Improvement of Fatigue Life of Steel Catenary Risers and Oil Offloading Lines in West Africa

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Abstract This paper describes a study of the methods for improving fatigue life of girth welds for applications in the demanding environment of West Africa such as offshore Oil Off-loading Lines (OOL's) and Steel Catenary Risers (SCR's). Investigations concerned the various methods for improvement of the fatigue life including root and cap grinding and internal root re-melting. The paper also describes the use of laser triangulation as a mean to verify root profile before and after improvement. Introduction Fatigue based designs and the need to deliver high performance welds are now common features of the flowline and riser systems required for the exploitation of the deep water reserves. Such requirements typically apply to steel catenary risers, oil off-loading lines and increasingly, as a result of the mitigation measures needed for flowlines that are subject to pressure and temperature fluctuations (e.g. where there is a risk of lateral buckling). The consequence of such designs is the need for fabrication methods and girth welding procedures which satisfy very demanding fatigue performance targets while offering acceptable levels of productivity. Recent industry experience has demonstrated that the some existing welding methods such as pulsed GMAW and GTAW are able to deliver results that exceed previous expectation for single sided welds but only when used with great care. Unfortunately this enhanced performance has only been possible by implementing very stringent control measures on weld geometry and allowable defects these in turn dictate requirements that are very close to the capability limit of the contributory processes i.e. pipe production accuracy, weld reproducibility and NDT system capability. The criticality is clearly demonstrated by reference to the figure 1 which shows that even a very small flaw (micro-fissure) can be implicated in fatigue crack initiation. In this example of a manually welded root in 28mm thick c-steel, the undetected cold lap had an initial height in the order of 100 micrometres. Objectives Based on the foregoing considerations, the author's Company initiated a programme of work to investigate fatigue life improvement techniques that could be used to complement existing high productivity welding methods. The objectives were to identify methods that could be used to be maintain the high performance but with greater repeatability and, ideally, with the relaxation of some of the controlling parameters.