API TR 2PY

Fixed offshore platform pile foundation design and assessment in the Gulf of Mexico (GOM) has been based on API RP 2A (various editions) and API/ISO RP 2GEO, 1st Edition. Over the years, post-hurricane observations of the platform performance in the GOM have shown that these standards tend to underestimate soil stiffness and capacity. This underprediction does not necessarily lead to conservative structural design and assessment. For example, fatigue design requires a realistic estimate of structural dynamics and strength design, with a clear definition of structural load path and failure mechanism.

Research work, consisting of small- and large-scale pile tests, laboratory testing, and numerical analyses, has been carried out for years to understand this conservatism and define best-estimate geotechnical criteria, i.e. best-estimate soil lateral reaction for pile foundation design. These efforts culminated in the proposed draft of API/ISO RP 2GEO, 2nd Edition, which systematically redefines the clay soil lateral reaction formulations for piles under monotonic, cyclic, and fatigue loads.

The goal of this study, sponsored by API SC2, was to perform structural analyses using soil models developed by 2GEO, 1st Edition criteria and 2GEO, 2nd Edition draft criteria to determine the effect of the new clay soil p-y formulations on the structural responses of these platforms. The predicted responses were then compared with measured or observed platform performance in field.

Three featured fixed offshore platforms in the GOM, i.e. a free-standing caisson structure, a jacket type platform, and a tripod platform, were selected as representative structures for the study. The selected platforms are actual platforms in operation in the GOM. The caisson structure and jacket platform were damaged in Hurricanes Andrew and Ike, respectively. Platform vibration of the tripod platform was measured and recorded in the field to verify the tripod structural integrity.

All relevant platform design, operation, and maintenance data of these structures are sufficient and available for the study.

Results of the analyses performed in this study demonstrate that structural models using the soil reaction formulations of the proposed 2GEO, 2nd Edition (draft), in combination with the best-estimate soil profiles interpreted based on the standard direct simple shear (DSS) tests, adequately predicted the platform responses observed in hurricanes and in normal sea conditions.