Drill string compensator, also known as heave compensator. This is an example of active heave compensator (AHC)

A drill string compensator (DSC) decreases the influence of the heave of a drilling vessel on the drill bit. Drill string compensators are also known under the more general name: heave compensator. Drill string compensators are used onboard drill ships and semi-submersible drill rigs. There are two major types of drill string compensators:

  • Drill string compensator between the traveling block and hook .
  • Top mounted heave compensator

Drill string compensators are just giant springs between the drill bit and rig. These consist of one or more hydraulic cylinders and air bottles. In between, a medium separator is sometimes used. If there is no spring element, movement of the drill rig would immediately place enormous forces on the drill bit and pull the drill bit off the bottom of the hole. A spring has a force/displacement curve. To get only small force variations on the drill bit, the hysteresis of the spring should be small and the curve should be horizontal, or the gas volume should be large. This is more difficult for a hook-mounted compensator than for a top-mounted one. Several patents are in use for a solution to provide this horizontal curve with a relatively small volume of gas. It will be clear that these (kinematic) constructions can in fact only be used in a top-mounted heave compensator.

Active drill string compensator

To improve this load/heave curve, an active heave compensator could be used. Such a device however requires an enormous amount of power and cooling. A semi-active system is a better solution. In this case, the largest part of the load is taken by the passive heave compensator and an extra active device, working in parallel, takes care of the increase of the load/heave curve and hysteresis.

The combination of PDSC (passive drill string compensation) and AHC (active heave compensation) are now in operation on the semi-submersible drill rigs Seadrill and West Sirius. The systems are installed on all of the new generation deep water drilling semi-submersible drill rigs.

Manufacturers of drill string compensators

  • Rucker Shaffer (US) (Hookmounted)
  • Aker Kvaerner MH (Maritime Hydraulics) (Norw) (Topmounted)
  • ICON Engineering Pty Ltd (www.iconeng.com.au)