Worldwide offshore contractor Subsea 7 has finally launched its new autonomous underwater vehicle (AUV). The ASV Geosub successfully completed a range of demanding and comprehensive sea trials to prepare for the harsh realities of operating in extremely hostile deepwater environments.

The final set of trials took place at Scapa Flow in Orkney and off Raasay on the West Coast of Scotland. Scapa Flow provided relatively sheltered waters, with the added advantage of containing numerous wrecks from both World Wars that provide suitable targets for testing the sonar and multibeam systems. The trials demonstrated that Subsea 7's vehicle is reliable, efficient, and capable of producing high-quality data gathered on a real offshore job.

A Three-Year Journey
The story of the ASV Geosub's development began in late 1999. Subsea 7's Research and Development team had been tracking the progress of AUV development for many years, looking for applications for the oil and gas industry. As it became clear that an active market in AUV usage for survey applications was predicted for the future, the company embarked on a review of vehicle options with regard to technical quality and commercial factors.

One of the potential partners for Subsea 7 was the Southampton Oceanographic Centre (SOC), which had commenced its AUV program in 1995. Since that time, its AUV had successfully completed well over 300 missions, accumulating over 5,000-mission-kilometers while engaged in scientific and environmental projects around the world. This review resulted in SOC's AUV, called the Autosub, becoming the initial AUV platform for Subsea 7 to base further development upon. Subsea 7 obtained a licence agreement for total access to SOC's technology for application in the oil and gas and subsea cable market for a period of 10 years.

Outfitting the Vehicle
ASV Geosub was designed to address increasingly demanding specifications for surveying and positioning in ever deeper waters. Initially, the applications for the AUV will be focused upon geophysical acquisition in association with site surveys and pipeline and cable route surveys.

The vehicle carries an array of sophisticated acoustic sensors to provide high-resolution seabed data, including the Simrad EM2000 multibeam echosounder and Edgetech FSAU dual-frequency sidescan sonar and chirp sub-bottom profiler. CodaOctopus provided a Survey Acquisition System that will time-stamp and record the geophysical and navigation data and additionally provide a software-configurable synchronization scheme to minimize acoustic interference between the many acoustic transmissions to and from the vehicle.

Ixsea-Oceano provided the navigation engine for the AUV. The Phins Inertial Navigator system incorporates an adaptive Kalman Filter that combines inertial, gyro, DVL, and acoustic positioning data to provide an optimal real-time estimate of the AUV's position.

Other positioning technologies are being explored (terrain matching, pipeline tracking, and repeat feature recognition) that may further enhance the positioning capabilities over time. The presence of the AUV on a survey vessel will allow dual-mode operation whereby the surface vessel can undertake additional tasks simultaneously with the AUV survey or inspection. The technology is also being developed further for applications such as intervention.

The vehicle is designed to operate in water depths down to 10,000 feet (3,000m). At 22 feet long, three feet in diameter, and weighing more than 5,000 pounds, this is a large vehicle and a superb platform for deploying a comprehensive suite of survey sensors.

General Mission Overview
The first phase of Subsea 7's sea trials took place in the sheltered waters of the Scapa Flow at the end of July. The ASV Geosub was launched from the survey vessel, Jean Charcot during the 10-day test period. With an excited team looking on, the vehicle executed surface and shallow water missions to assess the reliability of its sub-systems and survey and positioning sensors in depths down to 197 feet (60m).

The second phase of trials took place off Raasay at a depth of 492 feet (150m). All systems performed to expectations. "During the trials, Geosub consistently performed every task that was asked of it," said Murray Dick, Subsea 7's global remote technology group manager.

The key advantage of an AUV is its ability to acquire high-resolution, precisely positioned survey data. Traditionally, offshore geophysical survey is undertaken using survey sensors mounted either directly on the survey vessel itself or on a towfish platform towed behind a survey vessel. It is extremely difficult and cumbersome to navigate the towfish close to the required survey line. Due to the amount of cable deployed, it can take several hours to turn the towfish around for the next survey line in the opposite direction and an accurate result is not guaranteed. As oil and gas field developments around the world move further into deeper waters, the quality of survey data that can be acquired using this approach is being reduced significantly.

Using precise onboard positioning capability, an AUV navigates close to the seabed at normal survey speeds of approximately three to four knots, and exhibits a much tighter turning circle, leading to significant time efficiencies over traditional techniques.

Depending on sensor payload and environmental conditions, survey missions of 30 to 60 hours can be undertaken, powered by an array of rechargeable lithium ion batteries. The vehicle is launched from a survey vessel, and following navigation systems alignment on the surface, dives through the water column, typically levelling out about 100 feet (30m) above the seabed. The AUV then closely follows a pre-programmed survey data acquisition mission described by a number of waypoints, each with its own associated position, depth, and sensor configuration requirements. During this period there is the potential to allow the topside vessel to undertake other tasks while the AUV is deployed.

Upon completion of the mission, the vehicle returns to the surface to a pre-defined location and is recovered to the vessel, where the survey data is downloaded.

For the entire Subsea 7 team, seeing the vehicle they had worked so hard on, for so long, was a celebration.

"The trials team worked tirelessly to ensure the vehicle was put through the most stringent of tests. We are delighted to have reached the stage of taking Geosub to the commercial market and bringing a new dimension of the latest underwater technology to Subsea 7's customers," said Murray Dick.

Just the Beginning
Subsea 7 does not consider its present vehicle as an end product, but rather the start of a series of development projects. These will develop AUV concepts into a number of tools designed to maximize the benefit to the oil and gas industry.

This process will start by optimizing the three critical components of the current generation of survey AUVs: reliable all-weather performance, payload capability, and launch and recovery systems. A program of continuous improvement to the performance of ASV Geosub, its operation, and payload, is currently being addressed by Subsea 7's active and aggressive R&D program.