A drill string compensator 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
  Block and tackle
  A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift or pull heavy loads.The pulleys are assembled together to form blocks so that one is fixed and one moves with the load...
  
   and hook .
• Top mounted heave compensator

Drill string compensators are just giant spring

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

s between the drill bit and rig. These consist of one or more hydraulic cylinder

Hydraulic cylinder

A Hydraulic cylinder is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in engineering vehicles.- Operation :...

s 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

Drill bit

Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....

 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

Hysteresis

Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...

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)