Frederic Demanze

Fatigue Life Analysis of Polyurethane Bending Stiffeners

Following some experiences of bending stiffeners fatigue failures during full scale tests performed at Flexi France on flexible pipe and stiffener assemblies, Technip decided to launch in 1999 a major research program on fatigue life analysis of bending stiffeners made of Polyurethane material. This fatigue life assessment is now systematically performed by Technip for all new design of flexible riser bending stiffeners. This totally innovative method comprises a number of features as follows: Firstly fatigue behaviour of polyurethane material is described. The theoretical background, based on effective strain intensity factor, is detailed, together with experimental results on laboratory notched samples, solicited under strain control for various strain ratios, to obtain fatigue data. These fatigue data are well fitted by a power law defining the total number of cycles at break as a function of the effective strain intensity factor. The notion of fatigue threshold, below which no propagation is observed, is also demonstrated. Secondly the design used by Technip for its bending stiffeners, and most of all the critical areas regarding fatigue for these massive polyurethane structures are presented. Thirdly the methodology for fatigue life assessment of bending stiffeners in the critical areas defined above is discussed. Calibration of the strain calculation principle is presented versus finite element analysis. Based on all fatigue test results, the size of the equivalent notch to be considered at design stage, in the same critical areas, is discussed. Finally, a comprehensive calibration of the methodology according to full and middle scale test results is presented. The present paper is therefore a step forward in the knowledge of fatigue behaviour of massive polyurethane bending stiffener structures, which are critical items for flexible risers integrity, and widely used in the offshore industry. The confidence in bending stiffeners reliability is greatly enhanced by the introduction of this innovative methodology developed by Technip.Copyright

Towards an H2S Free Environment in Flexible Pipe Annulus Thanks to a New Anti-H2S Polymer Layer

Due to H2 S pressure build-up in annulus, the use of sour service steel grades is required in some cases. For ultra deep water fields where the flexible pipe top tension is critical, high weight of the structural steel layers combined with lower mechanical properties (about 950MPa) of the sour grade materials is a disadvantage. The generic flexible pipe design avoids any direct contact between the bore environment (production fluids, injection fluids, etc) and the steel layers of the flexible pipe. However, diffusion of active components such as water, carbon dioxide, hydrogen sulfide, etc, through the thermoplastic sheaths could lead to corrosion exposure of metallic layers between the thermoplastic sheaths. In order to reduce the weight of the flexible pipe structure to reduce riser top tension, high strength grade steel layers (>1200MPa) can be used with reduced thickness, considering that the steel layers will not be exposed to H2 S coming from the bore fluid. This is now achievable even when H2 S is present in the bore by use of a new reactive polymer layer, which will ensure a H2 S free environment in the flexible pipe annulus. The new “Anti-H2 S” layer will chemically react with the H2 S, leading to a complete absence of H2 S exposure to steel layers during the service life of the pipe. In addition, the “Anti-H2 S” layer makes it possible to propose flexible pipes for applications with high H2 S content for which no acceptable solution would otherwise be found. This paper will present the theoretical background supporting the new Anti-H2 S layer concept, along with the qualification works carried out both at laboratory scale and on full flexible pipe structures. It will also focus on the design/calculation methodology applicable to this new sheath. Introduction of the “Anti-H2 S” layer results in that: - Flexible pipes can now be proposed for applications with high H2 S contents; - Sweet service high strength steel grades can be used in flexible pipe annulus even with the presence of H2 S into the bore of the pipe; - The weight of the flexible pipe is reduced.Copyright

Thermal Stresses and Crack Propagation in PVDF Pressure Barriers Submitted to High Temperature Variations

During the use of flexible pipes, important thermal variations can occur leading to thermal stresses. As a result two aspects were identified, in the past, due to thermal stress cycling [1,2] : – pull out of the pressure barrier, – crack growth of the pressure sheath. Pull out of the PVDF pressure sheath has been stopped by the design of new end connections [3]. Nevertheless, in order to get more knowledge of thermal stresses, Technip-Coflexip has developed a theoretical approach which is able to calculate whatever the plasticizer content, the initial temperature, the final temperature and the temperature variation, the thermal stresses for flexible pipes using PVDF materials. Comparisons between theoretical predictions and full scales test results show very good accordance and allow confirming that the Technip-Coflexip designs of end-connections are suitable for the offshore industry. Crack growth through the pressure sheath has been experienced only during accelerated testing at SINTEF and has not so far been seen in risers in operations. Crack growth through PVDF pressure sheath can be fully predicted by a model developed by Technip-Coflexip. The two major conclusions of the work performed are: – failure experienced at SINTEF are only linked to rough hand-machining and very severe test conditions, – failure can not occur in normal operation due to the fact that thermal stresses generated, linked with a good surface machining, are below the threshold of crack propagation.Copyright