Work Package 2

Mooring loads assessment and reduction, shared mooring validation

Manufacture of elastomeric mooring tether, University of Exeter

Objective: De-risk innovations that lower mooring costs over 50% and enhance structural survivability

A mooring refers to any station keeping configuration to which a floating structure may be secured. The mooring provides to a large extend the restoring forces holding a floating structure secured to a location at the water surface whilst allowing some free movements.

A fundamental design question is the understanding of the mooring restoring forces that will allow to secure the floating structure but, in case for wave energy devices, not to impede the energy extraction modes. The capital cost of present mooring systems is estimated to incur about 10% of the capital cost of a typical marine energy converter installation. OPERA is addressing the design and cost challenge of the mooring through the introduction of a novel mooring component an elastomeric tether developed by the University of Exeter.

OPERA will reduce the mooring costs in wave energy through:

  • The use of shared mooring. In aquaculture shared mooring reduces mooring costs up to 50% and similar gains are expected in wave energy application.
  • Elastomeric tether. Laboratory results validated with at-sea data demonstrate a load reduction potential of over 70% for the elastomeric tether, which would translate to cost reduction of the same order for mooring lines. Elastomeric tether greatly enhances survivability with respect to a low-cycle fatigue failure mode for mooring connections. Low-cycle fatigue resistance is a central technical challenge for wave energy.

For these purposes, this Work Package will

  • specify the design requirements for a mooring condition monitoring system (CMS)
  • design assemble and incorporate the CMS and the elastomeric tether in the shared mooring
  • design, manufacture and bench-test a novel elastomeric tether
  • evaluate shared mooring open-sea operation with and without elastomeric tether

A key aspect subsequent to the field demonstration and bench testing is to validate the advanced design towards their use in marine environment (TRL 5-6), with respect to uncertainty arising from gaps in the understanding of components tested under laboratory conditions (TRL 3-4).

 

Task to be performed

Comprehensive dynamic behaviour study

This study will reduce uncertainties in the shared integrated mooring system and to specify design requirements for a mooring load and response monitoring system. This dynamic study will aid in the benchmarking for the open sea demonstration through a fully coupled simulation design assessment for both the shared mooring system with and without elastomer component, including slow drift motion, nonlinear coupled behavior, dynamic load characteristics and potential load reduction.

Shared mooring open-sea testing in two configurations

It involves a detailed design, the assembly and incorporation of the load and response monitoring system into the mooring configurations for open-sea demonstrations, as well as the integration of the elastomer component into the second demonstration phase. It is imperative to assess, validate and enhance the mooring configurations through the experience gained from open-sea demonstrations to obtain performance and reliability data related to dynamic motion characteristics, wear-out and fatigue properties.

Design, manufacture and bench test of full-scale elastomeric tether

This novel mooring tether will remove the conflict between axial stiffness and Minimum Breaking Load (MBL). Based on the design criteria identified for open sea demonstration within first task in this work Package, multiple full-scale elements of a novel elastomeric mooring component will be manufactured and bench tested prior to the integration into the shared mooring configuration. The bench test will be based on the advances made during Laboratory prototype (TRL 3-4) testing, allowing to de-risking the integration of an elastomer at sea environment (TRL 5-6).

Evaluation of shared mooring configurations

This evaluation will be based on data obtained from open sea demonstration campaigns. Data obtained on the previous tasks will be used to compare and validate the characteristics of the shared mooring configuration in its different configurations. Performance parameters, e.g. extreme loads and motion response to different sea states informing technology compliance, survivability & reliability, as well as economical improvements will be assessed.

Bench testing of elastomeric mooring tether, University of Exeter

Involved Partners

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654.444