Pipe ends machining (counter-bore) is one of the practical approaches for improving the fatigue performance of a welding pipe. This approach makes the pipe end dimension within the range of the target internal diameter (ID) and wall thickness (WT), in order to reduce the ID stress concentration factor (SCF) caused by misalignment due to the pipe manufacture tolerances.
The counter-bore approach is very efficient for improving the fatigue life on the girth welds. But it may introduce a more critical position on the counter-bore area other than on the girth welds if the counter-bore profile is not well designed, especially for the high performance girth welds.
This paper investigates all the possible factors which may cause the higher stress to appear in the base metal of the counter-bore area, and gives the key parameters which impact on it. Based on the fatigue test methodology and S-N curve based fatigue design procedure, authors propose a practical method/procedure to design a suitable counter-bore profile to ensure that this profile does not fail earlier than the girth weld. The implications of this paper should have significant relevance to the high fatigue performance welds design.
Due to the presence of a higher stress level and potential defects in the weld toe, stress concentration mitigation is always considered in riser fatigue design and construction, especially for the high fatigue performance girth welds. Considering manufacturing tolerances, pipe end misalignment will aggravate the effect of stress concentration. The ground flush of the weld cap is beneficial to eliminate the stress concentration on the outer diameter (OD), but it cannot apply on the ID. Generally, the methods used for eliminating the stress level on the riser ID can be mainly classified into 3 types (Mansour, 2021) as follows:
1). Pick and rotate pipe on the firing line of the installation vessel for best fit two welding pipes,
2). Survey and sort all riser pipes by certain joints in sequence. Marking and tracking pipes during transportation. Carefully pick up pair of pipes on the firing line of the installation vessel
3). Counter-bore the pipe end on the ID.
