An Improved Semisubmersible Work Vessel for North Sea Operations
Authors: Arps Jan L. 
Published: December 2007

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 and on maneuverability. For a work vessel operating in open water, sea-keeping 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 damage both cargo pitching, and rolling, which damage both cargo and vessel and make travel uncomfortable for those aboard. In addition, 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 in delivering platform. Experience has shown that in delivering supplies and equipment to offshore rigs considerable time is frequently lost, at a very high cost per hour, in waiting for weather to improve.

Design of the Twin-Hulled Semisubmersible Work Vessel
Five years ago the Trident Offshore Co. perfected the concept of the semisubmersible work vessel. Such a vessel would provide a stable platform under extreme sea conditions, have a large and dry work deck, and be able to maintain speed without pounding or slamming in rough seas. The design concept utilizes two torpedo-shaped lower bulls located well below the air/water interface. Analogous to two submarines at periscope depth, these bulls keep the major portion of the vessel's buoyancy below the area of maximum wave action. The hulls taper to narrow cross-sections at the water line to provide a low water plane - which further minimizes the effects of wave action - then widen again above the water line. A streamlined upper deck connects the hulls at a level high enough to clear the wave crests. Below the water line, the lower hulls are connected by one or more horizontal hydrofoils, which provide both structural strength and serve as heave, roll, and pitch dampers. Because of the excellent damping of the hydrofoils, the vessel's motions are very small and extremely gentle, whether on station or under way. Model tests run in the wave-making laboratory of the Netherlands Ship Model Basin at Wageningen, Holland, demonstrated that the roll, pitch, and heave motions for this kind of work vessel are significantly lower than those of comparable surface ships in the usually critical range of 8- to 15-second-period waves. Of major importance, it was shown that compared with conventional vessels, the Trident is able to maintain speed in a seaway that would reduce transit time significantly under adverse sea conditions.

The MV Duplus
In 1968 the Trident design was used in the construction of a 133-ft prototype vessel, the MV Duplus, to perform seabed investigations, underwater structural repairs, and construction support activities for the Netherlands Offshore Co. Since being launched in 1969, this 1200-ton vessel has served admirably as a floating test bed for the Trident semisubmersible design concept, and has aroused much interest and comment in the industry. During this period, her capabilities have been thoroughly tested, and her strengths and weaknesses evaluated. The ability to maintain speed in a seaway, to keep the work deck dry under the most adverse conditions, and to maintain a stable work platform have been proved conclusively. This is not to say that the MV Duplus performed perfectly in every respect. She had her problems. She was underpowered for the jobs she was called upon to do. Between the time the hull was designed and the ship was finished, considerable additional equipment, such as cranes, additional anchoring gear, and extra superstructure were added, which adversely affected draft and metacentric height. Finally, a number of mechanical and electrical bugs reduced her over-all effectiveness. After 2 years of operation, in an effort to compensge for me excess topside weight, the vessel's waterline area was increased nearly 50 percent by widening the hulls at the water line. Although this increased load-carrying capability, some of the desirable motion characteristics of the design were compromised (Table 1). Nonetheless, as a prototype vessel, the MV Duplus has proved that the design concept of the Trident semisubmersible work vessel is sound and that a vessel of this type can serve as an effective profit center for the offshore operator.
TABLE 1 - MV DUPLUS HULL CHARACTERISTICS
1971 1967 1969 NORTH SEA RATIO Original North Sea Operations of Increase Model Correlation After After Test Test Modifications Modifications Displacement, tons 948 1,174 1,288 1.1 Draft 15.5 17.0 17.0 1.0 GM, ft 5 3 9 3.0 Heave natural period, seconds 10.8 10.8 9.7 0.9 period, seconds 10.8 10.8 9.7 0.9 Pitch periods, Pitch periods, seconds 15.0 13.6 11.5 0.8 Roll period, seconds 13.0 19.0 9.5 0.5 Surface C.W.L., sq ft 1,225 1,370 2,270 1.66
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