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.- 1.3.1 Sea environment model.- 1.3.2 Hydrodynamic loading on the structure.- 1.4 Local Stress History at Joints.- 1.4.1 Structural model.- 1.4.2 Foundation model.- 1.4.3 Methods for determining the stress response.- 1.4.4 Stress concentration at joints.- 1.5 Fatigue Life Estimation.- 1.5.1 S-N approach.- 1.5.2 Fracture mechanics approach.- 1.6 Significance and Outline of Present Investigation.- 2 Hydrodynamic Loading.- 2.1 Sea Description.- 2.1.1 Short term model.- 2.1.2 Long term model.- 2.1.3 Simulation of random waves.- 2.1.4 Wave kinematics.- 2.1.5 Treatment of variable submergence.- 2.1.6 Wave current interaction.- 2.2 Load Description.- 2.2.1 Fluid loading on a tubular member.- 2.2.1.1 Drag force and its linearisation.- 2.2.1.2 Inertia force.- 2.2.1.3 Evaluation of nodal loading.- 2.2.2 Fluid loading associated with lumped volumes and areas at the nodes.- 2.2.3 Calculation of the load vector.- 3 Structural Modelling.- 3.1 Idealization of the Jacket Platform.- 3.1.1 Structural model I.- 3.1.2 Structural model II.- 3.2 Equations of Motion.- 3.2.1 Mass matrix.- 3.2.2 Damping matrix.- 3.2.3 Stiffness matrix.- 3.3 Computation of Natural Frequencies and Mode Shapes.- 3.3.1 Generalized coordinates.- 3.4 Reduced Equations of Motion in Time Domain.- 3.4.1 Generalized mass matrix.- 3.4.2 Generalized damping matrix.- 3.4.3 Generalized stiffness matrix.- 3.4.4 Generalized load vector.- 4 Foundation Impedances.- 4.1 Dynamic Soil Reactions.- 4.1.1 Soil stiffness and damping.- 4.2 Soil-Pile Model.- 4.3 Pile-Head Impedances.- 4.3.1 Vertical vibration of pile.- 4.3.2 Horizontal vibration of pile.- 4.4 Pile-Head Dynamic Stiffness Matrix.- 5 Fatigue Damage Evaluation.- 5.1 Evaluation of Structural Response.- 5.1.1 Frequency domain solution technique.- 5.1.2 Mode acceleration method.- 5.1.3 Nominal stresses at the joints.- 5.2 Local Stresses at the Joints.- 5.3 Fatigue Damage.- 5.3.1 S-N curve approach.- 5.3.2 Fracture mechanics approach.- 5.3.2.1 Stress intensity factor.- 5.3.2.2 Fatigue crack growth model.- 5.3.2.3 Weighted average range.- 5.3.2.4 Fatigue life estimate.- 6 Results and Discussions.- 6.1 Pile-Head Impedance Functions.- 6.1.1 Validation of the proposed analytical technique.- 6.1.2 Influence of various soil parameters on the pile-head impedance functions.- 6.1.2.1 Effect of soil’s shear modulus.- 6.1.2.2 Effect of soil’s Poisson’s ratio.- 6.1.2.3 Effect of soil’s material damping.- 6.1.2.4 Uniform versus linear distribution of soil’s shear modulus.- 6.1.2.5 Effect of soil-pile separation near mudline.- 6.2 Example Problem.- 6.2.1 Description of the structure.- 6.2.2 Description of the long term sea model.- 6.2.3 Mode summation method versus mode acceleration method.- 6.3 Fatigue Damage Characteristics of a Steel Jacket Structure.- 6.4 Sensitivity Study of Fatigue Damage.- 6.4.1 Uncertainties in soil parameters.- 6.4.1.1 Effect of soil’s shear modulus.- 6.4.1.1.1 Fatigue damage at joint J1.- 6.4.1.1.2 Fatigue damage at joint J2.- 6.4.1.1.3 Fatigue damage at joint J3.- 6.4.1.1.4 Fatigue damage at joint J4.- 6.4.1.2 Effects of distribution of soil’s shear modulus along depth and soil-pile separation near mudline.- 6.4.1.2.1 Fatigue damage at joint J1.- 6.4.1.2.2 Fatigue damage at joint J2.- 6.4.1.2.3 Fatigue damage at joint J3.- 6.4.1.2.4 Fatigue damage at joint J4.- 6.4.2 Influence of Hydrodynamic Parameters.- 6.4.2.1 Effects of current on the fatigue damage.- 6.4.2.1.1 Fatigue damage at joint J1.- 6.4.2.1.2 Fatigue damage at joint J2.- 6.4.2.1.3 Fatigue damage at joint J3.- 6.4.2.1.4 Fatigue damage at joint J4.- 6.4.2.2 Constant submergence versus variable submergence of structural members.- 6.4.2.2.1 Fatigue damage at joint J1.- 6.4.2.2.2 Fatigue damage at joint J2.- 6.4.2.2.3 Fatigue damage at joint J3.- 6.4.2.2.4 Fatigue damage at joint J4.- 6.4.3 Effect of Structural Modelling on Fatigue Damage.- 6.4.3.1 Fatigue damage at joint J1.- 6.4.3.2 Fatigue damage at joint J2.- 6.4.3.3 Fatigue damage at joint J3.- 6.4.3.4 Fatigue damage at joint J4.- 6.4.4 Effects of SCF and S-N curves on the fatigue damage.- 6.4.4.1 Stress concentration factors.- 6.4.4.2 S-N curves.- 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.