Crown Mounted Compensators (CMC)

The purpose of the NOV Crown Mounted Compensators is to apply a constant tension to the drill string and compensate for any rig movement. Correct weight on bit (tension in drill string) is extremely important to maintain efficient drilling and long drill bit life. The CMC is installed on top of the derrick. They all comprises of a pair of vertically mounted hydraulic cylinders whose function is to convert load to hydraulic pressure. The piston rods are attached directly to the crown block.

Since the crown block will move in relation to the derrick, some form of length compensation is required for the drill line. This is obtained by guide sheave and pivoted arms, or through our new Eccenter Hub.

The CMCs ar built in accordance with API 8C, PSL1 and supplementary requirements of SR-3 and SR-5 for the sheaves and bearings, and API 4F for the steructure and crown block assembly.

Compensator fluid is used on the cylinder side of the system in order for the crown block to be locked in any position. A safety valve in the circuit will automatically close, and shut off the compensator fluid, in case of a momentary drop in pressure at the valve port. This typically happens if the drill string breaks or due to sudden loss of load in the hook.

A skid mounted hydraulic power unit is included, consisting of a stainless steel tank for supply and storage of compensator fluid, a fluid filling/draining pump and a high pressure leakage pump. The unit is mounted at deck level.

• Minimized hook and load variations over the full stroke.
• Active compensator may be added.
• Proven design.
• Mounted on top of the water table beams and may be retrofitted to standard 18’x18’ or 20’x20’ derrick top.
• Plunger type compensator cylinders; no internal seals.
• Compensator may be locked in any position.
• Compensator cylinders are spaced in order to allow all standard top drives t be fully hoisted in between.
• Easy access to all service needing parts.
• Static load = Derrick rating (crown block on water table).
• The high end isolation valve reacts to any sudden failure. (i.e. drill string failure).
• The isolation valve is remotely operated to lock the crown block in any position.