5. Conclusions
Current design guidelines for ship collisions with offshore structures were developed thirty years ago for supply vessels of 5000-ton displacements with a raked bow. The guidelines need to be revised in view of significant increases in the supply vessel sizes and a wide variety of bow configurations (bulbous bows, X-bows etc.) that can exert significant collision forces over small areas on platforms. In addition, if ice-reinforced supply vessels are used to serve offshore platforms, significantly stronger bows will be used than that assumed in NORSOK N-004.
The expected increase in the requirement for the kinetic energy from accidents will place heavy demands on the energy dissipation. There are several scenarios for which it may be difficult to meet these demands without sizing platform members in the collision-prone area using shared-energy design or strength design such that the impacting vessel dissipates a significant amount of energy. This consideration is especially relevant for jackets for which there is a limited potential for energy dissipation in typical single braces. This study demonstrates the importance of accounting for interaction effects. A rigid bow is an overly conservative assumption for most scenarios and does not give appropriate credit to strengthening efforts. However, neglecting interaction effects may lead to an underprediction of the collision load exerted on the platform. For bulbous bow impact to floating platforms, the rigid bow assumption results in larger damage and energy dissipation at the forecastle deck level, whereas more dangerous penetrations of the platform by the bulb below sea level may be grossly underestimated.