Experimental study of Stability Limits for Semisubmersibe Drilling Platforms

ABSTRACT The existing stability criterion for semi submersible drilling platforms requires righting moment energy in excess of wind heel energy to allow for dynamic effects. Dynamic effects have been studied by testing a scale model of a semi submersible platform to evaluate the effect of wind velocity, center of gravity, air gap, and moorings ontendency to capsize in survival wave conditionsrelative motion between leeward deck edge and wave surface as a measure of down flooding tendency. FOREWORD At the occasion of the 1971 Offshore Technology Conference, the research panel on Offshore Mobile Platforms had its first meeting. Designated as Panel MS-3 of the Marine Systems committee, under the continuing research activities of the Society of Naval Architects and Marine Engineers, it has a se 1ect representation of drilling con tractors, oil companies, regulatory agencies and naval architects. Its purpose was set forth to identify and to promote research in such vital areas associated with mobile platforms towards more adequate design, construction and operation. By the following year, the panel had clearly identified the assessment of stability of mobile platforms as one of the most important and pressing problems, particularly in regard to the semi-submersible. The existing criteria, as promulgated by the u. S. Coast Guard and the American Bureau of Shipping (Reference I), were considered inadequate to properly assess the various dynamic influences on the platform and its inherent ability to resist them. A program of research was indicated. By OTC 1973, the program was on its way. Davidson Laboratory of Stevens Institute of Technology was selected to perform the initial research, and funds were being accumulated from solicitations through the industry, to where by mid-summer the first phase of experimental work was begun. This paper discusses the rationale leading to the panel IS conclusion that a research project of this type was both necessary and urgent, along with an outline of the full intended program. The gist of the paper, however, is the presentation of the experimental work done during the second half of last year, as a major part of this research, and which resulted in some valuable observations and a few surprises. EXAMINING THE PRESENT STABILITY CRITERIA Dissatisfaction with the present stability criteria as applied to semi-submersibles has existed since their adoption in the 1968 ABS rules. (It may be noted that many panel MS-3 members have also been serving on the ABS rules committee.) Based on a U. S. Coast Guard criterion for the stability of a surface ship in roll, somewhat empirically derived, it does not adequately account for the excitations nor responses of the semi-submersibleto the possible environment. Figure I shows the now familiar curves of requirement for stability; taken from the detail requirements as set forth in Reference I and as later amended in Reference 2. It shows an overturning moment curve, calculated for a steady wind speed, superimposed on the vessel IS righting moment curve of static stability, all in calm water.