principles.
Passive heave compensation (PHC) acts as a gas-loaded spring system, reducing the dynamic forces on the load. It does not require an input signal and is therefore also called passive constant tension.
PHC systems use a large hydraulic cylinder, which balances the load using nitrogen gas. The cylinder is connected via a medium separator to an accumulator, which is charged with the gas. There is no active control over the position of the load and the movement of the load during lowering depends on external forces, like drag and inertia.
PHC is reliable, simple and doesn’t require power to operate. This is a major advantage because the system will continue to operate even during a power failure. PHC, however, is limited because there is no control over the position of the load and it is less effective on objects with little drag.
Huisman introduced active heave compensation (AHC) on its cranes in 1984. AHC is position-control of a load. The vessel’s motions are measured using a real-time signal from a motion reference unit (MRU), which is used to actively control the position of the load, so that it remains at a selected position.
AHC is generally performed using electronic or hydraulic winches. By paying-out or reeling-in wire, according to the MRU measurements, the position of the load is controlled. The winches have virtually unlimited stroke and are simple to operate.
They do, however, require a lot of power and there is no back-up during power failure. Also wear of the wire rope on the drum (known as drum crushing) is a drawback of this application. However, AHC can also be designed using a cylinder.
Both active and passive heave compensation have their advantages and their limitations. So, to have the best of both worlds, Huisman designed and built a combination of both systems on our offshore cranes.
The system comprises of a large hydraulic cylinder working in parallel with two smaller cylinders. The larger cylinder serves as the major load-bearing component and is used for passive heave compensation.
The two small bore cylinders are used to actively control the position of the large cylinder, and consequently the position of the load.
The hoist system and compensating system are independent and the passive system provides back up during power failure.
With this system Huisman believes it has found a solution for the growing loads being installed subsea. It provides the accurate position control, with limited power usage, even for very large loads.
Both AHC and PHC systems have advantages and disadvantages, which should be considered and commercially evaluated by the operator.
We believe in the combination of both systems to increase the value of offshore installation vessels through improved operability.