Small scale experimental validation of a numerical model of the HarshLab2.0 floating platform coupled with a non-linear lumped mass catenary mooring system

Abstract

When focusing on mooring system numerical modelling, the efforts are focused on validating models that increase the accuracy and maintain the computation time under reasonable limits. In this paper an approach for modelling the interaction among supporting structure and mooring system is introduced through kinematic relations. The proposed approach has been validated with the experimental wave tank 1:13.6 scaled data of the HarshLab 2.0 platform, a CALM type buoy moored with a three-line catenary system and used as a floating laboratory for materials and corrosion testing, to be installed at BiMEP. The drag forces of the buoy as well as the Morison coefficients of the heave-pitch coupling, induced by the attached structure for ships bow landing, have been identified. Results of the mooring line tensions are validated with imposed displacements of the structure and, subsequently, with coupled simulations of the moored buoy in a set of realistic sea states. Sources of differences on the estimation of line tensions are found to be mainly due to uncertainties of seabed friction forces, a high sensitivity of line tensions to small swaying and a poor pitching performance of the numerical model, very likely due to a very non-linear pitching of the physical model.

Frequency domain modelling of a coupled system of floating structure and mooring Lines: An application to a wave energy converter

“…The floater and mooring coupled non-linear TD numerical model, used as the reference model in this work, has been introduced and validated in [18]. Motions of several diffracting bodies can be included to represent WECs, mostly made up of more than one diffracting body.…”

“…In contrast with the non-linear time domain model, the linearized FD model provides only the time-varying part of the solution. The structural damping, as already introduced in [18], is valid for velocities referred either to the absolute reference centre or to the mean position. However, the stiffness matrix needs to be redefined to work with the time varying motions, referenced to the mean position.…”

“…The mooring system for the model verification has been assumed to be made up of three catenary lines as specified in Table 2 and represented in Figure 4, in a water depth of 172m, 140m below the fairleads. The corresponding lines are made up of a single chain section with the properties specified in Table 3 and also assumed in the tank test validation introduced in [18]. The corresponding non-dimensional pretension of the lines is 1.43 [-] as defined in [26].…”

“…Mooring line motions are based on the non-linear lumped mass method and are coupled to the floater by kinematic relations. The non-linear floater and mooring system coupled model has been validated with tank test results, and introduced in [18]. In the present work all non-linear effects, such as drag force on the floater and line sections as well as the geometric stiffness, have been linearized through statistical linearization for random waves.…”

A Comparison of Numerical Approaches for the Design of Mooring Systems for Wave Energy Converters

“…An underprediction of the equivalent QSTD model of maximum line tensions of 60% to 70% under regular motions has been documented [5] with respect to experimental tests. In addition to the accuracy of different approaches, it has been found [6] that inaccuracies in structure motions are directly translated into differences on mooring line tensions. However, the QSTD approach has been successfully applied to estimate the umbilical cable influence on underwater vehicles [7,8], accounting for current forces on the umbilical.…”