Delivering A Clearer Picture Of The Seabed

Rapid turnaround survey delivers outstanding image quality.

In September 2012, BP asked NCS Survey to conduct a survey to support moving a jackup accommodation unit alongside the West Sole Charlie platform in the Southern North Sea. BP has since transferred owner- and operatorship of its Southern North Sea assets to Perenco.

A detailed investigation of the seabed was required to assess its suitability to support a jackup rig alongside the West Sole Charlie platform, and examine the site for the jackup rig and along the anchor corridors. Jackup legs cannot be positioned where there is a risk of debris puncturing the spud can or an uneven seabed that could result in the rig toppling.

The survey utilised NCS Survey’s Gavia autonomous underwater vehicle (AUV), which is equipped with high-resolution side scan sonar and multibeam echo sounder data acquisition systems and a precise Doppler-aided inertial navigation system to provide the best possible images of the seabed.

The aims of the surveying operation, which covered an area of approximately 2 km2, were to identify and measure any seabed debris or features that might cause problems during anchoring operations; reveal the shape and depth of any seabed depressions; and locate any evidence of gas seeping from the seabed or wells within the area.

Dick Whiting, senior project manager, NCS Survey, explains: “The initial approach BP considered for the seabed survey was a conventional survey vessel. However, the presence of such a vessel with a towing spread would significantly affect the local fishing community during the shellfish season. As a result, BP looked into using alternative AUV technology to reduce the footprint of the survey and, therefore, its impact on the local fishing industry.

The flexibility of the AUV option is that the system can be launched from and retrieved to most of the vessels that operate in the North Sea. Unlike many larger survey companies, NCS Survey does not operate its own vessels but turns available vessels into survey vessels for the duration of a job. This is the ideal model for quick mobilisation on smaller surveys.

The AUV option provides a cost-effective option when compared with either remotely operated vehicle operations or a topside multibeam echo sounder survey with a towed side scan sonar. In addition, using an AUV avoids the potential safety issues of operating towed or tethered vehicles close to an existing platform.

Whiting says, “The operation mobilised out of Great Yarmouth, UK, and lasted nine days. Operations in the North Sea in October can be challenging with high wind speeds and unfavourable sea states. Indeed, a significant proportion of this period was spent waiting on weather. However, the surveying work was completed efficiently in the two operational days available.”

Processing started on board the vessel during weather downtime and NCS Survey was able to supply initial images from the two days of surveying 24 hours after each day. There are many linear features close to the east side of the platform (possibly scaffolding poles) and it was thought that these might interact with the proposed spud can locations. However, no obstructions were found within the survey area to affect the installation of a jackup rig.

NCS Survey delivered an informative draft report to BP approximately a week after the surveying operation concluded. This is an unusually quick turnaround: the standard for seabed surveys of this kind can be four to six weeks. Whiting says, “This fast delivery reflects the investment we have made in processing capabilities. For example, in 2009, NCS Survey purchased the survey operations department of Sonar Research and Development, a company specialising in developing 3D visualisation software and multibeam echo sounder processing technology.”

However, the fast turnaround was not at the expense of image quality.

In seabed surveying, as in many aspects of the oil and gas industry, customers want a responsive and flexible service that can be tailored to their specific requirements. According to Whiting, “NCS Survey is a fairly small, but rapidly growing, company, so we have good relationships with our oil company clients and often work with them to extend our capabilities to answer a specific need. This may include funding to improve or develop a product and push the boundaries of what is possible. Our development processes are driven by customers’ needs.”

One way to meet their needs is by operating a portable and modular system. In 2010, NCS Survey shipped a Gavia AUV to Argentina: the whole system was packed into a small shipping container. The components of the Gavia can be stripped down to nothing longer than 0.5 m. It is essentially a lightweight, portable platform for various instruments. This modularity means that it can be deployed anywhere in the world within a matter of days.

It also means that NCS Survey can tailor the Gavia to each job. The company can swap modules in and out to give a customer exactly the combination they require in terms of sensors and data gathering options. This modularity also means that the vehicle’s capabilities can be added to and extended almost indefinitely. New sensor modules are under development all the time.

Seabed surveys are a crucial part of oilfield operations, particularly when operators are looking to add new infrastructure to existing field locations, but they have a fixed shelf life. Offshore operators have to repeat them when they plan significant changes to existing developments. A survey’s shelf life is typically three to six months for insurance purposes. Once that shelf life has expired, the survey is no longer valid and a new survey must recheck for the presence of debris in the area. AUVs provide a very cost-effective method for completing this work.

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