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Feature Focus: Offshore Innovations: Teaching Rover New Tricks PUBLIC ACCESS

Deepwater Vehicles are Shedding Weight and Adding Capabilities to Extend their Reach Toward the World’s Energy Stores.

[+] Author Notes

Senior Editor

Mechanical Engineering 124(05), 47-51 (May 01, 2002) (5 pages) doi:10.1115/1.2002-May-2

This article highlights that manufacturers have been designing remotely operated vehicles (ROVs) that are directly powered by electric motors to improve their energy efficiency, and to reduce their weight and size. Alstom Schilling Robotics developed the Quest to compete with 100- to 150-hp hydraulic ROVs in such underwater tasks as offshore construction support, remote tool deployment, object recovery, salvage, surveying, and mapping. The Quest’s Sea Net communication and telemetry system carries signals over a single optical fiber to and from modular hubs affixed to the ROV, TMS, and accessories, including sensors, cameras, lights, thrusters, and tools. The greater mass of hydraulic-powered ROVs gives them an advantage over electric vehicles in rough seas, where greater stability is needed. To operate in looser soil that cannot support the trencher, the tracks can be removed and replaced with skids, so that the trencher’s power can be directed to its thrusters.

The stars in the hunt for offshore sources of oil and gas are squat, multiton machines equipped with cameras, lights, sensors, and robotic manipulators. These remotely operated vehicles, known as ROVs, enable oil and gas producers to mine the seabed at depths in excess of 3,000 meters, far beyond the limits of human divers.

The vehicles used in offshore oil and gas exploration are aptly named work class ROVs. They are used to survey, construct, and support wells, pipelines, and ancillary equipment, such as cables. They carry cameras and lights to relay images to their remote pilots, and robotic manipulators to use tools.

Most ROVs use electric motors to drive hydraulic pumps to power their propulsive thrusters and robotic manipulator arms, but recently, manufacturers have been designing ROVs that are directly powered by electric motors to improve their energy efficiency, and to reduce their weight and size. One of the latest electric work class ROVs is the 100-horsepower-equivalent Quest, introduced by Alstom Schilling Robotics of Davis, Calif., in May of last year.

Weightier undersea tasks, such as laying cable in trenches, need the brawn provided by hydraulic power. The Venom ROVs designed by Hydrovision of Aberdeen, Scotland, fall into this category.

Alstom Schilling Robotics developed the Quest to compete with 100- to 150-hp hydraulic ROVs in such underwater tasks as offshore construction support, remote tool deployment, object recovery, salvage, surveying, and mapping. The vehicle’s standard operating depth is 3,000 meters, or 9,900 feet. It has an optional extended rating to 6,500 meters, or 21,450 feet.

Alstom Schilling Robotics designed its Quest ROV to support offshore construction, deploy tools, and survey down to 6,500-meter depths.

Grahic Jump LocationAlstom Schilling Robotics designed its Quest ROV to support offshore construction, deploy tools, and survey down to 6,500-meter depths.

Engineers provided the Quest with a simpler and lighter winch and gantry to launch and recover the vehicle; they eliminated much of the ROV’s hydraulic components and simplified and integrated others.

Grahic Jump LocationEngineers provided the Quest with a simpler and lighter winch and gantry to launch and recover the vehicle; they eliminated much of the ROV’s hydraulic components and simplified and integrated others.

The Quest’s design eliminates the weight of heavy hydraulic pumps, valve manifolds, and associated plumbing, according to Jason Stanley, an ocean engineer and project manager at ASR.

“The more an ROV weighs, the more powerful its gantry and winch need to be in order to deploy it in the water,” Stanley said. “Heavier, larger vehicles take longer to deploy in the water, and time is money for our customers when there is a job to be done.”

By eliminating hydraulic components and by simplifying and integrating others, the Quest’s designers shaved 30 percent off its gantry lifting requirements compared with similar 100-hp hydraulic ROVs, according to Stanley.

As a result, the Quest’s electric launch and recovery system (eLARS), consisting of a winch and gantry, is lighter, simpler to construct, and easier to use than typical winch-based launch systems. The eLARS, which was designed with mobility in mind, can be transported with the ROV and its tether management system within a single package.

Rather than swinging the ROV through an A-frame, as with hydraulic ROVs, the Quest eLARS operator lifts the entire ROV package off the deck using an armored umbilical cable, and the lift follows a track to the deck edge.

The Quest eLARS is powered by Alstom AC vector drives. The programmable drive system actively compensates for heaving decks in heavy seas, can recover cable at more than 1 meter per second, and is equipped with a Lebus-style grooved drum, fail-safe brakes, and automatic level winding.

