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Dynamic response of ship structures to impact loads Asadi, Ghasem Vaez-Zadeh


In this study the dynamic response of a ship structure to impact loads is investigated. The ship motion is fully three-dimensional and the ship structure is modeled as a three-dimensional elastic beam. Finite element methods are used to digitize the equations of motion of the system. The forces on the ship are interactive with the ship motion and position so that a full dynamic analysis is essential. Two main problems are considered: i) Estimation of hull damage when a ship collides with another ship, floating structure or fixed installation. A particular aspect of this analysis which has not previously been examined analytically involves estimating damage to the bottom of ship when it runs aground. Depending on the nature of the ground the ship may be pierced and significant amounts of steel may be torn, or the ship may ride over a sand bar without tearing but with noticeable denting and bending. In such grounding studies it has been necessary to introduce certain strength coefficients, realistic values of which have not been determined, but for which sensible estimates have been made. The results of a numerical study into grounding and collision damage illustrate clearly that ship speed is the major variable in the damage process. In particular the effect of subsequent angular motions incurred during a high speed collision can cause secondary but also significant collisions further aft. It is believed that these aspects of collision and grounding, and the related problems associated with collision whilst maneuvering, have not been investigated previously. ii) Bending stresses induced in ice-breaking ships during operation in ice. In this second class of problems two modes of operations are considered; continuous operation in level ice without loss of speed, and high speed ramming of ice ridges in which the ship is brought to rest. In the continuous ice breaking mode, the impulse loads are relatively low but periodic. The period of the impulse loads varies linearly with ship speed and also depends on the hardness and thickness of the ice. Since the ship is an elastic system with natural frequencies of the same order as impact frequency, some interesting response conditions have been identified leading to large flexural bending stresses in the ship. In the ramming mode,' two response states are of importance., The initial impulse at the bow of the ship, when contact is first made, causes the ship to respond primarily in its first flexural mode with possibly large bending stresses developing during the first second after impact. The ship then rides onto the ice in a "beaching mode" causing large quasi-static bending stresses in the hull which reach a peak after five seconds or so. Both of these peak bending situations have been investigated and their dependence on speed, hull" stiffness, bow angle, and ship speed has been established. In the past few years some data obtained from ships operating in the Beaufort sea has been released, both for continuous ice-breaking and for ramming. Whenever possible those data have been compared with the results predicted by the numerical method developed here. The agreement is shown to be very good.

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