Motion prediction of a Semi-Submersible Crane Vessel at inconvenient draft

Abstract

During specific operations, offshore marine contractor Heerema Marine Contractors reduces the draft of their Semi-Submersible Crane Vessel (SSCV) such that the floaters are submerged in close proximity to the free surface. This draft is also known as inconvenient draft. At this draft the motions of the vessel do not fully comply anymore with the computed prediction using linear diffraction theory. A proper motion prediction is required to ensure a safe execution of the offshore operation. The key issue is the submerged part of the floater, since discrepancies occur as soon as there is only a small water column on top of the floaters. Motion RAOs are computed via the hydrodynamic coefficients and wave loads. The reason why this leads to discrepancies in the motion RAO is not yet known. In addition, qualitative hydrodynamic data should be gathered for an object submerged in close proximity of the free surface.
Therefore, in this study the cause of the discrepancies in motion RAO is studied on an experimental basis for an object submerged in close proximity to the free surface. The scope is narrowed down to a two-dimensional cross-section of a SSCV-floater. Numerical simulations using linear diffraction theory are performed in WAMIT. The results are compared to experimental data obtained via model tests performed at the towing tank facilities at the faculty 3mE at Delft University of Technology.

Based on experimental data is concluded that linear diffraction theory does not predict physical phenomena that satisfy the boundary conditions and that underlie principles of the theory. Despite the fact that the experimental data does satisfy the boundary conditions of the numerical simulation, discrepancies occur at the hydrodynamic coefficients and wave load. As a result, it can be concluded that linear diffraction theory malfunctions at the inconvenient draft region, and therefore is not the correct theory to determine RAOs for an object on inconvenient draft.
Discrepancies in motion RAO are found to be dominated by the discrepancies the wave load, except for the frequency at which the added mass equals negative inertia of the body. Discrepancies for added mass are less significant than for the damping coefficient and wave load. At higher frequencies inertia becomes dominant over damping and damping deviations affect the RAO less.
The pitch motion about the center of the cross-section does not represent a rotational motion about same the degree of freedom for an SSCV. However, it allowed to experimentally investigate the global numerical extremes. In addition, it led to the finding that it seems that the rotational data is more affected than the heave data.

There is a strong suspicion that poles in the complex plane are the cause of the discrepancies in the hydrodynamic data, which result of the used Green's function by Wehausen and Laitone. This suspicion is based on characteristics of the numerical data in combination with findings in literature and the experimental data, accumulate.

Currently, an approach that makes use of free surface damping is applied to predict the motion of an SSCV at inconvenient draft. Based on experimental data can be concluded that the use of free surface damping without further modifications does not result in an accurate motion prediction.

Lastly, the effect of nonlinearities when increasing the oscillation or wave amplitude. The wave load is found to be more prone to nonlinear effects than the hydrodynamic coefficients.The force signal remained dominantly harmonic, but higher harmonic forces did show up. Furthermore, with the exception of most tests at the largest tested submergence, higher harmonic waves were measured and visually observed during the tests. These higher harmonic waves arose at the transition from the shallow to deep water regime and vice versa. These forces were found to have hardly any effect on the force signal.