Building the big one
Impressive statistics abound for the Thunder Horse PDQ, the largest production semisubmersible ever built, which in operation will have a displacement of 130,000 tonnes – a significant step beyond the previous largest production semi with displacement of 85,000 tonnes in the Åsgard field offshore Norway. But perhaps all things on a grand scale are no less than should be expected for a platform that for the next 25 years or more will be the oil and gas production hub for the largest hydrocarbon discovery to date in the Gulf of Mexico, tapping into a reservoir lying some 6000m beneath mud, rock and salt, topped by 1900m of ocean, which yields its hydrocarbons at pressures over 1200 bar and temperatures of 135˚C, conditions rarely encountered anywhere in the offshore world.
‘There were many technology gaps to fill when we started out, and we’ve pushed beyond existing limits on many fronts. The result is a world class engineering achievement, a pioneering step akin to those needed in the past to open up the North Sea or the north slope of Alaska, which will be of benefit to BP’s future deepwater projects and to the wider industry.’
‘The unique conditions of Thunder Horse have combined to give the PDQ its shape and scale,’ notes Baur. ‘A semisubmersible provides us with a large and stable installation which can carry the high operating loads imposed by the processing and drilling requirements, while at the same time supporting the additional loads of the dozen or so massive risers weighing around 400 tonnes each that will hang off the hull, and the 16 deepwater chain and wire mooring lines that anchor the semi in 1844m of water. The PDQ is also designed to survive a once-in-a-hundred-years storm, and withstand the hurricanes and high seasonal ocean currents which occur in the Gulf of Mexico.
‘Take all these factors together, and the result is this very large, robust platform, with accommodation for 229 persons, unique in both its size and the duties it will perform.’
Oil and gas from the field, expected by the end of this year, will be exported from the PDQ in separate pipelines connecting into the main Mardi Gras transmission system, operated by BP, and thence to shore.
Global operation
The Thunder Horse field was discovered in 1999, with the basic execution plan for the project and concept for the PDQ decided by early 2001 (Frontiers, September 2001). Detailed design of the PDQ began later that year, the hull being designed by GVA of Sweden, an established leader in semisubmersible vessel design, while the topsides modules were designed by Mustang Engineering in Houston. Construction of the topsides modules, beginning early in 2002, was carried out by J Ray McDermott in Morgan City, Louisiana – the fabrication yard was dedicated to Thunder Horse and BP’s other Gulf of Mexico deepwater projects, Holstein, Mad Dog and Atlantis, which were being progressed simultaneously. Construction of the semisubmersible hull began later in 2002 at the Okpo fabrication yard of Daewoo Shipbuilding and Marine Engineering (DSME) in South Korea. Integration of hull and topsides – the completion of the PDQ – was conducted at Kiewit Offshore Services in Ingleside, Texas, prior to the PDQ being towed to the field.‘The diversity of contractors spread around the globe presented its own set of interface management and logistics challenges,’ adds Baur. ‘The scale of the job meant we needed nine delivery teams, working in different cultures, which created many engineering interfaces – far more than usual – for which we had to ensure that technical specifications, dimensions and weight, and delivery schedule were all tightly controlled. Careful co-ordination between teams resulted in no significant mismatches – the modules were installed on the 15,000m2 deck at Ingleside to within an accuracy of 20mm, and the topsides weight came in a few per cent below contingency budget. Outstanding performance.’
‘Design and fabrication effort for the PDQ will amount to around 15 million manhours by the time it sails from Texas,’ says Devers. ‘At peak, we had teams of 2700 people working on hull fabrication and 1200 on the topsides modules. As on all BP projects, safety has been of prime importance, and at the outset we initiated a safety awareness and training programme across the contractors and vendors involved, with the result that in all these hours worked we have achieved an excellent safety record.’
Measured by the industry standard of number of incidents per 200,000 manhours worked, the PDQ has achieved a total recordables incident rate of 0.43, and a ‘days away from work’ rate of 0.04, ranking the project in the top industry quartile for safety performance.
