The performance of a deep water composite production riser from a system perspective is presented, and its advantages are articulated through comparisons with a typical steel riser under identical service conditions. The composite riser joints in this study were considered to be a part of a single Tension Leg Platform (TLP) riser string to be installed at a depth of 6000 ft in Gulf of Mexico. A series of numerical analyses –burst, collapse, fatigue, global and local – have been performed, and the capacities of the composite riser have been determined utilizing long-term strength properties.
The capacities associated with the hoop direction, i.e., burst and collapse, are limited by the presence of a steel internal liner whose function is to ensure pressure and fluid tightness. The collapse capacity of the riser can be drastically impaired by the presence of a debond between the liner and composite.
Due to the high strength to weight ratio of the carbon/epoxy composite, its response under combined axial tension and bending moment showed great safety margins, favoring pursuits of greater water depths. The study also constructed damage envelopes associated with axial tension and bending moment, which facilitate feasibility checks for expanding the use of the composite joints to other locations or systems.
The fatigue life of the composite body is expected to greatly exceed its design life, and the most critical element is the welds between the liner and metal end pieces. Since there is wide dispersion of S-N relationships for carbon/epoxy composites depending on the combinations of constituent materials, a parametric study was carried out in this study to suggest the range of acceptable S-N relationships.
The composite riser is estimated to offer only moderate damping, due primarily to its specially orthotropic lay-up. The study also demonstrates that the use of Rayleigh stiffness proportional damping may not be suitable for deep water risers. A series of forced excitation analyses show that the system in sea water does not show notable resonance due to fluid drag. When compared with the steel riser, vibration amplitudes at low elevations are much lower.
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