By use of the rigid-perfect plastic constitutive assumption, a simplified plastic model is proposed for the dynamic analysis of loose hatch plates in offshore platforms subjected to vertical impact of falling drill-collars. Dynamic P-wc curve is first developed for the estimation of critical impact energy and the corresponding critical displacement of the plate at the impact point with a satisfactory accuracy. A further simplified bi-linear P-wc curve is proposed for the description of the time histories of the interface force between the projectile and the target, the velocity and the displacement of the plate at the impact point As the results are given in analytical formulae, the proposed model is convenient for the design purpose. The present model has been proved to be satisfactory as compared with experimental results if the plugging criterion (i.e., critical interface force) can be accurately determined.
Among all kinds of dropped objects in offshore engineering, drill-collars have been identified as the most dangerous item because of its high kinetic energy and small contact area which may cause penetration on the deck structure of an offshore platform. The specifications of this kind of impact problems are large projectile mass, low impact velocity and small contact area. So far, research on this topic has been caried out by model or full scale experiments, e.g., the work reported by N. Ellis et al (1980), A. Wenger (l98la, b, 1982), M. Langseth (1988) etc. Few theoretical study has been found in available literatures except for those concerning circular plates (L. Cederfelt, 1982) or infinite plates (R. L. Woodward, 1987). It has been shown by scaled model tests that the dynamic response of a loose hatch plate during an impact by a dropped drill-collar can be divided into three phases: the transient phase which lasts to the activation of the support,