ABSTRACT

The North Sea is an extremely severe environment and so design and fabrication for fatigue considerations is very important at weld locations. Single sided closure welds can be particularly sensitive to fatigue, where the inspection of the root is limited and as such the fabrication fit-up and the quality of the weld cannot be guaranteed. Recently, this location has been the subject of more rigorous non-destructive testing (NDT) requirements, with more recently, radiographic testing. (RT) of the partially completed weld to detect small defects at the weld root. Although such RT is very effective, it can be very time consuming and costly. It would be virtually intolerable to impose such requirements for all single sided closure welds on a large jacket structure. This paper presents a method, developed and utilised for BP?s South East Forties jacket structure, to relate various standards of NDT procedures to the design process. Basically, the welds are grouped into classes depending on their fatigue life, which is in turn related to a maximum tolerable defect. For each maximum tolerable defect size, an appropriate NDT procedure is used. This design process thus places NDT effort into the areas where it is more relevant and, just as important, avoids unnecessary over-inspection of non-fatigue sensitive locations. It is shown that full use of analytical techniques can be made to ensure compatible and rational welding inspection during fabrication.

1.0 INTRODUCTION

Single sided closure welds necessary on most structures and are particularly numerous on large jacket structures. Typical locations are at brace to stub welds partway along brace members between nodes (Fig. 1). Such locations are particularly sensitive to fatigue considerations and can be further exacerbated by a thickness transition, which is very often located on the inside edge thus coinciding with the weld root (Fig. 2). Added to this are the normal fabrication tolerances for mismatch arising for example, from out-of-circularity and misalignment of the tubulars (Fig 3). Welding processes will also introduce defects, which increase the susceptibility of the weld to fatigue failure. This is particularly so for single sided closure welds, where missed edges, cracks, slag inclusions and other defects cannot be removed by work on the inside of the weld. Thus, it can be seen that weld root imperfections of various types will be very important in the fatigue assessment of such joints. There are varying levels of inspection and a number of NDT techniques that can be employed to detect such imperfections. It has been shown on recent jacket fabrications that very small irregularities at the root of a single sided weld can be detected by RT of the partially completed weld. The size of detectable defect is much smaller than that which can be detected by ultrasonic testing of the completed weld. It is not the purpose of this paper to present the merits of the various NDT procedures, but the above has now been seen on several fabrications at which John Brown Offshore Structures has had a presence. Fabricators, have at first been very sceptical that small irregularities, in the order of l mm would be detectable by RT of the partially completed weld, but after trials all have been convinced.

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