Analytical and experimental investigation on the fatigue behaviour of tubular joints for offshore monopod structures

Gowda, Siddappa Shankare (1983) Analytical and experimental investigation on the fatigue behaviour of tubular joints for offshore monopod structures. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This study is concerned with the analytical and experimental investigation of fatigue behaviour of monopod tubular joints in air and cold seawater environments. A total of seven joints were tested under cyclic loading. Two specimens - one in air and the other in water - were subjected to pseudo-random loadings. The remaining five specimens - two in air and three in seawater - were tested under constant amplitude sinusoidal loadings. The joints were tested both in as-welded and weld tow ground conditions. The corrosion tests were carried out in natural seawater at 0°C to simulate the cold ocean environment. -- From the measured strains during static loading at critical hot spot locations, strain and stress concentration factors were determined and compared with the finite element analysis. The MSC/NASTRAN finite element computer code was used using triangular and quadrilateral flat shell elements. Each element has five degrees of freedom consisting of two in-plane displacements, a normal displacement and bending rotations. -- The effect of seawater, weld toe grinding, the nature of applied loads and crack growth characteristics are studied in detail. The fatigue lives of the joints for crack initiation, propagation and final failure are determined and compared with those of T or Y-joints from the published literature. For the three joints tested in seawater, the fatigue life for wall penetration are compared with the analytical life estimation using fracture mechanics approach. The fatigue results of all the joints tested are compared with the relevant U.K. DD 55 Q and AWS-XX S-N curves.

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