Reliability of Suction Caissons for Deep Water Floating Facilities

With the extension of the offshore industry into deeper water, the types of offshore structures changed from gravity base structures and fixed platforms supported by driven piles to floating vessels supported with vertical tendons or catenary mooring lines. The anchors for these new systems also changed as increased water depths presented problems for underwater pile driving. Suction caissons then became the preferred foundation option for these facilities. A number of significant differences exist between how suction caissons and piles provide resistance and how the foundation system for a floating vessel performs compared to a fixed platform. One of the primary challenges with this new foundation system was the determination of the overall system reliability. An initial study for suction caissons by Clukey et al. (2000) showed that the overall system reliability for suction caissons was not as robust as for a fixed platform. The impact of a single suction caisson failing was found to be more severe and could more easily lead to a system failure. Based on this initial generic study, it was recommended that for extreme loading events the factor of safety (FoS) for Gulf of Mexico (GoM) 100-yr hurricane conditions should be increased from 1.5 (for piles) to 2.0 for suction caissons. This FoS is currently being used in API-RP2SK for vertically dominated loading. However, since that initial study significantly more reliability work has been done at the University of Texas, Det Norske Veritas, and the Norwegian Geotechnical Institute. These studies considered a wide variety of cases, including different floating vessel types, soil conditions, water depths, and loading conditions. The results from this more recent work are compared in this paper. These results show that the current design codes are robust and conservative. Based on the studies opportunities exist to make the design codes less conservative while still maintaining desired levels of reliability. In addition, the historical development of suction caissons is presented.