Deepwater surveying is the acquisition of positioning, geophysical and geotechnical data for reporting and mapping purposes. Surveying, often overlooked, is an integral part of the deepwater development.
Types and Uses of Subsea Surveys
Survey is the first and last thing done in a field. First is the geophysical and archaeological hazard survey performed before drilling. And the last thing in a field’s life is abandonment survey or as-left survey. In between, surveying is conducted along with every other facet of the field’s development.
After the hazard survey, and a successful well is drilled, plans begin to bring the field’s reservoir to market. A route survey is conducted to determine the flowline routing to a processing infrastructure. This survey includes, but is not limited to, geophysical aspects such as multibeam bathymetry, sub-bottom profiling, and magnetic detection. With this data, pipeline engineers can plan the flowline route and stresses it will see under installation and operational conditions.
In the installation phase, the survey is key to keeping the flowline and umbilical within the governmental mandated ROW (right of way) granted to the oil company for their use. It is at this point that the type of survey accuracy parameters to be used are investigated. There are technically three such types of surveying with different levels of accuracy: surface positioning, ultra-short baseline (USBL) and long baseline (LBL).
Surface positioning takes only measurements from the surface location of the vessel performing the work. It is useful only in shallow water. USBL is the most commonly used survey methodology. USBL has an inherent accuracy of 0.5% of the water depth, depending on the type of equipment employed. For example, in 1000m of water, your accuracy of data from USBL is 5m, but only if you are averaging a position over time. The most accurate type of survey methodology is LBL. It involves deployment of an array of transponders and calibrating them to grid reference on the surface. It is a time-consuming process but can result in very accurate (+/- 10cm nominally) positioning inside the array.
The choice of survey methodology is selected based on the field’s needs and engineering parameters. LBL is normally only used during installation operations. The derived positioning received from installation operations can be found again using USBL for inspection and maintenance operations.