Bluefin-21 AUV

Beneath the horizon

Deepwater Horizon oil spill showcases ROV technology.

The world waits with bated breath to hear whether the latest attempt to cap the Macondo well has been a success - but what exactly has been going on beneath the surface?

The subsea activities being undertaken in response to the tragic incidents aboard the Deepwater Horizon semi-submersible mobile drilling unit (MODU) have been performed by remotely operated vehicles (ROVs).

ROVs are underwater robots powered and controlled from a surface vessel or platform via an umbilical. The ROVs used are equipped with dual manipulators and are of a grouping known as 'work-class' (WROV) as they perform the work that would have historically been undertaken by divers in shallow waters. The water depth at the Macondo well is approximately 1,500m and, although well out of the reach of divers, is not now considered especially deep when compared with some of the other oil and gas projects already in place, or under development. All the WROVs in use at the site are capable of working in depths of at least 3,000m. Some autonomous underwater vehicles (AUV) are also being used to collect seawater samples and to map the extent of the oil in the water column.

At the time of writing, the situation at Macondo is that quantities of fluid and gas are being diverted to the Helix Energy owned Q4000 semi-submersible MODU via a subsea manifold connected to the faulty blow-out preventer (BOP) that should have activated and sheared the drill pipe and sealed the well. Another connection has been made from the top of the BOP to the Transocean owned Discoverer Enterprise drill ship via a cap that has been secured to the newly cut drilling riser that projects from the top of the BOP. Relief wells are being drilled by a pair of Transocean platforms. To reach this stage, many operator- and vehicle-hours have been spent observing to enable the response engineers to assess the damage to the subsea infrastructure and to determine possible solutions.

Among the first WROVs on scene were the two Schilling 'ultra heavy duty' (UHD) WROVs operated by C-Innovation, which were deployed from the Max Chouest ROV/platform support vessel and that attempted to activate the BOP. The attempt to activate the BOP involved the WROVs providing hydraulic pressure via a connecting hose that (if all was working properly) force the BOP shear to cut through or crush the drill-pipe. The UHD (Ultra Heavy Duty) has a 200HP hydraulic power-pack that is used to power its thrusters and has an additional power-pack of up to 75HP to power tooling. The 3m long, 2m high vehicles weigh 5,000kg in air and are available with depth ratings of up to 4,000m. Some 50 UHD vehicles have been built to date since starting production in 2007 and are predominantly owned by operators with large vessel fleets such as C-Innovation (the ROV division of Gulf of Mexico support company Edison Chouest Offshore), and Bourbon Offshore.

Maxximum WROV

The most powerful WROV on site is the Oceaneering Maxximum WROV, which has a 300HP hydraulic power-pack, and is hosted onboard the Ocean Intervention III ROV support vessel owned by Island Offshore on a long-term lease with Oceaneering. The other WROV on the vessel is a 150HP Oceaneering Millennium vehicle, and both have depth ratings of 3,000m.

Many WROVs are deployed directly from a host vessel with their power and control umbilical being supported at one end by the vessel and the other end by the WROV. In deep water, the necessary length and thickness of umbilical to reach from the surface and give the vehicle sufficient working range would mean that the vehicle would have to be equipped with extra buoyancy and thruster power. Instead, deep-water ROVs are connected to a lightweight umbilical (known as a tether) of up to 900m in length, which is in turn connected to the thicker, heavier primary umbilical that reaches to the surface vessel via a tether management system (TMS). The ROV is launched and recovered attached to the TMS either in a cage-type garage or attached via a locking latch system to a top-hat style TMS.

On modern, dedicated ROV support vessels such as the Ocean Intervention III, the TMS and ROV is deployed via an extendable arm that brings the vehicle from its resting position inside the vessel to a point well clear of the side of the vessel where there is less risk of impact during the launch or recovery process in heavy weather than would the case if a through the hull 'moon-pool' was used.

The WROVs at the site have performed a wide range of duties with their manipulators and onboard tooling but they are also used to position and provide power to tools that are suspended from the surface vessels. In the early days of the response, this included the insertion of a collection tube into the broken riser - the tube was connected to the Transocean Discoverer Enterprise drillship. The WROVs were then used to install a subsea manifold that connected to the BOP to allow the 'top-kill' and 'junk-shot' procedures to be attempted. A number of containment cap solutions designed to rest over the damaged riser and BOP were attempted but failed due to the build-up of hydrates that effectively blocked the flow.

