"The survey was very quick and we could see immediately how everything fitted together. During pre-engineering, when we were trying to devise a solution, it was valuable to have such an accurate visual of the problem."
“The survey was very quick and we could see immediately how everything fitted together. During pre-engineering, when we were trying to devise a solution, it was valuable to have such an accurate visual of the problem.”
That was Acergy project manager Scott Winning talking. The problem he was referring to was cracking in a caisson structure attached to the massive oil storage tank that sits on the seabed beneath DONG Energy’s Siri platform, which is situated in 70 m of water about 240 km off the Danish coast. The survey that provided his engineers with such a good visual was carried out using a remarkable new 3D sonar imaging system supplied by Seatronics.
It was in the middle of last year, during a routine platform inspection, when DONG established that it had a problem with the Siri platform and had to shut down production, not only from Siri but also from the neighbouring Nini, Cecilie and Stine fields. Following work by DONG engineers to determine the cause of the cracks in the caisson support, Acergy was engaged to devise a solution to the problem, one that would enable production to be restarted as quickly as possible.
Believing that having a good overall picture of the structure and the surrounding area would be important to help his team engineer the best solution to the problem, Winning asked Fugro, which had found the problem in the first place, to carry out a further remotely operated vehicle (ROV) survey. But, with visibility down to 5 m at the seabed, the challenge was how to obtain the topographical data Winning wanted.
Seatronics provided the answer: one of the latest miniature, multibeam sonar systems developed by BlueView Technologies, which is based in Seattle, USA. The sonar system was hand-carried offshore by a single sonar technician, and it then took less than a day to integrate and test the unit with the ROV. The survey to collect data took about 10 hours and involved setting the ROV down on the seabed at various selected locations. In all, eleven 3D sonar images of the area were taken and combined to provide the final result. Shown top right, the picture, which covers an area roughly 50 × 40 m, may be viewed on a computer screen from any angle, including looking upwards from the seabed right into the problem area.
As Winning said, the results of the BlueView survey were valuable in developing a solution to the problem on Siri, which involved installing a custom-designed support structure for the caisson sitting on two new mud mats strategically placed on the seabed. Acergy completed the remedial work in January this year, and Siri and its linked fields were back in production very soon afterwards.
Alistair Coutts, global business development manager for Seatronics, said, “We expected to get a good result with the unit following a trial we conducted in Aberdeen harbour last year. The quality of the information we obtained in the harbour and in this first commercial application in the North Sea were exactly the same. As word of the technology spreads, we expect to see demand for the equipment grow rapidly. It is the ideal tool for simple yet detailed inspections of smaller, defined areas where there is limited ROV access and poor visibility.”
Seatronics has an exclusive reseller’s agreement with BlueView Technologies under which the two companies work closely together to market BlueView’s miniature, multibeam sonar systems worldwide within the oil and gas industry.
How the Siri DATA was obtained
The Siri survey was carried out using a BlueView BV5000 mechanical-scanning, multibeam sonar system. The patented unit comprises a high-frequency (1350 kHz) multibeam sonar head coupled with a very precise pan-and-tilt mechanism. This enables a series of high-resolution 2D profiles to be collected from the same fixed location. A 3D image in the form of a standard point cloud is automatically generated by combining the 2D profile data. This image can be viewed and manipulated in all laser and sonar point-cloud viewing software packages. The BlueView system is unique in not requiring any other sensors to produce good-quality 3D data.
An image of a more extensive area is easily produced by overlaying the images collected from adjacent locations around a site. Using standard laser scanning software, adjacent scans are snapped together using like features in the models rather than precise location information. This is how the Siri image was built up.
The technology has the advantage of being very directionally sensitive, which reduces multi-path effects and results in unusually clean raw data. In the past, the time needed to clean traditional multibeam data from complex structures could far exceed that necessary for data collection. Normally, a technician can clean and combine BlueView scans at a rate of an hour for each hour of data collected.