Finding optimal operational up-time via assessment of a site's metocean conditions

O&M of offshore wind farms is relatively more difficult and expensive than equivalent activities in onshore wind farms.

Right from the planning stages, the project needs to consider action for combating challenges related to harsh winter conditions and accessibility for routine servicing and maintenance. In order to accurately assess the local environmental conditions for designing of offshore wind turbines, consideration should be given to site specific investigation through deployment of suitable metocean sensors, as well as analysis of long-term historical data sets and numerical models.

For example, it is vital to assess the impact on the basis of criteria comprising the wind profile including variation of wind speed with height and distribution of wind energy. The wind analysis includes desk-top assessment of the wind resource, the wind speed profile with height, and wind energy density spectra and turbulence intensity at nacelle height.  Also, there is information on wave climate in terms of height, period and direction, and its energy density spectra, in a form that allows ready input into integrated engineering analysis packages.

In terms of support for offshore wind farms, such services contribute significantly be it for installation process or reducing down-time. In terms of efficiency, there are software which allow the cost implications of weather down-time to be accurately assessed during project planning. They are used in offshore construction, installation and operational planning projects.

Sharing more details regarding the same with, Jonathan Duffy, offshore wind generation manager, Airtricity said the site metocean conditions should be understood in as much detail as possible so the range and probability of specific conditions at the turbine locations are understood. 

"This will include long term wind, wave, current and tidal range statistics together with site measurements. To choose a suitable transport and access system for moving people and parts from the quayside to the position on the windfarm where they are required to work, this information is required," said Duffy. 

He added that it is also important to have more than one way to access and egress making it easier to react to the ever changing environmental conditions while building some redundancy into the system.

"It is also important to do everything possible to avoid the requirement to access during the winter months. Scheduled and preventative maintenance work should be planned for the summer and resourced so that the work plans can be executed during the shortest possible time when the weather is right," he said.

Such studies involve assessment of metocean criteria for engineering design, fatigue analysis, and installation planning of wind turbine structures, other associated offshore structures, and seabed cabling.

For instance, when Metoc plc, an environmental design and risk management specialist, completed a Metocean Design for Scira Offshore Energy for the Round 2 Sheringham Shoal offshore wind farm in the southern North Sea off the North Norfolk coast last year, it produced information on prevailing and extreme conditions for wind, waves, current, and water level, including their joint probability of occurrence, to assist engineers with integrated loading analysis and project planning. Secondary variables, such as temperature, ice and snow, and marine growth, were also addressed.

Particular attention was paid to the calibration and validation of long-term hindcast model data in relation to site-specific measurements. The work also quantified the spatial variability in metocean conditions across the wind farm - specifically current, water level, and wave crest elevation.

One of the key deliverables from the study was said to be a long-term database of wind and wave time series which was produced to enable the project to perform further operability (installation and maintenance access) simulations.

A company like Fugro via its GEOS Weather Windows service compares the metocean constraints of an operation against historical or model data, to calculate the likelihood of successful completion within a given time frame.

Profiles reflecting the limits of operability of each parameter are constructed. Profiles can be simple such

as wind speeds less than 10 ms-1 during a three-hour operation, or complex profiles reflecting those experienced throughout a complete installation procedure. All relevant profiles are then brought together into a study configuration and tested against historical metocean data representative of the region. The software then works to find periods in the data when all operational limitations are satisfied, to derive all successful opportunities of operational up-time.

The number of hours/days that a lengthy operation would be expected to take, allowing for the influence of bad weather, are thus identified.