Energy absorption of offshore platforms during ship collisions
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- Institutt for marin teknikk 
Severe ship collisions are fortunately not a frequent occurrence, however they are present. There were 115 recorded collisions on the Norwegian Continental Shelf from 1982-2010, and in the more recent years between 2001-2010 there have been 26.Although none of the collisions have caused loss of life, the potential consequences of such accidents vary from minor structural damage to major threats to structural integrity, great economic loss and severe oil pollution. It is typically understood that when an offshore platform is struck by a ship, most of the energy is dissipated through plastic deformations, and little energy as elastic vibrations. This thesis investigate energy absorption of offshore platforms during ship collisions. Three different platform structures are examined: a four legged jacket an eight-legged jacket a jack-upAn impact model consisting of a mass-nonlinear spring system was established in the nonlinear FEA program USFOS. To get as accurate and realistic results as possible the local indentation characteristics of the ship was accounted for by using the force-deformation relationship obtained from a high-resolution local bow crushing analysis in LS-DYNA.An initial analysis of the three platforms were performed yielding reasonable results. The various platforms have very different structural integrity which leads to a distinct response. The two jacket platforms experience similar response but to a different extent as the eight-legged jacket is stiffer and stronger than the four-legged jacket. In the jack-up platform more energy was dissipated through elastic vibrations. Parametric studies were performed varying the impact direction and kinetic energy at impact. To examine if the natural period of vibration for the platform has a connection with the energy dissipation, the natural period of vibration was modified by scaling the tube thicknesses of the structural members. The difference in the natural periods for the jacket platforms were very small and only around 0.03-0.05s. It is unlikely that this small variation has any significant impact on the energy absorption of the platforms. The difference in energy absorption was rather believed to be due to change of structural properties which again cause different interactions between the platforms and ship.For the jack-up platform the variation of the natural periods were considerably larger with a relative difference of 3-5 seconds between the smallest and largest thicknesses. The parametric studies showed that there indeed is a possibility that the natural period of the jack-up platform has an impact on the energy absorption. As the natural period became larger, less energy was absorbed by plastic deformations of the ship and platform, and more energy was dissipated as elastic energy.