Numerical Calculation of Roll, Pitch and Heave Decay of a Gravity-Based Structure During Tow-Out

ExxonMobil Canada Properties and its collaborators are building a gravity based structure (GBS) in Newfoundland and Labrador to be installed on the Hebron Field offshore Eastern Canada. The geometry of the Hebron GBS is driven by the design constraints of a long operating life placed on the seafloor in the harsh environments at the Hebron field location. Prior to installation, the GBS encounters a number of significant floating phases — deep-water construction, towage, and installation. In each of these stages the hydrodynamic character of the unique Hebron geometry must be quantified to predict dynamic motions, assess tow performance, and define installation procedures and limitations. For the floating GBS, both heave and roll natural periods are long, and resonant responses due to wind gusting and vertical wave drift take place. Assessing the hydrodynamics by industry standard empirical methods is insufficient as regards capturing the viscous damping generated by eddy formation off the sharp edges of the submerged GBS structure. This paper focuses on the utilization of Computational Fluid Dynamics (CFD) to better understand and quantify this viscous damping. The Hebron GBS work has challenged the traditional methods for model generation and use of CFD for damping estimation, primarily because these techniques are most frequently used for traditional hull shapes. For example, application of the overset mesh technique, as successfully applied for ship-shaped vessels, does not provide the required accuracy in this case. Alternative numerical models are tested and found to work well: morphing techniques and sliding meshes for heave and pitch, respectively. Based on convergence and sensitivity studies, the results are accepted. Ultimately, the CFD work allows the project to quantify the expected increases in hydrodynamic damping and progress in the process of improving predictions of GBS motions and thus optimizing the operational planning of towing and installation.