Assessment of oil spill in ship collision and grounding
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- Institutt for marin teknikk 
The present work is concerned with problems related to ship accidents. Ship collisions and grounding continue to occur despite continuous efforts to prevent such accidents. With the increasing demands for safety at sea and for protection of the environment, it is of crucial importance to be able to reduce the number of accidents, assess their consequences and ultimately minimize or prevent potential damage to ships and the marine environment. This thesis focuses on the consequences of ship accidents. The study comprises an examination of the interaction of ships’ motions with surrounding water during a collision, a structural analysis of some collision scenarios and oil spill predictions based on damaged tanks. Further, some procedures are proposed to reduce the structural and environmental consequences. The thesis is composed as a collection of articles and consists of two parts. The first part is an introduction to the topic and summarizes the findings from my Ph.D. work. The second part presents the articles. A simplified analytical method has been used for external mechanics analysis to compute the motions of striking and struck vessels during collision, the energy released during crushing and the impact impulse. Sensitivity analysis was performed to study the relative influence of the inputs. A “one-at-a-time” sampling type of sensitivity analysis was conducted, which means that only one parameter was changed at a time, and the effects on the results were studied after each change. Based on the results, the worst cases among various scenarios were chosen for structural analysis. The striking ship was a large supply vessel, and a ship-shaped floating production, storage and offloading (FPSO) and columnstabilized platform were considered as the struck vessels. Analysis of the internal mechanism was conducted by application of finite element models to assess the strain energy dissipation and damage caused by collision for ship-ship collisions and ship-platform accidents. A series of large-scale finite element analyses has been conducted to model the event in which the stern and bulbous bow of the supply vessel collide with the side of the FPSO and the leg of the platform. The influences of boundary conditions, contact definition, friction coefficient, mesh sizes and structural design are investigated. On the basis of these simulations, force-deformation curves and pressure-area curves have been established for collision with columns of offshore platforms as well as with plane sides. A significant part of the thesis is dedicated to the oil spill process originating from damaged ships. The aim of the work is to provide a reliable estimate of the oil spill volume and out-flow time from damaged tankers. An analytical model is suggested for prediction of the oil loss volume and efflux time, which includes both instantaneous loss and subsequent loss seen in environmental effects. The instantaneous oil loss depends on the tank design, size of the hole and location of the damage, and the subsequent loss is attributable to wave, hydrostatic changes, and tidal variation. Experiments were performed to verify the analytical results. Based on scale analysis, a model tank was built at a scale of 1/30. Laboratory data of oil releases are collected. The experimental tests were conducted for releases above and below the waterline from a damaged tank. The tests were successful, and there was good agreement between the analytical and experimental results. To extrapolate the results from the model to real cases and tune the coefficients for real ships, numerical simulations have also been conducted. Two-dimensional multiphase oil spill models have been established, and the Volume of Fluid (VOF) technique and laminar flow assumptions were used. Two series of simulations were performed for the purpose of comparison between the experimental tests and more realistic full-scale conditions. The results of three different approaches were compared for various tank designs. There was good agreement between the results obtained by means of the different approaches. Finally, the study presents some active and passive methods to reduce oil outflow volume and/or increase out-flow time, such as vacuum systems, de-ballasting, oil transfer and application of an elastic membrane inside the damaged hull. Keywords: Ship Collision, Grounding, External mechanics, Internal mechanics, Non Linear Finite Element Analysis, Oil spill volume, outflow time, Experimental Tests of cargo outflow, CFD simulation, oil spill mitigation.