Drilling Technology:
Drillstring Motion Compensators

     With the advent of drilling offshore wells from floating drilling vessels in the late 1950’s came the inherent problem of how to keep the drillbit on bottom and drilling while the drilling vessel heaved up and down. The answer at the time was to use a tool developed by the oilwell fishing tool industry known as a “Bumper Sub”. This tool was simply a slip joint that was placed above the drill collars that would allow the drillpipe to move up and down while the drill collars and bit remained on bottom. Most Bumper Subs were limited to a stroke of only six to eight feet. The Driller had to continually estimate the heave of the drilling vessel as well as the drilling penetration rate trying to keep the Bumper Sub from bottoming out at either end of its stroke. If the sub bottomed out the bit would bounce off bottom with the entire weight of the Drillstring usually damaging and shortening the life of the bit. A good Driller became an artist at keeping the bit on bottom most of the time.

     However, there was one problem that the Driller could not overcome. That is the fact that the high-pressure mud seals in the Bumper Sub only had an average life span of around thirty to forty hours. This meant that the Drillstring had to be tripped to refurbish the Bumper Sub before the seals wore out causing a leak that would wash out the Bumper Sub and allow the Drillstring to part. Since the life of most drillbits exceeded the life of the Bumper Sub more than the usual amount of costly Drillstring trips were required. Over the decade of the 1960’s floating drilling had to contend with the Bumper Sub though new type seal technology did make for up to three times better life. There needed to be a better answer and much pressure was placed on the drilling equipment manufacturers to find another method of Drillstring motion compensation to replace the Bumper Sub.

     In about 1970 three companies began to put major effort into development of a tool that could be placed on the rig at the surface to compensate for and separate the Rig’s motion from the Drillstring. These companies were Ventura Tool Company (VETCO), Rucker Company, and Western Gear Company. Each chose a variation of the successful hydro pneumatic Marine Riser Tensioner technology developed in the early 1960’s. Vetco was the first to install a successful prototype system in 1971 on the Wodeco IV in the Santa Barbara channel for Humble Oil Company (now ExxonMobil).

Rucker and Western Gear followed this closely in 1972 with deliveries to customers of their first production Compensators. All three systems worked well and were a successful solution to the Bumper Sub problems.

     Though the three systems were somewhat different in design each is mounted in the derrick between the Traveling Block and Hook and all use compressed air as a spring force to hold positive tension on the Drillstring. Each allows for better control of the bit weight resulting in much better bit life and drilling penetration rates. Since they are mounted as part of the traveling system in the derrick not all of the older rigs prior to the 1970’s could be readily retrofitted with the new Motion Compensators. By the mid 1970’s all new floating rigs built were either equipped with or the derricks were prepared for later installation of Drillstring Motion Compensators

     The three original Compensator designs are still in existence with many improvements and much higher capacity systems though the ownership of the three original companies has changed. The Rucker Company Compensator product line, which has been owned by Shaffer since 1972 is now owned by Varco International. Control Flow Company who service and market both products now owns the Vetco and Western Gear Compensator product lines.

     Vetco, Rucker Company and Western Gear will be recognized as the originators of the technology and Varco and Control Flow will be recognized as the current technology owners.    

     Recognizing the following individuals and companies who contributed to the development of this technology: 

Arthur Hor Ting Chin, Bruce Duncan, James Hanes, Edward Larralde, Glen Robinson, Richard Sprague, Rucker/Shaffer (now Varco International), Ventura Tool Co. and Western Gear (now Control Flow Inc.).