Calibration of partial safety factors for offshore foundation design

The foundation of an offshore installation is designed to resist the static and dynamic loads it is subjected to during its lifetime. The design codes specify combinations of load factor on the static load, load factor on the dynamic load, and resistance factor on the soil resistance, with reference to the ‘characteristic’ loads acting on the foundation and the ‘characteristic’ foundation capacity. In principle, the designer aims at achieving a target annual failure probability. Whether or not the load and resistance factors specified in the design codes actually achieve a target annual probability of failure depends on the type of installation, the load characteristics, the soil characteristics, the analysis method(s) and the relative importance of static and dynamic loads. The paper calculates the load and resistance factors required for a target annual failure probability of 1 × 10. The calculated load and resistance factors are then compared with the values required by three offshore design guidelines and codes: API LRFD (2003), ISO 19902 (2007) and NORSOK (2004).