Note: This study is part of a broader project “A Comparative Risk Analysis of Composite and Steel Production Risers”, (MMS Project 490).
This study will analyze a single string composite riser system which is part of a TLP in 6000 ft Gulf of Mexico environment. The riser system consists of wellhead, stress joint, tensioner joint, and surface tree, in addition to riser joints. A limited number of steel riser joints will be used near the stress joint and mean water level, where severe local loads are expected. Fig. 2 shows a schematic of the riser system to be analyzed in this study. Various aspects of the composite riser will be analyzed with ABAQUS finite element software.
First, basic pressure capacities, i.e., burst and collapse, of the composite riser are estimated. In the burst analysis, the maximum internal pressure is applied to a riser section, and the stresses in all layers will be checked for failure. Then higher pressures are applied incrementally until fiber rupture occurs, and loss of stiffness resulting from initiation and progression of damage is applied to the finite element model. Collapse analysis is performed in a similar manner, but in this case the effect of debond areas between the structural composite body and internal liner will also be studied. When a debond exists, it may impair the collapse resistance of the composite riser. In addition, if sea water penetrates into the composite body building up pressure on the interface, local collapse of the internal liner may occur at a pressure far smaller than the collapse pressure of the riser section. Based on these scenarios, various debond geometry is incorporated in the analysis.
The combined loading responses are obtained in two steps: global and local. Analysis of a conventional metallic riser does not always require a local analysis. However, unlike isotropic materials, stresses and strains in each layer of the composite tubular are not conveniently calculated by simple formulas. In the global analysis, a few combinations of operational and environmental loads are applied to the complete riser system model. Based on the response data, a critical location along the span of the riser is selected, and the local load effects at this particular location, i.e., nodal forces and bending moment or displacements and rotation, are applied as the boundary conditions in the subsequent local analysis. Again, the failure analysis scheme used in the burst analysis is used in the local analysis.
In the fatigue analysis, wave fatigue at selected locations of the composite riser is estimated. Long term sea state is modeled by multiple sea states, and using the Rayleigh probability density function and S-N data, damage caused by each sea state is calculated. In addition to estimating total damage and fatigue life, representation of S-N data and effect of mean stress are discussed.
Lastly, the amount of damping in the composite riser is estimated based on the strain energy method, where damping is determined by the ratio of dissipated energy to stored energy. Lamina damping properties in terms of specific damping capacity or loss factor are used to obtain the total dissipated energy. The value of damping thus obtained will be incorporated in an excitation simulation where the entire riser in water oscillates at one of its natural frequencies. The response will be compared with that of a steel riser and the composite riser with its damping ignored.