The Quest ROV package consists of the vehicle and the cylindrical tether management system, or TMS, and the work skids. The TMS is reminiscent of a top hat, and named such by whimsical engineers. The work skids, basically additional aluminum frames, are bolted to the bottom of the ROV and contain tools for the ROV’s tasks.

The Quest TMS is a remote controlled submersible winch that deploys a soft tether mated to the vehicle when the package is lowered to its working depth. The pilot then uses the ROVs thrusters to fly the vehicle to its work site. Upon completing its tasks, the ROV is flown back to the TMS to be raised to the surface.

Oil and gas explorers prefer ROVs with a TMS like the Quest to reduce the effects of current on the vehicle, to deploy it rapidly, and to park it between assignments. The top hat configurations compactness enables operators to add tooling packages.

The Quest also can be configured to swim off tether and be connected directly to its deck winches by its umbilical. This design is typically favored for shallow water operations.

Each of the Quest’s seven thrusters is powered by electric direct drive motors that increase the vehicle's overall propulsion efficiency compared to similar hydraulic systems.

Grahic Jump LocationEach of the Quest’s seven thrusters is powered by electric direct drive motors that increase the vehicle's overall propulsion efficiency compared to similar hydraulic systems.

The ROV is propelled by seven thrusters, each powered by a 7.5-kilowatt electric ring motor. Each thruster weighs 55 pounds, or 25 kilograms, in water, and while turning at 1,000 rpm, provides a maximum thrust rated at 450 lbs., or 204 kg, in forward or reverse operation. The entire propulsion system contains only seven moving parts, one for each thruster, compared with 100 or more moving parts on most hydraulic work class ROVs, according to Stanley. “The thrusters’ electric direct drive increases the Quest’s overall propulsion efficiency by a minimum of 40 percent, compared to similar hydraulic systems,” said Stanley.

The electric motor is equipped with hydrodynamic bearings lubricated with seawater, eliminating the need for sealed, oil-lubed bearings that can be contaminated. Most of the motor’s parts are made of engineered thermoplastic to lighten their weight and make them corrosion resistant. The rotor magnets and stator windings are encapsulated in plastic.

The Quest’s Sea Net communication and telemetry system carries signals over a single optical fiber to and from modular hubs affixed to the ROV, TMS, and accessories, including sensors, cameras, lights, thrusters, and tools. Alstom Schilling designed these hubs to handle up to 30 serial and 8 simultaneous composite video channels each.

Modular communication hubs affixed to the Quest ROV, TMS, and their accessories receive and transmit signals over a single optical fiber.

Grahic Jump LocationModular communication hubs affixed to the Quest ROV, TMS, and their accessories receive and transmit signals over a single optical fiber.

Hubs can be added to accommodate additional gear. ASR eliminated the need for wire harnesses by routing signals and power directly from a printed circuit board to external connectors on the hub’s array of cable ports.

Electricity is sent to the True Wave high-voltage converters installed on both the ROV and the tether system. ASR designed the converters to be about one-fifth the size of a standard 60-hertz converter by using high-efficiency core materials and windings of rectangular wire. The converters are oil-filled, further reducing their weight and size compared with an air-filled enclosure.

The units convert 3,000-volt ac, 600 Hz, three-phase power into two dc voltages: 600 for the electric motors aboard the vehicle and tether system, and 26 for the controllers and accessories.

The efficiency of the Quest power system enabled engineers to reduce the diameter of the ROV’s umbilical to 27 millimeters from a typical 41 mm, reducing weight by as much as 8,000 pounds.

The Quest’s top hat is equipped with a two-stage docking skirt to cushion docking impact. The system is powered by the same electric ring motors used to power the vehicle’s thrusters. A shuttling drum in the TMS winds the tether without twisting or reverse bending it, to prevent damage to optical and electrical connections. Back tension limits and variable speed control wind the tether smoothly, at speeds up to 90 meters per minute.

The pilot regulates the tether recovery speed by means of a foot pedal or one of the operator touch screens. The depth of the TMS, as well as the direction, paid-out length, and speed of the tether are communicated to the pilot.

High-intensity discharge lights illuminate the Stygian depths so that the Quest’s video cameras can capture images. Electric actuators can tilt and pan the lights and cameras into desired position. Video images and all other data collected by the SeaNet’s instruments are displayed to the Quest pilot on a single, 72-inch Theater View display screen. Typically, six to seven screen segments display what the Quest’s cameras see, along with data readouts from sensors.

