ABSTRACT:

Load carrying capacity of a tubular member with a circumferential through-thickness crack is investigated. A series of pure bending tests is carried out. Based on the experimental results. a generation phase analysis is performed with the finite element method. in which both material and geometrical non-linearities are taken into account. Application phase analyses are also performed employing the J-integral and the Crack Opening Angle (COA) as parameters which rule the initiation and the continuation of a crack growth respectively. It has been found that a higher ultimate strength may be attained with higher critical values of J-Integral and COA and the decrease in load carrying capacity is strongly affected by the critical value of COA.

INTRODUCTION

Many types of offshore structures have been built and in service to get seabed resources such as petroleum and natural gas. However, the connections of tubular members sometimes experience a fatigue damage due to small welding defects, large strain concentrations and cyclic wave loads. The denting damages caused by supply boat collisions or object fallings from a platform deck may also act as a crack initiator because of the large strain concentration at the bottom of the dent. In general, a material with sufficient ductility is used for legs and bracing members in offshore structures and the initiation of a crack does not lead to the immediate breaking of the cracked member. However, a crack usually grows under cyclic wave loads, and a plastic collapse or an abrupt breaking of a cracked member may occur when the structure is exposed to severe sea condition. The collapse of a cracked member may further cause that of other intact members because of the redistribution of internal forces, which may finally lead to an overall collapse of the structure.

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