Stability requirements for Mobile Offshore Drilling Units (MODU's), particularly semi-submersibles, have been the focal point of discussion by numerous affiliations of the offshore community. Among the organizations that have comprehensively evaluated existing stability standards are the sub-committees under the International Maritime Organization (IMO). Based on several internationally sponsored research projects, and specifically two ABS Joint Industry Projects (JIP's), an alternative intact criteria as well as modifications to the damage stability criteria, incorporating a residual stability standard, has been adopted by IMO. This paper provides background on the recent developments of residual stability standards, methodology used in the ABS investigation to evaluate the susceptibility to capsize and downflooding, examples of analytically predicted capsize, and discussion on the relationship of dynamic response to static stability curve characteristics. The paper concludes with a summary of findings and the recommendation for a minimum residual stability standard in terms of residual righting arm and positive stability range.


IMO has completed its evaluation of the adequacy of the criteria for intact stability and residual stability after damage. The intact criteria proposed by the ABS JIP on MODU Stability [1,2,3,4] has been adopted as an alternative stability criteria by IMO. Further, IMO recommended in March 1988 that semi-submersibles should be provided with a margin of residual stability in a damage condition at least equivalent to that required for conventional ships under SOLAS, 1974. However, due to the lack of substantiating data available at that time, a decision concerning the appropriate level of safety was held in abeyance. Accordingly, at the request of several delegations, ABS undertook a Special Joint Industry Project (SJIP) to investigate residual stability after damage with the support and guidance of a joint industry project committee. This committee consisted of regulatory agencies, oil companies and drilling contractors.

Preliminary SJIP results were presented to IMO in July 1988. Based on discussions during this meeting the SJIP was continued in order to develop a larger data base of dynamic responses of semlsubmersibles possessing minimum extents of residual stability. Following completion of the necessary calculations and analyses, the relevant results and findings were presented to IMO in December 1988 and will be incorporated into the revised MODU Code.

This paper presents the essential results of the ASS SJIP. Particular emphasis Is placed on damage scenarios possessing minimum amounts of residual stability and an overall evaluation of safety margins Incorporated in the analytical procedure.


The following section details the approach taken in the SJIP to assess the safety margin of MODU's against capsize in terms of the residual righting moment curve and downflooding considering predicted dynamic responses due to realistic environmental conditions.

Three generic units (GU) of semi-submersible configuration, developed as part of the intact study of the ASS JIP on MODU Stability, were used as the basis for a data base of static stability characteristics.

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