Fatigue Life Assessment of Reeled Risers
Authors: Netto Theodoro A. , Lourenço Marcelo I. , Botto Adriana 
Published: October 2008

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 those 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 under pre-strained 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 prestraining.

Introduction

The most efficient and cost-effective method of pipeline installation involves pipe onshore welding and pipeline bending over a rigid circular surface on the vessel. During offshore installation, the pipeline is straightened and launched under tension into the sea. These methods have been successfully used in Brazil for installing pipelines up to 1,300 m (approximately 4,300 ft) water depth.

However, the bending, unbending, and straightening processes as applied on the vessels induce the pipe to bending-curvature histories, which are well into the plastic range of the material (e.g. Odegard et.al (1998)). Although the pipe is straightened prior to launch, distortions in the form of residual out-ofroundness, residual stresses, changes in material properties due to plasticity, and growing of eventual welding flaws may occur. These effects may have an influence in both the ultimate strength and subsequent fatigue performance of the line. Thus, in addition to ultimate strength design, the use of steel catenary risers (SCR's), which will be inevitably subjected to cyclic loading during operation, implies in a careful examination of the possibility of riser failure due to fatigue. In this context, the influence of installation methods involving pipeline material and geometric changes must betaken into account in the evaluation of the riser structural integrity during its operational life.

Fatigue life predictions based on S-N curves and fracture mechanics approaches can be employed in the preliminary stages of the design. Nevertheless, the necessary confidence to establish a consistent planning for SCR's installations using reel methods should be built upon laboratory fatigue tests using full-scale pipes and realistic girth welding processes. A test program including reeling simulation and fatigue tests on pipes with outside diameters (D) of 8.625 in and girth welds with induced lack of fusion and lack of penetration defects has been recently carried out at COPPE/UFRJ as part of a research project involving PETROBRAS and mostly funded by the Brazilian Governmental Financial Agency FINEP.