Fatigue Behavior Of Tubular Joints In Offshore Structures

ABSTRACT A comparative study of the potential fatigue behavior of tubular K-type joints using theoretically derived fatigue prediction curves is outlined in this paper using a cumulative damage approach. It is shown that under the same stress level the fatigue resistance increases proportionately with the yield strength for steels with yield strength lower than 60 ksi and remains approximately constant for steels with higher yield strength. Hence an overall economy in terms of extended service life can be archieved. INTRODUCTION In the template type offshore structures, tubular members are often arranged in such a manner that the entire load is being transferred from one branch to the other via the chord. Due to the radial flexibility of chord members, the stress picture at the welded joints is complex. The local stresses and the fatigue resistance of the welded joints are largely governed by the member sizes, the quality of welds and the wide variation of environmentally induced loading cycles. To improve design of such structures, fatigue resistance, ductility, fracture toughness, homogeneity, weldability, strength to price ratio etc., are to be considered for various types of steels which may be used in the construction of these structures. Based on experimental observations, Marshall (1) demonstrated that in case of static failure, the tubular joints have a tremendous reserve capacity beyond the first yield. Beyond yield, the connection deforms while the applied load continues to increase and finally at load levels of the order of 2.5 to 8 times that of the first yield, the joint fails either by pullout failure in case of tensile loads or by localized collapse of the chord for compressive loads. This extra capacity results from plastic behavior, triaxial stresses, strain hardening, load redistribution and large deformation behavior at the chord material. Welded tubular joints in offshore structures are usually subjected to a continuous spectrum of cyclic wave loadings which require considerations of cumulative fatigue damage. Where fatigue environments involve stress cycles of varying magnitude and varying number of applications, failure is usually assumed to occur when the Miner sum* (mathematical formula) (available in full paper) reaches unity. Stress fluctuations due to wave loadings are defined in terms of the peak-to-trough amplitude of the stress range of these fluctuations. Because of the induced residual stresses in the tubular joints during the welding process, the zero stress points usually remain undefined and therefore the contribution of the mean stress level in fatigue damage has not been considered in the present investigation. This paper outlines the fatigue performance of four different grades of structural steels ranging in yield strength from 36 ksi to 100 ksi. The fatigue performance of these steels are investigated by evaluating the cumulative damage of K-type tubular joints subjected to a typical wave loading in the Gulf of Mexico. The effect of yield strength on the fatigue resistance of tubular joints has been studied quantitatively and the results appear to indicate that if the low strength carbon steels in tubular joints are replaced with low-alloy high strength steels, a significant improvement in fatigue performance can be achieved. This effect of yield strength may be attributed to the role of plasticity in influencing the behavior of sharp fatigue cracks in structural steels under cyclic loading conditions.