What are the risks of adverse weather to the design, installation and maintenance of floating offshore wind farms, and how can they be managed?
As the world decarbonises to meet emissions targets, floating offshore wind (FLOW) is seen to have huge potential to contribute to the energy mix. FLOW turbines can be installed in deeper water and further offshore than traditional fixed-bottom turbines, opening up new areas for development with greater wind resource.
However, this increased distance from shore results in greater adverse weather and harsher meteorological and oceanographic (metocean) conditions, leading to a decrease in accessibility to site and additional challenges for design, installation, operation & maintenance (O&M) and decommissioning phases.
Adverse weather is a logistical and financial risk that must be considered when designing and planning projects or offering vessel services for the offshore wind industry. Indeed, marine operations represent a significant proportion of offshore wind energy costs, and with the industry’s focus on cost reduction and risk mitigation, weather risk is often a key consideration in project planning and commercials.
When taking on such risk, developers and installation contractors cannot afford to be without a detailed understanding of the impact of weather and the availability of weather windows on their projects.
In this article we explore why FLOW projects have the potential to be even more influenced by weather downtime than fixed bottom, and how the associated risks can be managed.
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ORE Catapult has recently highlighted the criticality of weather downtime on the choice of cable and mooring configuration for FLOW, with the inter-connectedness and inter-dependence of activities potentially resulting in spiralling weather downtime (and therefore cost) if delays occur.
The study explores how modifying mooring components and layout (such as the use of taut nylon moorings instead of large chain) and cable installation design (such as using wet stored dynamic cables instead of continuous end-to-end installation) can potentially mitigate these affects, leading to a reduction in required weather windows for vessel based installation. It emphasises further work is needed to translate assumed operability into accurate weather downtime durations through combined consideration of site specific metocean data, detailed weather limits, weather windows, alpha factors, weather spells analysis, and contingency plans.
The choice of anchoring technology will also determine the types of vessel required and duration of installation operations. Solutions which require large numbers of vessel movements and time at site have higher potential to be affected by weather, potentially incurring greater downtime. These vessel operations and any associated downtime may add significant additional expense, and must be factored into the overall cost of the anchoring solution.
Ports are required for pre-fabrication and assembly of foundations, as well as turbine integration before towage to site. Turbine integration may also be undertaken offshore for certain foundation types, though this technology is at an early stage. All these installation activities could potentially be undertaken in a single hub port or in multiple different ports with assembled turbines brought to the turbine integration port.
Transportation of assembled foundations to the turbine integration port will be subject to weather constraints, although the use of wet storage areas nearby to port should help mitigate programme risk through the creation of buffer stock capacity.
The choice of port for wind turbine integration is driven by proximity to site, since it is important to be able to leave port, tow to site and deploy within a 72-hour weather window. At a towing speed of approximately 3 knots, this is equivalent to around 200 nautical miles between port and windfarm site, though even this distance will likely be too far for many projects. This is because most locations so far identified for FLOW are highly exposed - such as the Celtic Sea and waters off northern Scotland - far more so than areas that have seen extensive development of fixed bottom projects, such as the southern North Sea and Irish Sea.
This will mean weather windows for installation will likely be narrower compared to fixed bottom. Internal analysis by ABPmer suggests that installation of integrated turbines to Celtic Sea Round 5 sites solely from French and Spanish Atlantic seaboard ports within a distance of 200 nm would be unviable, since installation would be restricted to an unacceptably small number of months in peak summer.
Turbine integration is carried out immediately before deployment of the completed floating turbine to the wind farm site. This is typically achieved with the floating foundation moored alongside a quay, and must be started and completed in good weather and with calm sea states in harbour. Accordingly, a detailed month-by-month understanding of likely variation in weather windows at the installation port is critical, and must include consideration of wind speed thresholds (which determine crane operability) and metocean conditions (including wave height, period and current speed which influence foundation stability alongside the quay). This information is key in helping determine the rate at which turbines can be deployed to site, therefore the overall likely duration of the project construction phase.
Across the FLOW industry there are different views on the most appropriate strategies for major component repair and exchange; the required depth of sites means jack-up vessels cannot be used. One alternative approach is tow-to-port – such as recently implemented for Kincardine Offshore Windfarm off the coast of Aberdeen. Another is in-situ repairs, which require heavy lift vessels or potentially self-hoisting and climbing cranes mounted on the turbine.
To arrive at the most cost effective solution, a wind farm operator will need to determine whether turbines can be towed back to shore by readily available and relatively inexpensive vessels for repair at the quayside using more cost effective cranage in onshore-type weather, or if the additional transit time required more than offsets the benefits of accessing milder wind and wave conditions and avoiding the need for expensive heavy lift vessels working offshore at the wind farm site.
Cost modelling of major repair strategies has been carried out by the Offshore Wind Innovation Hub, with support from ABPmer. This research finds that the optimum O&M solution varies by foundation technology, and highlights the criticality of weather limits and associated downtime (both for towage and cranage operations) in determining the optimum solution. For semi-submersible and barge foundations (most likely to be used in Celtic Sea Round 5 and ScotWind, Europe’s first commercial scale FLOW projects), offsite repair is found to be the most cost-effective approach. However, future innovations associated with onsite cranage may yet change this and - according to the World Forum Offshore Wind - may well become necessary given the long, suitable weather windows needed for tow-to-port operations.
Finally, harsh metocean conditions at FLOW sites will likely influence the vessel logistics strategy for maintenance. Early UK Round 1 & 2 (fixed bottom) projects typically rely on Crew Transfer Vessels (CTVs) to transfer technicians to site, with Round 3 and 4 projects, further offshore, generally adopting a Service Operation Vessel (SOV) model, with CTVs playing a supporting role.
SOVs are expected to play a central role in O&M for future FLOW projects, while the comparatively harsher metocean conditions may inhibit the use of CTVs, which typically require significant wave heights below 1.5 to 2 m for safe transfer. Detailed weather window analysis will be required to determine the suitability of these craft for FLOW projects.
It is impossible to eliminate the impact of weather on offshore operations, but understanding the likelihood of successful execution provides a means of managing the associated risks.
Developed in conjunction with offshore operators, ABPmer's Weather Downtime Express (WDTX) service gives developers, operators and contractors the insights needed to make informed decisions on weather downtime. A well-established tool, WDTX has been used and trusted by offshore installers since 2013. It can be used to inform decision making at all stages of the FLOW project lifecycle, and can consider any combination of home ports around the world. The tool is pre-populated with long-term records of metocean data on a global basis, so can be immediately applied to testing marine operational scenarios regardless of geography.
WDTX incorporates complex, multi-stage operations with multiple limiting parameters, and can process any number of scenarios, allowing rapid testing and decision-making. A worked example output from WDTX (showing a 3-turbine major repair campaign) is shown below:
Example programme for a 3-turbine on-site major repair campaign
FLOW technology is at a nascent stage, and current best estimates around key metocean limits (such as towing speed for integrated foundations and transfer criteria) are likely to change. WDTX is a powerful tool that can be used to explore programme sensitivity to variation in these constraint thresholds, improving understanding of the project risk profile and helping optimise decision making at each stage of the development.
Learn more about WDTX and sign up for a no-obligation demo at SEASTATES.net, or contact us to discuss bespoke weather downtime services.
Prepared by Tony Brooks, ABPmer renewable energy lead
This article was first published in August 2024, and has since been updated.
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