Flagship Greater Plutonio in its element
  from: Offshore Engineer
  by: Darius Snieckus
  Monday, April 07, 2008

Like its deepwater Angola brethren Girassol and Kizomba, BP’s Greater Plutonio development has necessarily been built around thinking big: a 1.77 million barrel FPSO capable of processing up to 240,000b/d of oil, 450,000b/d of water, and 400 million scf/d of gas, a 1258m-tall hybrid riser tower – the world’s largest to-date – and a sprawling subsea network made up of 43 wells and 260km of flowlines and umbilicals arranged over five fields in 1200-1450m of water. As the $4 billion complex came onstream last month, Darius Snieckus spoke with deputy project manager John Brownridge about BP’s ‘amazing journey’ with its first operatorship off Angola.

Glasses may well have been raised fractionally higher on 1 October as BP and Angolan state-owned oil company Sonangol celebrated production start-up from the $4 billion Greater Plutonio complex, for flow from the fivefield megadevelopment in the Lower Congo Basin, its four lead-off wells now onstream, carries with it wider resonances. Not only is the development the first to be brought onstream on block 18 as well as the UK supermajor’s maiden deepwater operatorship off Angola, but Greater Plutonio is also the offshore field that might well ultimately be responsible for pushing total crude production from the West African state past the 2 million b/d mark for the first time in its history, moving Angola into place behind Nigeria as the second-most prolific oil province in the region.

The FPSO-centred complex currently flowing upwards of 100,000b/d of 23-35°API crude on its way to a plateau production of 200,000b/d by late-2008 is of that order of magnitude the offshore oil and gas industry has come to expect in West Africa. Fully-developed and flowing at full spate, Greater Plutonio will encompass a far-reaching 43-well subsea network transporting production from the Galio, Cromio, Paladio, Plutonio and Cobalto finds up to a 1258m-tall riser tower and into a 1.77 million barrel storage capacity FPSO with facilities to process up to 240,000b/d of oil, 450,000b/d of produced and treated water, and 400 million scf/d of gas, with export to tankers in 1 million barrel cargoes via a catenary anchor leg mooring (Calm) offloading buoy.

Greater Plutonio’s story began in 1999 with the Platina discovery, drilled in 1400m of water by the Pride Angola drillship and flow tested at 6500b/d. The Plutonio find followed in July of the same year, flowing at 5700b/d on a restricted choke. Galio, the third spud drilled in block 18, and Paladio were discovered in 2000 in water depths of some 1200m, and flowed 4770b/d and 3980b/d, respectively. In March 2001 BP announced two further ‘significant’ oil finds via the Cromio and Cobalto wells, setting the seal on the cluster of fields that would come to be linked together to form Greater Plutonio.

‘Over a seven-year exploration period we made eight discoveries including those that now make up Greater Plutonio – our success rate on block 18 with exploration drilling has essentially been 100%,’ states Greater Plutonio deputy project manager John Brownridge. ‘To some large extent, this is a tribute to the seismic work we did.We shot a great deal of 3D seismic and spent a lot of time on high resolution processing and this has certainly driven our success on the block.’ Drilling triumphs, he notes, were one thing, finding a commercially viable development concept for the quintet of fields something else again.

Based on pre-FEED work by consultants Granherne, BP kicked off front end engineering and design at Kellogg Brown & Root’s Leatherhead, UK offices in 2003, under a contract that grew into an engineering, procurement and construction management deal, including detailed design for the FPSO topsides. Among the early visions for Greater Plutonio was a TLP-based concept similar to ExxonMobil’s block 15 Kizomba field – scotched due to fact that the long footprint of the BP development’s fields meant there was nowhere to anchor a facility with dry wells, necessitating a subsea system – and a hub-centred design – ruled out because of flow assurance issues related to ‘overlong’ distances between a central processing facility and the five fields.

