Fatigue is a possible mode of failure for offshore structures, particularly in deep water or rough sea locations. The problem of fatigue failure occurs at welded connections, where stresses are invariably greatest. The present paper defines this problem and describes the numerous and often complex variables which have made it difficult to design against this mode of failure. The goals of API-sponsored research regarding this topic are described and then compared with other research programs such as the United Kingdom Offshore Steels Research Project (UKOSRP), The API research has focused upon a critical aspect of this overall problem which has not been addressed directly by any other research program, i.e., the development of seawater corrosion fatigue test data in the 106 to 108 cycles-to-failure range. Results from the API project are also discussed with regard to recently developed API fatigue design criteria; and it is shown that designers, operators, and certifying authorities can place greater confidence in these as a consequence of this work. A need for further research is emphasized, and specific topics which have evolved as significant are listed and briefly discussed.
Failure of engineering structures and components due to dynamic or time variable stressing (fatigue) has historically been an important technological problem. In the case of offshore structures, a typical example of which is shown in Figure 1, fatigue is of concern from both design and operational standpoints due to wind and wave, current, diurnal, duty loadings as well as dynamic response (vibration). For jacket- or template-type structures fabricated from tubular steel members, if fatigue cracking and failure occur it will be at the welded tubular connections, as illustrated schematically by Figure 2. Stress a t such sites is concentrated due to structural (geometrical) irregularities, and metallurgical properties may be inferior due to influence of welding.
The offshore petroleum production industry has an excellent overall record with regard to avoiding fatigue failures. However, the recent thrusts into frontier type areas (deep water and rough seas) have resulted in more difficult operating conditions than in the past. Consequently, it is more important than ever that this fatigue problem be understood and that the design be based upon appropriate and realistic criteria that have been substantiated by experimental data. Accomplishment of this has been difficult, however, because of the fact that offshore structures in frontier-type locations may experience 107 to 108 cycles of relatively low-stress amplitude over the design life. Typical cumulative stress cycles for structures in various locations are compared in Figure 3.
The objective of the present paper is to discuss the fatigue problem with regard to the integrity of welded steel in seawater and to describe recent research efforts which have focused upon investigating this mode of failure and upon developing design criteria for mitigating this damage.