Highly Compliant Rigid Risers: Field Test Benchmarking a Time Domain VIV Algorithm

Abstract

Abstract This paper presents a time domain procedure for calculating the Vortex Induced Vibration (VIV) of risers, and compares the computer simulation resultswith large scale physical test data. The model test data was acquired by the Phase 1 HCR riser JIP [1]. That projectconducted large scale model tests of Highly Compliant Rigid Risers in about 800ft of still water with dynamic vessel heave simulated by a mechanical actuator. Most of the tests exhibited significant intermittent Vortex Induced Vibration(VIV) which impacts in-plane riser response and strength with in-plane dragamplification, and impacts riser fatigue with high frequency VIV bendingstress. The time domain method introduced here simulates the dynamic response(including VIV) of any arbitrary riser configuration to the combined action ofuniform or sheared current, waves and floating vessel wave-induced motion. Theanalysis results can be used to assess the riser's strength and fatigueintegrity. Possible riser types include vertically tensioned risers, SCRs, LazyWave SCRs and Compliant Vertical Access Risers (CVARs). Introduction A 1998 Joint Industry Project performed large-scale model tests and analysis ofthe 3 Highly Compliant Rigid (HCR) riser configurations illustrated on Figure 1through Figure 3. The risers were modeled with a 1.5 inch diameter pipe-stringin an 800 ft water depth, zero-current lake. Harmonic displacements wereintroduced at the riser top to simulate wave-induced vessel motions. Short, instrumented pipe joints (Pups) measured dynamic 2-axis bending andacceleration, and tension at locations with high bending. Pup response wassampled at 40 Hz which was rapid enough to capture the VIV response which wastypically in the 1 Hz to 5 Hz range. For analysis, pup response was resolvedinto components in and out of the plane of the riser. One of the most eye-opening model test findings was that for all 3 riser typesand for most of the excitation intensities, the risers exhibited significantintermittent VIV. This VIV is typically ignored by analysis. It changes thein-plane response by effectively increasing the in-plane drag coefficient, andit produces cyclic out-of-plane bending which reduces the fatigue life. Typical VIV analysis in the offshore industry uses frequency domain proceduresthat must assume a linear riser system. For linear methods to be valid, theriser tension and geometry must remain constant with time. These linearprocedures were originally developed to address vertical drilling risers thatsatisfy these restrictions with their constant tension and well defined flexjoint end conditions. However, recent attempts have been made to apply them tomore challenging SCRs.