The dynamic responses of any floating platform are dependent on the mass, stiffness and damping characteristics of the body as well as mooring system. Therefore, it is very essential to study the effect of individual contributions to the system that can finally help to economise their cost. This paper focuses on the effect of mooring stiffness on the responses of a truss spar platform, obtained by different grouping of lines. The study is part of our present researches on mooring systems which include the effect of line pretension, diameter and azimuth angles. The platform is modelled as a rigid body with three degrees-of-freedom and its motions are analyzed in time-domain using the implicit Newmark Beta technique. The mooring lines restoring force-excursion relationship is evaluated using a quasi-static approach. It is observed that the mooring system with lines arranged in less number of groups exhibits better performance in terms of the restoring forces as well as mean position of platform. However, the dynamic motions of platform remain unaffected for different line groups.
Mooring lines are the most commonly used station-keeping systems for floating platforms as they are easy to install and relocate. The mooring lines are usually pre-tensioned so as to use their energy absorption to reduce the platform motions and thereby, to lower the forces in the lines. To decide on the preliminary design of the platforms, it is necessary to investigate the restoring behaviour of the mooring systems for various parameters. In this study, two different mooring configurations with and without mooring line in wave heading direction are considered for determining its behaviour for various pretensions in the lines. A MATLAB code named QSAML has been developed using quasi-static approach to compute the restoring forces of the mooring system. The code is validated with experimental tests and used in this study. It has been observed that with increase in pretension of the mooring line, restoring performance of the mooring system can be improved. The maximum permissible excursions by mooring system in the wave heading direction are found to be more for relatively lower pretension values.
Moored systemsof floating platforms typically have mooring lines in groups,attached to hull of the platform in different configurations.Study on therestoring behaviour of these systemsfor variousparameters of mooring lines gives an insight to decide on the preliminary design of platforms.This paper includesparametric study on the behaviour of mooring systems for various symmetric, asymmetric configurations of the lines. To compute the restoring forces of the mooring system,quasi-static analysis has been adopted and a MATLAB code named QSAML has been developed, validated with experimental tests and used for the parametric study.It has been observed that, the restoring capability of mooring system reduces whenthe lines are placed away from the wave heading.For any wave heading, the variation in restoring behaviour of mooring system with symmetric configurations is insignificant up to relatively small excursions of the platform.
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