Bainbridge, C.A., M.Sc. C.Eng. M.R.Ae.S., Lloyd's Register of Shipping United Kingdom


The semi-submersible has been adapted to perform numerous functions offshore unconnected with drilling but still operating as a mobile unit. When used as an early production unit however, it is required to remain on a particular site for several years. The strength and fatigue of one such unit is considered for three different locations in the North Sea. For completeness, several other typical design features have been incorporated into the structure. Existing and proposed fatigue criteria are investigated and the results discussed. The principal conclusion is that major structural modifications will be required if the proposed changes are implemented.


In 1962, the first semi-submersible, the Blue Water began drilling operations in the Gulf of Mexico. A few years later, the semi-submersible Santo Fe Choctaw was designed and built as an offshore construction barge.

Since that time, the Offshore Industry has gradually utilized the potential of the semi-submersible unit to assist and perform various other operations offshore. To date, these include pipelaying, jacket installation, heavy lift, diving support, maintenance, firefighting, emergency support and temporary accommodation.

The latest chapter in semi-submersible usage, began in 1975 when the Transworld 58 was converted into an early production platform for Hamilton's Argyll Field in the North Sec. In 1981, this was followed by the Drillmaster conversion for the B P Buchan Field. Several of these early production units now operate Offshore Brazil. production units now operate Offshore Brazil. This paper investigates the structural effects of semi-submersibles used in a permanently moored condition where environmental factors are site specific.

The global stress levels hove been evaluated from an actual modern twin pontoon, column stabilized drilling unit designed for worldwide operation. pontoon, column stabilized drilling unit designed for worldwide operation. The unit has been subjected to three sets of environmental conditions typical for sites in the North Sea. For comparative purposes, several local details from a number of other semi-submersibles have been incorporated.

The variations in fatigue life estimations due to site environmental conditions, detail design and allowable S-n curves are presented and the results discussed.

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