Elastic Mooring Lines Research Articles

Short-term analysis of extreme wave-induced forces on the connections of a floating breakwater

In the design of floating breakwaters, assessment of the extreme wave-induced loads on the connections, their weakest element, is required to ensure structure survival. This case study provides guidelines for estimating the extreme wave-induced forces on the connections of a floating breakwater. A Boundary Element Method (BEM) solver was applied to obtain the time-domain response of an array of five pontoons anchored to the sea bottom with elastic mooring lines. The hydrodynamic behaviour of the structure was assessed for short-duration sea states with different wave peak periods and oblique wave directions. Two peak selection criteria were applied to obtain force distributions, and several different probability density functions (PDF) were fitted to the resulting data. The extreme wave-induced forces on every connection and sea state were estimated for two different exceedance levels during a typical 3-h sea state. Based on the results, combination of the Peaks Over Threshold (POT) method and generalized Pareto distribution results is proposed for estimating the wave-induced design forces on the connections of floating pontoon breakwaters.

Time-domain numerical modelling of the connector forces in a modular pontoon floating breakwater under regular and irregular oblique waves

Floating breakwaters are an interesting alternative to bottom-fixed structures due to their lower cost and environmental impact. Nonetheless, structural failures in the module connections are frequent. Accurate analysis of the hydrodynamic response is therefore crucial to their structural design. This article reports on numerical modelling of the forces on the module connectors in a modular pontoon breakwater. The study case was an array of five pontoons with elastic mooring lines under regular and irregular oblique waves. A 3D Boundary Element Method (BEM) solver was applied to find the velocity potentials and to characterize the frequency-domain response. The time-domain response of the floating breakwater is calculated along with the forces in the elastic mooring lines and module joints. Nonlinear hydrodynamic effects were reproduced by calculating the Froude-Krylov and hydrostatic forces over the instantaneous wetted surface at each time step. The response of the modular array under regular waves was then compared with previous experiments. After calibrating connector stiffness, the numerical and experimental results were in good agreement. Finally, the floating structure's response to regular and irregular waves was compared. The results revealed that simulation of only regular waves underestimates maximum connection forces by one order of magnitude.

Experimental investigation on characteristic change of a scale-extendable chain-type floating structure

The present paper reports an experimental investigation in order to understand the characteristic evolution of a chain-type floating system with increasing number of modules up to five. Semi-submersible modules are fabricated, connected by hinges and constrained by elastic mooring lines for experimental tests in a wave flume. The module responses of floating structures in various configurations are measured and analyzed in different wave periods and wave heights. In general, the response in longitudinal direction of multi-modular floating structure can be significantly improved by the increasing number of modules, while the effects of increasing modules on the heave and pitch responses are negligible. Based on the rigid module flexible connector (RMFC) model, validated by the experimental data, the connector loads of the experimental prototypes are assessed. The results indicate that longitudinal hinge loads increase doubly as modular number is added up to four, and afterwards increases very slow for further additional modules. The maximum longitudinal load usually occurs on the interior connectors, not on the connectors near the bow and stern. The level of vertical shear load mainly depends on the wave period rather than the number of modules in the floating platform. More detailed remarks are listed in conclusions.

Reducing hydroelastic responses of pontoon-type VLFS using vertical elastic mooring lines

This paper is concerned with the reduction of hydroelastic responses of pontoon-type very large floating structures (VLFS) via the use of vertical elastic mooring lines connected at the fore and aft edges of VLFS to the seabed. These vertical mooring lines are used in addition to the mooring dolphin-rubber fender system that is used to restrict the horizontal movement of VLFS. The considered rectangular shaped VLFS is modeled as a Mindlin plate floating on an ideal fluid in which the linear potential theory is applicable. The hydroelastic analysis of VLFS with vertical mooring lines modeled as vertical elastic springs is performed in the frequency domain by using the hybrid finite element–boundary element (FE-BE) method. This study examines the effectiveness of vertical elastic mooring lines in reducing hydroelastic responses for various wavelengths, incident wave angles and aspect ratios. The stiffnesses of the mooring lines are optimized for maximum reduction in the hydroelastic response by using the Differential Evolution algorithm. It was found that hydroelastic responses of VLFS could be significantly reduced by using vertical elastic mooring lines with optimal stiffnesses.

