Interest arises on verifying the SCR girth welds fatigue response to a more representative loading spectrum of the actual in-service conditions and after reel-lay deformation. It is important to determine if the actual riser component’s qualification, without pre-straining and under constant amplitude loading, evidences discrepancies with in-service conditions, in terms of fatigue strength.
This situation has motivated the full scale S-N fatigue performance evaluation of SCR girth welds under constant and variable amplitude loading, and after reel-lay simulations. A CMn steel X65 pipe 10.75” outside diameter (OD) and 25.4 mm wall thickness (WT) was chosen for this program. The Welding Procedure developed for girth welds manufacturing involved the use of the Lincoln STT® process for the root pass and the GMAW process for the fill and cap passes. Reeling trials were performed at Stress Engineering Services, Houston, U.S.A.. A dedicated commercial software was used to simulate the variable amplitude loading spectrum, which is representative of a SCR Touch Down Point (TDP) in West of Africa at a water depth of 1,200 m (3937 ft) and a FPSO as production platform.
The experimental approach was focused on estimating the damage introduced by reeling and by loading cycles of various magnitudes in the riser service time history. Results of strained and unstrained specimens, tested at constant and variable amplitude, have been compared, and the cumulative damage rule typically used by Riser fatigue designers has been evaluated (i.e. Miner’s linear cumulative damage rule). Systematic fractographic investigations were performed on all the samples after testing to identify their fatigue failure initiation causes.
Fatigue Characteristic of Designed T-Type Specimen under Two-Step Repeating Variable Amplitude Load with Low-Amplitude Load below the Fatigue Limit