ASR engineers designed the Quest system to be operated using two identical touch screens to provide pilots with a comfortable, ergonomic control station. They replaced the joystick typically used to control ROVs with a hybrid joystick/PC mouse called a puck.

“The puck replaces a host of toggles ordinarily used in ROV control rooms that make it resemble an airplane cockpit,” Stanley said. There are two pucks, mounted on the right and left, to suit the manual dexterity of the pilot, or to divide the operation of the ROV.

Pilots aboard ship use handheld pucks to direct these electric actuators in order to tilt and pan the Quest ROV's lights and cameras into the most advantageous position.

Grahic Jump LocationPilots aboard ship use handheld pucks to direct these electric actuators in order to tilt and pan the Quest ROV's lights and cameras into the most advantageous position.

Once on the job, the Quest uses its robotic arms, or manipulators, to grip and manipulate tools and parts. The Quest ROV’s eighth electric ring motor powers a modular, hydraulic power unit that pressurizes fluid through solenoid valves and pressure control modules to give movement and direction to the anodized aluminum, or titanium, arms.

ASR fabricates different model arms to suit different ROV tasks. The Quest is typically equipped with a five-function, rate-controlled manipulator called the Rig-master on the port side of the vehicle. The Rigmaster is used for heavy jobs that require more muscle than finesse, such as picking up a cutting tool, cable, wire rope, or a large-diameter container.

For finer manipulation, such as plugging a hydraulic tool into a fluid connection or inserting bolts, the Quest is equipped with the seven-function, spatially correspondent Orion manipulator, typically mounted on the starboard side of the vehicle. Because the Orion arms usually require closed-loop precision for these tasks, Alstom Schilling magnetically couples and seals position sensors inside laser-welded titanium housings.

Hydrovision equipped its Venom ROVs with hydraulic tools to handle muscular undersea tasks, such as opening and closing hydraulic valves.

Grahic Jump LocationHydrovision equipped its Venom ROVs with hydraulic tools to handle muscular undersea tasks, such as opening and closing hydraulic valves.

The first Quest ROV was deployed in the Gulf of Thailand by Canyon Offshore Inc., a Houston-based undersea robotics company working under contract for Clough Unithai Engineering Ltd., from May to September last year. Canyon Offshore deployed the ROV from Clough Unithai’s multipurpose vessel ,Java Constructor, to help build a half-dozen four-leg wellhead platforms and six submarine pipelines totaling 25 km, as well as risers and subsea tie-ins.

The Quest first supported pipelaying in the Unocal Erawan oil and gas fields, in currents approaching 3 knots. The ROV surveyed the sea bottom, monitored pipeline stress, surveyed the finished pipeline, and cut wires.

Engineers used the Quest to help install drill jackets, single-point buoys, and piles and mooring. The ROV also monitored pile driving, conducted metrology with its sensors, and delivered equipment to divers working at the wellheads.

Canyon Offshore was acquired by Houston neighbor Cal Dive International, an undersea oil and gas construction firm that wasted no time putting Quest 1 to work. It is being fitted to Cal Dive’s MV Eclipse vessel to build oil and gas pipelines in the Gulf of Mexico and Brazilian waters.

Canyon Offshore purchased a second Quest ROV that it used last summer and fall to help locate and salvage the Ehime Maru, the Japanese trawler sunk by the submarine USS Greeneville off Hawaii in February 2001.

ASR is building eight more Quest ROVs for Canyon Off shore—two will be delivered this spring—for work in the Gulf of Mexico and the North Sea. International oceanographic research concerns have also expressed strong interest in the vehicles, Stanley said.

The greater mass of hydraulic-powered ROVs gives them an advantage over electric vehicles in rough seas, where greater stability is needed. Their hydraulic thrusters and tools bring stronger mechanical muscles to bear for tasks, including inspecting and changing blowout-prevention ring seals, manually opening and closing hydraulic valves, and deploying and recovering acoustic beacons. Those are among the primary duties of the Venom work class ROVs manufactured by Hydrovision of Aberdeen, Scotland. The company also designs and markets all-electric ROVs through its subsidiary, Seaeye Marine.

Hydrovision builds 100-and 150-hp versions of the Venom ROVs to operate at depths of 3,000 meters. All Venoms weigh approximately 3,500 kg and are equipped with a proprietary Curve-tech power pack matched to the ROV’s horsepower. The Curvetech is a three-phase, 3,000-volt, 60-Hz power pack motor connected to the vehicle’s main Rexroth hydraulic pump, which powers the propulsion and tooling systems.

