Implicit – Explicit – Implicit, An Investigation of a Ship Impact on an Off-Shore Installation

The initialisation of stresses due to gravity or manufacture can often be ignored when the item concerned is subjected to an impact such that the stresses induced by the impact are significantly higher than those from any pre-stress. In items such as nuclear transport containers the assumption of no pre-stress in fastening bolts can predict greater separation than would be achieved in practice and hence be considered as a conservative approach. In a large structure, such as an off-shore oil or gas platform, both the stresses due to gravity and the displacement can be significant. In the cases considered here, an additional structure is required to be added to an existing off-shore platform. This structure must be analysed to ensure both it and the existing structure can withstand a 14MJ ship impact from any direction and at a number of heights depending upon the sea level. It is further necessary to ensure that this post impact, damaged structure can withstand a 100 year storm loading. In general this would not cause too many problems, but the analysis model used also combines loadings from DNV-Sesam which is applied to a beam model. This beam model is required to be translated to a LS_DYNA model and certain beams are then represented by a far more detailed shell and solid element model.