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Drill String Compensators (DSCs)
Drill String Compensators (DSCs)
Motion Compensation

The Shaffer Drill String Compensator (DSC) is the most popular system of its kind in use today.

The Shaffer Drill String Compensator (DSC) is the most popular system of its kind in use today. The motion compensator is mounted between the traveling block and hook, and is connected to the deck-mounted air pressure vessels (APVs) via standpipes and a hose loop. The DSC isolates the heaving motion of the vessel from the drill string. It minimizes wear between the drill string and the blowout preventer, marine riser and casing strings. Shaffer DSCs, like other Shaffer motion compensation equipment, use the hydro-pneumatic operating principle. DSCs are furnished with operating stroke lengths of 15, 18, 20 or 25 feet, depending on rig design and application. They can be manufactured to meet ASME, ABS, USCG, DNV, and other applicable codes. Operation of the DSC is performed both manually and electronically from the deck-mounted control console.

During drilling operations, the Shaffer DSC keeps the drill bit on the bottom of the hole within the weight limits set by the driller. For other operations, the system compensates for rig motion, maintaining a position relative to the ocean floor.

As the rig heaves upward, the compensator cylinders retract and the hook moves downward relative to the drill floor, while remaining at a constant level relative to the earth. The cylinder piston compresses the air through the hose into the air pressure vessels to maintain the preset tension level. As the rig heaves downward, air from the air pressure vessels expands into the compensator cylinder, and the system works in reverse.

Shaffer DSCs, by compensating for vertical heave motion of the drilling vessel, minimize wear inside the BOP stack and on the marine riser and casing string. Wear on the ram BOP seals and annular BOP elements is also minimized. There is virtually no motion of the drill string while the blowout preventers are closed. In addition, the BOP stack can be landed without abrupt motion changes, and the setting of the packers and drill stem test tools is easier.

Design Features:

Constant Bit Weight Control:

The driller can increase, decrease, or maintain at a preset level the drill bit weight by controlling the compensator pressure. The need to remove the drill string from the hole to change bit weight is eliminated when drilling with a Rucker DSC. Constant bit weight control yields higher penetration rates and longer drill bit life.

Faster Response Time:

The Shaffer air system, in contrast with a hydraulic system, eliminates system shock pressure and permits faster compensation action. It was decided that the technique of energy storage and transfer utilized in our tensioner systems would be used in the compensator system. That is, compressed air would be used not only to store energy, but also to transfer energy to and from the compensator. The primary requirement for a drill string compensator is to provide a relatively high force at high velocities representing considerable power. For example, 400,000 lbs at 300 ft/min is 3600 horse power. As seen in Figure 1, the air pressure is applied directly to the compensator pistons, rather than by use of a liquid interface fluid. This allows higher flow rates at lower pressure drop through smaller lines due to the lower mass and viscosity of air compared to hydraulic fluid. A further feature is that the pressure drop is less at lower pressures, whereas liquid pressure drop remains constant for all pressures for a constant flow rate.

Complete Control by Driller:

Console at driller's station provides full control of compensator load and monitoring of stroke position. Mechanical lock bar is activated at the console to lock the hook and traveling block together- a foolproof method to prevent accidentally dropping the load. The bar cannot be disengaged unless the tension setting exceeds the hook load.

Extended Stroke Operation:

The DSC design achieves a long compensating stroke with a reduced vertical height by using a two-part chain and roller system. The system delivers 25 feet of compensation from 12 feet of cylinder stroke, reducing installation height and the wear on bearings and packing.

Full Rated Compensation at Lower Pressure:

Although other systems require 3,500 psi to equal Shaffer system ratings, the Shaffer system achieves 400,000; 600,000 or 800,000 lb. full rated compensation with only 2,260 psi. Shaffer air pressure vessels and system components are manufactured to operate at 2,400 psi.

Upgrade From 400K to 600K:

The 400K Rucker Drill String Compensator (DSC) can now be upgraded to achieve a compensating capacity of 600,000 lbs. in a 400K frame.

Options for an Upgrade:

Options for upgrading a 400K DSC have historically included installing a 600K DSC or a CMC. The 400K to 600K upgrade was developed as a less expensive option to consider. This upgrade maintains the 1,000,000 lb. mechanically locked hook load capabilities of the existing compensator. It increases the rated hook load capacity from 400,000 lbs. to 600,000 lbs.

How is it Done?:

The upgrade package (Slimline, SL) is designed to increase the hook load capacity to 600,000 lbs. while minimizing the number of components being replaced. The significant components replaced include:

  • Cylinders of a larger diameter installed within the existing main frame.
  • Chains are replaced with 500K chain.
  • Fork blocks, chain adjustment rods, cross beam blocks, sheaves, and hook frame lifters are upgraded.
  • A new hook load gauge reflecting the higher capacity is provided for the driller's console.
  • Upgrade piping.
  • Miscellaneous hardware.

The upgraded system continues to operate at the existing design pressure. The new cylinders incorporate fewer component parts, special low friction packing, and rod end volume designed to use the existing air-oil reservoirs.

Please contact your Shaffer sales representative to further discuss upgrading your 400K DSC to 600K.

Integral Block Drill String Compensators:

The Shaffer integral block drill string compensator has a specially designed traveling block integrated with the DSC main frame. Less derrick height is required with this design because it has 4 feet more clearance than a DSC attached to an existing traveling block. The integral block design is available for all compensator stroke lengths- 15, 18, 20, and 25 feet. For the 400,000-lb DSC, the integral block consists of six 60" diameter sheaves with a 1,000,000-lb static capacity. For the 600,000-lb DSC, there are seven 60" diameter sheaves with 1,500,000-lb static capacity.

Traveling Block Mounted Drill Compensators:

The rigid block connector on the traveling block mounted DSC eliminates the need for an additional flexible link in the traveling equipment by rigidly pinning the main frame of the drill string compensator to the traveling block. This improves the stability of the system and eliminates the need for the guide dolly system to be designed to take out the relative motion between the main frame and the traveling block.