Fatigue Behaviour of Offshore Structures

and Literature Rev iew Chapter 1 1. INTRODUCTION AND LITERATURE REVIEW The exploration for oil and gas in ever increasing water depths has given an impetus to research efforts on the behaviour of offshore structures under ocean environment. These structures are continuously subjected to environmental loading because of waves, wind and current. A response analysis is required to assess the safety of offshore structure under severe storm conditions as well as for estimation of damage caused by less severe but more frequently occuring sea states. A majority of the reported failures in the life time of offshore structures are in fact fatigue failures. The offshore structures are usually built in the form of welded tubular structures. The joints of these tubular members experience the fatigue damage mainly due to small defects in welding which act as crack initiators, high stress concentrations and the variable loads. The variable loads due to the ocean waves cause cyclic stress variation in the structural members and the accumu­ lated effect of these stresses results in the fatigue failure.

1 Introduction and Literature Review.- 1.1 Mechanics of Fatigue.- 1.2 Fatigue in Offshore Structures.- 1.3 Sea Environmental Loading.- 1.4 Local Stress History at Joints.- 1.5 Fatigue Life Estimation.- 1.6 Significance and Outline of Present Investigation.- 2 Hydrodynamic Loading.- 2.1 Sea Description.- 2.2 Load Description.- 3 Structural Modelling.- 3.1 Idealization of the Jacket Platform.- 3.2 Equations of Motion.- 3.3 Computation of Natural Frequencies and Mode Shapes.- 3.4 Reduced Equations of Motion in Time Domain.- 4 Foundation Impedances.- 4.1 Dynamic Soil Reactions.- 4.2 Soil-Pile Model.- 4.3 Pile-Head Impedances.- 4.4 Pile-Head Dynamic Stiffness Matrix.- 5 Fatigue Damage Evaluation.- 5.1 Evaluation of Structural Response.- 5.2 Local Stresses at the Joints.- 5.3 Fatigue Damage.- 6 Results and Discussions.- 6.1 Pile-Head Impedance Functions.- 6.2 Example Problem.- 6.3 Fatigue Damage Characteristics of a Steel Jacket Structure.- 6.4 Sensitivity Study of Fatigue Damage.- 6.5 S-N Curve Versus Fracture Mechanics Approach to Fatigue Damage Analysis.- 7 Conclusions and Recommendations for Future Work.- 7.1 Conclusions.- 7.2 Recommendations for Future Work.- References.