On accidental limit state design of spherical type LNG carrier structures against ship collisions
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The modem design of steel structures uses limit states classified into four types, namely serviceability limit state, ultimate limit state, fatigue limit state and accidental limit state. The aim of this paper is to fill an important existing design need, by proposing and illustrating a procedure to confirm that the accidental limit state based safety level of a spherical type LNG carrier designed to carry liquefied natural gas of 135,000 m 3 capacity is sufficient against ship collisions. An assessment procedure based on the energy dissipation capability relating to structural crashworthiness can in this case be applied to protect spherical LNG cargo tanks from fracture in minor or moderate collision accidents as shown in the paper. In the procedures developed, while the loss of kinetic energy during the collision accident is approximately estimated by applying the momentum equilibrium of the two implicated ships, a series of relatively sophisticated numerical computations are undertaken to simulate the structural crashworthiness of the object ship in the full load condition at standstill, varying the loading conditions and types of the striking ship, when the LNG carrier side structure is considered to be struck by the bow of either a VLCC or another LNG carrier. Based on the computed results, the collision resistance indices (CRI), which in the present study are defined as a function of the ratio of the structural energy dissipation capability to the initial kinetic energy loss are calculated for the object ship for various collision scenarios. Some important insights useful for preventing collision damage of the spherical LNG cargo tanks of the ship are also discussed.