Surface Motion Compensation
WiJIiam L. Clark
A motion arrestor for nullifying vessel heave in wireline operations, especially in well logging operations, is available. This compensation device hangs below the hook and uses a working wireline reeved from the top of the riser pipe around the motion arrester working sheave, and to the drill floor. The logging sheave and working sheave are connected to the same yoke, which is suspended from the motion arrester by a pneumatic spring (special tensioner).
As the rig heaves upward, the working sheave and the logging line are retained in a relatively fixed place in space (as controlled by the two-part line). As the rig heaves downward, the sheaves again remain in a relative place in space as controlled by the pneumatic spring. This compensation motion has the effect of nullifying relative motion between the rig and earth generated by heave of the vessel. This eliminates the rig motion that would otherwise be superimposed by the data read by the logging sonde.
This compensation device is not required when a drill string compensator (DSC) is installed. The DSC has the capability of performing this function in addition to many others.
The major application for a drill string compensator is to nullify rig heave that would be imposed on the drill string. This motion nullification significantly improves the operation of the following procedures:
The DSC maintains a virtually constant bit weight, unaffected by rig heave. It improves penetration rates and significantly improves drill bit life. The DSC permits instant and easy drill bit weight changes at the surface, without tripping the drill string, thus eliminating many trips of the drill string.
b. handing BOP Stack
The DSC permits a relatively soft landing of the BOP stack and/or marine riser, not only in a safer manner, but in rougher sea conditions or higher rig heave conditions than would otherwise be possible.
c. Landing Casing
The DSC permits safe landing of casing in sea conditions and/or rig heave conditions that would otherwise be impossible or unsafe.
d. Sa/ety Control
The DSC can eliminate the motion of the drill string in the BOP stack. This eliminates wear of BOP seals due to rig heave and drill string motion incurred when closing the rams or annular BOP's on the drill pipe.
e. Miscellaneous Operations
Operations that would otherwise be hampered or impossible with the drill string moving with the rig heave are now possible because the DSC eliminates drill string motion during many operations.
All present drilling rig DSC's are an air spring tensioning device. They are passive devices that function based on the difference in the suspended weight of the drill string and the tension level set in the DSC.
It is possible to manufacture an active-servo controlled de sign, but at this time only one or two such units have been manufactured. They have been used on coring vessels. The following discussions cover the popular passive systems which are predominantly in use.
The weight on the drill bit equals the weight of the drill string less the DSC tension setting. Tension levels are controlled in the DSC exactly the same as in a riser tensioner. The basic techniques and technology developed for riser tensioners were applied to drill string compensators.
During drilling, the drill string weight is supported by the hydraulic-pneumatic cylinder of the DSC and the drill bit weight on the bottom. The cylinders are interconnected to the air pressure vessels (the same as the riser tensioners). Control of the air pressure in the air pressure vessels determines the tension level. Proper DSC drilling techniques always require a DSC tension setting to be less than the weight of the drill string.
As the rig heaves upward, the support cylinders must stroke to extend the DSC and thus to compress air from the cylinders into the air pressure vessels. The large volume of the air pressure vessels controls the variations in pressure due to air compression, the same as in the riser tensioners. This cylinder stroking maintains the preselected support load (tension) and thus maintains virtually the same weight on the drill bit.
As the rig heaves downward the support cylinders retract the DSC. Downward heave tends to place more weight on the drill bit; but as the cylinders are maintained at the preselected air pressure level, the cylinder retracts the DSC to maintain the preselected load (tension) and thus maintains the drill bit weight. During retraction of the DSC, air expands from the air pressure vessels to the cylinders maintaining the desired pressure level.
Ratio of the cylinder volume and air pressure vessel volume is important. The volume selections by each manufacturer optimize the equipment required versus the benefit derived from large volume air pressure vessels. An explanation of these effects can be seen in the section on Pressure Control in Cylinders in the "Tensioning Systems" section.
The DSC application requires closer control than the riser tensioners. The load variation of the DSC, as related to heave stroking or compensation stroking, is a percentage of the total supported weight. However, the desired control is the variation in drill bit weight, which is a fraction of the drill string weight.
The stroking variation equals ± 6% of 200,000 lbs. or ± 12,000 lbs., which is 24% of the desired drill bit weight.
The example above also illustrates the importance of installing the pneumatic and hydraulic system in accordance with the manufacturer's specifications. Pressure drop due to air/oil flow through the piping system causes a direct degradation of performance. System designers have optimized their system requirements and the applicable specifications should be followed.
Basic Types of DSC's a. Deadline
A tensioner can be mounted in the deadline of the draw-works system to control the weight on the bit. This technique has been used on one rig and functions well as a safety device, but was found to be too mechanically inefficient for drilling operations.
b. Crown Block
A tensioning device supports the crown block and thus the drill string. By supporting the crown block with controlled tension the compensator becomes a motion nullification device by raising and lowering the crown block. This then raises and lowers the traveling block and hook to nullify motion or to isolate rig motion from the drill string. This technique is presently in operation on several drill rigs and has proven to be effective. The principles of operation and control used in this design are similar to the traveling block DSC.
c. Traveling Block DSC
A tensioner device to support the drill string is connected between the traveling block and the hook to become a motion nullification device. Its tension level is controlled by techniques identical to a riser tensioner.
