This version is available at https://strathprints.strath.ac.uk/60458/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output.1 | P a g e ABSTRACTAs the search for oil in the Gulf of Mexico (GoM) moves into deeper waters, floating platforms such as semi-submersibles are in increasing demand. As semi-submersibles increase in size, the effect of vortex-induced motions (VIMs) becomes a significant problem in their design. VIMs stem from transverse forces caused by the current affecting the platform, with vortexes moving downstream on either side of the structure. The loop/eddy current phenomenon in the GoM leads to a constant current being present in the area, with speeds of up to 1.8 m/s. The accurate prediction of the vertical and transverse motions of semi-submersibles is crucial for the design of the riser systems. It is therefore beneficial to investigate the hydrodynamic forces acting on the geometry, and means of reducing these forces. A common method of reducing transverse forces is the addition of column appendages, such as helical strakes. In this paper, full-scale computational fluid dynamic analyses are carried out to examine the transverse forces caused by this vortex shedding using realistic current velocities in the GoM. Helical strakes are attached to the geometry to break up the coherence of the vortex shedding and the performance of these strakes is investigated numerically.