Feature ArticleConducting a Multibeam Survey At a Gulf of Mexico Excavation Site
By Dixie Poché
Public Relations Manager
Fugro Chance Inc.
Excavation sites on land are often a beehive of activity, with excavators, bulldozers and backhoes digging up soil and rocks to ready the site. But when the excavation site is at a water depth of 300 feet with many unknowns, these excavations become a greater challenge in which detailed mapping is critical to success.
Fugro Chance Inc. was contracted earlier this year to collect baseline data at one such site in the Gulf of Mexico for a decommissioning project of an approximately 500-foot-tall structure. The structure stood in 475 feet of water, and each of its four legs was approximately 7 feet in diameter.
Normal removal of the four-pile structure could not be conducted by using jet strings because the location of brackets on the structure prevented easy access to digging underneath. Removing the structure instead called for the four legs to be cut below the mud line, meaning divers would need access to the cut points on each leg. Excavation around each leg was required for safe diver operations. The holes for the leg needed to be dug about 25 feet below the natural seafloor. A further challenge was that each leg had a different elevation because the jacket was sitting at an unlevel position.
In addition to collecting baseline data, Fugro Chance monitored the excavation as it progressed toward the desired 'bowl' pattern, which would prevent sidewall mud collapse. Through advancements in a rotary-mounted multibeam system, Fugro was able to gather accurate imagery of the site throughout the project, providing safe operation for the divers and allowing the client to determine if it needed to change its excavation strategy.
Choice of Multibeam Sonar
For many years, Fugro has utilized a method of scanning sonar to determine topography and to map features on the seafloor. However, scanning sonar is two-dimensional, and as such, cannot show elevation change. In contrast, multibeam sonar provides a true 3D perspective. It provides a fan-shaped coverage of the seafloor, typically 160°4, and it has the ability to direct the beams toward inaccessible areas such as breakwaters and shoals.
Data from multibeam sonar are different in another significant way. Scanning sonar employs a single beam with a narrowly focused horizontal aperture and a wide vertical aperture; each ping continuously plots returned intensities of the echo, but it cannot discriminate where those returns are in the vertical plane. This results in what is effectively a two-dimensional intensity map. The multibeam system deployed by Fugro consists of an array of 256 discrete beams, each having a focused conical footprint. Coupled with a motion sensor, the range and angle from each beam can be used to georeference each beam target in a 3D space, resulting in a 3D point cloud.
Rotary-Mounted Multibeam System
Because of the many sources of interference that occur in a congested working field, it is not practical to have more than one vessel running multibeam lines. Being unable to find an existing solution, Fugro engineers came up with a number of potential designs in July 2010. A prototype was manufactured and then tested in a tank before heading to a lake for full system tests.
Fugro has used various off-the-shelf multibeam sonars for the rotary-mounted system, which weighs about 350 pounds and is about 4 feet long and 20 inches in diameter. It attaches to a 25-foot, 8-inch-diameter pole, allowing a continuous swath of data to be collected in a 360° circle from a stationary vessel. During the collection process, an operator controls the multibeam rotation speed, direction and other factors. With the capability to view the resulting 3D point cloud in real time, the area of interest is mapped without voids.
This digital terrain model image sequence compares the original seafloor (top) progressing toward the completed excavation (bottom).
The control program for this new system was developed in conjunction with the rotator and utilizes a custom user interface that graphically illustrates the position of the multibeam system in relation to the vessel. The rotator can be positioned anywhere within a 350° window or be commanded to scan a particular range. The real-time display allows the operator to perform scans in an efficient manner, focusing on the desired areas.
The rotator was designed be compatible with Fugro's over-the-side mount deployment system, which is comprised of a swiveling vessel-mounting plate, poles of various lengths (to accommodate vessel draft) and sensor-mounting brackets. Vessels that have carried the rotating multibeam system range from 100-foot work boats to oil rigs. The system is also ideal for barges because of their limited mobility.
Through planning on the decommissioning project, which began in April and lasted five months, it was deemed necessary to jet a 15-foot hole around each of four leg structures, which would allow divers access below the mud line.
Equipment used to dig the holes included a suction tool to suck up the mud and an instrument that blows out the hole using pressure. These tools were used because the location of brackets on the structure prevented easy access to digging underneath. To ensure diver safety and avoid the threat of sidewall mud collapse, the hole underneath the platform was sloped.
Fugro was directed to monitor the excavation process around the four legs to ensure that a three-to-one slope was maintained for diver safety. Spoil removal volumetrics were monitored by comparing the progress of the most recent scan to an earlier scan. From this, the client could recalculate the volume removed and the quantity of spoil still needing to be excavated. This showed the progress of the changing seafloor, allowing the client to oversee and estimate a completion time for dredging. As the project progressed, the size of the bowl pattern could be improvised.
A 3D model of the structure and seafloor was created on-screen through data processing. Once the data were gathered and initially processed on site, post-processing spatial filters cleaned up outlier data points such as fish, plants and acoustic noise. Profiles were developed to show the client the landscape of the seafloor where it had been cut, for example, to indicate if the structure was under the mud. With different visual views available, the depth of the leg holes could be identified to further determine the depth below the natural seafloor.
Previously, this type of project would be, in effect, digging a hole 'in the blind' without the advantage of on-site visualization of the project and its time-lapsed progress. With ongoing monitoring at the excavation site, the client was able to determine if changes in dredging strategy were necessary, thus preventing reworking of an area. To continue this article please click here.
Dixie Poché is the public relations manager for Fugro Chance Inc. She has been with the company for more than 20 years in public relations and corporate communications. She is a graduate of the University of Louisiana at Lafayette with a bachelor's in journalism.