چگونگی توزیع انرژی در اثر برخورد کشتی با سازه جکت در انرژی بالا  Energy dissipation in high-energy ship-offshore jacket platform collisions

The ship collision evaluation often includes impact events between the offshore installation and nearby vessels subject of off-loading strike. A risk assessment usually needs to be carried out for potential collision events that are screened according to their risk for the structure and also to their likelihood. For the assessment process, the failure shall be considered for members individually or by means of the overall performance of the facility, i.e. keeping the structure functional after, for instance, rupturing of a brace or denting of a leg. During the impact, the kinetic energy of the striking ship is converted into strain energy of the vessel and the facility (that can be either fixed or floating). Some of the energy might also remain associated with the motion of the structures after the impact (rebound). It is therefore important to account for the plastic deformation and failure of structural members that are affected by the collision since they will generally be associated with the primary structural effects. Fixed platforms are typically lower in redundancy than the floating ones and also constitute the most representative offshore structures [1,2]. Likewise, the acceptance criteria defined for each type is also different. As for the energy amounts specified for the collision assessment, these are derived from both vessel size and impact speed. Collision events involving supply vessels are currently predicted for ship sizes up to 5000 ton [3], although these have significant variations in size according to the region they operate [4,5], while for the impact velocity these might range from 0.5 m/s for low-energy collision to 2 m/s for drifting supply vessels [4]. The combination between these two factors can actually result in large amounts of energy especially if the incidents involving passing vessels (reported in Ref. [6]) are considered, as well as a plausible increase in the number and average size of the world’s fleet in the coming years. For the evaluation of the structural damage via energy balance, the internal energy consists of contributions from both the vessel and installation strain energy. Such contributions might vary upon the relative strength between the two structures. The methods used to estimate the strain energy in the current design practice can be very conservative because of neglecting the ship-platform interaction through the assumption of the ship to be rigid and the entire strain energy from the installation deformation, or less conservative, by analysing ship and platform being collided by a rigid body separately. For the latter case, the strain energy from the vessel, as well as the associated damage to it is usually underestimated same is the correspondent applied load. To improve the prediction accuracy, the high fidelity FEA provides a mean to perform the coupled analyses by simultaneously considering deformations of both the facility and ship, and including their interaction. This approach gains significance, in particular for cases where greater energy amounts than those currently predicted by the design practice, since a better accuracy could allow for a less conservative solution.