The Curvetech can also power an optional, auxiliary Rexroth pump if a client wishes to separate the propulsion and tooling functions. “By using a single power pack for both pumps, we simplify the Venom’s overall design,” noted David Cruickshank, technical sales manager at Hydrovision.

Cruickshank said that the Rexroth pumps enable the ROV’s hydraulics to be run at full system pressure or to be remotely switched to standby pressure. “Full system pressure means the pumps automatically optimize the flow and pressure to maintain the requisite horsepower as demand varies,” he said. “Operators switch to standby pressure—typically set at 300 psi—when the Venom is idling between tasks on the seabed, and to offload the electric motor during startup. This economizes the motor’s energy and increases its components’ life.”

The Venom’s main pump delivers fluid power to a thruster control unit via a stainless steel check valve and a 3-micron filter. The control unit, in turn, feeds servo valves connected to the seven thrusters that propel the vehicle through the water. Four thrusters are mounted to provide horizontal control, while another four enable the vehicle to dive, surface, pitch, and roll.

Each thruster is equipped with fixed displacement, bent axis, aluminum motors specifically designed for Hydrovision by Rexroth to increase hydraulic efficiency and power, while reducing thruster weight. As a result, the thrusters supply more than 750 kilos of bollard pull at 100 hp.

The Curvetech communications system feeds data on the ROV’s movements via fiber optics to a control system. “We used commercially available industrial process control hardware and software so that the vehicle can be directed by a personal computer,” Cruickshank noted.

“Canyon Offshore has purchased a Venom ROV for drilling support and general underwater engineering support operations. Hydrovision is also constructing two other Venoms for the Russian Navy, which will use them to improve its underwater search, location, and salvage capabilities, in the wake of the Kursk sinking,” remarked Cruickshank.

The submarine Kursk, which sank in the Barents Sea in August 2000, was raised last fall by a Dutch joint venture working for the Russian government. An article on that operation begins on page 52.

The latest member of the Venom family is the Venom 3K-450 with a trencher module, designed to lay, inspect, and repair submarine cable. The new trencher works down to 3,000 meters. It provides up to 450 hp, or 335 kW, at that depth.

The trencher’s LARS consists of a telescopic A-frame mounted on a base, a rotator, x/y axis damping system, and control console. Operators use the control console to direct the main lift umbilical winch to lift the trencher over the side and lower it into the water.

The winch can hold up to 3,500 meters of 46.5-mm-diameter umbilical cable on its Lebus-style drum. The cable is protected by steel wire armor and contains both the high-voltage wire that powers the trencher, and the optical fiber that communicates between the ROV and the surface controls.

At its nether end, the umbilical feeds into a termination box that sends power and signals to the appropriate subassembly.

The Venom 3K-450 ROV can be configured with a top hat-type TMS so that the core ROV can be detached from the trencher module to perform separate tasks.

His Majesty Prince Andrew, the Duke of York, launched the first Venom 3K-450 trencher in February. The vehicle will be shipped to NSW GmbH in Nodrenham, Germany, later this year.

Grahic Jump LocationHis Majesty Prince Andrew, the Duke of York, launched the first Venom 3K-450 trencher in February. The vehicle will be shipped to NSW GmbH in Nodrenham, Germany, later this year.

During trenching mode, the pilot guides the Venom 3K-450 to lay a length of subsea cable in‘a trench as it is dug.

The trencher digs with two devices called jetting swords, which are located at the base of the ROV. Each jetting sword contains two types of vertically angled waterjets. High-pressure jets, with flow rates up to 230 cubic meters per hour at 15 bar pressure, are used to dig in densely packed soil. Lower-pressure jets, each providing about 10 bar pressure, remove loosely packed soil.

As the jets remove soil, a mechanical tool plows the furrow into which the cable is laid. The trench is then covered naturally by the seabed.

When trenching on denser soil, the pilot typically drives the Venom unit on caterpillar tracks. These tracks also can be used to move the trencher about on deck. The tracked Venom can make more than 3 knots in forward and reverse.

As the trencher traverses the sea bottom, its sensors and accessories transmit the location of the cable and the vehicle to the surface so they can be tracked. In addition, engineers attached a fifth wheel to the trencher to provide feedback of the ground distance traveled back to the control system, which also confirms that the tracks maintain seabed traction on the desired path.

To operate in looser soil that cannot support the trencher, the tracks can be removed and replaced with skids, so that the trencher’s power can be directed to its thrusters.

The first Venom 3K-450 trencher was launched by the Duke of York in February. The ROV will be shipped to NSW GmbH in Nodrenham, Germany, later this year. NSW is a manufacturer and installer of submarine fiber optic cable systems.

Copyright © 2002 by ASME
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