Highly Compliant Rigid (HCR) Riser Model Tests and Analysis

Abstract

Abstract A 1998 Joint Industry Project performed large-scale model tests and analysis of three Highly Compliant Rigid (HCR) riser configurations. The tests were conducted in about 800 ft of water. The objective was to reduce operator cost and risk by improving and verifying riser analysis methods. This was accomplished by a 3 step sequence of pre-test analysis, physical model tests and comparison of analysis with test results. The pre-test analyses were performed by 13 independent organizations using 11 riser analysis computer programs. The subsequent model test results were used to improve and verify the analysis procedures. The model tests taught many lessons including the importance of the seafloor soil and intermittent vortex induced vibration (VIV) to the overall riser response. Riser analysis programs available today have very limited capability to model the soils, and are unable to model intermittent VIV. Introduction The cash flow from industry's future "billion dollar" deepwater field developments will depend on risers. In very deep water, complex steel/polymer flexible risers ("flex risers") may not be technically or economically feasible, especially in larger diameters, because of their relatively high weight and cost. A reliable alternative would be attractive. "Flex risers" are typically specified because of their capability to bend in response to floating platform wave motions without incurring excessive fatigue damage. Their bending flexibility also simplifies installation. However, in deep water where vessel motions are a relatively small fraction of water depth, simple rigid pipe can be configured into overall shapes that provide the compliance needed to absorb platform motions. Their key design basis parameters are diameter, internal/external pressure, water depth, platform motion characteristics (e. g., ship, spar, semisubmersible, TLP) and wave environment. HCR risers have clear benefits over "flex risers" but present new design and analysis challenges. One of these challenges derives from the cyclic tension introduced by platform heave motions which makes bending stiffness highly variable and makes the problem highly non-linear. The cyclic tension can produce short periods of compression and negative bending stiffness that causes difficulty for some solution schemes. While there are several riser analysis programs available to the industry that address these computational requirements, there is little model or full-scale test data available for their validation. A key objective was to determine if the buckling predicted by some analysis programs is an artifact of the modeling or if it really occurs. If it does occur, then it was an objective to assess the accuracy of programs in predicting the onset of buckling. Another objective was to determine the magnitude of the effect of sea floor soils on riser response.