One of those veterans is Steve Byatt, a ‘founder member’ of the Thunder Horse team working on the design of the hull, who went on to manage the construction of the hull and drilling rig by DSME at Okpo in South Korea.
‘An important aspect of the design phase of the PDQ was to ensure that we were getting constructability into the design so that it could be built using modern shipbuilding methods,’ he recounts. ‘The overall plan called for the hull to be divided up and fabricated as separate blocks, including internal equipment, which would then be assembled in dry dock. The skill lay in selecting the boundaries of the blocks so that they could be handled and lifted individually at the fabrication yard.’
Some 330 blocks were required to complete the hull’s rectangular pontoons, four corner columns and deck box. The latter is a 10m-deep steel structure spanning the columns to form the 136m-long by 111m-wide main deck of the PDQ and ‘locks’ the 60,000 tonne hull together. The deck box, which houses around 20,000 tonnes of utilities and drilling equipment and most of the living quarters, has a watertight base section, an added safety precaution that would ensure – in the highly unlikely event of the double-skinned columns and compartmented pontoons being seriously damaged and flooded – that the PDQ would stay afloat.
The Thunder Horse PDQ was built at the gated end of DSME’s massive dry dock in parallel with four newbuild ships. Despite the capacity of the dock, the scale of the PDQ demanded some clever juggling at the yard.
The PDQ hull was erected in two seven-week batches: pontoons and columns first, followed by the deck box – to support the deck box from below during its erection, 3000 tonnes of temporary towers were brought into the dock.
Blocks of up to 700 tonnes were lifted into place using the dock gantry crane, while to reduce assembly time some ‘superblocks’ weighing up to 2600 tonnes were fabricated and lifted into place over the dock gate by a floating crane – the time-saving advantages of creating superblocks were subsequently passed on to the building of the semisubmersible hull for BP’s deepwater Atlantis field by DSME.
Bringing together so many blocks and internal components carried the risk of dimensional mismatches, adds Byatt. But such was the accuracy of working on the PDQ that this did not occur. He points to the closing of the rectangular pontoon, 11.5m deep and between 21m and 25m wide, which despite having a diagonal dimension of around 115m, was set to within 19mm – within the thermal tolerance alone – while internals such as stiffeners and pipework all lined up.
Once the PDQ hull structure was completed, it was moved from the dock to the quayside for outfitting with cables, piping and equipment, and the installation of the drilling rig, which was shipped from its UK fabricators in ‘kit form’ in 70 containers and erected on site – during construction, the survivability of the PDQ was put to an early test as two very fierce typhoons, one rated as a once-in-a-hundred-years event, passed over the shipyard. By the time the PDQ hull was ready to begin its journey across the world to Texas, the Thunder Horse project had received two BP Helios Awards, acknowledging both the safety performance in South Korea and the successful contractual partnership with DSME.
Epic voyage
Instinct might suggest that an unpowered floating vessel such as the PDQ would move across the oceans of the world by being towed by tugs. But faced with a voyage of almost 29,800km from Okpo to Ingleside, other factors came into play says Fred Agdern, transportation and installation manager.‘In a wet-tow sea voyage for the PDQ hull we may have averaged two to three knots over the 16,077 nautical mile journey,’ he points out. ‘It would have lasted many months longer than the dry transport we opted for, and would have slowed down the schedule to achieving first oil.’
However, for that ‘dry transport’ option, for which the 60,000 tonne PDQ would be carried clear of the water onboard another vessel, there was a significant challenge to address – no vessel existed that was large enough to carry the PDQ.
‘The largest dry transport at that time had been the 41,000 tonne P40 production semisubmersible moving from Singapore to Rio de Janeiro,’ adds Agdern. ‘The industry was just getting comfortable with this scale of dry tow, and then Thunder Horse came along. To move the PDQ we needed something even bigger.’
‘Despite the ship being widened, the PDQ overhung the vessel by some 20m at each side,’ says Agdern. ‘We were too wide to go through the Panama Canal, we would fit in the Suez Canal but were too high to go under a bridge, and so we headed towards the Cape of Good Hope and across the Atlantic.’
