What lies beneath: Making way for offshore windfarms - By Gary Thirkettle, Chief Commercial Officer at N-Sea

In recent years there has been an increasing focus on renewable energy sources with it now being considered a mainstream energy source. There has been a growing appetite for offshore wind farms as global energy demands increase and last year the total number of connected offshore windfarms in Europe reached 105 which equates to a total of 4,543 connected turbines across 11 countries.

Currently, with 37 active offshore windfarms and a total of 2,016 turbines generating 8,483.420 MW, the UK is one of the biggest offshore wind contributors with windfarms in the North Sea accounting for 70% of Europe’s offshore wind capacity.

Additionally, investment in new projects across Europe continues with 12 projects reaching Final Investment Decision last year.

Like all construction projects, there is a substantial amount of planning and work which needs to take place prior to installation. A crucial part of the offshore windfarm construction process is clearing the site where the turbines will be installed, including the route where cables will be laid, to ensure there are no obstacles, some of which could be dangerous.

During the Second World War approximately 600,000 to 1,000,000 naval mines of all types were laid. Clearance campaigns began after the war however almost 75 years later, there are still thousands of unexploded ordnances (UXO) present offshore. UXOs are often found during offshore projects, which pose a potential threat to work and vessels, especially when laying cables in the seabed.

A cost-effective and efficient risk-based solution for the mitigation of UXO is therefore a priority for offshore projects.  UXO management is split into three distinct activities: geographical survey to search for magnetic contacts; visual identification of any contacts to determine if they are UXOs, debris or boulders for example; and then the removal of the object and disposal of any UXOs found.

N-Sea, a leading integrated subsea infrastructure services provider, delivers a comprehensive range of survey and IMR (inspection, maintenance and repair) services to the oil and gas, renewables and civil contracting communities.

Specialising in subsea surveys, N-Sea has an impressive track record of UXO campaigns which includes the investigation and mapping of potential targets using state-of-the-art survey technologies, ROVs and divers. New technologies are enabling safer and more efficient clearance campaigns and as such the company’s survey expertise and wealth of experience in this area has been used to develop the next-generation of UXO detection equipment – MagSense.

MagSense is an innovative vertical gradiometer array system which delivers highly accurate UXO campaign data within the renewables and oil and gas sectors. Developed by N-Sea in collaboration with EIVA, MagSense collects and records high resolution data in magnetically noisy subsea environments and hostile conditions, delivering greater efficiency, unprecedented accuracy and enhanced safety. The systems manoeuvrability and flexibility have been designed specifically to reflect modern-day imperatives in offshore construction and can be tailored to individual client requirements.

The pioneering system was first adopted during the installation of offshore windfarms in the German Bight. Pre-installation research had identified the work sites as high-risk areas for UXO; therefore, installation could only commence after clearance work had taken place across these locations.

Unfortunately, UXO are not always in plain sight and for a clearance campaign to be successful, all suspected targets must be identified and removed – even if they are buried. The difficulties of identifying and extracting buried targets are well documented; however, by engaging vertical gradiometry, MagSense is able to better discriminate between potential UXO and other obstacles therefore removing the need for further target investigation. This is highly beneficial as it results in less excavation, less time and ultimately, less cost.

During this project, a challenge presented itself when the expected targets were not only extremely small in size and weight, but were also buried in the seabed. Add to this an already challenging North Sea environment with strong currents, and a rapidly changing seabed and poor visibility, the need for a high-tech survey solution becomes quickly apparent.

MagSense was deployed not only because it could perform with greater accuracy than competitors’ systems, but it could do so with maximum efficiency. The technology enabled vessels to operate at a higher survey speed, which meant each campaign was successfully completed in less time and delivered a 50% reduction in offshore operation time when compared to previous projects. This project proved MagSense to be invaluable in producing more efficient and accurate UXO survey results in highly challenging conditions.

MagSense has since been used on numerous offshore renewables UXO projects, including clearance campaigns on several windfarms across the UK and Europe.

So, what makes MagSense ideally suited to offshore windfarm construction projects? Aside from the highly accurate data which is obtained using this technology and enhanced project safety, MagSense has been specifically designed to conduct wide seabed surveys thereby covering a larger area than other survey technologies and is suitable for use in all environments.  What is also unique about MagSense is the added ability to collect high quality, high density gradiometer data in shallow tidal areas.

Used alongside the EIVA 3D Scanfish, the MagSense frame can be towed through water in ways previously not possible. This means even the most uneven and difficult terrain is surveyed accurately and consistently.

Additional sensors improve control and the 3D steering of the frame significantly reduces the amount of infill to be budgeted for which further reduces operational time. MagSense also features a unique launch-and-recovery system (LARS) which significantly reduces risk to personnel by keeping manual handling to a minimum and removing the need to swing loads which can have serious HSE implications.

MagSense has vastly enhanced the safety of UXO projects and is an example of N-Sea’s commitment to delivering safe, sound and swift operations. The LARS system further exemplifies this and is being further developed to ensure it can operate in even harsher sea conditions than it currently operates to further improve efficiency without compromise to safety.

As the drive for renewable and sustainable energy sources continues it is imperative to know exactly what lies beneath the surface as the seabed is cleared to make way for offshore windfarms. Engaging the services of experts in the field of UXO clearance who have innovative technologies at their disposal to accurately, efficiently and safely identify and clear work sites of UXO and other obstacles remains essential.