Opportunity knocks for floating wind development

The future is floating

The current global installed capacity of floating wind is around 180 MW. The market is predicted to reach 16 GW by 2030. Floating wind will play a key role in helping the industry and governments to achieve carbon neutrality by 2050. The benefits of floating wind include:

• Suitability for deeper waters – this is where 80 % of the world’s offshore wind resources are found. At depths beyond 60 metres, fixed foundation turbines become less viable for installation;

• More wind potential – the further offshore you go, the stronger and more consistent the wind becomes;

• Access to untapped markets – island nations and countries or states with a small continental shelf will be able to generate their own wind power and gain more energy independence.

What you need to consider

1.     Floating platforms

The most common options are:

• Barge – has a large surface area that sits on the water surface;

• Semi-submersible – sits a little deeper in the water and has multiple partially submerged columns creating a lower centre of gravity for improved stability;

• Spar – a giant cylinder with a heavy underwater ballast enables it to be suspended vertically in the water;

• Tension leg platform (TLP) – has radial arms tethered to the seafloor by vertical tensioned steel cables or tendons that restrain the platforms vertical movements and excessive motion. This type of platform is more practical in very deep waters.

2.     Anchor and mooring design

The most common types of mooring for floating installations are:

• Catenary mooring:

- Flexible chain mooring lines are connected from the anchor to a barge, semi-submersible or spar platform;
- Part of each cable lies on the seabed, using its own weight to provide resistance and stability;
- Typically unsuitable for uneven or unstable seabed conditions, environmentally sensitive areas or areas with anthropogenic constraints.

• Semi-taut mooring

- Mooring lines are almost entirely suspended in the water column between the anchor and floating platform;- Small seabed footprint;
- Chain or cable inclination angle provides a higher level of vertical loading on the anchor than a catenary mooring.

• TLP mooring

- Held in position by three or four tensioned steel cables or tendons that extend vertically into the seabed anchor;
- Provides excellent vertical stability;
- Allows some lateral movement to accommodate environmental loads;
- The most expensive option – suited to very deep water.

3.     Foundation concepts

The main installation and handling options available for floating wind are:

• Drag anchor

- Easy and cheap to deploy and retrieve;
- Not suitable for gravelly or rocky soils;
- Some drag designs have low resistance to vertical loading making them suitable for catenary moorings only;
- Anchors can move after the installation embedment.

• Gravity anchors

- Simple installation and retrieval;
- Unsuitable for undulated seafloor (anchor sits on the seabed);
- Low resistance to vertical and horizontal loads;
- Potential to slide from installation position;
- Suitable for wide range of sediment types.

• Suction anchors

- More complex and longer installation time, with pump and large crane required;
- Provide good horizontal and vertical resistance, so are fine for catenary, semi-taut or TLP moorings;
- Unsuitable for rock, gravel or dense soils, or some types of sandy soils.

• Driven pile anchors

- High installation complexity – pile-driving hammer and large crane needed;
- High noise levels and vibrations during installation;
- Provide good horizontal and vertical resistance – can be used for catenary, semi-taut or TLP moorings;
- Suitable for a wide range of sediment types, including hard or dense strata;
- Non-retrievable.

• Drilled and grouted pile anchors

- Extremely complex and long installation, involving drilling the pilot hole and grouting the anchor in place;
- Suitable for a range of hard/cemented or rocky strata;
- Unsuitable for soft soils or loose sands due to open-hole instability or wash-out;
- Provide good horizontal and vertical resistance – can be used for catenary, semi-taut or TLP moorings;
- Non-retrievable.

4.     Geohazards and constraints

As floating wind technology expands into new areas and deeper waters, some geohazards and constraints become more prevalent and could seriously impact anchor performance if overlooked. Examples include:

• Seabed instability – sediment transport or turbidity flows could undercut anchors, cause downslope anchor drag and damage cables and mooring lines;

• Fluid expulsion and shallow gas pockets – caused by natural hydrocarbons migrating upwards along fault conduits or accumulations of biogenic gas;

• Seismic activity – earthquakes can trigger slope failure and sediment transport flows, fault displacement and liquefaction of granular soils;

• Challenging soils – island nations often have carbonate or volcanic soil conditions that may have higher compressibility and be susceptible to rapid strength degradation;

• Complex metocean conditions – waves, deepwater currents and extreme environmental conditions can cause fatigue or failure of the mooring systems.

Early engagement with an expert is essential

It is critical to engage early on with a data expert and experienced consultant. We have a long history of advising and planning offshore geophysical and geotechnical investigations and providing asset performance analysis for the offshore energy industry. Here’s the typical process that our consultants apply:

1.     Initial site screening – gathering and integrating data from existing expertise and publicly available sources into an early-stage earth model;

2.     Integrated digital site characterisation – designing an optimum data acquisition and testing programme that provides real-time access to the data and analysis, to minimise risks and uncertainty;

3.     Analytics, design and advice – updating the earth model and optimising the mooring and anchor designs using numerical and physical modelling;

4.     Installation – monitoring positioning, drilling, suction pile and pile driving. Recommending the correct equipment for installation and delivering accurate estimates of the soil resistance and anchor capacities.

Did you know?

Fugro has been providing metocean, geotechnical engineering and design services for deepwater anchoring systems for over 40 years.