‘Because we were looking at a project that involved multiple deepwater fields, geographically widely spread, our initial cost estimates came out pretty high,’ explains Brownridge. ‘So we went back to the drawing board using a multidisciplinary approach looking at a range of concepts – TLP, Spar, and FPSObased. We decided on the large FPSO, single train, subsea well concept, with single hybrid riser tower, and a combination of reeled and J-laid flowlines as the best solution.’

‘For this kind of development, an FPSO was the only way forward,’ he continues. ‘Had this been one field with the same reserves as our five, then a TLP would have been fine. But we needed subsea wells, the storage that an FPSO affords you, and to be able to offload in large 1 million barrel parcels. In the end, the concept we arrived at looked very much the obvious solution for Greater Plutonio given its layout.’ It is also a ‘clustered’ field architecture that BP had experience of using West of Shetland, notes Brownridge. ‘This was clearly the right way for us to go at the time.’

Crystallisation of the Greater Plutonio development plan was helped by BP’s involvement as a partner on the Totaloperated Girassol field on block 17 and ExxonMobil’s block 15 developments, he acknowledges. ‘We were able to draw in general on our experience on these projects and more specifically when it came to designing the riser tower – and we even ended up with engineers on BP staff that had worked for contractors on Girassol so that gave us confidence that we had a workable solution.’

By 2004, BP had set about letting the main contracts for the project. Hyundai Heavy Industries was awarded fabrication of the FPSO hull and topsides, along with final integration work, out of its yards in Ulsan, South Korea under a $350 million deal. FMC received its ‘single largest order to-date’ in the form of $400 million contract to supply wellhead systems and guidebases, production and injection trees, retrievable choke modules, manifolds, and a raft of connection, control and monitoring systems. A consortium fashioned by Acergy and Technip specifically for Greater Plutonio took home supply and delivery of the umbilicals, flowlines and risers, a turnkey URF order worth over $900 million all-in. And Transocean won the drilling assignment for the development, upgrading its fifth generation semisubmersible, the Sedco Express, into a fully-fledged development driller for the five-year job.

A further key contract later went out – through Acergy – to Single Buoy Moorings for one of its Calm buoy offloading systems for the export side of the development.

‘Interfacing between these four main contracts was quite complicated,’ underscores Brownridge, ‘because although, for instance, HHI built the topsides and the hull, KBR had designed the topsides – and then there were interfaces between the hull and the mooring system, which was part of the URF contract, and the riser tower itself, and the offloading system. So there were thousands of interfaces to be managed and this was one of central challenges during the project execution phase.’

Subsea and subcontracts

Sprawling across more than 600km² of seabed on block 18, the subsea production architecture for Greater Plutonio is made up of 43 wells – 20 producers, 20 water injectors and three gas injectors – set into nine manifolds that are networked into southern and northern production systems with 120km of flowlines and 100km of instrument and control umbilicals. In line with the project’s local content strategy, FMC fabricated six of the field’s nine manifolds through Angola’s Sonamet – building a 1400m2 pipework fab hall in Lobito to complete the contract – and 42 of the production guide bases were delivered by Luandabased Algoa. On top of this, though manufacture of the components for the field’s 45 subsea trees was handled out of FMC’s Dunfermline, Scotland facility, 25 are being assembled as ‘superkits’ at the Norwegian contractor’s Luanda base.

Tree deliveries are slated to continue through the next two years, as ongoing drilling on Greater Plutonio adds wells to the 17 completed so far – which Brownridge enthuses ‘mostly came in with zero skin’, that is with no measurable pressure drop across the completion – and the full complement of 43 scheduled to be online in 2010.

The four production wells that have been brought onstream to this point, all of which lie on the Plutonio field proper, are flowing through the southern production system, a 12in syntactic polypropylene insulated loop linking together five offline manifolds – connected to the inline tree assemblies with tie-in spools – that allows for reverse flow for pigging/flooding operations. The development’s northern system is comprised of a fully-insulated production line and a non-insulated carbon steel service line that gives the operator the option of closinng the loop and displacing with diesel to avoid hydrate formation after a shutdown, particularly important as water cut increases over the production timeline.

