A cooperative analytical and experimental technical program involving three organizations is described which is leading to the capability to predict the service life of dynamic flexible risers. The motions and loads on flexible risers require knowledge of static mechanical properties of the pipe Wellstream has developed a family of simple and accurate methods for calculating static failure and stiffness properties of flexible pipes. Degradation of risers may result from wear of the tensile armor wires as a result of slip during flexure. Wellstream has derived a formula for the magnitude of slip of the wires across each other during dynamic bending, and a criterion for a contact pressure that will prevent slip. These are being incorporated, with the help of personnel from the University College London, into a model of wire stress increases occurring over a bending cycle. This model considers non-slip as well as slip regimes during bending, and permits calculation of failure by metal fatigue due to stress increases with time as metal cross sectional area decreases due to abrasion. Zentech Consultants is encoding the degradation model as a module auxiliary to the riser analysis program Flexriser 4 so that a life prediction can be made as an output for a given user design study, for the expected lifetime current and wave excitation spectrum. Wear of the plastic barrier layer and of the other plastic layers as a result of flexure is also being addressed, and algorithms for predicting plastic degradation will be added to the model. In the interim, available public domain wear data is being used.
The major limitation to flexible user life IS the degradation due to wear and fatigue that may occur in the two contrawound layers of tensile armor under the action of bending that results from wave and current action.