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Foundation Ex,
Tech Sessions Q&As

During the Tech Sessions the audience have got stuck in and helped generate excellent debate

Here we’ve taken more time to answer the big technical questions 

How much is the wind turbine performance affected by the floater movements?

The behaviour of floating offshore wind turbine (FOWT) is complicated under the combined effects of aerodynamic and hydrodynamic loading, as the floater has six rigid-body modes of motion: heave, sway, surge, roll, pitch, and yaw. The tilt angel of the FOWT changes the effective area of the rotor blades, leading to a reduction in power output. A recent study from RMUTT indicates that reduction percentage occurred at a beta angle between 3.5°-6.1°, resulting in loss of performance ranging from 22%-32% at various wind speeds.

To have a good understanding of the impact of floating movement on the FOWT performance, experimental models and computational fluid dynamics (CFD) simulations can be developed. The integrated load analysis using the coupled aero-hydrodynamic approach is recommended to analyse FOWT’s structural and turbine performance.

Which software (combinations) are most common for integrated design of floaters and could you cover some of the pro's & con's?

An Integrated Loads Analysis (ILA) of floating offshore wind (FOW) needs to incorporate the coupling effects between the floater motion and the turbine aerodynamic forces, and between the floating and the station keeping system. A fully coupled analysis of FOW combines aerodynamic and hydrodynamic effects simultaneously. There is a number software and software combination options available for FOW analysis in the market, such as OrcaFlex+FAST, HAWC2, SIMA, SLOW, etc. The tool selection depends on the structural type, stage of design, and designer’s experience/preference. Normally, verification analysis uses a different tool to validate the original design analysis. Table 4-2 of DNVGL-RP-0286 (Edition May 2019) presents a comparison of market available software for FOW analysis.

What do you think is the biggest obstacle facing further expansion and commercial deployment of floating offshore wind?

Further expansion and commercial deployment of FOW will need significant reduction of LCOE. To accelerate the FOW deployment, the following areas need to be considered:

Design optimisation to further reduce the structural size and weight and consider the construction;

Innovative fabrication methods to overcome the limitation of the current available construction yards;

Supply chain development to reduce the fabrication, transportation and installation cost;

Innovative O&M approach to mitigate health and safety risks of transferring personnel to and from FOWT and performing maintenance work on FOWT due to the inherent dynamics of floating structures.

How is the energy transported to shore? electric cable or hydrogen?

The ways of transporting energy to shore depend on the location, market, and foundation of the FOW farm. If a FOW site is close to shore, connecting power grid using electric cables is a common solution of energy transfer. However, Hydrogen is certainly an attractive option. FOWT can convert seawater to hydrogen and export the gas via a pipeline to shore. It would be independent of the grid, leading to significant cost reduction of energy transmission and storage would be hugely advantageous in addressing the variability of energy production and energy demand.

Can you talk about the benefits and limitations of serial production for floating foundations?

The serial production for floating foundations will help achieve economy of scale and reduce LCOE. However, to achieve serial production, the supply chain of FOW needs to be further developed. One of the major obstacles in Europe is the limitations of the existing construction yards’ capacities of fabrication and construction of a large amount of FOW units.

Is there potential to repurpose old semi-subs for FOW or are the stability requirements too unique?

The existing FOW semi-subs installed in one site can be potentially moved and installed in another area, given a detailed site-specific assessment has been carried out and approved the feasibility. It would be not practical to use existing Oil & Gas semi-subs for FOW as the they have completely different design requirements.

Do you think FOW will accelerate development of integrated design? Do you think this will have any knock-on implications to fixed bottom offshore wind structure design?

No doubt that the integrated design will provide more optimised and efficient solution. Design optimisation for both floating and bottom-fixed wind requires close collaborations between floater designers and wind turbine OEMs.

What the main challenges for the large size monopile design in high seismic areas?

The main problem actually is certification, not design. The lack of examples for certification bodies in high seismic areas can lead to over-conservatism. Having said that, high seismic areas often come with other extreme design conditions – such as large breaking waves or typhoons/Hurricanes and these can be the critical cases.

What about asymmetric (oval) monopiles? This could be orientated for each position to take advantage of the dominant wind/wave fatigue around the clock position. High SCF details could be placed on the "flat" side of the oval.

We think it could be done – would have to consider fabrication issues etc. with rolling an oval

What driving hammer types are required to install a 12 m diameter monopile? Are they available already? And if the USA becomes a 'monopile market', how to deal with the strict environmental requirements (i.e noise)?

Noise is going to be a very big factor indeed. Hammers and anvils can be developed, I believe we have reached around 10m already. Noise mitigation through bubble curtains may not be good enough, so options like vibrohammers may have to be considered

What about the MP to TP flange interface? Is it going to work for larger diameter piles or we need a novel solution?

It becomes more and more difficult as we move towards large numbers M72 bolts and all the weight and torqueing problems! It is likely a novel solution may be found to be more cost-effective once developed and there is a focus on researching this in some circles already

So moving into the thick plate probabilistic fracture mechanics territory - how big of a risk is that to the structural design? Is that something that was overcome earlier as monopiles first entered XXL space?

This will certainly become more of an issue as we move into thicker and thicker plates. I have not gone into fracture mechanics level on any Concept designs so far but certainly some investigations may be required – especially at welds.

Bigger turbines in deepwater will see larger hydrodynamic loads. Do you think the impact of this will overtake turbine loading? I think determining loading will become more and more complex – but whether it passes the turbine loading before floating or jackets become preferable solutions I’m not sure. Isn't this exactly an item to optimize in the future - that loads will never be the same from all directions?