Fatigue Behaviour of Offshore Structures

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 accumulated effect of these stresses results in the fatigue failure.

The first step in the response analysis procedure is the mathematical modelling of the sea environment and the loading imposed on the structure. The sea environment may be characterized mainly by overwater wind, surface waves and currents. Overwater wind during severe storm conditions could significantly influence the design of offshore structures because of the large forces it can induce on the exposed parts of the structure. The wind loading, however, has not been considered in the present work. For the fatigue analysis of offshore structures hydrodynamic loadings caused by the surface waves are of major importance. As the fatigue is known to be low stress high cycle phenomenon, even the low to moderate sea could significantly add to cumulative fatigue damage. Finally, currents may also appreciably contribute to the total forces exerted on the structure. The following sections describe the formulation of the mathematical model of the sea environment and the hydrodynamic loading induced on the structure by waves and currents.

The steel jacket platforms are constructed of tubular members joined together to form three dimensional frame system. The platform is supported on piles driven deep into the sea bed. The jacket structure is a continuous system having an infinite number of dynamic degrees of freedom. For the purpose of dynamic analysis, it is generally adequate to represent such a structure with a lumped parameter model consisting of discrete masses located at nodal points of a stiffness network.

The response to wave loading of a steel-jacket platform supported on flexible foundations may be significantly different from that of the platform supported on rigid base. This happens because the natural frequency and mode shape in the two cases can be quite different from each other and the frequency parameter has important bearing on the dynamic magnification of the structural response. The resistance of pile to vibratory motion is provided by the steel pile along with its interaction with the surrounding soil medium. While in the case of vertical motion the resistance developed by the pile is given by end bearing, skin-friction or by a combination of the two, in case of the pile subjected to horizontal forces the resistance is provided by horizontal bearing of the pile against the soil. The vibratory motion of the soil-pile subsystem is restricted because of significant loss of energy in soil medium. The damping of the pile motion occurs because of: (a) the radiation of wave energy into the far field and (b) the loss of energy due to internal friction of soil. Thus the accountancy of both the radiation damping and the material or hysteresis damping are essential in an accurate estimation of dynamic response of pile-supported structures.

The plane frame version of a steel jacket platform is analysed by mode acceleration method. The local stresses in the joints of an offshore structure subjected to random wave/current load are derived from the nominal stress response of the structural members using the stress concentration factors. The fatigue life estimates for welded joint are obtained by both the S-N curve approach alongwith Palmgren-Miner rule and the fracture mechanics approach.

The present study involves the fatigue analysis of a pile supported steel jacket platform subjected to random sea waves and current loading. The structure is analysed using the substructure approach. The structure subsystem and the foundation subsystem are modelled separately. The jacket structure is modelled as a plane frame. The contribution of the members in the orthogonal frames of the steel jacket to the total mass, damping and hydrodynamic forces are also accounted for in the analysis of the idealized structure. The foundation subsystem is replaced at the pile-structure interface by equivalent linear springs and viscous dampers characterized by pile-head impedance functions. These properties are obtained by analyzing the pile by the proposed transfer matrix approach taking soil-pile interaction into account. The dynamic response of the steel jacket platform is expected to be sensitive to the pile-head impedance. It is difficult to get a correct estimate of the pile head impedance because of the inherent uncertainties in the soil properties. Therefore, it is needed first to study the influence of soil-pile parameters on the pile head impedance functions. In the subsequent sections the study involves specific investigations into the sensitivity of fatigue behaviour of a steel jacket platform to several uncertainties in pile foundation characteristics, hydrodynamic loading, structural modelling and fatigue damage model.