Coping with offshore weather: why statistics just don’t cut it anymore

Last winter brought in heavy rain, large waves and relentless wind gusts to the coastal regions of northern Europe. What is being done to mitigate offshore damage, costs and health and safety concerns?

Severe offshore weather conditions can cause consequences ranging from a disruption of electricity transmission, to shut-down of energy production, and even an entire platform sinking into the sea in a worst case scenario, therefore, extensive...

By Heba Hashem

During the peak of the 2013/14 winter storm season, Europe was affected by extreme weather, putting at risk both offshore wind construction activities and existing installations.

While windstorm/cyclone Xaver brought gusts up to 142 mph to upland Scotland, through to the lowlands in the United Kingdom and across northern Europe, it also unleashed heavy storms and a spring tide over North England, Germany, Denmark, Belgium, Poland, the Netherlands and the southern fringes of Sweden and Norway; Xaver winds spread far and strong, up to 81 mph (130km/hr) onto Nissum Fjord in Denmark.

Then there was windstorm Christina, which also brought strong winds and high waves to the coastlines of the UK, highlighting again the growing rate at which exteme weather is a major part of any project's planning and health and safety regime.

But what is considered a peculiar twist is the recently found knowledge that offshore wind farms are in fact mitigating onshore damage caused by hurricanes. A Stanford University study published on suggests that hurricanes are causing increasing damage to many coastal regions worldwide and that offshore wind turbines can mitigate hurricane damage. The study uses an advanced climate–weather computer model that correctly treats the energy extraction of wind turbines, finding that large turbine arrays (300+ GW installed capacity) may diminish peak near-surface hurricane wind speeds by 25–41 m s−1 (56–92 mph) and storm surge by 6–79%.

According to the study, benefits occur whether turbine arrays are placed immediately upstream of a city or along an expanse of coastline. The reduction in wind speed due to large arrays increases the probability of survival of even present turbine designs.

Implications on offshore projects

These windy locations are, not coincidentally, where most of Europe’s operational offshore wind farms are located, and where new ones are being constructed. For projects under development, operations could be delayed over and over again due to high seas with construction doubling in some cases.

"Offshore construction works are weather sensitive. Some works are mainly wind-speed sensitive, others are more wave-height sensitive, while others again are more water-temperature sensitive”, says Ole Bigum Nielsen, director of Project Execution Offshore at Sweden’s state-owned Vattenfall.

But with consequences ranging from a disruption of electricity transmission, to shut-down of energy production, and even an entire platform sinking into the sea in a worst case scenario, extensive advance work must be done to schedule the use of special vessels and equipment.

“Mitigation actions are typically used in the offshore wind industry – for example, by choice of installation vessels, time of the year for installation and planning of the work in general. Despite all mitigation actions, the weather is still an important factor,” says Nielsen.

Factoring in the weather

Indeed, weather is a crucial factor to take into the planning phase. After all, wind power units are being erected deeper into the sea and further off the coast, making the process logistically and financially challenging.

But since weather predictions will not always be absolutely accurate, it is wise to extend the timeframe for some project activities such as cable installations, which are more weather sensitive.

Another way to avoid weather-related delays in construction and the resulting costs increases – given that most of the equipment is usually hired – is through design considerations.

For instance, build-time at sea could be limited by constructing as much as possible onshore, including the foundations, which are usually installed before the turbines. However, foundation designs, especially for deep water, are still evolving.

One of the latest innovations in offshore wind foundations is the Seatower Cranefree Gravity foundation. The new technology, now verified and approved by DNV, tackles several of the issues associated with offshore wind turbine foundations, including the risk and cost of weather delays.

Self-floating before installation on the seabed, and designed using regular concrete and steel, Seatower’s foundation requires only towing vessels for its transportation and installation, enabling offshore wind farms to be erected in deeper waters without expensive installation vessels.

“When we started developing the Seatower technology, one of the absolute requirements was to be able to install the foundations in maximum waves up to 4-5 meters,” Petter Karal, CEO of Seatower AS, told Wind Energy Update.

Seatower Cranefree Gravity foundation will soon be installed by MT Højgaard as part of a development project related to the future offshore wind farm ‘Parc éolien en mer de Fécamp’ in France.

The multi-patented foundation – based on principles from the offshore oil and gas sector – will be manufactured by French construction company Eiffage Travaux Publics, after which it will be towed by tug boats about 25 km from Le Havre to the offshore wind farm site in the English Channel, and placed in its position where sea depth is 35 metres.

Using tugs to transport the foundation is expected to reduce costs dramatically compared to using a large installation vessel, which is far more susceptible to weather conditions. It should also extend the operational weather window and improve the speed at which the prototype foundation is installed, enabling it to be completed by the end of 2014 as planned.

“We expect that this type of foundation combined with the method of installation can be a game changer in the market and can lead to considerable installation cost savings”, says Kim Andersen, Vice President, MT Højgaard, Offshore & Steel Bridges.

Forecasts – how reliable?

Regardless of the technologies opted for, planning offshore construction works is normally based on weather statistics, often 20-year data, which should provide an indication of the type of weather to expect. Yet according to Nielsen, there is no guarantee: “A statistic is nothing more than an average.”

Extreme weather, he adds, is not taken into account within the existing weather statistics and introduces by then a new risk for the wind industry. “Even the best planning can turn out to be unreliable if the extreme weather continues as a trend – not necessarily a big issue but something to be aware of.”

Traditionally, the summer in Europe has been seen as one long and stable offshore construction installation period, as opposed to the winter half of the year.

However, recent years have shown a tendency to less stable periods, as Nielsen points out: “The months of November and December have been seen sometimes even calmer than the months of April and May.”

Karal similarly states: “Bad weather is a recurring and increasing event in offshore activities, and weather risk must be managed actively. During a multi-year offshore project, you can be almost certain to encounter protracted periods of such weather. Technologies must be such that the whole project isn’t derailed by it.”