Single-Stack Wellhead System for Jackup Drilling Rig

Abstract

Phillips Petroleum identified surface wellhead and blowout preventer (BOP) nipple up and nipple down time as an area where significant improvements in safety and savings in rig time could be made on the Audrey field in the southern North Sea UK sector. With a view toward future use on central North Sea high-temperature, high-pressure exploration and development wells drilled from heavy jack-up rigs, a unitized wellhead system designed for use with a single 18-3/4" BOP stack .and a mudline suspension system has been fabricated and used successfully on three development wells on the Audrey Phase II Project and on one exploration well in the southern North Sea. INTROPUCTIONSubsea wellhead systems have evolved to the point where there is a standard configuration in common usage. This configuration uses an 18-314" 10M or 15M wellhead housing run on the 20" casing string with mandrel hangers for the 13-3/8" and 9-5/8" casing strings. The 18-3/4" BOP has a large enough diameter to permit drilling 17-112" hole without underreaming and a sufficiently high pressure rating for drilling to TD, yet is still small enough to be References and illustrations at end of paper handled without difficulty, particularly since most floating rigs are equipped with good BOP handling equipment. Because running and retrieving the BOP stack is time-consuming, a test stump for the BOP is provided to allow the stack to be tested on the surface. Use of a single stack keeps rig time spent running and retrieving BOPs to a minimum. All operations on the subsea wellhead are performed quickly using downhole tools run from the surface because of the prohibitive cost of manned intervention and the high cost of rig time. Despite the complexity of the equipment used in drilling with a subsea wellhead, this system has proven to be reliable and fast.In theory, nippling up and nippling down surface wellheads and BOPs should be much easier than running subsea wellheads and BOPs, since much less equipment is required and the wellhead and BOP are readily accessible. In practice, this is not the case. A major obstacle is that most jack-up rigs are equipped with two BOPs, a 21-114" 2M or 20-3/4" 3M BOP and a 13-5/8" 10M or 13-518" 15M BOP. Changing BOPs at least once is necessary on most wells in order to have a sufficiently high pressure rating for drilling to TD along with sufficiently large internal diameter to pass the drilling assemblies for the upper hole sections. Consequently, the limited availability of large-diameter, high-pressure BOPs on jack-up rigs has hindered the development of jack-up wellhead systems analogous to those currently in use on the vast 235