This paper presents a procedure for evaluating the collision response of fixed offshore structures exposed to high energy collisions with Offshore Service Vessels (OSV) or other floating units. A combination of traditional non-linear frame analyses for screening purposes and more detailed non-linear FE analyses using an explicit code is proposed. In the detailed integrated FE analyses, both the fixed offshore structure and the impacting vessel are explicitly modeled as deformable bodies. The progressive changes in contact and the resistance of both deforming objects are fully accounted for, enabling for more accurate results compared to methods where the resistances of the two objects are assessed separately. Results from case studies are shown. The calculated resistances for the striking objects are compared to the load-indentation curves presented in DNV-RP-C204 [1] and NORSOK N-004 [2].

Non-linear time domain irregular wave simulations have been performed for the Kvitebjørn jacket platform located in the North Sea with the aim to quantify the dynamic amplification. The jacket is a slender structure installed in about 190m water depth. For each of the selected extreme sea states both quasi-static and dynamic response simulations have been carried out for several wave realizations using different seeds. Based on the quasi-static response and dynamic response, equivalent dynamic amplification factors (EDAFs) were calculated for different response measures in the jacket. EDAF is the factor one has to multiply the q-probability quasi-static response with in order to obtain an adequate estimate of the q-probability dynamic response. The EDAFs are to be used in ultimate limit state (ULS) and accidental limit state (ALS) analyses of the platform. Simulations were performed applying a Gaussian wave with Wheeler stretching and a second-order wave model. This paper focuses on the selection of wave kinematics method and on the establishment of the EDAF analysis procedure.

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This paper deals with how structural integrity can be documented by use of non-linear analysis methods. The focus is on structures that are exposed to extreme environmental loads. In order to prove the structural integrity for aging offshore installation, non-linear analysis methods are finding increased applications. The codes ISO 19902, API RP 2A and Norsok N-006 give recommendations for use of non-linear methods for capacity checks of existing structures. This paper discusses the different safety formats given in these codes. Also presented is how practical acceptance criteria that have to be applied in conjunction with such analyses can be formulated. The topics that are dealt with are issues that the analyst performing such analyses needs to address like: definition of failure, representation of joints, issues on cyclic loading, instability of cross sections and member buckling. In cases of novel design solutions or special structural details, non-linear analysis methods may also find application when planning new structures.