The Norwegian Deepwater Program (NDP) has over several years working on conceptual development and design of new steel riser solutions for deep water and harsh environment. For the steel lazy wave riser (SLWR) design, the buoyancy section is a critical segment. When subjected to current, both the buoyancy elements and the riser may experience vortex induced vibrations (VIV), which can lead to fast accumulation of fatigue damage and amplified drag loads. As part of developing robust SLWR, it is crucial to understand the current induced loads and responses on various staggered buoyancy configurations.
In order to investigate the interaction of bare pipe section and buoyancy elements and its effect on VIV, hydrodynamic model tests were carried out in MARINTEK’s ocean basin in 2014. The test pipe is 38 m in length and 3 cm in diameter. The length of each buoyancy element 0.15 m and its diameter is 0.15 m. The ratio between buoyancy element diameter and riser diameter is 5. VIV response may depend on the spacing ratio of the length of the buoyancy segments and the gaps between two adjacent buoyancy elements, aspect ratio of the buoyancy element, and coverage percentage of buoyancy elements. Hence these parameters were varied and 6 different staggered buoyancy configurations were tested in uniform flows. The use of strakes to suppress VIV was also evaluated. The primary objective is to determine a range for these parameters that leads to the smallest VIV curvature (and hence an optimal riser fatigue design).
The shedding frequency of the bare riser section is significantly higher than the buoyancy element due to its smaller diameter. Therefore, the bare riser section will excite much higher modes. As observed from the present tests, the fatigue damage is dominated by the bare riser component for all of the test configurations. The CF displacement of cases with the highest buoyancy coverage (50%) is often dominated by the vortex shedding of the buoyancy element. The use of stakes can effectively suppress the vortex shedding and leads to lowest fatigue damage in the test. In addition, the non-dimensional frequency of the buoyancy element is low (about 0.087–0.122) due to its small aspect ratios (1/1 and 2/1) in present tests. The vortex shedding of the buoyancy element seems to be weakened when the spacing ratio is larger than 1/1, which is different than earlier tests using buoyancy element with larger aspect ratio (≥5/1). The impact on the SLWR design is evaluated based on both present and other relevant model tests results.