While progress on Exxon's Natuna project remains stalled, other consortia are pressing ahead with Indonesian gas export schemes. The major offshore undertaking at present involves a new long distance subsea pipeline carrying gas south to Singapore from three separate production sharing contracts (PSC) in the West Natuna Sea.
Conoco is the driving force behind the project, and its share of the development is also the most innovative, featuring a movable steel gravity production platform. The other two operators, whose fields lie further north along the pipeline route, are adopting a more conventional solution involving fixed and subsea installations.
West Natuna movable steel gravity platform.
In two cases, the gas was an unwanted by-product of exploration drilling close to oil production complexes. The idea of harnessing it finally came about two years ago at a meeting in Singapore, attended by Conoco's Tom Bundy. There a Sembawang executive happened to confide his company's ambitions for gas-fired power.
Commercial discussions then ensued between SembCorp Industries, Pertamina, Conoco, and the other two gasfield operators on the projected route (Conoco's reserves alone looked insufficient to meet the scheme's needs). A gas sales contract was concluded this past January, under which 325 MMcf/d will be supplied through a new 28-in., 650-km subsea pipeline over a period of 22 years, starting July 2001.
West Natuna group
The three PSCs pooling their gas are working together as the West Natuna group. Natuna Sea Block B, operated by Conoco Indonesia, in partnership with Inpex and Texaco, will provide 43.1% of the supplies. Block A ,operated by Premier Oil Natuna Sea, will send through 36.9%, and the Kakap PSC, led by Gulf Resources/ Kakap, will deliver 20%. Reserves contracted over the 22 years amount to 2.5 tcf.
This June, J Ray McDermott was awarded the pipeline installation. Its' KP I and DB 26 vessels will share the work, likely to be completed before the monsoon season gets going in late 2000. Much of the line's route is through Indonesian waters, in depths of up to 300 ft.
Conoco performed preliminary engineering studies with Intec Engineering in Houston. Currents and seabed features are not considered problematic. The main perceived difficulties are the approaches to the termination point on the island of Paula Sukra, through the congested Singapore Straits, where there are numerous flowlines and cables that must be avoided.
All the gas has been contracted to Pertamina, with SembCorp Industries the ultimate buyer. It will be used for power generation and also as feedstock for petrochemical plants in Singapore.
Product routing
Conoco has produced oil from Block B since 1979, beginning with the now depleted Udang field and currently through fixed platforms on the Belida Field. Recoverable reserves on the block were initially estimated at 160 million bbl, but have been steadily re-evaluated upwards to the current figure of 400 plus million bbl EUR. In 1986, Conoco also deployed the production barge San Jacinto to drain progressively some smaller fields elsewhere in the PSC. This vessel currently is producing 5,000 b/d from the Sembilang Field, having already depleted Kepiting and Ikan Pari.
Over the years, ongoing exploration continued to reveal pockets of dry and wet standalone gas. For West Natuna, another moveable production system seemed the likely outcome. According to David Tuturea, Conoco's Marine Project Engineer for this project, "We had our minds set initially on a spread-moored barge, but then anticipated problems with the associated mooring system and large diameter flexible risers."
Moveable jackup
Thoughts then switched in 1997 to deploying a converted jackup - at a time when rigs of all types were still in short supply. "We approached drilling companies, but at the time they were actively engaged in their mainline business," says Tuturea. "It looked like we'd have to go for a newbuild, using an existing or a new design." At that time, newbuild costs were still inflated. But if the platform could operate continuously, offering a stable environment for the compressors, the advantages appeared to outweigh the cost penalty.
Conoco invited studies from four engineering groups based on a moveable jackup, as opposed to a mobile unit switching frequently between locations. The winner was London-based Arup Energy's MOgPU (movable offshore gas production unit), selected in March 1998.
A further series of studies and model tests ensued before the contract was finally awarded this past March. The unit will be a self-installing jackup with four lattice legs, connected to a steel gravity base, supporting a rectangular stiffened plate steel deck. The legs will be located outside the deck, thereby facilitating use of a conventional deck equipment layout.
