Study of Snap Loads for Idealized Mooring Configurations with a Buoy, Inextensible and Elastic Cable Combinations for the Multi-Float M4 Wave Energy Converter

Abstract

There has been considerable modelling and wave basin validation of the multi-mode, multi-float, moored wave energy converter M4. The 6 float (2 power take off) (PTO) configuration is considered here with mooring from a buoy with light inextensible cables. Large mean mooring forces and very large peak or snap forces were measured in large waves while the rotational response about the hinges (for power take off in operational conditions) was predominantly linear. Modelling has been extended with elastic mooring cables connected directly to the base of the bow float and to the buoy. The experimental mean force is input to the linear diffraction/radiation model. The device response is effectively unchanged. The peak mooring force and tensions remain large with direct connection to the base of the bow float but are only slightly greater than the mean forces with elastic cables to the buoy, and an elastic hawser provides a further slight reduction. For the largest waves measured, the force was about 10% of the dry weight of the platform. The idealized efficient modelling may inform more detailed design while efficient methods for determining highly nonlinear mean forces remain to be established.

Experimental study of mooring forces on the multi-float WEC M4 in large waves with buoy and elastic cables

“…The mooring is assumed to be quasi-steady and only the buoyancy of the buoy assuming still water level is included. The mooring formulation given in (Stansby and Carpintero Moreno, 2020b) is precise within this context. The comparison results with the experimental data are shown in Fig.…”

“…There are some experimental data for a single buoy on a taut elastic cable (Hann et al, 2018) and for the hinged attenuator-type 6-float wave energy converter M4 with a single point mooring in uni-directional waves with inelastic cables (Stansby and Carpintero Moreno, 2020a). An important observation for M4 is that in large waves, as well as small-to-intermediate waves, the pitch response is almost linear (Santo et al, 2017), while the mean mooring force is highly nonlinear, and snap loads can be massive with inelastic cables (Stansby and Carpintero Moreno, 2020b). A preliminary numerical model investigation suggested that snap loads may be almost eliminated with elastic cables when combined with a single-point buoy (Stansby and Carpintero Moreno, 2020b).…”

“…An important observation for M4 is that in large waves, as well as small-to-intermediate waves, the pitch response is almost linear (Santo et al, 2017), while the mean mooring force is highly nonlinear, and snap loads can be massive with inelastic cables (Stansby and Carpintero Moreno, 2020b). A preliminary numerical model investigation suggested that snap loads may be almost eliminated with elastic cables when combined with a single-point buoy (Stansby and Carpintero Moreno, 2020b).…”

High-Capacity Wave Energy Conversion by Multi-Float, Multi-PTO, Control and Prediction: Generalized State-Space Modelling With Linear Optimal Control and Arbitrary Headings

“…It should be noted that, while angular response was effectively linear, the mooring loads are highly nonlinear in extreme waves. Optimum design for moorings is in progress [21].…”

Modelling and Optimisation of Wave Energy Converters

“…Some specific studies on the performance of the six-float M4 device in terms of wave power absorption, dynamic response, energy yield, mooring forces, etc. can be found in [742,106].…”