Manifolds are equipped with Framo multiphase flowmeters for ‘a simpler subsea configuration and more frequent testing’ than would be possible with a test separator, Brownridge remarks.

Laying of the field’s umbilicals and flexibles was performed by the Acergy Eagle subsea construction vessel, while the Acergy Polaris laybarge – in 2006 outfitted with a new Huisman J-lay tower for the Greater Plutonio job – handled installation of production lines. Technip’s Deep Blue field construction vessel laid the water injection lines, loading up at the Dande spool base. Some 98km of umbilicals and flexibles were supplied by Duco and KOP in the US, and Angoflex in Angola; Tenaris delivered the flowlines.

The ‘beating heart’ of Greater Plutonio is its riser tower. Part of the Acergy- Technip consortium’s far-reaching workscope, the 1258m-tall tower houses 3 x 12in production risers, a 12in gas injection line, a 12in service line, 3 x 3 1/2in gas lift lines, 2 x 12in and 1 x 14in water injection lines, and a gaslift umbilical all arrayed around a core pipe. The 4200t unit was fabricated in five 300m-long bundled sections by Sonamet at its Lobito yard, with the riser tower’s 27-compartment buoyancy tank supplied by France’s Tissot.

‘Different from the riser towers for Girassol, which were fabricated in a splayed “goose foot” fashion and then brought together, our riser tower was manufactured in sections onshore and then each section joined together as the whole assembly floated out in the Bay of Lobito,’ explains Brownridge. Fully assembled, the tower was surface-towed out of the bay and then submerged for transport to field this May, where it was upended alongside the waiting FPSO and installed by the Acergy Eagle and Acergy Legend vessels with the help of the Maersk Asserter.

The riser tower is connected with flexible lines to the FPSO on the vessel’s port side, while the starboard riser porch has been left free for future tie-ins.

Floating the all-electric FPSO

Spread-moored in 1310m of water southeast of the Platina discovery, Greater Plutonio’s 300m-long FPSO gives the first impression of looking much like most any other deepwater floating production vessel at work today in West African waters: VLCC in size, with near 2 million barrel storage and high-capacity processing topsides. Outward appearances suggest the Greater Plutonio FPSO to be a close relation to the Girassol floater, and indeed that producing Total’s follow-on Dalia development as well, but there are a number of marked differences about Greater Plutonio’s, topmost of which is that it is ‘all-electric’.

Power ‘to everywhere that is needed’ on the FPSO, including all pumps and injection compressors, is provided by four Rolls-Royce RB211 turbine-generators with the result that the vessel’s annual greenhouse gas emission is expected to be cut from 1.1 million tonnes to 700,000t, a ‘significant reduction’, as Brownridge underlines. ‘When you look back, you wonder why we as an industry ever did it any other way. It just makes so much sense to go all-electric.’ The Greater Plutonio FPSO’s ‘optimal energy efficiency and minimal emissions’ design in fact somewhat anticipated BP’s new ‘environmental requirements for new projects’ initiative – a scheme started in 2004 that sets out nine environmental impact management processes that are undertaken at different times in the life of a project and a series of 12 environmental performance requirements that need to be met with every development.

‘There is what we build in to a vessel’s design and then there are the controls set on things like our flaring policy – we use low pressure gas recovery off the various systems to ensure the flaring is minimal – and our sensitivity to the environment in which we operate, which means our survey boats have marine mammal observers onboard and we have had members of the Southampton National Oceanography Centre on our rigs making use of our ROVs during downtime for research purposes.’

