New technologies are required to cope with the cold and pressure as oil companies head out into deeper waters in their quest for oil.
As oil companies venture deeper into the oceans, they are having to install production equipment in dark, freezing conditions and under crushing pressures on the seabed. Commercial divers cannot operate in such a hostile environment, so expensive remotely controlled underwater vehicles - operated via an umbilical cable that stretches perhaps a couple of kilometres from a support vessel to the sea floor - must be used to maintain ever more complex equipment.
However, a more cost-effective solution for light intervention tasks may soon be provided in the form of vehicles that ‘fly’ to work sites without connection to the surface.
Subsea production systems are designed to be as simple and reliable as possible. They are built up from modules, which can be retrieved for repair. Problems with hydraulic systems, which are susceptible to fluid leaks and hydraulic contamination, are a particular worry, because of the environmental damage they cause. But finding and repairing a leak in 3,000m water depth - where some of the deepest wells are now being drilled - is no simple task.
So, one aim in recent years has been to replace hydraulics with more reliable electric actuation systems. There has been steady progress on this front, and the installation of the world’s first all-electric subsea tree system in late September 2008 marked an important milestone. However, complete elimination of hydraulics remains some way in the future, as hydraulic actuators are well suited to the harsh working conditions found deep within well bores where sub-surface safety valves are installed. Halliburton is reportedly among the companies seeking to develop an electrically actuated valve for this purpose.
Shell and Perdido
Increasing water depth and the growing complexity of subsea systems have piled on the problems for subsea inspection, repair and maintenance (IRM). One important innovation was the introduction of process systems that can separate oil, water and gas fractions on the seabed. The history of this technology dates back to 1989, but it was only with the successful commissioning of the Tordis subsea separator in 2007 that this technology was field-proven. Subsea separation has since been adopted for the Perdido and Pazflor projects, which are both ground-breaking in their own right.
Shell set a world water-depth record at the end of 2008 when it completed a well in 2,851m of water for the Perdido project, which is located 320km from Houston. The Pazflor project is notable for its vast scale, with subsea hardware supplied by FMC Technologies, under a $980m contract, which is said to be the largest order ever placed for equipment of this sort.
Subsea compression is the next challenge, and this may be adopted for the Ormen Lange field, 120km off mid-Norway, which supplies 20 per cent of the UK’s gas needs. If pilot trials are successful, a subsea station would be installed in place of a compressor platform, at a depth of 1,100m. The 60MW compression station, which would be used to counter falling reservoir pressure, would comprise four units, each similar to the pilot unit which is to be tested later this year. Aker Kvaerner has just opened a new subsea hall at its yard in Egersund, west Norway, where the pilot unit is to be assembled and tested, prior to a basin trial at the Ormen Lange terminal, in Nyhamna. The pilot unit will measure 35m × 6.5m × 13m and weigh 1,100t when assembled.
Seabed drilling rig
Even greater innovation is likely to occur in the future. Stavanger-based, Seabed Rig AS is taking on a truly awesome challenge in developing a submerged autonomous seabed drilling rig. “Our approach is to remotely control the sea-floor drilling system from a surface ship,” says Kenneth Mikalsen, chief technology officer of Seabed Rig. “We encapsulate the system to make it environmentally friendly with no discharge to the sea.”
Seabed Rig has spent the last three years developing this technology with assistance from the oil-giant Statoil, which has also taken a stake in the company. In May 2009, Seabed Rig licensed technology and contracted with Energid Technologies Corporation to create a robot control system for the subsea operations.
The Energid Technologies Corporation, which has experience with the space agency Nasa, says that the practical operation of a drilling rig on the hostile ocean floor is only possible using robotics. The robots maintaining the rig must be versatile and able to perform many tasks, from drill pipe insertion to maintenance and repair. They must be mechanically advanced and intelligently controlled.
Energid Technologies will use its robotics software Actin, to simulate and control the complex robots used by Seabed Rig. The software will be responsible for coordinating the motion of multiple robots, each with many moving parts. It will allow the undersea kinematically redundant robot arms to reach around obstacles, optimise for strength, and smoothly avoid joint limits. “Actin will maximise performance and make control from the surface easier by allowing direct control over hand placement,” says Mikalsen.
Speaking about progress on the project Mikalsen says that the iron roughneck - a piece of equipment to make and break the joints within the drill string - was “tested with great success before summer this year at Iris, in Stavanger. The next machine to be tested is the robot for the drill floor. This machine has just been assembled, and the testing will start in the near future.”
Total and Pazflor
The scale of some recent deepwater projects is illustrated by Pazflor, which lies off the coast of Angola, and is operated by the French oil giant Total. In addition to subsea separation, this project may use autonomous underwater vehicles to assist with subsea intervention for the first time. This would be a major step-forward.
Pazflor is the third in a series of giant oil developments in Angola ‘Block 17’ license area; the earlier developments were called Girassol and Dalia. Each of the previous projects involves clusters of wells connected by a web of pipes and cables to a gigantic floating production and storage unit. The subsea equipment within this license area is currently maintained by two dedicated field support vessels, with a third vessel due to join by the time Pazflor comes into production in early 2011.
Pazflor extends over an area 600 sq km - roughly the area of Paris - in depths ranging from 600m to 1,200m. It will be developed by 49 subsea wells spread across the reservoir, with the produced fluids piped to a colossal floating production unit capable of storing two million barrels of oil. But the project is considerably more complex than its predecessors because some of the produced fluids are to be processed on the seafloor.
