This paper described a hybrid method to assess large diameter spool VIV and FIV, and applied it to a 36" complex-shape spool study. The hybrid method employs empirical lift coefficients, and imposes the lift force to spool structural beam model to obtain the time domain dynamic response, and extracts results for design criteria check. Spool boundary conditions at the PLEM end was modeled separately in FEA software to give the equivalent bending stiffness. Internal slug flow induced force was assessed and converted to time history using an equivalent work technique, which enabled the superposition of the slug force and lift force. The hybrid method was validated on a straight pipeline span, with comparison to Shear7 results. It is then further used to assess the 36" spool vibrations under both installation and operation conditions. For all external current induced vibrations (VIV), severe resonance vibrations have been observed, and the results are in good agreements with Shear7 results. For internal slug flow induced vibrations (FIV), the vibration amplitudes are small, and has negligible effect on spool dynamics. Based on the assessment results, FIV is not a concern for the spool system, however, additional free span supports are required to alleviate the risks of VIV. It concluded that the hybrid method is valid for large diameter 3D spool VIV and FIV analysis, and it is also applicable to other similar pipeline and spool free span analysis.


In a recent project offshore Bengal, large diameter (36") pipelines have been designed and installed, for crude oil and high-speed diesel transportation between an offshore single point mooring buoy and onshore facility, and a 36" pipeline spool was used to connect the pipeline and PLEM, as shown in Fig. 1. The spool end flanges have an elevation difference of approximately 5m, with lower end flange connecting to pipeline, and higher end flange connecting to pipeline end manifold (PLEM) pipe. Middle segment of the spool is coated with concrete coating. Spool total dry weight is about 40Te.

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