The vessel’s 21,000t topsides are made up of 12 modules, each weighing around 2000t, including units for gas and water injection – needed for pressure maintenance and voidage replacement in the complex’s various field reservoirs including gas reinjection into the Plutonio structure until Angola’s first offshore gas-gathering scheme, Angola LNG, is up and running, likely in 2012. The FPSO’s 80,000t hull was launched in July 2005 at HHI’s Ulsan shipyard, being transported round to the facility’s offshore yard in January 2006 for the start of the module installation. Following an ‘effectively flawless’ programme of module lifts, BP – ahead of project schedule – spent a further four weeks on commissioning work to ‘get ahead of the game’. Last November, towed by a trio of Semco tugs, the Greater Plutonio began its 78-day voyage to the field.

Design of the layout of the Bureau Veritas-classed vessel is ‘inherently safe’, according to Brownridge. ‘One of the key safety features is the blast gap between the compression modules and the rest of the plant, and the lay down area which forms a second blast gap, effectively, between the plant and the accommodation unit. The layout of this FPSO has made it possible for us to space modules out in the safest way we know.’ Unlike many FPSOs afloat in the world’s deepwater regions, the Greater Plutonio floater also features a curved bow: on tow from Korea to Angola speeds of 7knots were reached, saving transit time and money.

Remotely monitored 2km off from the FPSO, the Calm offshore loading buoy is connected to the vessel via twin 18in lines, with offloading to tankers taking place at around 6000m3/h. Until the Angola LNG terminal is operational, associated gas from Greater Plutonio is being reinjected. The FPSO’s gas system has been designed with a subsea gas export regulating manifold, allowing the operator to inject gas at a higher pressure than will be used for export, when the time comes. ‘This means we can balance the two – if once the LNG terminal is online it happens to be down for a while and you can’t keep backing up the pipeline, then we will be able to switch back over to reinjection using this regulatory manifold,’ explains Brownridge.’

Pumping crude at a rate of some 100,000b/d right now through Greater Plutonio’s southern system, in line with BP’s planned two-stage ramp up, production will continue at this level for the next few months as the gas system is commissioned. Then 200,000b/d or better beckons, with a four-year production plateau predicted.

Subsea 7’s Sevenseas Angola subsidiary has been awarded the $80 million contract for life-of-field support services on Greater Plutonio, a three-year deal that will cover project management and engineering for the maintenance, repair and light construction work required on the field, as well as the tie-in of future production flowlines and water injection lines.

Long term planning has been foundational to Greater Plutonio, according to Brownridge. ‘We believe in being what we term a “local energy company”,’ he stresses. ‘We don’t want to be oil company that rolls in, develops the field, and pulls out all its capability and goes on to work in another part of the world. Our local content strategy on Greater Plutonio was to maximise where sensible. And this matches up with Sonangol’s desire to ensure that each new project in Angola is building capability. Particularly on the subsea contracts we were looking for opportunities to build in Angola – and it worked out to be a quite a tonnage of equipment – and potential employment of Angolans both in our company and for our contractors.

‘There is also the desire to put money into local training initiatives, including the Essa training centre,’ he continues, ‘and we instituted technician training programmes that involved educating candidates at our various facilities around the world as well as offshore in the North Sea, US Gulf and on BP tankers too.’

Moving forward on Greater Plutonio and its surrounds, BP is now looking at development scenarios for Platina- Chumbo in the west and for the 2004 Cesio find, which lies at a distance of 22km from the installed FPSO. Having broken ground on BP’s newest ‘profit centre’ with Greater Plutonio, the oil company’s Angolan vision is bifocal: bringing the current complex up to plateau production in a timely fashion to ensure the greatest profitability over its 25-year field life, and casting ahead to the block 18 fields next in line for development – and indeed those on its operated ultra-deepwater block 31 where it has made 14 finds to-date.

‘It has been an amazing journey accomplishing what we have, all together, on this development, building for Angola and working with the Angolan people on what is a truly global project,’ concludes Brownridge.

‘It is has been a massive integration task that has all come together with the start of operation early last month.We have learned a great deal on this project, much of which we are taking forward – both in terms of design and experience – to future projects off Angola.’ OE

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