A SLACK MOORING FOR A LARGE SHIP UNDER BEAM SEAS IN PORT

As an empirical and traditional ship mooring method, a pretension mooring (a hard mooring) or slack mooring (without pretension in mooring line) is performed for large ship or small ship, respectively. However, the slack mooring is applied to a large ship under beam seas in port to avoid large roll and sway motions, and excessive loads in mooring lines and fenders. In this study, the mechanism and validity of the slack mooring for large ship is investigated by a physical model test and a numerical simulation method of the moored ship motions and mooring loads under beam seas.

The Snap Tension Analysis of Taut-Slack Mooring Line with tanh Method

The continuous model is introduced, and the nonlinear partial differential equations of taut-slack mooring line system are transferred to nonlinear algebraic equations through tanh method, and four solitary solutions are obtained further. At the same time, to express the results clearly, the curve surfaces of strains, displacements, and tension are plotted. The results show that there are four different solutions in the system. With the pretension increasing, the tension changes from one solitary solution to snap tension, and when the pretension is increased further, the curve converts to continuous line, until straight line, which is corresponding to the taut mooring line. In the process of increasing of pretension, the mooring line transfers from slack to taut, accompanied with tension skipping, which is reduced by the system parameters, and different combination of parameters may introduce different tension in line, and the uncertainty may cause the breakage of mooring system. The results have an agreement with experiment, which shows that the calculating method in this paper may be believable and feasible. This work may provide reference for design of mooring system.

The Analytical Solution of Taut-Slack Mooring Line With Tanh-Method

In this paper, the continuous model is introduced and solitary solutions of the nonlinear partial differential equations of taut-slack mooring line are obtained by tanh method, and the plots of results are drawn. The results show that, with the continuous increasing of pretension, the tension curve convert from snap tension to continuous line, until to multi-peaks line. In the process of increasing the pretension, the mooring line transfer from slack to taut, until to breakage, accompanying with tension skip, which is reduced by the system parameters, and different combination of parameters may introduce to different tension in line, and the uncertainty may cause the breakage of mooring system.

Hydrodynamics of Multiple Floating Point-Absorber Wave Energy Systems With Inter-Body and Bottom Slack-Mooring Connections

Point absorbers constitute an important class of offshore wave energy converters. If employed in the extensive exploitation of the offshore wave energy resource, they should be deployed in arrays, the distance between elements in the array being possibly tens of meters. In such cases, it may be more convenient that the array is spread moored to the sea bottom through only some of its elements (possibly located in the periphery), while the other array elements are prevented from drifting and colliding with each other by connections to adjacent elements. This kind of mooring arrangement is addressed in the paper in a simplified way by considering an array of two buoys. Two opposed slack-mooring lines connect the floater pair to the bottom, while a third line, from whose mid-point hangs a weight, connects the two buoys pulling them towards each other. The centres of the buoys and the mooring lines are in a vertical plane parallel to the incoming wave direction, so that body and mooring motions are two-dimensional. The whole system — buoys, moorings and power take-off systems (PTOs) — is assumed linear, so that a frequency domain analysis may be employed. In the numerical simulations, two identical hemispherical buoys oscillate in heave and surge, acted upon by the waves, the mooring system and their PTOs. The PTO consists of a linear damper whose force is proportional to the heave velocity. Results from numerical simulations, with regular and irregular waves, are presented for the motions and power absorption of the converters, for different mooring and PTO parameters. Comparisons with the simpler cases of one single buoy in the moored and unmoored situations show significant differences.

Station Keeping Study for WEC Devices Including Compliant Chain, Compliant Hybrid and Taut Arrangement

This paper presents results from the third phase of large scale mooring investigation at the Scapa Flow in Orkney, Scotland, for wave energy applications. It considers the suitability of three slack mooring line configurations, namely, i) compliant chain, ii) complaint hybrid and iii) taut fibre rope arrangement. This investigation is extended with a numerical study analysing the station keeping criteria for the three slack line configurations and that of a taut system for wave energy converter installations. The results are used to discuss the suitability of the individual mooring line configurations and that of the taut arrangement. This paper is partially to provoke a discussion on the advantages of a taut mooring system and raise the issues that need to be solved if they are to be practicable.