The Direct Line Compensator system with integral travelling block (Figure 1) is designed to minimize the effects of drilling vessel vertical motion (heave) on the fixed and rotating drill string and bit. The motion compensator is in the derrick and is suspended by the wire rope reeved over the crown block and connected to the travelling beam. The cylinder assembly is connected to air pressure vessels (APVs) by a standpipe and hose loops. The DLC is powered by compressed air and can be remotely controlled from the derrick-mounted electric control console. National Oilwell Varco's motion compensators are manufactured to API 8C specifications.
Figure 1: Shaffer DLC
DLC Composed of Single Compression-Type Air Cylinder:
The DLC is composed of a single compression-type air cylinder pressurized by power air pressure vessels. The compensating force (pull on the drill string) is determined by the magnitude of the system air pressure and is affected by the APV volume, piping and the cylinder stroke and position. As the drilling vessel heaves upward from wave action, the compensator cylinder rod extends and the rod moves downward. This downward rod motion is relative to the drilling vessel but in actuality is fixed relative to the earth. As the rod extends, air is transmitted through large hoses back to the APVs. When the drilling vessel motion is reversed, the air flows from the APVs to the cylinder which retracts the rod. During operation, the compensator works at approximately midstroke, and the only movement relative to the drilling vessel is the drill string, top drive and cylinder rod. The traveling block, hoses and main cylinder body remain motionless relative to the drilling vessel.
Compensator Developed for Full-Time Drilling and Other Operations:
The compensator was developed for full-time drilling and other operations, which require the elimination of drill string motion in the hole and through the blowout preventer (BOP) stack. In addition, the driller can increase, decrease, or maintain at a preset level the drill bit weight by controlling the compensator pressure. The DLC eliminates the need to remove the drill string from the hole to change the bit weight.
Compensator Cylinder Dampened and Lubricated with Low Pressure Fluid:
The compensator cylinder is dampened and lubricated with low pressure fluid on the blind side of the piston. This fluid also provides cushioning, and in conjunction with the speed control valve, prevents a runaway condition if the drill string breaks. The rod side of the cylinder has high pressure fluid for rod end lubrication and to assist in effecting a cushion in conjunction with the cushion collar assembly.
Direct Line Compensator Assembly:
The assembly is attached between the crown block and the top drive, and contains the traveling block, cylinder assembly, mechanical lock assembly, and rod eye assembly. The compensator assembly is rated for 650,000 pounds of load when compensating and 1,500,000 pounds when fully retracted and mechanically locked.
The traveling block assembly is rated for 500 tons and is integrally connected to the DLC cylinder blind end. Its sheave cluster contains six (6) sheaves with two (2) 12" diameter single tapered roller bearings that are designed to accommodate 1 â…œ" wire rope. Coplanar guide dolly mounting pads are included on the traveling block side plate.
Soft Landing DLC:
The Soft Landing Direct Line Compensator, DLC, (Figure 2) is an in-line compensator system mounted in the drill string for orientation and soft landing of subsea components. The system consists of a cylinder assembly made up in the drill string, a service loop to the derrick, two 312 gallon air pressure vessels, a control manifold to regulate pressure, and a control panel to operate the manifold. National Oilwell Varco's motion compensators are manufactured to API 8C specifications.
The system operates on 2,400 psi maximum clean air or nitrogen and is capable of compensating loads of up to 650,000 pounds in heave conditions of Â± 2 feet. The bottom connection of the cylinder is designed with an integral swivel to allow manual rotation of the supported load.
The control panel allows monitoring of load, pressure and stroke position of the cylinder as well as fast and slow increase and decrease of pressure to control the position of the load. Electrical is 120 VAC explosion proof in panel and at manifold and 24 VDC at cylinder.
Figure 2: Shaffer Soft Landing Direct Line Compensator