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Windtech International July August 2024 issue





Use of Real-Time Wave Measurements for Operational Decision Support

ImageThe influence of waves on any offshore operation is a significant consideration for safety and cost. A detailed understanding of the wave climate specific to a particular site is of crucial importance. For more than 7 years Emu Limited’s MetOcean Section has been producing systems to make wave data understandable and easily available in real time, assisting operators to make informed decisions. The company is continually developing a range of tools and procedures for clients to enable tailored data to be accessed rapidly, directly by the staff responsible for offshore operations. In this article the author details how real-time data can help operators and gives examples of projects where clients have been able to save several million pounds, with indirect savings of even greater value, while at the same time enhancing operational safety.

By Robin Newman, Principle Oceanographer, Emu Limited, UK

{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}As the wind farm industry operates further offshore into ever deeper waters, particularly in the UK with the Round 3 developments, a detailed understanding of the wave climate becomes essential. Wave data is complex and can be confusing. Even when of mild appearance, the state of the waves can dramatically affect logistics as well as health and safety issues. The provision of understandable real-time information to support operational decision-making is vital. Emu Limited’s spectral interface has been specifically developed and tailored to show wave data in graphical format for easy interpretation by operational staff.

Opening the Weather Window
The ability to deliver real-time wave energy data direct to the desktop is a vital tool in decision-making for offshore operations. Complex wave climates can be simplified using a meaningful graphical representation. This provides clients with a deeper knowledge of current sea state and allows operators to:

Reduce health and safety risk
  • Realistic sea state workable levels can be set
  • Preparation of reasonable site-specific risk mitigation strategy
  • Restrict operations to sea states within safety limits
Reduce delays
  • Maximise weather windows
  • Minimise weather downtime
  • Real-time data can eliminate delays while awaiting improved forecasts
Reduce costs
  • Eliminate unnecessary vessel mobilisation
  • Informed choice of vessels most suitable for site and wave climate
Tailoring to Suit Requirements
Knowing the safety limits for wavelengths specific to particular vessels and/or rigs enables operators to decide whether to launch or not. Using vessels/rigs that ‘fit’ with the wavelength, even if the waves are high, means they will not be destabilised with each wave. Even if the waves are small, a disparity between wavelength and vessel/rig can result in stability being adversely affected.

Analysing Waves
Decisions made on real data have greater value than those based on forecasts. Techniques developed by Emu can help define wave climates. Wave energy can be spectrally split to show the energy associated with differing wave periods and their directions. Further forcing factors can be defined, which would be lost under normal wave statistic analysis. Energy is the primary focus – construction engineers need to know all incoming wave energy sources.

Potential to save millions of pounds
As an example, Nexen Petroleum managers on the Buzzard oil platform in the North Sea used Emu’s real-time system for monitoring sea states in their operational decisions when moving the jack-up rig Galaxy III into position alongside the Buzzard platform. This involved data telemetry systems, which had been installed within 7 days from the initial enquiry, with data available through a bespoke website using receiving stations on Nexen’s platform and rig. A wave period of 6 seconds was determined as the limit for the repositioning operation. The real-time spectral interface was tailored to isolate longer swell energy from locally generated wind waves. This system allowed engineers to move the rig successfully, by taking advantage of weather windows that would not have been identified otherwise. The use of Emu’s MetOcean’s system contributed towards the financial savings achieved by Nexen Petroleum on this project.

Two-Dimensional Spectral Splitting
Similarly, Emu Limited’s MetOcean team has examined wave behaviour for the London Array project and undertaken complex secondary analysis of wave climates, extracting additional information from the data. Historical data was reassessed using a two-dimensional spectral splitting approach, which allowed the wave direction from a very complex sea to be established. Again this has led to substantial savings in cost. In addition, work on deriving specific spreading factors (engineering design criteria used to reduce the effect of the wave-induced orbital current on structures) for the site has been possible.

Aid to Coastal Defences
The new real-time spectral splitting system results from research carried out by Emu’s MetOcean team, which enables information on wave energy spectra to be available to non-technical users. Additional joint research with the Channel Observatory (CCO) in Southern England into the effect of long-period waves on beach defences has recently been published in ‘Proceedings of the International Conference of Coastal Engineers 2008’. This collaboration led to the adoption across the CCO network (more than 23 sites, extending from Kent to Cornwall) of the new system of monitoring sea states to warn of potential sea defence failures.

Aid to Offshore Wind Industry
The latest application for the spectral splitting approach will be a decision-making tool for the offshore wind industry with particular application for reducing the hazards when attempting to perform complex operations such as transferring from a vessel to a fixed structure. Trials in conjunction with wind farm developers have also shown a potential for significant cost savings.

Biography of the Author
Robin Newman has been in the industry for 17 years. He is the Principal Oceanographer with Emu Limited and Manager of the MetOcean team. He oversees the UK’s largest hydrodynamic data collection network. He has developed comprehensive and bespoke oceanographic software for user-friendly data processing, analysis and display, including spectral data.{/access}
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