Global Developers
Here Fraser showed a slide depicting the principal developers of wave-energy technology and the maturity of each type.
This was followed by a comparison of the mooring requirements and operation of oil-and-gas operations and the wave-energy conversion industry.
Item Oil and Gas Wave-energy Conversion
Operational intent
Catenary Retain a specified watch circle Limit loads and promote energy extraction
Tension mooring Distance between drill floor and Limit loads and promote energy extraction
seabed
Human monitoring Continual onboard None load cells
Maintenance intervals Annual (DNV) Limited (3-5 years)
Environmental risks High (Hydrocarbons) Low (commercial)
Comparative mass <=5% >=25%
Materials Chain/wire/synthetic Chain/wire/synthetic
Storm survival Rise up/cut and run Remain and survive
CAPEX (% Project cost) >5% <50%
Revenues ~$billions ~$20m over 25 years
Challenges
Challenges faced by wave-energy conversion companies include the fact that they are often first of class; they have additional degrees of freedom, there is a non-linear response of the power take-off, the OWC chambers vary the system response, the elative unit size and mass compared to wavelength. In addition, they are following the cash-rich oil and gas industry, but without the cash reserves. There has been a history of perceived failure of wave-energy systems, and the operational range and survival peak is high.
The curved line for design waves shows the cumulative percentage of occurrence for a specific location in the UK, and the diagonal line shows the DNV requirement.
The curved line for wave power shows the cumulative percentage of occurrence for a specific location in the UK, and the diagonal line shows the DNV requirement.
Mooring Process
After checking the site parameters and deciding a physical design, the mooring process includes the following steps:
1. Quasi static analysis and concept design.
2. Uncoupled motion analysis (software).
3. Model testing, structure and mooring (physical).
4. Coupled analysis (software).
5. Final design, bill of materials and mooring manual.
There is feedback from Step 3 to Step 1, and from Step 4 to Step 3.
Input data for the mooring process includes information for the concept design including geometry and hydrostatic parameters, site data including bathymetry, met-ocean data including DNV-RP-C205, wave climate (significant wave height vs period and direction), the design wave contour, wind and current.
Analysis
Input data for Step 1, the quasi-static analysis, includes the mooring stiffness characteristic, the mooring length and bearing, the total mean steady forces provided by wind, wave and current, and the vessel excursion. The output of this analysis gives the fairlead mooring-line tension.
In Step 2, the uncoupled analysis, hydrodynamic modelling of the structure is done by HydroStar or SESAM. Inputs include vessel geometry and hydrostatic properties, and the output gives the hydrodynamic motions.
The output of Step 3, the scale model trials, gives the motions, loads and a high-speed video recording.
Step 4, the uncoupled analysis, includes a parametric analysis and design sensitivities, and output gives the loads.
In Step 5 the design is finalised and a bill of materials produced.
Mooring Designs
What do the moorings look like? Fraser showed diagrams of the principal types of moorings. These included the OPT-PB150 with a three-point taut float for a 300 t unit, the Orecon-MRC quasi-tension mooring for a 1750 t unit, and the Orcaflex for a design wave with a return period of 100 years. A video of the Orcaflex unit showed the scope of oscillations of the unit and the mooring lines due to motions.
Five hull model mooring variations were tried. These included
A a standard 100 m two-bridle mooring;
B up-weather, with Legs 1 and 2 of 140 m, and Leg 3 of 60 m;
C as B but with mooring attached to float only;
D1 lengths as in B but with long up-weather bridles to spar and float, and down-weather to spar only;
D2 as in D1 but with lower pre-tension; and
E as in D1 but with all moorings attached to the spar.
The mooring problem for the Orecon-MRC unit is that the mooring load is provided by both the cross-sectional area of the unit and the cross-sectional area of the chambers themselves. Fraser showed a video of a model of the unit in Toulon, France, in operational conditions, and then in storm conditions. In storm conditions the structure bobs and nosedives, not due to the mooring system, but due to the air pressure in the chambers. The mooring loads go through the roof, and a gravity mooring is not possible. This is because the equation for the natural heave period is dominated by the term for the chamber stiffness on the bottom line.
The MRC pre-tension (900 t) acts to oppose the negative pressure within the chambers, thus drawing air through the turbine. The anchors act as a reaction mass to oppose positive pressure within the chambers, thus forcing air out through the turbine. The final mooring system uses a combination of forward and aft moorings, together with vertical tension moorings to the sea bed.
Fraser then showed videos of the sonton wave and the Orcaflex model.
Conclusion
Wave-energy conversion systems have developed from an idea to an industry, and have come of age. The design of moorings for the various systems are also developing, based on the experiences of the various companies involved. There are various types, and these can now be designed to cope with the loads imposed by storm conditions, which is what they have to be able to survive.
Questions
Question time was lengthy, and elicited some further interesting points.
Failure mode and effect analysis comprises a large part of what mooring designers do in developing designs.
The water depth depends on the type of device; deeper water, in general, means more energy. A depth of water of about 50 m seems to be about the optimum. A distance of 50 m from shore is about the closest. In South Australia there is an installation about 1.5 km off the beach, so people don’t even notice.
Erosion of the sea bed due to the converter is minimal; it cannot be measured at model scale.
Maintenance currently has to be done at least yearly to maintain class, but operators want to extend this to three years between lift-out for inspections.
Crystal balling the future, development in Australia may depend on ACRE, the Australian Cooperative research Centre for Renewable Energy, which has released $130million for research. In three years, we could see 3 MW in South Australia. The UK has a large number of OEM companies involved, e.g. ABB, Siemens, and Alsthom. For progress, ther have to be no failures. Young, bright people from university are needed to make it happen. We will see more-rigorous energy regulation, and no failures will see it go ahead, e.g. 20 MW within five years, and then it will explode.
The vote of thanks was proposed, and the “thank you” bottle of wine presented, by Syd Cullen.
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