RIASSUNTO
The need to drill in deeper water depths is increasing. Also, the frequency of drilling in established deepwater depths continues to rise. Water depths of 2500 to 3500 feet for many of the dynamically positioned drilling vessels have become commonplace. And drilling in these water depths has not been limited to calm and protected water sites. Extensive drilling programs have been continuing off the East Coast of Canada and Western Australia in deepwater for the past few years.
One disadvantage of drilling in deepwater is the loss of practical diver backups should anything happen on the ocean floor. For backup, the contractors have to rely on remote means of mechanical manipulation using mechanisms suspended from the drilling vessel by tubulars or by sophisticated manned or remote controlled submarines. Neither of these backups provides an ideal solution to be counted on at all times. Therefore, the emphasis in deepwater drilling has turned towards the development of sophisticated and reliable equipment for underwater use. Some of the new technology includes superb multiplexed electrohydraulic control systems, rubber and metal laminated flex joints, high strength riser couplings, sonar and TV reentry systems, guidelineless wellhead systems, and much more.
However, as with anything mechanical and being controlled by humans, mistakes and accidents do occur. Such an accident occurred with the SEDCO 471 drill ship while it was drilling off Western Australia in 3145 feet of water. The SEDCO 471 is a dynamically positioned drill ship with guidelineless subsea equipment. The controls for the BOP consist of a multiplexed electrohydraulic system. The vessel is capable of drilling in 6000 feet of water and the 3145 foot location was well within its limits. The rig had run 30"" and 20"" casing and the permanent guidelineless guidebase was in place with the 16-3/4"", 10,000 PSI wellhead. The BOP was run to bottom and latched to the wellhead. During the BOP function test it was determined there was a leak at the connection between the upper and lower BOP stacks.
The leak could not be corrected and the decision was made to pull both stacks. As the riser and BOP were being retrieved, the riser string jumped and a loss of 160,000 lbs. was noticed on the weight indicator. It was not too difficult to correlate the fact that the lower BOP weighed 160,000 lbs. in water and the weight loss on the weight indicator was the same.
The external TV's on the upper stack verified there was no lower stack. The lower stack was dropped because of a malfunction in a closed hydraulic circuit used for pressure balance in a vent part of the hydraulic connector between the upper and lower stacks.
The rig then fabricated an onboard hook assembly to try to grab a major structural part of the frame in an attempt to retrieve the stack. The attempts were unsuccessful and caused superficial structural damage to the frame. The rig moved back over location and began to ready their complete backup BOP onboard.