A Reliability Assessment of Collapse Safety Factors for Tendon Design

Since its first edition in 1987, the tendon system design section of The Recommended Practice for Planning, Designing, and Constructing of Tension Leg Platforms, API RP 2T, has not been materially revised. An entirely new draft of that section, including both strength and fatigue recommended practices, has now been completed. One key aspect for future designs is the maximum depth in which tendons can be installed while still maintaining adequate and consistent reliability. The new strength guidance recommends the axial tension-hydrostatic collapse interaction equation currently used by API RP 2A LRFD, coupled with a working stress design format with explicit tension and collapse safety factors. The latter factor controls the water depth applicability. To establish a basis for a hydrostatic collapse safety factor, ExxonMobil performed an independent reliability study to arrive at a safety factor consistent with the new interaction equation accuracy and current fabrication and design tendon practices. This paper presents the data gathering, analysis procedure, and interpretation of results leading to a safety factor that is consistent with standard practices. The analyses were conducted via Monte Carlo simulations for a number of design cases across a range of safety factors. Results indicate that a collapse safety factor of 1.67 for the Category A operational load case that controls tendon design in deep water yields levels of reliability consistent with those historically sought by design codes such as AISC and API.