Each of these subsea processing units will weigh 400t and measure 25m × 15m × 15m. During the process, the liquid/gas fractions will be separated so that they flow to the floating production facility in different pipelines. The removal of gas from the liquid stream prevents cavitation problems in the seabed oil booster pumps. Each seabed processing unit incorporates sophisticated sensors, closed-loop controls, pumps, valves and sampling points. There is therefore considerable potential for IRM work.
Conventional subsea inspection
In ocean depths where diving is impractical, IRM is routinely carried out by a remote operated vehicle (ROV) controlled from a field-support vessel, with individual modules retrieved to the surface by crane for repair. It can take a couple of hours to deploy a vehicle to the seabed. The recently commissioned $50m Normand Subsea provides an example of a state-of-the-art deepwater ROV support vessel.
This vessel, which is owned by offshore contractor Subsea 7, has commenced her first project for Shell in the Ormen Lange field in the Norwegian sector of the North Sea. Amongst other facilities, this vessel has a 150t heave-compensated crane, and an extensive spread of two work-class ROVs and four smaller ROVs designed to work in depths up to 1200m. It is also fitted with an integrated module handling system capable of deploying or recovering a 35t module in water depths of down to 1,200m.
The ROV’s propulsion system and its manipulator arms are typically controlled by a three-man crew from a console onboard the field support vessel. Graphic displays and video images provide information for the operator, with joysticks for fingertip control of thrusters and manipulator arms. The umbilical cable would typically have a diameter of around 30mm, and neutral buoyancy.
It transmits electric power, data and control signals to and from the support vessel. Fibre-optic data transmission enables high-quality video images to be transmitted to the surface.
The ROV needs to hover for prolonged periods over the worksite, and this can present a problem in very deep water, because ocean currents exert fluctuating drag forces on the long umbilical cable. This problem is usually mitigated by a tether management system.
The tether management system consists of a submersible housing containing a winch system, which is lowered to the working depth of the ROV, and acts as a base for the ROV operations. A short length of umbilical is then paid out horizontally under constant tension from the winch as the ROV moves to the work site.
However, the high cost of ROV operations has led some engineers to look at the possibility of using autonomous underwater vehicles (AUVs) for the less demanding tasks, such as visual inspection and light intervention work.
AUV subsea intervention
The first generation of AUVs were designed to ‘fly’ along predetermined underwater paths without any connection to the surface. They were used to survey seabed terrain. But more advanced versions have since been developed, which are more compact, and incorporate intelligent navigation for tracking features such as pipelines. SeeByte is a well-established company in this field, and supplied a vehicle that carried out an uninterrupted 10km inspection along Talisman’s pipeline off the Orkney Islands, during a BP-sponsored trial. This was a world record for a two-man portable unit when it was performed in June 2008.
This underwater vehicle employed Autotracker software which paints a picture of the seabed and interprets that picture so it can instruct the AUV to maintain a constant offset from the pipeline. The module is capable of accurately tracking a single pipeline amongst multiple pipelines and on varied terrains. In addition, Autotracker includes advanced search routines that enable it to recover the pipeline track after an unexpected pipeline burial.
The offshore contractor, Subsea 7, announced a joint venture collaboration with SeeByte in February 2008 to develop an autonomous vehicle capable of both inspection and light intervention on the seabed.
Swimmer: the Hybrid AUV/ROV
The French company Cybernetix has previously built a prototype AUV capable of light intervention work on the seabed. The prototype was put through tank and shallow field tests off the south coast of France during the autumn of 2003. The work was carried out under the European Commission (EC) ‘Alive’ programme. Cybernetix was project coordinator with partners IFREMER, Hitec and the Ocean Systems Laboratory of Edinburgh University.
During the prototype trials the AUV docked onto a subsea structure and successfully carried out pre-programmed tasks, such as opening and closing valves, with its hydraulic manipulator arm.
The vehicle was equipped with video cameras and sonar systems to identify its position with computer systems to guide it to its docking location and operate its manipulator arms.
It was recognised however, that the amount of intervention work that the unit could perform would be severely limited by the battery storage capacity of the autonomous vehicle.
This led Statoil, FMC Kongsberg Subsea and Cybernetix to join forces to develop an AUV/ROV hybrid. This concept is now marketed under the trade name ‘Swimmer’, and involves using an AUV to transport a work-class ROV to the seabed, where the ROV would be connected to a docking station. The ROV could then be deployed to work without the need for a field-support vessel.
A paper at the OTC conference in 2009 outlined how Total could make use of this ‘Swimmer’ hybrid technology for the Pazflor project. While no firm decision has been made, the paper outlines the technical obstacles that need to be overcome, and strongly suggests that the technology will be implemented. The paper makes the point that it would be sensible to incorporate docking points on the subsea manifolds for the implementation of schemes of this nature.
The docking points would receive power and data from the floating production facility, with control via the permanently installed production umbilical. Once the docking operation is complete, the work ROV would be set free to perform work within the local area, controlled via a short (perhaps 100m long) umbilical. It’s anticipated that the ROV would be left underwater for a period of perhaps three months. During that time, the AUV could recharge its batteries, and carry out further tasks, such as pipeline inspection, prior to returning the ROV to the surface.