Corrosion Fatigue Strength of Offshore Monopod Tubular Joints in Cold Ocean Environment

Abstract

ABSTRACT This paper describes the fatigue behavior of monopod tubular joints in air and cold ocean environments. The test program includes testing of seven joints under constant amplitude and pseudo-random loadings in as-welded and weld toe ground conditions. The corrosion tests were carried out in natural seawater at 0°C. The effect of stiffening rings has been studied on one joint under constant amplitude loading. The strain and stress concentration factors determined from strain measurements are compared with the finite element analysis which correlate well with the experimental results. The effect of seawater, weld toe grinding, the nature of applied load and the application of fracture mechanics are discussed in detail. The fatigue lives of the joints for crack initiation, propagation and final failure are compared with the relevant U.K. DD 55 Q and AWS-XX S-N curves. INTRODUCTION Research of fatigue of steel tubular joints has heralded by the petroleum industry due to increasing use of tubular structurals in the design and fabrication of offshore exploration and production platforms. The majority of exploration and production platforms are of the basic jacket-type supporting a platform deck. A monopod platform in Cook Inlet, Alaska, and a monocone structure in the Beaufort Sea have been installed in shallow waters. Heerema Engineering Company has recently proposed a fixed type Tripod Tower Platform, (Fig. 1), for a water depth of 375m, which could also be used in ice infested regions[1]. Monopod structures offer minimum exposed frontal area to reduce wave and ice actions [2]. Analytical and experimental investigations of monopod tubular joints subjected to both static and fatigue loads are scarce in the literature, compared to planar joints such as K, T, X, Y, in which extensive research has been carried out by many investigators (3,4,5,6,7). This paper describes the experimental and analytical investigation of the fatigue behavior of monopod tubular joints with and without stiffening rings in are and cold seawater environments. The joints were tested under constant and variable amplitude loadings in as-welded and weld toe ground conditions. The fatigue lives for crack initiation, propagation final failure of the joints tested in 'wet? conditions are compared with those of a companion 'dry? test specimen and the relevant published data. The linear extrapolation of the strain ranges was used to determine the hot spot strain range. The measured strains and stresses were compared with those obtained by the finite element analysis. The stress intensity factors and the corrosion fatigue crack growth data relevant to the application of fracture mechanics theory are discussed in detail. The crack growth behavior in air and water is determined and the fatigue life of the joints compared with the U.K. DD 55 Q and AWS-XX S-N curves. EXPERIMENTAL PROGRAMME Design and Fabrication of Specimens The specimens consist of one joint with stiffening rings (Fig. 2) and six joints without stiffening rings (Fig. 3). The material used for the specimens was of ASTM A53 and ASTM A36 specifications with a yield strength of 372 N/mm2 and 248 N/mm2.