Offshore Engineering is a track that is growing up considerably during last decades. Offshore wind turbines, oil platforms and other projects situated on the sea are good examples of marine constructions. These types of structures are normally subjected to cyclic loads that may produce significant changes in the properties of the material during the life of the structure. This effect is also known as fatigue. Fatigue is a process of progressive and permanent structural change occurring in a material which is subjected to loads which produce time fluctuating stresses and strains. In offshore structures, the fatigue damage is more important because the special conditions of the sea and the marine environment that may accelerate the degradation of the structure. The thesis deals with the response of prestressed concrete elements subjected to fatigue damage in marine environment. A review of the most important codes for calculating the fatigue damage is presented, comparing several requirements and regulations. Finally, a methodology for calculating the fatigue damage in offshore concrete structures is proposed, considering corrosion and the mechanical response of the structure.

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.

Over the past 20 years, the Transport and Road Research Laboratory has carried out a co-ordinated programme of fatigue testing, including work on the fatigue performance of reinforced and pre-stressed concrete beams. The research has led to a better understanding of the fatigue behaviour of plain concrete, the various types of reinforcing bars in air and concrete, continuous welded, lapped and coupled bars, and the effects of corrosion. The work of TRRL and many other organizations is reviewed and a summary of current design rules with recommendations for assessing the fatigue life of new structures in service is given.