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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16 st / September 2017 www.sea-technology.com photo (10 megapixels) generates files of 6 MB on average. The amount of pho- tos depends on the number of pipelines within the area, but in general 2,000 photos (with 50 percent overlap) are needed per kilometer of pipeline (i.e., 12 GB/km). In order to achieve the required photo resolu- tion, the acquisition of bathymetric and sonographic data is done separately from the acquisition of the photos. The former is conducted with the AUV 40 m above the seafloor, while the photos are taken with the AUV at an altitude of 6 m above the seafloor. As in any other hydrographic survey, quality control dur- ing data acquisition is extremely important. Procedures re- garding QC of bathymetric data, side scan data and subsea positioning should be well-defined and followed in order to assure data quality in accordance with the goals of the project, reducing rework and optimizing ship time. Good- quality data will also positively affect the data-processing time. Another important issue is data management. Because of the huge daily rate of data collection, the organization of the data packages (including security backups) allows a better planning of the processing activities, resulting in more precise delivery time for the final products. The data-processing phase involves processing bathy- metric and sonographic data to generate 3D mosaics, which will be the base for the mapping of the subsea structures in the area. It is important to have a well-defined processing routine. Parallel processing of bathymetric and sonographic data saves time, but integration between the two groups is Concerning data quality, questions emerged such as: How would the noise around the oil rig affect the data; would the resolution of the data be acceptable for the purposes of the project; by using two bathymetric systems simultaneously, what would be the differences in depth between measure- ments from each system; how would the effects of acoustic positioning impact the resolution of the final results; and what would be the time needed to process the data and generate reliable mosaics that could be effectively used for planning the decommissioning activities? Survey Process The survey process can be divided into four interdepen- dent phases: data acquisition, data processing, mapping of subsea structures and planning of decommissioning activi- ties. Data acquisition comprised gathering bathymetric and sonographic data, as well as photographs of the installations in order to generate 3D high-resolution mosaics of the sea- floor and all subsea facilities. This phase is one of the most critical concerning data amount and quality. HISAS and EM 2040 systems alone generate data of approximately 3.5 GB per hour and 2 GB per hour, respectively, resulting in a to- tal of 132 GB every 24 hours of operation. Moreover, each (Top) Combined view of the interferomet- ric bathymetry, SAS mosaic and photo survey products: interferometric bathym- etry (left), synthetic aperture sonar image (right) and photograph (center). (Bottom) SAS mosaic and final product of the map- ping.

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