International concern In the area of safe operation of Mobile. Offshore Drilling Units (MODUs), particularly semisubmersibles, has generated additional requirements Intended to enhance safety margins of stability. Recognizing the need for an In-depth review of the existing MODU stability criteria In order to assess this concern, the American Bureau of Shipping (ASS) has, over the past several years, been examining the validity of current standards with the support and guidance of a joint Industry project committee. This committee is comprised of representatives of regulatory agencies, oil companies, designers/builders and drilling contractors. This paper describes the research comprised of tasks on environmental conditions, model testing and analytic correlation, theoretical analysis, assessment of the existing criteria and proposal of a new criteria format. Inconsistencies afforded by the existing criteria are quantified and discussed. Finally the specifics of a new criteria format which Include dynamic response characteristics are addressed with respect to ultimate stability or capsizing, as well as the downflooding, are presented. The paper concludes with a discussion regarding the Impact of the proposed criteria on existing semi-submersible designs based on changes in deckload carrying capacity.
The fundamental stability criteria for MODUs (i.e., the intact area ratio) has been applied by every regulatory body since the criteria's formal Introduction by ASS, two decades ago. It is necessary to point out that this criteria, which was derived on an empirical basis considering service experience accumulated over many years for ships, is a standard, based only on the hydrostatic restoring capabilities of a design relative to a mean wind force. In order to judiciously assess the additional requirements set by several administrations, ABS initially sought to analyze the underlying principles of the existing stability criteria with respect to semisubmersible MODUs. This research program, referred to as the Phase-1 Study [1,2] demonstrated the validity of the analytical tools and, through a limited parametric study, identified influential parameters characterizing the dynamic behavior of semisubmersibles In moderate seaways. Phase-I concluded that neither the area ratio nor the Initial GM effectively guarantee a uniform safety margin against downfloodlng. Further, additional investigations utilizing dynamic analysis would be necessary so that concrete conclusions regarding the justification or modification of the stability requirements could be formed [3].
In view of the above conclusion and the desire expressed by several organizations to achieve a rational assessment of the stability standard, ASS presented a project proposal to industry soliciting financial as well as technical support. The ASS Joint Industry Project (JIP) on MODU Stability formally commenced on 25 June 1986 with the first meeting of the 17 participants that comprise the (JIP) Committee. Accomplishments of the project are the basis of this paper.
In light of the above and to evaluated the margins of safety against downflooding and capsize, the JIP established three objectives:
Parametric analysis of semi-submersibles behavior in realistic environmental conditions to determine geometric and operating parameters effecting the dynamic motion of semi-Submersibles.
