Fatigue Performance of Reeled Risers

Abstract

One of the most effective installation methods of metallic risers is the reel-lay process, in which pipe segments are welded onshore and subsequently bent over a cylindrical rigid surface (reel) in a laying vessel. During installation, the line is unreeled, straightened, and then laid into the sea under tension. In this process, material properties change and eventual weld defects may increase, thus reducing the fatigue life of these joints under operational loads. Therefore, welded joints must be manufactured based on strict weld acceptance criteria. These criteria shall guarantee reliable standards regarding the fatigue life of the joints while not impairing the feasibility of weld manufacture (high cost). In this work, the reeling process is initially simulated through a nonlinear finite element model that incorporates weld defects. The results are then used as guidelines to experimentally obtain fracture mechanics parameters of typical weld and base materials under prestrained conditions. The fatigue life of as-welded and reeled joints with different defects (lack of fusion and lack of penetration) are subsequently estimated via a finite element model that accounts for the changes in the material properties due to pre-straining. The results are compared and used to suggest guidelines to the design of reeled steel catenary risers.