Numerical modelling of seabed impact effects on chain and small diameter mooring cables

Abstract

Catenary mooring lines experience liftoff from and grounding on the seabed when undergoing large dynamic motions. Numerical line mooring models account for this interaction using 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. In this work, the root cause and conditions that lead to the production of the large dynamic tension fluctuations is determined. The effect of line discretisation and seabed model on the tension fluctuations is investigated using the widely used spring-mattress approach and a modified seabed reaction force model. 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.

Uncertainty Quantification in Mooring Cable Dynamics Using Polynomial Chaos Expansions

“…The oscillations in low tension regions are exaggerated in the numerical model. This is due to (i) the problem being ill-posed at slack conditions [40] and (ii) the ground interaction being modeled with the traditional elastic seabed approach (the so-called spring-mattress method), which is known to introduce spurious oscillations [41]. For each period, we analyze four different scenarios: one analyzing the effects of uncertainty on all the coefficients at the same time, and three which analyze the effects of uncertainty in each coefficient individually.…”

Modeling and Numerical Simulation of Semitensioned Mooring Line under Taut-Slack State

“…A mixed distribution extreme anchorage tension and fatigue damage analysis model was proposed, and the parameters of the mixed model were estimated by the expectation-maximization algorithm, and the limit tension analysis was carried out by depolymerization method to investigate the infuence of the polymerization process setting on the prediction of limit tension. Low et al [21] studied the efect of line dispersion and the seafoor model on tension fuctuation by using the spring-pad method and improved seafoor reaction model. Hsu et al [22] proposed a comprehensive probability distribution model to predict the impact load on the mooring line when the fan experiences large wave and wind-induced motion.…”