Fatigue Testing and S-N Data for Fatigue Analysis of Piles

Design S-N curves in design codes are based on fatigue test data where the stress cycle is under external tension load. It is observed that during pile driving most of the stress cycle is compressive and the design procedure used for fatigue analysis of piles might therefore be conservative. In order to investigate this further it was proposed to perform laboratory fatigue testing of specimens that are representative for butt welds in piles under relevant loading conditions. In the present project 30 test specimens made from welded plates were fatigue tested at different loading conditions to assess effect of compressive stress cycles as compared with tensile stress cycles. During 2006 the Edda tripod in block 2/7 was taken ashore. This platform has been in service since 1976 and the piles are considered to be representative for the piles installed in the North Sea jacket structures during the seventies. Therefore it was suggested to investigate the pile weld at the sea bed in detail to learn about the actual fabrication, the residual stress at the weld after pile driving and 30 years of in service life and residual fatigue life of the pile. 6 test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.