Offshore platforms during their pre-service and in-service stages are under cumulative damages due to repeated application of time-varying stresses known as fatigue damages. Ocean wave loads usually have the highest contribution as time-varying loads on the jacket part of fixed platforms and therefore fatigue design of jacket structures to wave loading is one of the main structural design challenges. Spectral fatigue analysis is a widely accepted method for fatigue design of fixed offshore platforms mainly for the dynamic sensitive ones. Stress range transfer functions are needed in spectral fatigue analysis and can be obtained using time domain or frequency domain analysis. Time domain analyses usually lead to more accurate transfer functions but they need much more computational efforts compared to frequency domain analyses. Calculations for frequency domain transfer functions are much faster and usually lead to more conservative fatigue results. This paper compares spectral fatigue design results of an example platform using time domain and frequency domain transfer functions. Additional adjustments on the frequency domain analysis to make them less conservative and as close as possible to time domain results are also discussed in detail.
Structural joints subjected to large numbers of time varying load cycles may fracture even at very low nominal stress levels. This is known as fatigue damage and is taken into account as one of the most important considerations in design of offshore platforms. Usually cyclic stresses due to ocean wave loads have the highest contribution in fatigue life of the jacket part of fixed platforms (ISO, 2007). S-N curves (number of allowable loading cycles, N, for a given stress range value, S, based on experiments on different types of specimen) or fracture mechanics techniques (for the assessment of in-service cracks and selection of the inspection methodology) are usually used for fatigue design of joints (Baltrop, 1991).
