The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community
Issue link: http://sea-technology.epubxp.com/i/875429
www.sea-technology.com September 2017 / st 15 T he decommissioning phase of an oil field occurs at the end of the field's active life, and it is a critical phase, with potential for oil and chemical leakage that can damage the environment, which is why it is regulated by legislation. The environmental impacts of decommissioning oil and gas production facilities were first highlighted in 1995 with the attempt to sink the Brent Spar structure in the British North Sea. At that time, the British policy for decom was to sink the platform at its location. Environmentalists feared an accumulation of polluting material in the sea, and after they protested, the platform was taken ashore and disassembled, and the steel structures were reused in the construction of a Norwegian wharf, confirming the possibility of reusing the material from old exploration platforms. This fact incited oil-producing countries, Brazil included, to create rules to regulate the decommissioning process of oil fields. The complexities of the activity planning and decision making and the extremely high costs involved in the de- commissioning, abandonment and removal of obsolete off- shore installations led to the need to use new technologies and to develop consistent survey procedures to map the seafloor around the sites. One of the most critical steps in this process is to determine the exact location of the sub- sea structures to be removed, such as pipelines, wellheads, equipment, power cables, etc. This information is the base for decommissioning planning and management. Pilot Project With the expected shutdown of 75 oil platforms in the Campos Basin in southeast Brazil in the next several years, a pilot project was conducted from summer 2016 to 2017 to test a new approach using sonar techniques to update the position of all subsea facilities in some priority areas for fu- ture decommissioning. Due to the limitations for navigation around oil rigs by conventional survey vessels with towed equipment and the need for high-resolution data (limited when using low-frequency, hull-mounted systems), the use of an AUV platform, instead of an ROV, was the natural choice. The Kongsberg HUGIN 3000 AUV was equipped with a state-of-the-art suite of imaging systems, including a Kongs- berg HISAS 1030, an interferometric synthetic aperture so- nar system capable of providing very high-resolution images and detailed bathymetry of the seabed at a speed of 4 kt., with resolution of approximately 3-by-3 cm out to a dis- tance of more than 200 m from both sides of the AUV. Ad- ditionally, a Kongsberg EM 2040 multibeam echosounder and a black-and-white still camera with LED lighting were installed on the AUV. The EM 2040 system was used to cov- er the nadir area with bathymetric data with resolutions up to 4 cm, while the camera was used to take pictures of the subsea structures for inspection purposes. This project represented a paradigm change in the inspec- tion operations of subsea facilities, and some complications were expected. The first issue was choosing an AUV instead of an ROV as the survey platform. While an ROV operates at an average velocity of 0.5 kt., an AUV can run surveys in speeds up to 4 kt. This positively affects the information acquisition rate, increasing the efficiency and reducing the time to survey a specific area of interest. The second issue was that, using sonar techniques, the information would no longer be "visual" but based on a sonographic mosaic, the quality of which can be affected by environmental and op- erational factors. Seabed Mapping with HISAS Sonar For Decommissioning Projects High-Resolution Surveying for Decom Planning By Dr. Arthur Ayres Neto • Geraldo Pinto Rodrigues • Igor Drummond Alvarenga Phases of the survey process.