Techniques of Traveling Block Drill String Compensator a. Tension Type Cylinder
This technique applies to a cylinder(s) with high pressure on the rod side of the cylinder(s) between the traveling block and the hook (Figure 2-22). In order to provide lubrication to the cylinder an air/oil accumulator is used on the rig floor or on the derrick (mast) and oil flows through the hose loop to the cylinder. Pressure in the cylinder controls the DSC tension level in the same manner as in a riser tensioner. The weight on the drill bit is determined by the weight of the drill string minus the tension setting of the DSC.
In the event of a drill string breakage the oil flow input to the cylinder must be instantly shut off to prevent damage to the cylinder, surrounding equipment, and personnel.
b. Compression Type Cylinder
This technique applies a cylinder with high pressure air on the blind side of a cylinder between the traveling block and the hook (Figure 2-23). Lubrication and safety control are achieved by the low pressure air/oil reservoir on the rod side of the cylinder. Chain reeved around the cylinder in two parts of line delivers a compensation stroke twice that of the cylinder stroke. In other words, a 9 foot cylinder stroke delivers 18 feet of compensation motion. The use of a low pressure air/oil reservoir permits use of high pressure air directly on the blind side of the cylinder, so that air is flowed through the stand
pipe and hose loop for a reduced pressure drop and reduction of shock loading.
Compression loaded cylinders (Figure 2-24) support the crown biock. The crown block is mounted on a framework-supported dolly atop the derrick, which allows vertical motion of the crown block. Controlling the pressure in the support cylinders determines the tension level or load support level of the crown block.
The weight on the drill bit equals the weight of the drill string minus the tension setting of the DSC.
The crown block DSC moves the compensation device to the crown block rather than locating at the traveling block. This location eliminates the hose loop required by a traveling block DSC, but present crown block DSC designs are significantly heavier.
Articulated out-rigger sheaves are mounted on the crown block support mechanism. These sheaves are connected to nullify motion of the traveling block which would otherwise be generated by raising and lowering of the crown block relative to the draw-works. This causes motion of the drawworks wireline system and would otherwise be seen as motion in the traveling block.
Operation of a Drill String Compensator a. Tension Level
The tension level of a DSC is determined by the air pressure in the cylinder or the oil pressure controlled by the high pressure air in the accumulator.
Bit weight = drill string weight minus DSC tension When a DSC is in normal operation, the large volume air pressure vessels are directly connected through piping, the stand pipe, and a hose loop to the DSC cylinders (with an air/oil interface on the tension type cylinders). The tension level is increased by compressing more air into the closed air system (air pressure vessels). It is reduced by venting air from the air pressure vessels into the atmosphere. Control of the air pressure in the air pressure vessels and DSC system is identical to techniques utilized in the riser tensioner systems.
b. Commence Drilling
After setting the desired tension level in the DSC and making the drill string connections, lower the drill string into the hole until the drill bit contacts the hole bottom. At contact, continue to lower the traveling block to permit the DSC to stroke about its midpoint. Note that the tension level of the DSC is always less than the total weight of the drill string. Therefore, when picking the drill string up from the slips, the compensator will extend its full length before the drill string is lifted from the slips.
When reaching the bottom of the hole and landing the drill bit, the compensator will begin to retract and support weight of the drill string equal to its tension level. This leaves the difference in the weight and DSC tension on the drill bit as previously calculated. Lowering the traveling block to approximately half the stroke of the DSC will set the DSC to operate about its mid-stroke. As the bit drills off, the driller continues to incrementally lower the traveling block to maintain the DSC stroking about its midpoint. As long as the DSC is operating within its stroking limits, the weight on the bit will be controlled by the DSC and will be maintained at its preselected load.
c. BOP Stack Landing
A relatively soft landing of the heavy BOP stack can be achieved utilizing the DSC. One common technique that has been successfully used is to set the tension level of the DSC a few hundred pounds less than the weight of the package to be landed on the ocean bottom. When beginning the initial lift from the drill rig (slips or spider), Taise the traveling block to extend the DSC to its complete length (the weight exceeds the tension setting). When reaching the ocean floor with the package, the initial contact on the ocean bottom with the package will allow the compensator to begin retracting in its stroke.
However, the DSC will continue to support most or almost all of the load (an amount equal to its tension setting), leaving the ocean bottom structure supporting only the difference in the tension setting and the weight of the package, which can be adjusted to very light loads as compared to the total weight of the BOP stack. This technique permits landing and retrieving the BOP stack in much rougher sea conditions, and in a safer manner, than would otherwise be possible.
d. Casing Landing
Landing casing is accomplished in the same manner as described in the BOP stack landing. This relatively soft landing of casing permits the operation to take place in much higher rig heave conditions without damage to hangers and hanger seals than could be achieved without such a device.
The DSC is a tool which permits more efficient drilling operations in normal sea conditions, and in rougher sea conditions it permits drilling to continue when they might otherwise be impossible. A DSC saves rig time, and thus money, on each location.
Time on location is reduced. The amount of time reduced or saved varies between locations due to area conditions, rig design, weather conditions, etc. However, use of the DSC can always reduce time. Drilling of the hole can be accomplished faster with less days, fewer trips, and fewer drill bits. Setup and relocation times are reduced and can also be accomplished in rougher sea conditions. Thus, the total time for a single hole completion is reduced, sometimes saving as much money on one location as the entire cost of the DSC system.
Drill string compensator (400,000 lb. unit) traveling block type. (Courtesy of Vetoo)
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