Vortex induced vibration (VIV) occurs when a body is exposed to a fluid flow that causes an unsteady flow pattern due to vortex shedding at or near the structure's natural frequency leading to resonant vibrations. This phenomenon frequently happens in slender elements such as risers used in deepwater oil and gas developments. Such vibrations may reduce the fatigue life of the riser, potentially causing catastrophic damage to the integrity of the system and the environment. Installation of VIV suppression devices onto risers will affect the vortex shedding patterns and is thus able to mitigate VIV. A model test was carried out to study the effectiveness of a newly developed U-shaped fairing in reducing drag and VIV of risers.

The U-shaped fairing is an alternative to conventional spiral strakes and tear-drop-shaped fairings for riser VIV suppression. The model test findings offer useful information for its industrial application in suppressing riser VIV.

The model test was performed in a towing tank facility in Malaysia located at Universiti Teknologi Malaysia (UTM). Current flow was simulated by towing a vertically submerged scaled-down riser section underwater with a moving carriage. The riser model was attached to a custom-designed test rig to replicate a section of the actual riser and to simulate its 2D motions. The model riser was tested in three different conditions, i.e. (a) bare riser without fairing; (b) riser with weathervaning fairing (normal condition) and (c) riser with fairing stuck at different orientations (abnormal condition). The test results show significant reduction of VIV on the model riser with fairing under normal condition. In addition, it is found that the drag load for the riser with fairing is significantly lower compared to that of a bare riser or a typical straked riser.

This test marks a significant collaboration between a local research institute and the offshore oil and gas industry, aligned with the initiative of increasing local capabilities. Effective mitigation of VIV of risers using suppression devices could lead to improved riser fatigue life and overall a more economical platform design.

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