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.