Comparative Performance of Semisubmersibles in the North Sea
Authors: Burnell A. John 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
This paper was prepared for the SPE-European Spring Meeting 1975 of the Society of Petroleum Engineers of AIME, held in London, England, April 14–15, 1975. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
The first part of this Paper discusses the North Sea environment and the sources of information that provide motions data to the designer. The motion characteristics are then established of two units typical of designs being used in the North Sea today. One has its buoyancy primarily in isolated footings (i.e. SEDCO K) and the other in continuous hulls (i.e. SEDCO 700). Theoretical predictions are compared with experimental model tests and then with observed motions during the 1973–1974 Season. Evidence available for other units of each type (e.g. PENTAGONE 81 for the first group and the PACESETTER for the second group) is then presented and discussed.
The Paper continues with a discussion of the application of these results to design, development and future operations. The emphasis throughout is practical rather than theoretical, on the collection and interpretation of data rather than the mathematical equations.

Introduction
Semi-submersibles began to operate in the North Sea in the mid-sixties and papers describing their characteristics began to appear shortly after. A majority of the Papers listed here were prepared by people in the Shell Group and the British Petroleum Company using the SEDCO 135 type for their chief example. 1967–1968 produced three useful Papers. West, for Shell, produced three useful Papers. West, for Shell, discussed offshore drilling unit design using the SEDCO 135 type and the Staflo as examples. Avery, for British Petroleum, again discussed offshore drilling, this time with rather more emphasis on the drilling system, using the Sea Quest (SEDCO 135 Type) as a particular reference. Watts and Faulkner, for Shell again, reviewed the performance of the SEDCO 135F off the West Coast of Canada. 1970–31971 produced a second set of three Papers. produced a second set of three Papers. Starink, for Shell, led off with useful performance information on Staflo, SEDNETH I performance information on Staflo, SEDNETH I and the SEDCO 135 type. Bell and Walker, for British Petroleum, concentrated on member stress measurements made on their Sea Quest while Hammett, for SEDCO, did the same thing for the SEDCO 135F. After this, there was a lull in the activity whilst people in the offshore business concentrated on other matters such as designing all the recently completed twin-hulled units. Then in 1974, Bell, for British Petroleum again, produced valuable data from Petroleum again, produced valuable data from more recent observations on the Sea Quest.