INTECSEA has been supporting a client in designing, modelling and assessing a proposal to add additional riser capacity to an existing platform through spare drill slots in the jacket structure. Traditional empirical analysis indicated that the spans between the guides would be subject to Vortex Induced Vibration (VIV), prompting the use of VIV suppression strakes. Analysis was performed to quantify the effectiveness of strakes and to optimise where they are required to minimize jacket loads and operational maintenance and inspection requirements. The current models for VIV have been developed for pipelines and long dynamic risers. Traditional rigid risers installed into a traditional jacket are not well covered.
Computational Fluid Dynamics (CFD) modelling was undertaken using ANSYS CFX on bare riser sections, along with strakes of various characteristic parameters based on vendor specifications. A range of current conditions were modelled to stimulate the onset of VIV. Validation of the performance against shedding frequency and mesh independence was performed. This work demonstrated the performance of the strakes in reducing cross flow force amplitude as well as the fluctuating component of the in-line force. The frequency of the force response was also shown to be more random and act on a longer timescale than the bare riser. The increase in drag forces was also found to be of the order reported by the vendors contacted as part of this project.
Fluid Structure Interaction (FSI) simulations were performed for the full riser length to better capture the interdependence of the vortex shedding and the motion of the riser by two-way coupling of the CFD and an FEA model built within ANSYS. Simulations also examined the splash zone where only part of the span was submerged. Larger simulations were performed to quantify the effect of damping due to the adjacent spans which have lateral constraints at the guides within the drill slots. This has been a challenging modelling activity as the CFD model is very large and required High Performance Computing (HPC) to run the model in parallel.
The work has shown that the application of strakes is an effective method of addressing VIV phenomena within this application. The FSI simulations have examined the dynamic damping along the full riser length. Further modelling work can be performed to better understand the conservativeness of these mitigation strategies saving operators significant cost and time in installation and operating costs.