For AUV eyes only

Heather Saucier investigates how wireless technologies used by AUVs can make sea-bottom surveillance more cost-effective and time-efficient.

A multibeam sonar system  (EM 2040) image taken by an AUV indicates debris encroaching on a pipeline. Images from Kongsberg Maritime.

Initially developed in the 1950s to study diffusion, acoustic transmission and submarine wakes, autonomous underwater vehicles (AUVs) have made their way into the oil and gas industry and, some say, are on the verge of reaching game-changing status.

Primarily used for detailed hydrographic surveys, pipeline and seismic array inspections, and environmental monitoring in both shallow and deep waters, AUVs are slowly replacing the need for surface vessels and, in some cases, remotely operated vehicles (ROVs).

While new technology is often eclipsed by the tried and true, especially during times of economic distress, those on both the sales and consumer sides of AUV technology insist that the vehicles can trim costs substantially by covering vast amounts of subsea terrain in one-fourth the time of traditional surface vessels.

“New technology is always daunting to clients,” says Simon Goldsworthy, EURA Sales and Marketing manager of the United Kingdom-based UTEC NCS Survey, which often employs AUVs for its clients in the petroleum industry. “However, the use of AUVs is picking up momentum. AUVs have proven themselves in a variety of oilfield markets, and clients are looking at them as a game-changer for reducing vessel hours or even days on site.”

Anatomy of an AUV

A close-up photo shows the debris imaged above taken by an AUV.

While many types of AUVs exist, they all operate without a person on board, like an ROV; however, an AUV is independent, unlike an ROV, with no cables connecting back to a vessel, thus the AUV relies on navigation software that allows users to choose how it will perform pre-programmed tasks.

Equipped with a multitude of sensors including compasses, synthetic aperture sonar and “sniffers,” AUVs can conduct precise ocean-floor surveys in the place of less effective bathymetric surveys, as well as detect the presence of oil plumes and other chemicals that could threaten the environment, says Arnt-Helge Olsen, vice president of Subsea Sales for Kongsberg Maritime, which is the manufacturer of some of the largest AUVs – the Hugin, Munin and Remus – which can reach depths up to 6000m, travel as fast as 6 knots, and operate in arctic and tropical waters.

Using wireless technology to communicate, AUVs provide better data because they use high frequency sonar and can get ultra close to ocean floor – their sensors relatively undisturbed by rough currents.

Most AUVs are battery powered, and most use propeller-based thrusters to move, while buoyancy engine-driven underwater gliders can operate for a year and cross oceans.

Small AUVS can be launched off a beach, while larger units are housed in landers on the seafloor, Olsen says, adding that they are primarily used in North Sea, Gulf of Mexico and offshore Brazil.

Array of uses

Men prepare a HUGIN AUV for its next mission in the North Sea.

When performing site surveys after decommissioning a rig in shallow water in the US, traditionally the trawl method has been used to ensure proper clean-up. Vessels drag nets to catch remnants, and boat captains record items removed from the seafloor.

This type of operation can take multiple days and the cost is more than US$10,000 for each site, says Tim Taylor, CEO and president of Tiburon Subsea, which rents AUVs to companies to perform the same task.

“However, with an AUV, we can survey 8-10 sites in the same amount of time and cut the cost by half,” Taylor says, whose company ships the 90-pound vehicles to clients with pre-trained operators. “And the imagery and data is so much more advanced. We have a full mosaic map of the area and exact GPS geo-referenced data on it.”

Routine pipeline and seismic array inspections benefit from AUVs as well. The vehicles can survey the ocean floor for threatening obstacles prior to the laying of pipelines and seismic arrays. They also can detect corrosion or damaged pipelines from seabed slides, as well as seismic arrays that have damaged nodes or that are entangled with adjacent arrays.

Calling AUVs a “force multiplier,” Arnar Steingrimsson of Teledyne Marine, which manufactures low-logistic AUVs that are both propeller driven (the Gavia) and equipped with buoyancy engines (the Slocum G2 Glider), explains that one small surface vessel can launch multiple AUVs that each perform specific tasks – cutting costs tremendously by eliminating the need for a large surface vessel, tow spread or ROV.

A HUGIN AUV is prepared for launch in the North Sea.

AUVs are ideal for platform area surveys, as they can encroach within the 500m exclusion zone without stopping production, Goldsworthy says. FAUVs also have successfully performed scouring surveys between platform jacket legs.

Furthermore, some AUVs, such as the Hugin, can retrieve data – such as currents, temperatures, salinity, mammal life and even oil seeps, methane and carbon dioxide – by performing full water column dives.

“If you have a rig in an area with high currents, it could affect rig operations,” Steingrimsson says. “Furthermore, if hydrocarbons are detected in the water column, that’s an indication of a possible leak that you would have difficulty detecting from the surface. This enables you to detect problems in advance and greatly mitigate risk.”

Traditionally, when collecting sonar data from deep-tow or remotely operated towed vehicles in 4000-5000m water depth, roughly 9km of cable is needed, requiring days of work from a surface vessel, Olsen says. “It takes hours for a boat to turn around as opposed to minutes,” he says of an AUV’s capabilities. “There are no cables. All the sensors are on board.”

In the future, AUVs will be able to lay seismic arrays as well, Olsen says. “We are working to develop a seismic node that will swim out and deploy on the seabed,” he says. “The AUV seismic nodes would be lowered in a sub-sea cage to the seafloor and swim out to the proper place. They would no longer be lowered from the surface. But this function will take years to fully mature.”

(Editor’s note: See “Making nodes fly” on page 42 of this issue for more information about flying nodes.)

In the meantime

Roughly 10 companies sell AUVs on the international market, and Taylor believes more are in demand.

“It’s a busy ocean and it’s getting crowded down there,” he says. “You’ve got to have eyes down there, and these AUVs are the eyes.”

And, don’t forget about the No. 1 force fought by the industry: the weather.

“The North Sea is the perfect example. One-hundred percent of the days in the winter are unusable,” Taylor says. “So get as much done in the summer as possible. Launch four vehicles and get four times the data.”