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Evaluation of the nonlinear structural response of any structure is a challenging task; a range of input parameters are needed, most of which has significant statistical variability and the evaluations require a high degree of craftsmanship. Hence, high demands are set forth both to the analyst and the body in charge of verification of the results. Recent efforts by DNVGL attempts to mitigate this with the second edition of the DNVGL-RP-C208 for determination of nonlinear structural response, in which guidance or requirements are given on many of the challenging aspects. This paper discuss the various challenges and the direction to which the RP-C208 points compared to published research. Parameters affecting the plastic hardening, strain-rate effects and ductile fracture are discussed separately. Then, the combined effect of the range of assumptions is evaluated to assess the resulting level of safety. INTRODUCTION Design of offshore structures for infrequent actions such as collisions and wave impacts is challenging. The nature of the problem is such that it involves numerous non-linearities and uncertainties related to material response, structural geometry and scenario. To make the analysis even more complex, evaluation of extreme responses are often flavored by the analysts own background and experience. In the case of evaluating the response of a collision event, the common approach goes by use of nonlinear Finite Element analysis (NLFEA). Assuming that the analyst has identified the most critical and relevant accidental scenario and established a decent numerical model, the next step is to analyze the accidental response of the structure. This involves determining the plastic deformation and potential for load redistribution until the system reaches unstable failure, e.g. penetration hull, cargo tanks or a set of compartments. The way to establish the combination of material parameters and the subsequent onset of fracture has until now been much up to the analyst and his design team to decide provided that the rationale behind the analysis is sound. With the recent update in DNV-RP-C208 [1], the recommendations to ductile fracture capacity and restrictions to yield level are revised. This may enhance safety with respect to design against collisions, but if not exercised with care may very well lead to unnecessary conservative accidental design. Compared to other industries in which nonlinear response is of importance, the maritime and offshore industry does only to a very limited degree utilize material testing for calibration of the NLFEA simulations. Rather, the material parameters are calibrated to lower, mean or upper bound limits to which the material batches have to be grouped. The connection between the actual material and the simulated material is thus lost. Combining this with the knowledge that the plastic response of the material is determining where and how strains will localize (and thereby where fracture will initiate, see Storheim and Amdahl [2]) raises a concern w.r.t. the accuracy of the simulation results. Further, when individual material parameters are assessed separately rather than combined, e.g. by minimizing the strain to failure and ultimate tensile strength separately, the resulting behavior may deviate significantly from the reality. The material response is further complicated when fracture may occur. First, a reasonable strain localization is a prerequisite for a reasonable fracture prediction. Second, the complicated process of fracture should be included as 1 Copyright © 2017 ASME Proceedings of the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway

Evaluation of Nonlinear Material Behavior for Offshore Structures Subjected to Accidental Actions