The authors' company has been engaged in the design of semisubmersibles since the 1960s, with experience ranging from mobile offshore drilling units (MODUs) to multi-service construction vessels (MSVs). The design philosophy and approach taken to stability has evolved from a very simple basis for the first-generation units of the 1960s to the more sophisticated methodology employed in the third-generation units being built today. A brief history of the evolution of the semisubmersible concept is presented, together with the basic objectives for a successful design


At the time of writing this paper, over 180 semisubmersibles have been built worldwide, with approximately 70 located in what are considered harsh-environment areas, for instance the North Sea and the East coast of Canada The experience to date in operation of all types of Mobile Offshore Drilling Units (MODUs) is approaching 4000 rig-years worldwide and nearly 600 rig-years in the North Sea alone. The reason there are so many semisubmersibles in operation, and that so many continue to be built even in the depressed economic conditions of the past several years, is that semisubmersibles are the only practical solution to the operational problems of exploratory drilling m deep and hostile waters. In the 1970s, the lower limit of waterdepth for semisubmersibles was considered 90 m (300 ft); now jack-ups are capable of being operated in waters up to 140 m deep (450 ft) The requirement for semisubmersibles, however, remains strong, as exploration is now concentrating on waterdepths in the 300450 m (1000–1500 ft) range


The purpose of this paper is to discuss the stability requirements for modem semisubmersible units from the perspective of the designer. The designer's objective is to develop a configuration that fulfills the owner's operational requirements at a reasonable cost without compromising safety, strength, or performance The paper will describe the many design elements that either influence, or are influenced by, the stability characteristics chosen for the design.

Scope of Paper

The paper is limited to a discussion of stability design practices for semisubmersible drilling units, although principles will be applicable to other semisubmerslble types as well A brief history of the semisubmersible design concept is presented to compare design practices of the 1960s with current practice. A discussion of the history of stability rules for semisubmersibles is included for a thorough understanding of the issues being presented.


The history of early offshore drilling may be of interest, to contrast the equipment and procedures used in the first ventures offshore with those used offshore today. There has been tremendous growth in offshore drilling since the first MODUs appeared m 1950 Figure 1 shows the chronological growth of the worldwide MODU fleet from the 1950s to the present The following sections highlight the technological advances in MODU design over the 1950–1985 time period Design and construction considerations of early MODUs are discussed in detail in References 1 through 4.

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