For the loadout operation, the PDQ was towed to a location 17km from shore at Okpo and ballasted down to a draft of 8.5m. The Blue Marlin was ballasted deeper still – only the forward bridge and aft towers remained above water – allowing the PDQ to be towed into position above the ship, after which the Blue Marlin was deballasted to raise it up and lift the PDQ clear of the water.
‘A lot of planning went into the dry tow,’ Agdern notes, ‘including extensive model testing of the voyage in a wave tank in the Netherlands to help predict behaviour in a variety of sea states and the resulting loads the structure would experience. We had to ensure internal stiffening would not bend – vessels in dry tow experience more severe forces than when they are floating. We knew we might have to head directly into 9m waves round the Cape of Good Hope.’
The PDQ set sail from Okpo on 23 July 2004, taking a course across the Sea of Japan, around southeast Asia and across the Indian Ocean to the tip of South Africa (see map below). From there, the Blue Marlin and its giant cargo headed across the Atlantic to the coast of Brazil and on into the Gulf of Mexico. The voyage took 62 days, refuelling only once, burning up to 60 tonnes of fuel a day and touching 14 knots with a good following wind.
‘There were many challenges involved in fabricating modules of this size and complexity,’ says Lee Pantermuehl, BP’s topsides delivery manager during construction and now responsible for offshore hookup and commissioning. ‘Because so much of the equipment in the modules is large – for example, the low pressure separators are almost 4m in diameter and 20m long, and there are 11 large vessels like this in the production module – there could be only one construction sequence for building the module. The engineering design was tailored to meet this sequence, rather than adopt the traditional Gulf of Mexico approach of designing by layers. Mustang Engineering did a tremendous job on detailed design, and McDermott delivered a high quality product. The safety record at the yard was excellent, recording no lost time incidents in almost 4 million manhours.’
Oil, gas and water will arrive at the PDQ topsides from the reservoir at pressures of around 275 bar when flowing – this pressure can climb to over 800 bar if the topsides systems are shut in. Large choke valves housed in the deck box reduce the pressure to 90 bar, with temperatures around 120˚C. The wellfluids will then be separated into oil, gas and produced water streams in two parallel processing trains (see flow diagram below). Stabilised oil will be pumped for export at 220 bar and gas compressed to 180 bar, both of which will rate as the highest volume and export pressures in the Gulf of Mexico.
Much of the power is consumed by electric motors driving the facility’s unusually large rotating equipment – principally pumps and compressors – such as the three 2425kW vapour recovery units helping to ensure no hydrocarbon vapours are released to atmosphere, two 6340kW gas booster compressors, and three 4850kW gas export compressors. And rotating machinery needs to be cooled, demanding the intake of 7000m3 per hour of seawater to act as the cooling agent for the platform’s circulating glycol-water cooling medium.
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‘Depending on what is needed in the reservoir, we can inject water at 200,000bpd at 585 bar, or 300,000bpd at 435 bar,’ Pantermuehl points out. ‘At the beginning of the project, no pumps existed that could deliver this duty. We held a design competition among specialist manufacturers which led to Sulzer of the UK being selected to develop the new centrifugal pumps. We have four of these onboard, each rated at 9700kW.’
Installing and testing the pumps brought its own challenges, adds Saha. ‘These discharge pressures require pipework 50mm thick, and special 24-hour-long welding procedures to complete welds on the 300mm diameter carbon steel discharge lines.’
Big lifter
The list of facts and figures associated with the topsides is long and impressive – the modules contain 50km of pipework, 14,000 interconnecting pipe spools, 12,000 instrument loops and 250km of cabling – so too is the challenge of bringing all of these systems and their interfaces together. Getting the massive modules onto the high deck of the PDQ hull was the necessary first step in the integration process at Ingleside.The modules were shipped to Ingleside on a barge purpose-built to allow efficient loading and offloading of large modules – the power generation module is 32m wide and 81m long, with the other two modules coming in at 35m wide and 51m long.
