Thunder Horse Back on Course
In July 2005, a dramatic sequence of events in the Gulf of Mexico threatened BP's Thunder Horse platform. Hugh Kidwell reports on the rescue of the structure.
The BP Thunder Horse Production-Drilling-Quarters (PDQ) platform is a marvel of modern offshore engineering. With a displacement of 130,000t and a weight of 59,500t, it is the largest offshore semi-submersible installation of its kind in the world.
Worth over $1bn, the platform is due, for the next 25 years, to be the hub of the largest hydrocarbon discovery ever made in the Gulf of Mexico (discovered in 1999). The field (75% BP owned and 25% ExxonMobil) has estimated reserves of 575 million barrels of oil equivalent.
The platform will provide access to a reservoir lying over 6,000m beneath mud, rock and salt, and beneath 1,900m of ocean. The hydrocarbons at these depths are yielded at extreme pressures of over 1,200bar and temperatures of 135˚C.
The Thunder Horse is specifically designed for the high-pressure / high-temperature reservoir conditions and the very deep water of the location. The platform will accommodate 11 direct access subsea wells and 13 remote subsea wells, and will include some of the world's longest deviated wells.
The platform is supported by four 23m-wide columns to below the water line where they join the rectangular pontoon of the hull, which forms the submerged base of the platform. At this level there are over 150 watertight ballast compartments, which can be adjusted to control the stability of the structure, particularly in heavy weather.
The Thunder Horse is well equipped for its activities in the Gulf of Mexico (GOM); it has twin drilling derricks which reach to 130m above the decks. The main derrick has a hook load capacity of 1,000t and the second is an auxiliary which saves time spent making up drill strings and supports completions and work over operations.
The platform is state-of-the-art, with new technology used for ultra-deep, high-temperature and high-pressure production. The platform will be able to capture waste heat through heat recovery units to be used in the production process.
In addition, to safeguard the environment and prevent routine overboard water discharges, which could be contaminated, the water produced by the platform will be mixed with seawater and reinjected for reservoir pressure maintenance.
LARGEST PLATFORM OF ITS KIND
The Thunder Horse is thought to be the largest platform of this type ever built. The hull, drilling section and quarters were built at DSME in South Korea. It was built in Okpo, Korea, and was floated around the Cape of Good Hope off South Africa to Corpus Christi Bay in Texas (16,077 nautical miles).
Here it was outfitted by Kiewit Offshore Services in Ingleside before being placed on station in the Gulf of Mexico Mississippi Canyon (Block 778), 150 miles southwest of New Orleans, by the Blue Marlin (the Norwegian-owned semi-submersible heavy lift ship) in April 2005.
The topside modules were constructed in Morgan City, Louisiana, by J Ray McDermott Inc. The topside arrangement consists of three modules: production, generator and compression.
The production and compression modules are three main deck levels high: the dimensions are approximately 115ft wide x 170ft long x 90ft high and they weigh 5,669t and 6,130t, respectively. The generator module has two main continuous deck levels dimensions, which are approximately 292ft x 104ft x 35ft, and weigh 7,658t.
The PDQ platform was designed to remain on location in a 100-year hurricane event, moored with a 16-point chain-wire-chain taut-leg system (moored by 16 chain, wire and suction pile anchors with a weight of approximately 82,543t); BP engineers define this hurricane event as a storm with 120mph sustained winds, a 74ft wave and 5kt currents occurring simultaneously.
The platform was scheduled to start production in late 2005 of 250,000 barrels per day of oil and 200 million cubic feet of natural gas. Oil and gas, when the platform is operational, will be transported to existing shelf and onshore interconnections via the Mardi Gras Transportation System.
By July 2005 the rig was undergoing commissioning and final installation of production modules prior to the start of production from the Thunder Horse field.
On 8 July 2005, the platform was evacuated due to the anticipated close proximity of Hurricane Dennis over this period – the rig was secured and the personnel left. It is not known whether a valve in the ballast system failed or was accidentally left unsecured.
On 11 July a passing vessel reported a problem with the platform and BP personnel returned to find it in a distressed situation with a 20°–30° list; it was in danger of capsizing and sinking into deep water.
BP acted immediately in conjunction with the US Coast Guard (Coast Guard cutter Pelican was on station to aid the recovery operations) and the US Minerals Management Service (MMS) to create a response plan. Several salvage contractors were employed, including T&T Bisso and SMIT Salvage Americas in conjunction with BP Engineering to stabilise the platform.
The problem was assessed by an underwater Remotely Operated Vehicle (ROV) team and no damage to the hull was found. Late in the evacuation procedure, a 10,000t winch was lost overboard and it was thought this could have caused damage to the flotation system, but this was not the case.
The first salvage team onboard the platform included BP Engineers, who checked the platform condition, restored the emergency power, sounded tanks and made an assessment of the platform's vessel management system. The team made attempts to utilise the onboard ballast system (intelligent pumping system) to bring the platform upright, but this was unsuccessful.
The cause of the problem was determined to be with the ballast system isolation valves in the two port columns, which had flooded, including the pump rooms at the base, and caused the platform to list to one side. An assessment was quickly made by the BP Houston Crisis Centre, which had to determine how long the platform would remain stable.
Explosives experts from T&T Bisso provided and installed explosive chain cutters on the platforms mooring chains in case it was necessary to cut the installation loose. The platform was seen to remain stable, but with another hurricane (Emily) threatening the area in four days the speedy progress of the operation was of the utmost importance.
SAVED BY PORTABLE PUMPS
Two high-displacement portable pumps were installed at the platform by lowering them through the elevator and pipe shafts to the affected areas. Pumping operations started on 13 July and were maintained around the clock.
The water levels inside the 185ft columns were seen to drop rapidly as the pumps gained the upper hand. After 24 hours the platform was clear of the sea by 12ft–15ft and was starting to recover a stable position.
Due to the closing in of the hurricane, BP hired a Dutch salvage company to provide an additional pump for the aft port column. By 15 July the platform was stable and had 25ft of clearance from sea level.
Following this, remedial operations were started to complete the platform's return to a stable position. The ballast tanks were carefully drained into the columns and the excess seawater pumped overboard. Work was continued to seal ballast water inlets and plug instrumentation ports. Following other sealing and weather-proofing activities the platform was secured.
A week later, having avoided the worst of the next hurricane, Thunder Horse was upright, with only a slight list, which was later corrected by fine tuning of the ballasting system.
THE CURRENT SITUATION
Due to BP responding rapidly and the cooperation of the US Coast Guard and US MMS in over flying Thunder Horse to assess the damage and the severity of the situation, the Thunder Horse facility was stabilised and restored to normal trim during July 2005.
Since that time the platform has successfully weathered both Hurricane Katrina and Hurricane Rita.
The whole incident involving the platform becoming unstable has been thoroughly investigated by BP and they concluded that it was not storm-related, but caused by problems with the ballast system, which have since been corrected and repaired offshore.
Thunder Horse is now expected to begin production in the second half of 2006, but the exact scheduling still depends on the weather in the Gulf of Mexico. Repair of the platform is continuing offshore and is expected to cost $250m. While repairs are continuing, BP has also resumed commissioning activities aboard the Thunder Horse so that it will be ready for production activities.