Conventional flaw assessment procedures are not explicitly developed for situations with large cyclic plastic strains, e.g. for pipelines installed by the Reeling method, and if used indiscriminately may give un-reliable results. In order to resolve this dilemma DNV, TWI and Sintef conducted a Joint Industry Project (Fracture Control for Installation Methods Introducing Cyclic Plastic Strain – Development of Guidelines for Reeling of Pipelines) to provide guidelines on testing and assessment procedures that can be employed by the industry. The project included Materials Testing, FEM Analyses and Validation Testing of Pipe Segments as well as Full Scale Pipes. This paper summarizes the Guideline document that was developed in the project.

Crack growth under cyclic loading has been studied by the finite element method. The calculation was made for plane stress conditions. The crack tip zone was modelled as a cohesive zone.The displacement of the free crack surface during unloading was found to be governed by the surrounding continuum and was independent of the details in the fracture zone. This means that crack closure upon unloading is directly related to the ultimate separation, of the cohesive zone, which in turn controls the residual plastic deformation left in the wake of the growing crack.If the distance over which closure takes place is rather small. closure may be very difficult to detect by the compliance technique.

Duplex stainless steels are widely used in subsea petroleum production installation. In many instances such steels are a robust solution to the design and manufacturing challenges. However, a number of failures have occurred, which were attributed to Hydrogen Induced Stress Cracking (HISC). Several failure investigations and extensive testing have been carried out over the last 5-10 years, and a design guideline has been established; DNV-RP-F112. The article presents HISC as a failure mode, with a brief summary of the necessary conditions and failure characteristics. The testing and research effort into HISC is described. Finally DNV-RP-F112 "Design of Duplex Stainless Steel Subsea Equipment Exposed to Cathodic Protection" is introduced and the main features are outlined.

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. In 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 1970s. Therefore it was suggested to investigate the pile weld at the sea bed in detail to assess the stress due to fabrication and 30 years of in-service life and the residual fatigue life of the pile. Six test specimens made from the Edda pile were fatigue tested. The results from the assessment and the fatigue testing are presented in this paper.