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Going with the FLOW

Driven by ambitious targets and the drive for energy self-sufficiency, the offshore wind industry is gathering pace. As it grows, the developing sector faces challenges. Amongst these is the installation of turbines in deeper waters.

Many areas planning offshore wind developments do not have the relatively shallow waters encountered in the North Sea region, where most projects have been carried out to date. Even there, with so much installation in recent years, developers are looking to construct wind farms farther from shore in deeper waters.

The solution is floating wind turbines. However, these come with their own unique challenges. For one thing, conventional installation using a jack-up vessel is not an option. A new generation of wind turbine installation vessels is required.

Meeting the challenge head on
Damen Shipyards Group, headquartered in the Netherlands, is taking the challenge head on. Damen has reached out to industry stakeholders including vessel operators, wind farm developers and equipment manufacturers to get their input on the vessel of the future. From this, the shipbuilder has developed an exciting new concept – the Damen Floating Offshore Wind Support Vessel (FLOW-SV), a vessel that aims to build bigger, faster and with increased efficiency.

The vessel’s primary function is the transportation and pre-lay of mooring spreads. Additionally, it can perform tow-outs and hook ups, meaning the entire process can be carried out with a single vessel.

Efficiency gains
The concept’s arrival is timely; there are plans in place to install many floating turbines in coming years.

“There are expected to be thousands of floating turbine installations in the coming years, including 1,800 in 2030,” says Wijtze van der Leij, Damen Sales Manager Offshore Wind. “A recent test case demonstrated that installation of a single turbine takes some 34 vessel days. Even with a generous allowance for efficiency gain, this is not likely to reduce below 11 vessel days. This means more than 100 vessels will be needed to install the turbines planned for 2030 alone.”

Certainly, current capacity does not allow for this. The vessels most suited to the job – large anchor handling tug suppliers (AHTS) – are largely already committed to projects in a resurgent oil and gas market. Besides, the capacity of such vessels is sufficient to handle just one mooring spread per voyage.

“The planned installations will require some 3.6 million metres of rope and chain, as well as 5,400 anchors. Plus, the chain required for a floating turbine spread is significantly larger than that used in the oil and gas sectors,” explains Wijtze. “Most vessels from the current AHTS fleet would be required to return to shore each time it had installed a single spread.”

Increased capacity
To increase efficiency, Damen’s FLOW-SV concept features significantly increased capacity, including a deck area of 1,600m2 and 3,000m3 chain locker space. With this, the vessel can transport three floater mooring systems in a single voyage.

The FLOW-SV does not stop there in its bid for increased efficiency, however.

Relieving the tension
“The traditional way to proof load mooring lines is to use the vessel’s bollard pull. The FLOW-SV offers more than 400 tonnes of bollard pull, which is significantly higher than most AHTS vessels, but with floating turbine spreads we are talking about proof loading lines up to 1,000 tonnes. Typically, the solution would be to use a tensioner, but this is a time consuming process. We heard from industry stakeholders that there is a lot of interest in an alternative method for proof loading.”

Damen came up with an innovative approach to tackle this. The process begins with the vessel anchoring itself at the bow. Following this, the turbine anchor is released over the transom. The vessel then combines its bollard pull and forward winch pull to embed the anchor and apply the proof load forces necessary to secure the line for the forces it is required to handle.

To facilitate this novel approach, the vessel features a unique propulsion arrangement. In addition to its twin azimuth thrusters for normal operations, DP and reaction anchor embedment, the FLOW-SV has two booster lines to ensure its ability to reach maximum bollard pull.

Green vessel for a clean sector
The development of the FLOW-SV is a logical step for a company committed to becoming the most sustainable shipbuilder. Damen is keen to support the offshore wind industry in reaching its full potential.

This goal of increased sustainability runs through the concept. It features, for instance, battery-hybrid power for peak shaving and spinning reserve, reducing emissions during DP operations. Furthermore, the vessel has been prepared ready to operate on carbon neutral methanol fuel in the future.

Looking ahead
The concept, as advanced as it is, does not represent the final word on the matter, however, as Wijtze explains.

“We started the process by reaching out to stakeholders and aim to continue in this way. We are reaching out to the industry, looking for partners who can help us to take this to the next stage. What we see today is just the beginning of a new generation in offshore wind.”

Vessel specifications:

• Length: 145 metres
• Beam: 32 metres
• Bollard pull: 400+ tonnes
• Forward winch: 600 tonnes pull, 1,250 tonnes brake
• Aft winch: 600 tonnes pull, 1,250 tonnes brake
• AHC crane: 250+ tonnes
• ROVs: 2
Source: Damen

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