This paper presents an analytical scheme for predicting the collapse strength of a flexible pipe, which considers the structural interaction between relevant layers. The analytical results were compared with a FEA model and a number of test data, and showed reasonably good agreement. The theoretical analysis showed that the pressure armor layer enhanced the strength of the carcass against buckling, though the barrier weakened this effect. The collapse strength of pipe was influenced by many factors such as the inner radius of the pipe, the thickness of the layers and the mechanical properties of the materials. For example, an increase in the thickness of the barrier will increase contact pressure and in turn reduce the critical pressure.
A flexible riser is typically protected from over-bending at the hang off region by a bending limiter, i.e. a bend stiffener or a bellmouth. The pipe will be forced into contact against the bending limiter at one side during bending. As a result, the side contact load may deflect the pipe cross-section and induce some additional stress on the pressure armor layer. Such additional stress is dynamic in nature and is anticipated to induce fatigue damage in combination with other dynamic stresses caused by tension variation and pressure for example. This paper highlights the mechanism governing potential pipe cross-section ovalization, and subsequent stress alternating and fatigue damage of pressure armor under dynamic loadings. Also presented in this paper is a procedure and technique for capturing such induced pressure armor damage in the pipe design process. It is noted that the pressure armor typically consists of strips with a profiled cross-section wound almost circumferentially. A theoretical approach is presented to evaluate the pipe cross-section ovality by considering the resistance from governing strength components such as carcass and pressure armor, and to calculate such induced stress variation and subsequent fatigue. This theoretical approach taken has been validated by FEA models and calibrated against a number of full-scale dynamic tests with pipe samples ranging from 4.75 to 16-inch ID. The methodology was third party reviewed and approved and deployed for riser system design and analysis.
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