The Role of the Semi-Submersible Work Vessel In Offshore Production Operations

Abstract The semisubmersible work vessel concept represents a major improvement over conventional designs. The inherent stability and large deck area of the twin-hulled semisubmersible work vessel make it an ideal vehicle for servicing offshore production and drilling operations in severe ocean environments. The 133-foot m.v. "DupIus" has served as a floating test bed for this design, and four years of operating experience with this vessel in the North Sea has proved the basic advantages of its noveI design concepts. As a result of this experience, a second generation of semi-submersible work vessels has been designed with characteristics even better than those of the m.v. "Duplus". Introduction The advent of the North Sea as one of the major oil and gas provinces of the world has focused new attention on the bad-weather performance of the heavy-duty offshore work vessel. North Sea weather conditions put emphasis on sustained speed in a seaway, maneuverability, and sea keeping ability. For a work vessel operating in open water, seakeeping ability is a prime requirement, yet when the sea kicks up to little more than a mild chop, most offshore work vessels today are unable to maintain speed without severe pounding, pitching, and rolling, which damages both cargo and vessel and makes travel uncomfortable for those aboard. Also, most offshore work vessels are usually very wet. It is not uncommon for the sea to inundate the wheelhouse, as well as everything on deck. On location, it often becomes difficult, if not impossible, to unload cargo from a rolling, pitching supply vessel onto the drilling or production platform. Experience has shown that considerable time is frequently lost, at a very high cost per hour, in delivering supplies and equipment to offshore rigs while waiting on weather. It is not unusual, for example, for North Sea supply and support operations to be curtailed as much as twenty-five percent of the time, due to adverse sea conditions. Also, in servicing underwater completion and salvage operations, one of the greatest operational problems results from the design of the average diving support vessel. When this vessel rolls severely during launch and recovery, it causes difficulties to the occupants of the diving capsule. This has limited diving operations with standard supply and support vessels to sea conditions no worse than sea state five. As a result, equipment too large and expensive for the job has frequently had to be used because of the inability of conventional work vesseIs to operate satisfactorily in rough seas. For example, it has from time to time been necessary to otherwise immobilize a large derrick barge to provide a stable platform for diving operations, whereas a much smaller work vessel would have been equally satisfactory, were it able to provide a similarly stable platform.