Efficient Spectral Wave Fatigue Assessment of Modules: Utilizing Response Amplitude Operators and Generic Stress Concentration Factors

Fatigue failure occurs due to repeated loading, and it is important to consider cyclic loading from wave action in the design. This paper describes an efficient spectral fatigue procedure that utilizes response amplitude operators (RAO), generic stress concentration factors (SCF), and structure analysis computer system (SACS) using its sub-programs, called Tow, Fatigue and Joint Mesher. Hot spot stress spectral fatigue analysis of topside modules, consisting of non-tubular joints, is usually complex and cannot be easily performed. An integrated model of the topside module and floating production, storage, and offloading (FPSO) vessel can be used in fatigue analysis. The hull and topside module engineers work with independent models due to contractual obligations. The hull engineer analyzes the FPSO vessel with a finite element (FE) model. The topside module engineer uses beam elements without meshing. The topsides module engineer can utilize RAO information provided by the naval architect. This was considered as an efficient alternative to developing an integrated model. In a hot spot stress approach, a large portion of the labor hours for fatigue analysis were normally allocated in generating the SCF. A default SCF equal to six is typically assumed for axial, in-plane bending, and out-of-plane bending in fatigue screening of non-tubular joints. This default SCF is not effective when there are many modules to analyze, and a local FE model is needed to further evaluate joints. In this paper, an efficient approach is proposed. The approach is using generic SCF of typical joint for the module. The approach has been applied on FPSO topside modules, which were planned to be installed in the North Sea and Gulf of Mexico. An example application of the procedure for a module is discussed. It also includes other considerations when performing fatigue analysis of modules. The usage of RAO information instead of an integrated model, and generic SCF to derive hot-spot stresses allows convenient application of different modules or projects that are based on typical or standard joint details. It increases the efficiency of resources, i.e. labor, computation, in performing spectral fatigue life assessment. The approach appears to be an improvement compared to using a default SCF equals to six.