‘The original project plan to lift the modules onto the hull offshore using a heavy lift barge was superseded by the building of an onshore crane by Kiewit Offshore Services at Ingleside,’ says Agdern, responsible for PDQ installation. ‘The net result is a more cost effective operation and one that is not subject to the vagaries of offshore weather.’
Deep drilling
One major item of equipment already installed on the PDQ deck prior to arriving at Ingleside was the twin drilling derrick.‘The drilling rig on the PDQ is state-of-the-art technology,’ explains John Frase, hull and rig commissioning manager, another of Thunder Horse’s much valued ‘veterans’ who has been with the project since the beginning, including construction in South Korea.
‘To be able to drill the wells below the PDQ in 1900m of water and then down to 8000m or so subsurface vertical depth, we have a main rig with a hook load capacity of almost 1000 tonnes. The second derrick is an auxiliary which allows us to save time in making up drill strings, and to support completions and workover operations. This means we will be able to drill one well and workover another simultaneously.’
During those operations there will be no need for personnel to be on the drill floor itself. The rig is fully automated and all operations can be controlled by two people in the driller’s control cabin. To access the dozen wells below the PDQ, gathered in two banks on the seabed, the semisubmersible will move itself around within a 100m radius by pulling on the chain jacks attached to its mooring lines.
Frase reflects on the construction of the rig in South Korea, noting that over 40% of the effort there was attributable to installing the drilling equipment.
All drill cuttings produced in the Thunder Horse field will be cleaned and returned to shore for disposal, one of several environmental protection measures the development has committed to – there will be no routine flaring, all produced sand will be cleaned and taken to shore, and produced water will be cleaned to contain less than the maximum regulatory allowed oil content level before any overboard discharge, although the primary operation calls for the produced water to be reinjected into the reservoir rather than discharged.
‘Commissioning is part of the final integration workscope,’ says BP’s Earl Dague, in charge of completing the PDQ before it sails for the field. ‘Bringing all the parts, large and small, together in a supersized production and drilling facility with a lot of interdependent equipment and systems presents its challenges.’
Dague has responsibility for the day-to-day decisions on who gets priority. Although the work was planned a year ago with BP’s delivery teams, the demands of a 1000-strong workforce, plus 500 contractors and vendors at the site, inevitably throws up simultaneous demands. And on top of the engineering work, there are the less obvious demands – providing hundreds of meals each day for round-the-clock working, or moving people up onto the PDQ and down again on the two elevators, an operation which he estimates has moved a million people during the work at Ingleside.
But there are major operations too.
‘As the PDQ is classed as a floating vessel, it must meet US Coast Guard approval before it can leave the yard and become operational in the field,’ he explains. ‘The approvals run into hundreds as we demonstrate that all safety systems are operative, the crew has been trained, design codes are rechecked, documents correct and so forth. But perhaps the largest action was the inclination test of the PDQ.’
‘All part of this once-in-a-career project called Thunder Horse,’ Dague adds with a smile.
And so the engineering epic of ‘building the big one’ draws to a close. When the PDQ is manoeuvred along the channels and out to sea by a battery of tugs under precision control on its way to installation in the deep waters of the Gulf of Mexico – ultimately to become the largest producing field in the region – so begins the next chapter for Thunder Horse. But that, as they say, is another story.
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Copyright in all published material including photographs, drawings and images in this magazine remains vested in BP plc and third party contributors to this magazine as appropriate. Accordingly neither the whole nor any part of this magazine can be reproduced in any form without express prior permission, either of the entity within BP plc in which copyright resides or the third party contributor as appropriate. Articles, opinions and letters from solicited or unsolicited third party sources appearing in this magazine do not necessarily represent the views of BP plc. Further, while BP plc has taken all reasonable steps to ensure that everything published is accurate it does not accept any responsibility for any errors or resulting loss or damage whatsoever or howsoever caused and readers have the responsibility to thoroughly check these aspects for themselves. Any enquiries about reproduction of content from this magazine should be directed to the Managing Editor (email: terry.knott@uk.bp.com).
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