Corrosion Fatigue Crack Propagation Tests On Steels For Offshore Structures

ABSTRACT The importance of research work on fatigue of deepwater offshore structures is discussed in the context of the special operating conditions for such structures. Problems in establishing realistic conditions for fatigue tests, and the possibilities afforded by the application of fracture mechanics are indicated The need for more research in several areas is emphasized. Experimental investigations, in which the effect on crack growth of cathodic protection and narrow band random loading were studied, are described. The experimental conditions and an unconventional test specimen containing six cracks are discussed. Some early test results are given. INTRODUCTION Oil production in severe weather areas by means of deep water structures has greatly increased the importance of research into corrosion fatigue of steel structures with the aim to improve the basis for design. The design against fatigue failure in offshore structures requires a knowledge of:Loading data in the form of magnitude and number of stress cycles that contribute to the degradation of fatigue strength, i.e. load spectra,. and physical data describing the conditions for corrosion, for instance surface potential and temperature.Design S-N diagrams for appropriate loading and environmental/physical conditions, i.e. S-N curves giving the fatigue strength of components in such a manner that the computed fatigue life (by a standardized procedure) gives an adequate margin of safety against fatigue failure.A procedure for calculating fatigue, i.e. a cumulative damage rule such as the Miner-Palmgren rule. Extensive research work is necessary to reduce the uncertainties related to the above aspects of the fatigue design process. Some of the most central problems are being treated in coordinated projects at Det norske Veritas (DnV), The Norwegian Institute of Technology (NTH), and The Foundation of Scientific and Industrial Research at the Norwegian Institute of Technology (SINTEF). This paper is concerned with work that is being carried out at SINTEF. Both in the current research work and in planned projects height priority is given to basic investigations aimed at creating a better understanding of corrosion fatigue in general as well as obtaining design data for welded offshore structures, using a fracture mechanics approach. The fracture mechanics approach to fatigue, involving measurement of low rates of crack growth in test specimens and the subsequent integration of such data to yield fatigue lives i.e. S-N data is considered to be a viable method for obtaining design data in the high cycle region of the S-N curve. The classical method to obtain fatigue data is to run the specimens until fracture and to present the results in a S-N diagram, as shown schematically in Fig. 1. The major drawback of this method is that no information is obtained about the damage development prior to fracture.