a b s t r a c tA large-scale model test of a free-hanging water intake riser (WIR) is performed in an ocean basin to investigate the riser responses under vessel motion. Top end of the WIR is forced to oscillate at given vessel motion trajectories. Fiber Brag Grating (FBG) strain sensors are used to measure the WIR dynamic responses. Experimental results firstly confirms that the free-hanging WIR would experience out-of-plane vortex-induced vibrations (VIVs) under pure vessel motion even for the case with a KC number as low as 5. Meanwhile, comparison between numerical results and experimental measurements suggests a significant drag amplification by out-of-plane vessel motion-induced VIV. What's more, further study on WIR response frequencies and cross section trajectories reveals a strong correlation between vessel motion-induced VIV and local KC number distribution, owing to the small KC number effect. The presented work provides useful references for gaining a better understanding on VIV induced by vessel motion, and for the development of future prediction models.
Water Intake Riser (WIR) offers the potential to convey a large amount of cold water from deep ocean to improve the topside process efficiency and reduce overall cost of a FPU, FPSO or FLNG vessel. Its large diameter (>1m) and high flow rate presents unique design challenges and performance uncertainties. A model test campaign of a large diameter WIR concept has been planned, designed, and executed in an offshore model basin on a fast track basis. The pipe was carefully scaled and fabricated to meet the test objectives. The model test setup included a riser model instrumented with optical strain sensors, a planar motion mechanism which simulates the vessel motions, and an internal water flow system. More than 200 cases were performed in the model basin including sinusoidal and random motion tests in combination with various flow rates. This paper discusses the challenges, observations and lesson learned during the model test, and some of the key results in comparison with analysis models. The successful execution of this project demonstrates that model testing is an important tool in advancing complex deepwater riser concept.
A model test campaign of a large diameter water intake riser (WIR) has been planned, designed, and successfully executed in an offshore model basin. The objective of the model test is to better understand the global dynamic behavior of WIR, and thus advance its design. The scopes of the model test are to measure the response of the riser under floater motions; investigate the effect of the internal water and flow rate; and observe any vortex-induced-vibration (VIV) and axial instability due to motion and / or internal flow. The paper presents the model test results and the numerical calibrations and validations. The WIR pipe was carefully scaled and designed to meet the test objectives. The WIR in model test scale is 150 mm in inside diameter and approximately 36 m in length. The model test setup includes a fully instrumented riser, a planar motion mechanism (PMM) which simulates the vessel motion and an internal water flow system (IWFS). The riser was instrumented with Fiber Bragg Grating (FBG) strain sensors along the pipe length and circumference. The WIR was hung-off from the PMM inside the deep basin pit. More than 200 cases were carried out in the basin including the sinusoidal motion tests and random motion tests with different flow rates. The model tests collected a wealth of data of the WIR dynamic responses under the vessel motions and the internal water flow conditions. As expected, WIR global bending responses are highly dependent on the pipe excitation modes and their corresponding mode curvatures. These responses can be predicted well by numerical software through a calibration process. The axial response of WIR due to motion and/or internal flow is much more complex. The amount of internal water coupled with the pipe depends on the vessel motions and internal flow fluctuation. This is important for axial stability prediction and seawater lift system design.
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