Wideband acoustic system keeps field development on track
Subsea installation can involve coordinating several vessels and engineering activities across a field, and accurate positioning is a crucial part of the process. In a recent North Sea field development operation, Seatronics, working in conjunction with Sonsub, showed how wideband acoustic systems can contribute to the smooth running of such complex projects.
Acoustic underwater positioning is an established technology. Tone-burst systems based on analogue electrical circuits and simple, pulsed, narrowband, continuous-wave signals have been regularly used since the mid-1970s. Over the years, water depths and installation complexity have increased. Now, the limited range and few channels available with tone-burst systems and the effects of interference from similar systems nearby mean the time necessary to acquire sufficient data to ensure positional accuracy is extending project times and, therefore, increasing costs. In recent years, more companies have used wideband systems to overcome these limitations.
In a wideband system, the transmitted message is spread over a broader frequency bandwidth than the information it contains, so the transmission looks like a wide range of frequencies rather than a signal on a discrete carrier. This wideband signal architecture effectively eliminates interference from neighbouring operations. In addition, the long duration of a wideband signal contains a large amount of energy to mitigate noise at the receiver yet the signal still retains bandwidth. A wideband system also makes many more channels available and enables users to operate 10 or more unique channels using the same tone frequency. A simple analogy is that of a digital television, which delivers significantly more channels than its analogue predecessor.
Fast and accurate
The Ettrick project is operated by Nexen Petroleum U.K. Ltd. It is a field development comprising eight subsea structures (including a centre manifold), a nine-point mooring system for a floating production, storage and offloading (FPSO) facility, seven spool pieces and over 21 km of flexible pipeline and umbilical. Sonsub, the installation contractor, needed a coordinated approach that would minimise the time required for the operation while ensuring accurate surveying and positioning of all the subsea assets. The company found that Seatronics had an extensive inventory available that could supply all the subsea positioning equipment required.
The Sonsub survey department and Seatronics worked together to develop a programme that included the use of ultra-short baseline (USBL), long baseline (LBL) and combined service (LUSBL) positioning to ensure accurate deployment of the equipment.
Adrian Mitchell, project surveyor, Saipem UK Ltd (Sonsub Division), described the work done: “The first step was to fit the Ettrick drill centre manifold (EDCM) with an attitude package we developed specifically to be both compact and manoeuvrable. It included a gyro with inbuilt motion reference unit, a battery, MK5 telemetry and a subsea display. The data from the attitude package was transferred using MK5 Compatt telemetry from the structure to a Sonardyne RovNav unit mounted on a remotely operated vehicle (ROV). The EDCM and the trees, which were subjected to a dimensional control survey while at the fabrication yard, were lowered through the water column using USBL positioning, and then installed in position with reference to an LBL array of transponders on the seabed.
“Once we had successfully installed the EDCM, we began the critical process of measuring the horizontal distance and the vertical difference between each pair of flange faces. This is a process known as metrology. Metrology operations involved first taking tree and EDCM heading and inclination measurements using a Seatronics attitude package, an ROV-mounted Octans gyro and a manipulator-held Valeport intelligent system sensor. Then, multiple wideband LBL observations were taken between the dimensionally controlled Compatt units on the EDCM and the tree and all the other Compatt units within the array. This data was processed to calculate the spatial relationship of each spool flange pair with respect to horizontal distance, height difference, spool-route bathymetry, flange heading and pitch.
“A second LBL array based around the FPSO facility’s location was used to calculate the cut-to-length positions on each mooring chain to enable the FPSO subsea buoy to be held in the correct place. The specifications called for high-level accuracy of ±0.2 m, which was provided by the LBL wideband system,” Adrian concluded.
The increases in noise tolerance, range, accuracy and speed of measurement provided by the wideband system enabled Sonsub to save a great deal of time. The supply of reliable equipment, experienced personnel and support by Seatronics were key to the successful installation of the FPSO buoy and spools.
Seatronics continuously invests in the most up-to-date and technically advanced equipment from the world’s leading manufacturers to ensure that it can supply its customers with a comprehensive range of equipment from its global offices in an effort to improve project efficiency.
Selecting the most appropriate technology for this complex multi-vessel operation enabled the Sonsub survey team and Seatronics to perform a more accurate job significantly quicker. “We were able to deliver the project because of the breadth and range of acoustic monitoring available from Seatronics,” said Adrian.