Moored offshore structures - evaluation of forces in elastic mooring lines

In most situations, the high frequency motions of the floating structure induce important effects in the mooring lines which affect also the motions of the structure. The experience accumulated during systematic experimental tests and calculations, carried out for different moored floating structures, showed a complex influence of various parameters on the dynamic effects. Therefore, it was considered that a systematic investigation is necessary. Due to the complexity of hydrodynamics aspects of offshore structures behaviour, experimental tests are practically compulsory in order to be able to properly evaluate and then to validate their behaviour in real sea. Moreover the necessity to carry out hydrodynamic tests is often required by customers, classification societies and other regulatory bodies. Consequently, the correct simulation of physical properties of the complex scaled models becomes a very important issue. The paper is investigating such kind of problems identifying the possible simplification, generating different approaches. One of the bases of the evaluation has been found consideringtheresults of systematic experimental tests on the dynamic behaviour of a mooring chain reproduced at five different scales. Dynamic effects as well as the influences of the elasticity simulation for 5 different scales are evaluated together. The paper presents systematic diagrams and practical results for a typical moored floating structure operating as pipe layer based on motion evaluations and accelerations in waves.

Three-dimensional behaviour of elastic marine cables in sheared currents

This paper presents the formulation and solution of governing equations that can be used to analyse the three-dimensional (3D) behaviour of either marine cables during installation or the response of segmented elastic mooring line catenaries as used for floating offshore structures when both are subjected to arbitrary sheared currents. The methodology used is an extension of one recently developed for analyses of marine cables when being installed on the seabed or being towed. The formulation describes elastic cable geometry in terms of two angles, elevation and azimuth, which are related to Cartesian co-ordinates by geometric compatibility relations. These relations are combined with the cable equilibrium equations to obtain a system of non-linear differential equations, which are numerically integrated by fourth and fifth order Runga–Kutta methods. The inclusion of cable elasticity and the ability to consider arbitrary stored currents are key features of this analysis. Results for cable tension, angles, geometry and elongation are presented for three example cases—the installation of a fibre optic marine cable, the static analysis of a deep water mooring line and the response of a telecommunications cable to a multi-directional current profile.

Stability of single point mooring systems

A mathematical model for the horizontal plane slow motions of ships connected to a mooring terminal through a single elastic line is developed based on the ship manoeuvring equations and a non-linear elastic mooring line model. Slowly varying excitations from current, wind and drift forces are included. Local perturbation analysis about the critical points of the corresponding autonomous system reveals the asymptotic stability characteristics of the mooring system. Time simulations using a tanker and a barge confirm the theoretically predicted long term behaviour and show the short term system behaviour. The mooring system may exhibit instability in sway, yaw and surge or reach a limit cyclic depending on the properties of the critical points, vessel type, propeller, initial conditions, current and wind.

On the failure to lay the submarine cable

This paper presents the formulation and solution of governing equations that can be used to analyse the three-dimensional (3D) behaviour of either marine cables during installation or the response of segmented elastic mooring line catenaries as used for floating offshore structures when both are subjected to arbitrary sheared currents. The methodology used is an extension of one recently developed for analyses of marine cables when being installed on the seabed or being towed. The formulation describes elastic cable geometry in terms of two angles, elevation and azimuth, which are related to Cartesian co-ordinates by geometric compatibility relations. These relations are combined with the cable equilibrium equations to obtain a system of non-linear differential equations, which are numerically integrated by fourth and fifth order Runga–Kutta methods. The inclusion of cable elasticity and the ability to consider arbitrary stored currents are key features of this analysis. Results for cable tension, angles, geometry and elongation are presented for three example cases—the installation of a fibre optic marine cable, the static analysis of a deep water mooring line and the response of a telecommunications cable to a multi-directional current profile.