High cycle fatigue failure is likely to occur when a rather undamped offshore installation is operated over a long period of time. Especially welded parts are vulnerable to fluctuating stress ranges, if the structural components, such as the small bore side branches, are loaded in resonance. In particular, the welded connections at e.g. tie-in points can be critical. Therefore, it is important to know what the current status of existing welded connections is and how long they can still be operated at any specific operating condition without failure.
This paper describes an advanced fatigue assessment method to determine the cumulative damage in the critical parts of the system. The method is developed for industrial applications where the available information is limited. The method comprises of a mechanical analysis and a fatigue assessment based on a fracture mechanics crack growth model. The method enables determination of inspection intervals and at the same time layout optimization for a long term safe operation. This paper pays special attention to complex, braced small bore side branches. Standard fatigue stress assessment methods can be conservative for the fatigue assessment of high frequent stress ranges with relatively low stress ranges. The current paper shows the effect of the fracture mechanics procedure for an example of a braced side branch connected to the main piping: both a fracture mechanics model and a detailed 3D FE model are made. With these models, the hot spot stress in the welds is evaluated and tailor-made S-N-curves are derived. With information of the operating history, the cumulative damage is determined for the weld detail. This knowledge on the current damage status is crucial for future operations.