|عنوان فارسی مقاله:||چگونگی توزیع انرژی در اثر برخورد کشتی با سازه جکت در انرژی بالا|
|عنوان انگلیسی مقاله:||Energy dissipation in high-energy ship-offshore jacket platform collisions|
|رشته های مرتبط:||مهندسی عمران، سازه و سازه های دریایی|
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|نشریه||الزویر – Elsevier|
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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 , 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 . The combination between these two factors can actually result in large amounts of energy especially if the incidents involving passing vessels (reported in Ref. ) 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.
2. Energy absorption Even though the elastic stiffness of the structures involved in the collision can affect the energy dissipation process, for high energy collisions the plastic deformations will absorb most of the initial kinetic energy, considering that the ship rebound will not be significant. Besides the global elastic vibrations of the installation, different plastic mechanisms can be formed locally on both ship and offshore facility depending on the collision scenario. The contribution of each of such modes is normally determined upon simplified hand calculation methods that can be found throughout the literature.
In this study the results of a series of detailed FEM simulations of the impact between vessels and fixed offshore steel platforms are discussed with respect to the energy dissipation. Predicting the size increase of supply vessels in the near future, as well as possible ship impacts involving higher energy amounts than those usually considered by the current design practice, both loading curves and simplified equivalent systems representative for ships up to 25,000 DWT were derived from the FEA. Aspects such as the steel grades or the scantling size variation were considered in the analyses for the purpose of broadening the scope of ship types/categories. Because the platform response to strong impacts might involve yielding of other members besides those directly affected by the contact with the ship, different scenarios were defined involving different plastic mechanisms and possible combinations among these mechanisms. It has been revealed that the plastic energy absorbed by the platform can be evaluated and will mostly depend on the impact area, but also include contributions from deformations that can take place in adjacent members. The thickness of the installation tubes, in particular of the struck tubes, has been shown to have some degree of connection to the platform internal energy after-impact and therefore the plastic moment of the tube walls can be used to describe the platform relative strength, depending on the platform dimensions as well, since elastic strain energy gains importance for big steel platforms and for joint impacts. The interaction between the vessel and the platform appears to be important, particularly in cases in which the energy share is closer to 50/50 corresponding to significant energy amounts and significant plastic deformation/damage in both structures. For the ship range considered, these are observed mainly when the tube thickness is in 45 mm < t < 70 mm. In conclusion, if plastic analysis is to be considered in the design practise against extreme accidental loads from higher energy ship impacts, the present results might provide some good indications especially regarding the global integrity of the structure and the ship-installation interaction. However, this can be better complemented with additional investigation on the individual failure of members based on energy absorption since that is only addressed generally in the current study.