Evaluation and mitigation of modelling errors in numerical simulations of mooring line and seabed contact

• Main Author: Low, Chee Meng
• Other Authors: Ng Yin Kwee, Eddie
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/93531; http://hdl.handle.net/10220/48561
• Institution: Nanyang Technological University

Time-domain mooring simulation is a direct and effective method of evaluating mooring line loads and floater motions, in which nonlinear effects including drag forces and seabed interaction are readily accounted for. The formulation of seabed models affects the accuracy of line loads, including maximum tension as well as fatigue results, and also floater motions in coupled simulations. The interaction between the seabed and discrete line structural models is challenging in time-domain numerical simulations and a myriad of seabed models have been proposed in the literature. However, issues such as the selection of suitable seabed force coefficients and the application of suitable line discretisations, to achieve a balance between accuracy and computational cost, remain challenging problems to be resolved. This thesis investigates the shortcomings of existing numerical methods in accurately capturing the dynamic mooring line-seabed interaction and proposes new solutions. The line-seabed interaction problem is pertinent to catenary mooring lines which experience liftoff from and grounding on the seabed when undergoing large dynamic motions. This interaction is accounted for by various seabed models and it is known that the action of liftoff and grounding may lead to large dynamic tension fluctuations. These fluctuations may be spurious due to the inability of discretised mooring models to adequately account for the effect of the seabed on the mooring line. The effect of line discretisation and seabed model formulation on the tension fluctuations is investigated using the widely used spring-mattress approach. An in-house mooring code was developed to perform these investigations. For code validation and benchmarking, and to illustrate the existence of the tension fluctuations problem due to nodal grounding in existing mooring line simulation codes, comparisons are made to a commercial software. This work aims to contribute to the field of numerical mooring line modelling by identifying the root cause and highlighting the conditions leading to the production of the large dynamic tension fluctuations that tend to occur due to impact loads imparted by spring-mattress type seabed models on discrete line structural models. In particular, these tension shock waves are found to be caused by strain discontinuities manifesting in the vicinity of the touchdown zone as a result of discrete line elements and nodes coming into contact with the seabed. The likelihood of occurrence of the strain discontinuities is determined to be dependent on the impact speeds and orientations of line elements in relation to the seabed; higher impact speeds and close alignment between the seabed normal direction and the axis of line elements increase the tendency of occurrence of the strain discontinuities. In light of these findings, a novel seabed reaction force model that inhibits the production of strain discontinuities in a discrete line model is proposed. The results from this research further show that using a suitably fine discretisation for the mooring line model, while incurring a higher computational cost, is a generally applicable approach to improve result accuracy. Specifically, the modelling of line touchdown-liftoff effects requires a finer mooring line discretisation at the touchdown point to avoid the production of spurious line tension fluctuations. The numerical experiments performed in this research demonstrate that, for the purpose of ameliorating the effects of propagating stress waves arising from rapid line grounding, a hybrid discretisation employing fine elements within a limited span of line close to the touchdown zone and coarse elements elsewhere is sufficient and preserves the accuracy of predicted peak tensions. The results of the discretisation refinement study in this work further suggest that specifying a numerical chain element size close to the physical chain link size within the locally refined zone is sufficient even under severe excitation conditions which promote the production of the tension fluctuations. The disparity in element sizes within and outside the local refinement zone gives rise to a stiff dynamical system. This work introduces an approach for applying adaptive discretisation to the line structural model with a non-uniform mesh, and dual-rate time integration for the resultant stiff dynamic system. The results in this research suggest that a dual-rate time integration approach for the proposed hybrid spatial discretisation of the line structure significantly reduces computational times for smaller refined zone line spans as a proportion of total line length. As the touchdown point changes with time, the proposed adaptive discretisation procedure enables the locally refined zone to shift in tandem in order to limit the spatial extent of the refined domain, thereby preserving the computational efficiency gains of the dual-rate time integration scheme even as the touchdown point changes significantly. Hence, in this context, this thesis presents a method to achieve high accuracy at lower computational expense. Future studies can extend the proposed method to cable models with additional degrees of freedom such as bending and torsion.