Session: 09-01-12 Experimental Methods

Paper Number: 79834

79834 - Experimental Analysis of Mooring and Power Cable Dynamics When Using Elastic String Models 

This work analyses the mooring and power cable dynamics in large-scale experimental tests carried out in the wave-current-tsunami flume (COCOTSU) facility at IHCantabria. The analysis is based on scaled elastic string models for a single chain-nylon mooring line and the dynamic cable of a 15MW floating offshore wind turbine (FOWT) supported by a concrete semi-submersible platform (ActiveFloat) in Gran Canaria Island (Spain). Both scaled concepts in the 100 m deep site are developed within the framework of the project COREWIND. All the test campaign is planned to be fully monitored; hence two overlapped video cameras register the line kinematics while the tensions are recorded in its two extreme points.

The most difficult characteristic to fix in an elastic material at laboratory scale 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, including a calibrated spring in the line as in an 'all chain' mooring configuration is not possible anymore, because the nylon has the limiting stiffness and this one is moored at the fairlead. The first problem lies in finding a material capable of replicating the real stiffness with an acceptable hysteresis. The second issue consists in knowing the axial stiffness of the selected elastic material for each imposed oscillation, as it depends on the loading velocity. On the other hand, the limiting mechanical characteristic of the lazy-wave cable is the bending stiffness, as adopting Froude scaling laws of similitude it is reduced at model scale by a power of five. In order to make the bending stiffness tests, we have previously built our own three-point bending flexural tester. 25 materials have been characterised to identify the best one for testing. From this analysis a database of materials has been derived.

For each of the configurations analysed, a set of 40 forced oscillation tests are planned to reproduce either mooring fairlead or power cable connector movements in surge. An initial tension-deformation test is followed by 28 combinations of harmonic excitations with two origins (initial static position and with a 7.5 m mean drift), two amplitudes (2.5 m and 5 m) and seven periods (6 s, 7.5 s, 9 s, 10.5 s, 12 s, 14 s and 17s), and 11 irregular time series obtained from 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 freely available to the wind energy sector. The ambitious reduced scale tests proposed provide enough cases to deliver a benchmarking database for numerical models calibration including forces at anchor and fairlead, as well as line shape.

Presenting Author: Miguel Somoano IHCantabria - Instituto de Hidráulica Ambiental de la Universidad de Cantabria

Authors:

Miguel Somoano IHCantabria - Instituto de Hidráulica Ambiental de la Universidad de Cantabria
David Blanco IHCantabria - Instituto de Hidráulica Ambiental de la Universidad de Cantabria
Alvaro Rodriguez-Luis IHCantabria - Instituto de Hidráulica Ambiental de la Universidad de Cantabria
Raul Guanche IHCantabria - Instituto de Hidráulica Ambiental de la Universidad de Cantabria