What is now been seen as one of the most successful operations in the response is the cutting and removal of sections of the damaged riser pipe from the BOP so that a more effective collection cap could be fitted, allowing for much greater recovery volumes. The lower marine riser containment (LMRP) cap contains methanol injection lines that are used to reduce the build-up of hydrates, and the structure has a rubber grommet that prevents leaks when compressed.

The first cut on the riser was made using a set of hydraulic shears from Genesis Attachments that weigh around 25t and can exert 29,000kN of force. The second cut - at the top of the BOP - was achieved using a diamond-coated band saw from Cutting Underwater Technologies. This technology is already used in the decommissioning of redundant oil and gas platforms. The cutting tool was manoeuvred into place by a WROV and locked itself onto the BOP. The continuous diamond-coated cutting band is mounted on a series of pulleys that are moved into position once the tool is securely in position. Hydraulic power and control for the tool is provided via the ROV.

The LMRP cap was installed on 3 June. It took oil and gas to the Discoverer Enterprise drill-ship where oil was collected and gas flared. A second system, which began operations on 16 June, took oil and gas from the BOP via the subsea manifold to the Q4000 semi-submersible where both oil and gas were flared. This followed calls for increased redundancy in the recovery operations that resulted from a shut-down caused by a lightning strike on the drillship.

Recovery operations were interrupted again when a WROV accidentally closed one of the pressure relief vents on the LMRP during a manoeuvre. This required the LMRP to be brought to the surface, repaired and reinstalled during a ten-hour operation on 24 June. The next step was to install a buoyed riser system that would be disconnectable in the case of severe surface weather conditions, and will recovery oil and gas to the Helix Producer 1 floating production vessel (owned by Helix Energy Solutions) from the subsea manifold currently used by the Q4000. This was then followed by the replacement of the leaking cap.

Public pressure

The physical demands on both WROVs and their operators in this response are far greater than would normally be the case for drill support operations due to the sheer amount of dive time needed and the understandable pressure from the public and the US government. Whilst the provision of live feeds from ROV cameras has allowed a rare glimpse into the previously hidden and unknown world of subsea operations, any interruption in the broadcast is met with media-fuelled cries of cover-up and conspiracy, which puts even more pressure on the personnel involved. Only once the leaking well is finally sealed, and an analysis of both the incident and the response undertaken will the real implications on the people, environment and industry in the Gulf of Mexico be understood.

A number of scientific AUV have been deployed in an attempt to map the spill and to take samples from within the water column to detect and measure levels of oil and chemical dispersants. The Monterrey Bay Aquarium Research Institute is working with the US National Oceanographic and Atmospheric Administration (NOAA) and has deployed a Bluefin Robotics' 'Bluefin-21' AUV from the NOAA Ship Gordon Gunter fitted with a 'Gulper' water-sample acquisition system. The Gulper is a syringe-like instrument which has a 2l bottle actuated by an electromagnetic trigger which can obtain a sample in about two seconds, and was developed in association with Strathclyde University.

The Mote Marine Laboratory has deployed three Teledyne Webb Slocum Glider vehicles (in association with Rutgers University and the University of Delaware) to patrol Florida's Gulf coast and detect signs of oil - in essence to act as an early-warning system. It is likely that additional gliders that are fitted with optical phytoplankton sensors will be added to operations in the near future.

Waldo

Phytoplankton form the basis of the entire food web and changes in the phytoplankton community could signal widespread disruption for other larger marine species from fish to mammals. Gliding AUV have potentially long ranges and high endurance but cannot perform well in locations with strong currents. One of the Mote Marine Laboratory gliders (known as 'Waldo'), was sent on a mission to patrol the continental shelf for oil just north and west of Key West and was tasked with looking underwater for evidence of the spill in an area that oil might be expected to appear if it is carried south in the Loop Current.

Waldo has a science payload containing an optical backscatter system, a dissolved organic matter sensor and Chlorophyll-a sensor as well as a conductivity, temperature and depth sensor. However, when it entered the pass between the Dry Tortugas and Rebecca Shoal, it couldn't swim against the strong currents there. It stopped sending data to shore and was recovered and subsequently relocated to a region with lower currents.

Other gliders have been deployed by the US Naval Oceanographic Office, the University of Southern Florida (all Slocum Gliders), the University of Washington (an iRobot SeaGlider) and a Bluefin Robotics Spray glider from the Scripps Institute of Oceanography. The Naval-owned gliders are operating in the restricted zone around and to the south of the Macondo well, while the academic vehicles are operating along the coastline to the east and west. This coordinated use of gliders and AUV is perhaps the first demonstration of ocean observation system technology that has crossed from the academic into the public domain.

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