Experimental analysis of mooring and power cable dynamics when using elastic string models

Abstract

This work analyses the mooring and power cable dynamics in large-scale experimental tests carried out in the wave­ current-tsunami flume (COCOTSU) facility al JHCantabria. The analysis is based on scaled elastic string models for a single chain -nylon mooring line and the dynamic cable of a J 5MW floating offshore wind turbine (FOWT) supported by a concrete semi-submersible platform (ActiveFloat) in Gran Canaria Jsland (Spain). Both scaled concepts in the 100 m deep si/e are developed within the framework of the project COREWJND. Ali the test campaign is p/anned to be ful/y monitored; hence two overlapped video cameras register the line kinematics while the tensions are recorded in its two extreme points. The most di.fficult characteristic to flx in an elastic material at laboratory sea/e is the combined reproduction of axial and bending stiffness. On the one hand, to replicate the real axial stiffness in a chain-nylon mooring line, the flrst problem lies in flnding a material capable of doing it with an acceptable hysteresis. The second issue consists in knowing the axial stiffness of the selected e/astic material for each imposed oscillation, as it depends on the loading velocity. On the other hand, the limiting mechanica/ characteristic of the lazy-wave cable is the bending stiffness. To make the bending stiffness tests, we have previously built our own three-point bending flexura/ tester. 25 materials have been characterised to identify the bes/ one for testing. From this analysis a database of materia/s has been derived. For each of the configurations analysed, a set of 40 forced oscillation tests are planned to reproduce either mooring fairlead orpower cable connector movements in surge. An initial tension -deformation test is followed by 28 combinations of harmonic excitations with two origins, two amplitudes and seven periods, and 11 irregular time series obtainedfrom the resulting surge displacements of the platform when simulating in OpenFAST extreme and severe Design Load Cases 1.3, 1.6 and 6.1. Experimental data obtained are stored in an online repository to make it free/y available to the wind energy sector. The ambitious reduced sea/e tests proposed provide enough cases to deliver a benchmarking database for numerical models calibration inc/uding forces at anchor and fair/ead, as well as line shape.