Active heave compensation for offshore missions

Active heave compensation for offshore missions
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July 2015

 

Technical assistance is essential whenever equipment weighing many tons is to be placed on the seabed. SBM puts its faith in active heave compensation made by Rexroth.

The yellow leviathan makes its way purposefully into the twilight of the deep sea. The blowout preventer is on its way to a wellhead on the seabed. By the time it has reached precisely its specified position, it will have covered about 3,000 meters – without deviating from the planned course a single time. There is none of the same kind of quiet at the surface. The sea is lashed by waves several meters high, causing the installation ship operated by the Dutch offshore company SBM to bob up and down. To ensure that this expensive equipment descends with no turbulence whatsoever, that the cable does not fail, and that the load is set down safely – all this requires a special technical trick. This is known as active heave compensation.

Cleverly defying nature

This unique concept does its work below decks, at complete reliability. The principle is quite simple. A Motion Reference Unit (MRU) continuously registers the vertical motions to which the ship is subjected. An extremely quick-reacting control processes these data and, in turn, forwards the results to the hydraulic winch which pays out the marine cable. The cylinder then extends or retracts so that the cable, reeved over a reversing sheave, is kept still in reference to the sea floor. In this way the system can compensate for at least 90 percent of all vertical motion, handling loads of up to 150 tons. When compared with other concepts, active heave compensation made by Rexroth differs in one highly significant point: The design joins in a very special fashion both a large hydraulic cylinder and a pneumatic cylinder.

The best of both worlds

The two technologies complement each other perfectly. The pneumatic cylinder – the passive element – supports and compensates for the static load while its hydraulic mate, as the active component, supplies the power necessary to compensate for dynamic effects such as friction losses or inertia. Required here is just a fraction of the hydraulic power which the winch would otherwise consume. This means significant energy savings for SBM. Reliable operation and the safety of both the operators and the load are always ensured, even under harsh marine conditions. The large hydraulic cylinder is shielded with the Enduroq 2200 surface finish. Two layers of protection offer exceptionally high resistance to wear and superior resistance against corrosion. This process was developed by Bosch Rexroth especially for offshore applications. Over and above this, the critical components in the system are designed for dual or multiple redundancy levels. To ensure that these backups are never called into action, three sensors installed directly in the cylinder – the Cylinder Integrated Measuring System (CIMS) – monitor ongoing operations.

Support from A to Z

The entire concept was developed by Bosch Rexroth especially for SBM. Almost all of the needed system components are produced in-house. This includes the drive section with the hydraulic power unit and cylinder, the compressed air supply unit, and the control system – comprising the software, the switchgear cabinet and the control panel. The second delivery to SBM software includes even further features, among which is the option for remote monitoring. The seagoing vessels, the equipment and the user will thus always have a calm and steady hand.