'Major risk': shortage of high-powered mooring vessels could imperil North Sea floating wind plans

A study of installation methods for floating wind farms in the North Sea context has warned a limited availability of high powered anchor-handling vessels is emerging as a key bottlenecking risk to the UK's ambitions of deploying gigawatt-scale arrays.

The study– a collaboration between Aberdeen-based consultancy Apollo, offshore vessel contractor DOF and the UK's Offshore Renewable Energy Catapult – offered some hope for reducing costs and boosting efficiency in installation operations, but only after offering a stark description of some of the challenges to be faced if floating wind technology is to be scaled up on a commercial basis.

The study covered the tow-out and mooring of floating offshore wind turbines (FOWTs) as well as installation of dynamic cables, and used as its base case a 900MW array from the UK's ScotWind lease round, at a water depth of 100 metres.

The study started with a scaled-up version of current practices for demonstrator floating wind arrays with shared suction piles installed by an offshore construction vessel, and catenary mooring lines with large diameter chains.

It featured 15MW three-legged semi-submersible turbines towed to location and hooked-up to the mooring lines.

The 60 FOWTs were configured in five clusters of 12, each cluster with two strings of six, pointing to a planned installation period of two years for the whole wind farm.

'Highly constrained'

A global shortage of high-powered anchor handling tug supply (AHTS) vessels quickly stood out as a major bottleneck, especially when considering the requirement for vessels with the highest pulling power, measured in the maritime industry as bollard pull .

For mooring pre-lay, tow, and hook-up operations, the base case required at least two vessels capable of handling 175mm chain with at least 300-tonne bollard pull, plus a third with at least 200-tonne bollard pull for positioning.

The report noted that the available fleet of such vessels is “highly constrained” with only nine vessels in the North Sea above 300-tonne bollard pull at present and only six vessels worldwide that could feasibly handle 175mm chain.

The study was based on a single wind farm in Scotland but, with conservative estimates for the UK floating wind ramp up suggesting that 10GW will be constructed between 2030 and 2040, AHTS capacity appears to be far below the required level for such ambitions.

"At least 2GW will be constructed concurrently each year, with 3-4GW expected to be deployed some years," the report stated, predicting that it may be necessary to dedicate between four and eight high-capacity 300-tonne bollard pull AHTS vessels and the same number of 200-tonne bollard pull vessels for the UK alone to meet its floating wind ambitions for this decade.

The Apollo report further predicted that demand from the oil and gas sector, as well as growing floating wind demand in other markets will squeeze the AHTS market further, raising vessel day rates by up to a factor of four.

“Unless the global fleet of highly capable AHTS vessels grows substantially in the next decade, it may prove impossible to source vessels to handle the required construction and tow to port demand at acceptable cost," it warned.

Major risk

Further vessel demands were factored for cable installation: the study's base case featured tethered wave-dynamic cable sections installed from a turbine platform using a construction vessel with a cable lay spread.

A team deployed from a W2W vessel would support cable pull-in, representing yet more vessel demand.

“The major risk posed by bottlenecking or other delays is for schedules to run past the installation window and into the winter season, driving up costs and other project risks,” the authors stated.

“Achievement of a two-year installation campaign depends on favourable weather and no major unplanned delays to any of the operations. Delays to one installation activity can impact the next, which can bring the mooring and cable connection operations into mid-winter, which in turn results in spiralling weather downtime."

O&M

Once the first large arrays have been installed, the report warned that incremental vessel demand for maintenance operations is expected, with "a large number of major component exchanges are expected to be required"

The Apollo-led study predicted that component replacement would settle at a rate of about 10% per FOWT per year.

By 2035, the authors predicted that 30 FOWTs could require tow to port (and back) every year in the UK context, with incremental tow-to-port and hook-up operations requiring the dedicated use of four 300-tonne bollard pull AHTS annually.

This will be followed, eventually, with even more demand due to decommissioning, the report pointed out.

The report said one method of reducing the dependence on hiring vessels at peak day rates is to install components off the critical path, and wet store in the field for later connection.

"A mooring pre-lay and FOWT hook-up campaign over three years will also ease the pressure on AHTS vessel demand," it said adding that scale of the installation challenges might persuade some floating wind developers to consider long term charter or even ownership to smooth out day rate spikes and de-risk the installation costs.

Future hope

At the heart of the study was an attempt to show “step changes which have the potential to address the critical installation bottlenecks”.

To do this, the authors presented two alternative scenarios comparing the economic impacts of current and emerging technologies

The report drew on experiences from demonstrator floating wind arrays and floating oil and gas facilities, as well as installation of inter-array cables for fixed-bottom wind farms and put forward some options to accelerate schedules and reduce vessel demands.

A second mooring scenario, called the “sensitivity” case, included application of a six-line drag anchor system and drag embedment among its features, allowing a reduction in chain size and thus potentially lower vessel specifications.

Despite these apparent gains, the installation study found the drag anchor case to be 22% more expensive than the base case.

This was largely due to the complexity of test tensioning the anchors to high loads on a six-line system to high loads, requiring two vessels in tandem, and a greater number of lines overall, resulting in an overall 27% uplift in vessel costs.

The study nevertheless noted there are "clear opportunities to follow a smarter risk-based approach to anchor tensioning requirements that could accelerate the pre-lay schedule by six weeks per year and reduce the mooring installation cost by 15%”.

A third mooring scenario, featuring taut nylon mooring system was selected as what was called a promising technology to replace the large diameter chain with much shorter lengths of lightweight synthetic rope, coupled with an anchor-mounted tensioning system to reduce wear, corrosion, and fatigue issues associated with top chain connections.

The study stressed that nylon mooring lines are easier to transport, reducing the overall demands on vessels, and requiring less frequent mooring chain loadouts, therefore accelerating pre-lay.

Although this suggested scope for significant gains, the study also found that time savings were partially offset by a four week longer suction anchor installation campaign.

Some technical challenges were noted, such as the fact that synthetic ropes can stretch over time after initial installation. “Whilst this can be managed via re-tensioning the system after the first year of operation, risks associated with synthetic rope creep require further study,” it stated.

According to the authors, the risk associated with long term wet storage of nylon rope also requires further qualification.