As an offshore contractor Heerema Marine Contractors (HMC) is specialized in the transportation and installation of offshore structures such as jackets, topsides and subsea infrastructure. Semi-Submersible Crane Vessels (SSCVs) are a part of the fleet currently operated. These vessels are mainly used for heavy lift and subsea construction operations in deep water. However, in recent years heavy lift operations in shallow waters have been increased. In very shallow waters the lift operation cannot be performed on the standard deep operational draft and shallower drafts are required, to maintain sufficient bottom clearance.
Diffraction programs using potential theory are a quick and effective method in calculating wave forces and ship motions. However for shallow drafts, where only a small layer of water is present on top of a submerged body, diffraction calculations typically overpredict motion and wave load RAOs. This shortcoming of diffraction programs is well known and observed after conducting model tests on a free floating SSCV at a shallow draft. As such, from a numerical point of view, these shallow drafts can be considered “inconvenient drafts”.
To verify whether CFD is able to predict the SSCV motions at these inconvenient drafts, CFD simulations are performed on a SSCV in waves. The SSCV is free to heave, roll and pitch. The focus will be on beam incoming waves with the SSCV at different drafts ranging from a deep draft, where potential theory is able to predict the motions very well to shallow drafts, where potential theory typically overpredicts the motions.
To limit the simulation domain an Euler Overlay Method is used where in the vicinity of the SSCV the domain is simulated using CFD by overlaying an Euler solution defined at the outer domain. To allow the SSCV to heave, roll and pitch within the domain the overset mesh method is used.