Apex hydraulics has over 30 years’ experience at the forefront of hydraulic cylinder design and manufacture. As well as designing bespoke hydraulic cylinders for fabrications across multiple sectors, Apex are able to improve on Original Equipment Manufacturer (OEM) design after cylinder failures; adapting failed cylinder designs to be longer lasting, more robust and able to cope with the rigours of a difficult environment. One such difficult environment is in the offshore sector, which presents a number of challenges with regard to the design and manufacture of hydraulic cylinders. A particularly complex branch of offshore hydraulic cylinder design is in that of Active and Passive Heave Compensating Cylinders.

Heave compensating cylinders are utilised in applications where a cylinder is required to compensate for movement; such as the movement of waves on the ocean.

One such offshore application is for Compensated Coil Tube Lifting Frames (CCTLF) which are used to maintain the steadiness of the machinery when drilling out a pipeline for an oil or gas company. It is vital that the drill must remain steady in relation to the seabed.

Heave compensating cylinders are also used for offshore cranes, lifting items out of the water, while being buffeted by waves themselves. While there are specialised cranes that already incorporate a heave compensating cylinder, it is also possible to convert a standard offshore lifting crane into a heave compensating crane by installing a bespoke motion compensating cylinder.

The principal of a heave compensating cylinder is similar to that of a car suspension; but with the opposite function. On a car, the wheels move up and down to keep the car relatively still. A motion compensating cylinder keeps the tool still while the vehicle itself moves around.

Also known as a Drill String Compensator, there are two main types of PHC. The first type is without electronics and used for simple applications; it can perform either a splash zone crossing OR a simple subsea compensation task. The second type of PHC comes with an electronics package that allows the compensator to do multiple tasks by allowing manipulation of the internal gas pressure (which can be increased or reduced based on changing external influences).
Features:

• Capacities from 20 to 2000 tonne.
• Stroke typically 3 to 5 m.
• Common for all passive heave compensators are that they consist of a hydraulic cylinder and at least one accumulator. A valve package to control damping is optional.
• The size of the gas volume dictates the dynamic amplification that will be seen. Typically, it can be limited to static weight plus a few percent.
• Under normal circumstances, PHC cannot compensate for motion in air

Uses:

PHC are used to limit dynamic loads due to heave motion; for example, during a splash zone crossing. If a vessel goes up a wave, while trying to lift high resistance objects (such as suction anchors or big GRP covers) it risks the crane overloading due to the extra weight of water caused by the sudden change in lifting speed. In this way, the heave compensators act a little like a rubber band; allowing the vessel and crane to move without breaking the crane or stretching the cable.

With a PHC, the vessel can be disengaged from the item that it is lifting, allowing the sea to do the moving. The “passive” nature of the cylinder is that it is only compensating for the outside forces of the sea movement.