Fatigue Analysis of the Cognac Platform

Abstract

This paper describes preliminary and detailed fatigue analyses of the 1,000-ft-deep Cognac platform. Dynamic structural analysis is used to generate local member stress transfer function data, and probabilistic spectral techniques are employed to evaluate long-term stress statistics and fatigue lives using Miner's cumulative damage rule. Directional spreading of wave energy is considered in the analysis. Introduction The Cognac platform is located in 1,025 ft (310 m) of water in the Gulf of Mexico. Fatigue damage considerations played an important role in the structural design of the platform. Many of the heavy-wall joints in the structure were controlled by fatigue rather than by punching shear under the design wave. The reason is that the natural period of the Cognac platform is in the 4- to 5-second range, whereas the natural periods of other structures in the gulf are generally less than 2 seconds. Thus, the Cognac platform responds dynamically to everyday waves in the gulf, which typically have zero-crossing periods between 3 to 5 seconds. In comparison, other Gulf of Mexico structures tend to respond statically. To make matters more difficult, the hydrodynamic damping due to low waves is very small. Recent studies by Ruhl indicate that the total damping - including structural, foundation, and hydrodynamic contributions - may be only about 20% of critical for platform response in low sea states. Low damping values result in large dynamic amplifications when the period of the forcing function (the wave period) is close to the natural period of the structure. The combination of low damping and a natural period close to that of frequently occurring Gulf of Mexico sea states results in a fatigue-sensitive structure .The steps taken to ensure the integrity of the structure against fatigue damage consisted of (1) a preliminary fatigue analysis that served as a basis for the design of the structure and (2) a detailed member-by-member check of the final design using probabilistic techniques described here. Preliminary Fatigue Considerations The overall strategy of fatigue design consists of a two-stage approach:initial design of members and joints using allowables derived from generic analysis, andfinal hot-spot by hot-spot fatigue analysis, which requires that all details of the structure be established a priori.Of necessity, the detailed analysis is done after all other design work has been completed and often after fabrication has begun. If any areas with undesirably short calculated fatigue lives are found, these can be upgraded - either by grinding welds to improve fatigue performance, locally building up the section thickness to reduce cyclic stress, or making other minor modifications.This general approach has been used on several platforms in the North Sea, the Gulf of Mexico, and offshore southern California. JPT P. 374＾