Subsea/Surface Systems Technology
The Pride Africa drillship had great success in the recovery of its deepwater riser string offshore Angola.
Pride picks up the pieces
The Pride Africa drillship had great success in the recovery of its deepwater riser string offshore Angola. The vessel dropped its riser and blowout preventer (BOP) stack in 5,400 ft water depth last November when there was a parting of the drill line. Since then, the vessel has been repaired and re-certified. The vessel recently returned to the scene of the accident and recovered all the riser joints lost in the incident. As this was being written, the Pride Africa was conducting excavation operations around the BOP prior to recovery.
The BOP was saved for last. When excavation began, silt was blown around the sea floor, and if the riser joints were still there, the debris would cover them up. Even before work began on recovering the riser joints, the wellhead was protected. Pride said the wellhead is in no danger from the work going on and was not damaged in the accident.
When the riser was dropped, it broke off from the BOP, leaving a joint and a half sticking out of the stack. The BOP sank about 25 meters into the seabed, where it rests upright in the mud. This position was a break for the owner. Had the BOP stack fallen over on its side, it would have been more difficult to recover.
Pride will use a combination of turbo prop fans and seabed excavation tools to clear away the silt and sand around the BOP. There are three possible recovery options. The BOP stack could be fished, using casing, riser, or drillpipe, depending on how successful the excavation is. Regardless of the level of success achieved on recovering the BOP, Pride Africa will return to drilling in April. The vessel has replaced the lost equipment and is not relying on the success of these efforts to continue operations.
Tetherless, high-pressure pipeline isolation tool
Plugging Specialists International AS (PSI) of Stavanger, Norway has installed two high pressure, remotely operated tetherless pipeline pressure isolation tools. Called PSI SmartPlugs, this tool allows communication and data transmission through the pipewall, using extremely low frequency electromagnetic signals. An antenna is lowered to the seafloor near the pipeline at the plug location. Instructions are sent through the pipewall using a laptop computer topside. Once the tool is set and holding pressure, the communication system continues to monitor all plug and pipeline pressures.
Previous pipeline isolation plugs were actuated using hoses running from the plug, through the launcher, to a hydraulic system outside the pipeline. This design limited piggability, pressure rating, and the distance the plug could travel. The hydraulics and pressure monitoring equipment are inside the SmartPlug, coupled to an onboard computer and communication system. The elimination of tethers means there are no holes in the launcher, and the pigging distance is virtually unlimited.
The SmartPlug system conducts plugging remotely without insert cabling.
ONGC contracted Rockwater (Halliburton) to replace a 26-in. gas riser and 30-in. oil riser. Without an isolation plug, the lines would have to be shut down, stopping all production from downstream platforms which export into these lines. Tethered plugs could not negotiate the bends involved in the project or cover the pigging distance required. Without this new technology, the pipelines would have been bled to atmosphere, shutting down all downstream production, taking days and wasting the entire pipeline contents. Water slugs would have been required, eating up several more days as the lines dried. Each of the pipelines is 250-km long, and has two other platforms producing into it downstream. That would mean wasting 500 km of contents, and over 30 days of lost production.
A 26-in. tetherless SmartPlug was inserted, pigged into position, and actuated via communication through the pipe wall. The installation took about 1.5 hours from insertion to setting. Downstream operating pressure was maintained at 80 bar, while upstream pressure was reduced to atmosphere. The riser and valves were changed-out in only eight days, allowing all downstream platforms to continue production at full pressure. PSI says it is looking into adapting this system to valves, internal sensors, or pigs, which could be computer monitored from anywhere in the world via satellite telephone.
Flange connection repaired remotely
A seabed flanged connection repair was conducted by a remotely operated vehicle in 500 meters water depth at BP Amoco Foinaven field. The repair was required for a flange in the flowline termination assembly at the base of the Foinaven water injection riser. The riser feeds injected water to different parts of the reservoir. Corrosion, caused by ingress of seawater, was threatening the capacity of the system and pressure had to be reduced as a precaution. The open space around the leak was too small for a repair using a clamp that completely enclosed the flange joint. Furmanite engineered a solution involving a smaller, lighter, bespoked clamp designed to cover the flange connection, and the injection of a specially developed sealing compound to complete the seal. The solution was successful and allowed oil production to continue through the line at capacity by allowing sufficient water injection to be restored.
The repair at 500 meters water depth was a first, according to the manufacturer. Typically, such a patch involves mixing the sealing resin on the surface and installing it using divers. Another problem was the size of the gap. The gap was so large that it required a specially formulated compound. This compound had to be fluid enough to inject, while also being of the correct consistency to bridge the gap without extruding into the pipeline. The operation required extensive testing to assure completion by remote control. Testing was undertaken at Furmanite's UK office in Kendal, at the National Hyperbaric Centre in Aberdeen (whose specially designed ROV equipment was used for the repair operation). It included testing the resin mixture's reaction to seabed temperatures and a perspex model of the flange allowing engineers to observe the resin flow in the flange. Final testing was completed in the field 24 hours before installation.