Sea Technology

SEP 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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Page 17 of 68 September 2017 / st 17 important to solve ambiguities within the data. The mosaics were generated with 6-cm pixel resolution, allowing practically all kinds of structures to be recognized. As expected, there were differences between the two bathym- etries from HISAS and EM 2040. These differences were at the order of 10 to 25 cm and basically observed on the outer-most part of the EM 2040 swath and created some undesirable arti- facts on the seafloor. By analyzing the causes of these discrepancies, it was observed that most of them were the result of inappropriate QC procedures for calibration of the systems. The quality delivered by synthetic aperture sonar is excellent, and using conventional interpretation techniques it is possible to identify pipelines with different diameters, subsea equipment, debris, free spans, overlaying and bur- ied pipelines with good resolution. Small offsets between the pipelines' positions were expected. This is mainly caused by variations in the precision of the underwater acoustic positioning system, which is affected by different factors such as offset angles, timing er- rors, hydrophone alignment and INS integration. These differences, how- ever, were smaller than 2 m and com- patible with the precision of the un- derwater acoustic positioning system (Kongsberg HiPAP). There is no con- clusive evidence that the noise around the platforms has affected the quality of the AUV positioning. However, the geometry between vessel and AUV when operating in shallow water (less than 200 m) can have huge impacts. Mapping is the most time- and re- source-consuming phase. It demands a relatively large team and powerful computers capable of processing large amounts of data. First, all pipelines (rigid and flexible) and cables (water, electrical, etc.) are digitized and indi- vidually identified. This is done based on the 3D mosaics generated in the processing phase. The next step is to select the photos located above each of the identified pipelines. In order to speed up the process, some in-house IT solutions based on categorical and numerical attributes were developed. For this specific task, a routine that automatically separates the photos ex- actly above each mapped pipeline and organizes them in a specific folder was developed. Considering that complex areas may need up to 240,000 photos (or 1.5 TB), this tool alone reduced the required time to validate the pho- tographic coverage of the pipelines from months to hours. At this stage, all pipeline crossings (describing which pipeline is above/below the other), equipment, debris (area or point) and environmental factors (e.g., presence of corals) are mapped and classified. Events that are not clearly identifi- able or that may indicate damages on the structure are separated in a differ- ent class. These locations are selected for later inspection with ROVs. These tasks are done manually by individual inspection of each photograph. Each event is plotted separately in specific layers on a GIS system that allows the visualization of the events individually or in any combination. The result is a complete inventory of all equipment and structures located on the seafloor. Finally, another computer routine de- fines the priority sequence for the re- moval of the pipelines and equipment. This list can be updated after every single pipeline is removed and the re- moval sequence is revalidated. With all the products in hand (ba- thymetry, SAS mosaic and interpreta- tion files), the last phase is the visual- ization of the data and the planning of the decommissioning operations. The main customers of this information are engineers, ROV operators and project managers. In order to facilitate the un- derstanding of the real conditions on the seafloor, the data are presented in a virtual-reality setting. These envi- ronments, traditionally used for visu- alization of 3D seismic cubes, were adapted for visualization of environ- mental data based on a GIS platform (ArcScene). Tridimensional visualiza- tion is the most advanced and intuitive way to quickly and comprehensively interact with complex data. It assists in understanding problems that require a fuller consideration of facts and cir- cumstances. Indeed, this is one of the main ad- vantages of using HISAS. It allows 100 percent of the area to be inspected— not only the pipelines—including en- vironmental factors that may affect decommissioning activities, while in- spections with ROVs are limited to the visual field of the cameras. Using a 3D visualization environment, it is pos- sible to gather the various stakeholders of the project, favoring interdisciplinar- ity, helping to clarify the uncertainties and enabling a better definition of the problems. 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