Helical strakes can suppress vortex-induced vibrations (VIV) in pipelines spans as well as risers. Pure in-line (IL) VIV is more of a concern for pipelines than for risers. To make it possible to assess the effectiveness of partial strake coverage for this case, an important gap in the hydrodynamic data for strakes is filled by the reported IL forced vibration tests. Therein a strake-covered rigid cylinder undergoes harmonic purely IL motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean's towing tank. These tests cover a range of frequencies and amplitudes of the harmonic motion, to generate added-mass and excitation functions are derived from the in-phase and 90° out-of-phase components of the hydrodynamic force on the pipe, respectively. Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007), the IL VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure IL VIV.
Excitation and added mass functions determined from forced vibration tests of a rigid cylinder undergoing harmonic motion in the flow are used in the semi-empirical software VIVANA to predict the VIV response of pipelines.
An advantage of this approach, as opposed to the more-commonly-used response function approach, is that it can account for changing conditions along the length of the pipe, like changing current velocity, seabed proximity, and/or pipe diameter. This makes it useful for pipelines as well as for risers when such changes occur. Further, for pipelines, travelling wave effects play less of a role than for risers, so the VIVANA approach can be simplified by assuming the phase angle of the harmonic response is constant along the span.
The interactions between cross-flow and in-line response that complicate the prediction of cross-flow VIV by the excitation function approach, do not arise for pure inline VIV. For the latter case, using the pure in-line forced vibration test data of Aronsen (2007), it is found that both VIVANA approach and simplified ‘SIVANA’ approach thereof predict VIV amplitudes consistent with experiments on flexible pipe (Ormen Lange umbilical VIV tests), and the DNVGL-RP-F105 response function for a range of structural and soil damping values.
In a companion paper, this approach is applied partially strake-covered pipeline spans, to show that a relatively small fraction of well-placed strake coverage is enough to suppress in-line VIV.
A series of experiments is performed in which a strake-covered rigid cylinder undergoes harmonic purely in-line motion while subject to a uniform “flow” created by towing the test rig along SINTEF Ocean’s towing tank. These tests are performed for a range of frequencies and amplitudes of the harmonic motion, to generate added-mass and excitation functions are derived from the in-phase and 90° out-of-phase components of the hydrodynamic force on the pipe, respectively.
Using these excitation- and added-mass functions in VIVANA together with those from experiments on bare pipe by Aronsen (2007), the in-line VIV response of partially strake-covered pipeline spans is calculated. It is found that as little as 10% strake coverage at the optimal location effectively suppresses pure in-line VIV.
Further advantages of strakes rather than intermediate supports to suppress in-line VIV include: strakes are not affected by the scour which can lower an intermediate support (in addition to creating the span in the first place). Further they do not prevent self-lowering of the pipeline or act as a point of concentration of VIV damage as the spans to each side of the intermediate support grow again.
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