MOgPU was developed by Arup Energy from their low cost platform concept called ACE. From Conoco's viewpoint, the apparent ease of fabrication was a plus. "It can be assembled like a barge," says Tuturea. "The legs are a jacket-type construction, without the need for very tight tolerances. The whole thing can be built in pieces, and in different places." The concept allows maximum completion of the topsides facilities prior to installation.
Hyundai won the open EPCI and the platform will be build at Ulsan, Korea, with much of the work subcontracted to Indonesian companies. The MOgPU will then be dry towed, fully assembled and pre-commissioned, to the West Natuna site 4,000 km away in March 2001.
Strand jacking
The steel platform's leg-deck interfacement along with the riser positions are shown
Once the platform reaches location, it will be floated off the transportation vessel to effectively become a barge itself, floating on its own buoyancy. When the self-installing unit has been jacked down into position, it will be able to withstand impact from 100-year storm waves (up to 9.2 meters high).
The installation, however, will require a weather window of at least four days with waves not exceeding 1 meter. "It's not in a typhoon area," says Tuturea. "The big problem here is a potential long-period wave swell, although that should be fairly easy to predict through studying movements of organized storms in the distance."
Conventional jackups, requiring more frequent relocation, have integral hydraulic or rack and pinion lifting systems, which must be maintained throughout the rig's operating life. The MOgPU, however, will be installed using a rented strand jacking system, which will allow the steel gravity base to be lowered from beneath the deck to the seabed, with the deck subsequently jacked up to its operational height. The deck is then suspended on the lifting cables while welding onto the lattice legs is effected. Once weldout is completed (probable duration around 10 days), all temporary equipment for the strand jacks can be returned to the rental company, with the lifting cables are also removed. When Conoco decides to relocate the MOgPU to another location, the jacking system can simply be re-hired for the repeat operation.
Multi-condition platform
ACE platforms are designed for installation in clay or sandy soils, in water depths of 30-100 meters. This initial location is 82 meters deep. According to Ove Arup's Brian Raine, "The most challenging aspect is that the soft clays extend to a considerable depth, although stiffer clays do occur in places."
Following laboratory analysis of soil samples by Fugro in the UK, an optimum foundation design was identified that could support the MOgPU over 25 years of production, if required. Platform hydrodynamic analyses were conducted at the IHL laboratory in Surabaya, Indonesia, with assistance from Marin in the Netherlands.
Conoco's West Natuna gas development project, including product transport connections, is shown.
The 4,500-ton deck will house two gas turbine driven compressors manufactured by Cooper Rolls/Dresser Rand, each capable of compressing at a rate of 125 MMcf/d. Processing requirements are minimal for the early phase of development, as the incoming gas will be very clean and dry - a dehydration package is the basic requirement. Other equipment will include two gas turbine power generators initially, with provision for a third if more power becomes necessary. This would be the case if, for instance, chemical injection were required to counteract hydrates. The deck will also support two 25-ton cranes, a helideck, and accommodation for up to 24 people at peak. Normally, a crew of 16 should suffice.
Initially, production will come from two subsea wells on the Tembang field tied back to the MOgPU via a 14-in. steel flowline and production riser. Gas from two new wells at Belida, along with gas flared currently, will also flow 31 km to the MOgPU through a 12-in. line, for mixing and blending with supplies from Tembang.
Further development
Belida is an oilfield developed through three platforms and an FSO. Associated gas produced through the drilling and production platform "A" (DPP-A) and wellhead platform "B" (WHP-B) has been exploited over the past few years partly as a lift mechanism, but also as fuel gas for the other installations. Further gas pockets in this area will be tapped for onward export to the MOgPU (along with the quantities currently flared). Belida is now expected to stay onstream for an additional six years. Without the gas scheme, it may have been abandoned earlier.
These initial fields are expected to begin depleting by 2003-2004. The plan then is to tie-in dry gas from Buntal to the West, which could keep the MOgPU on the same location through 2007-2008. Thereafter, Conoco will consider whether to tie in additional dry gas fields nearby such as Bintang Laut or move the MOgPU North to Bawal.
There are also the wet gas fields to the East, including Belanak, Hiu, and Kerisi. Conoco will have to decide at some point whether to exploit them using fixed platforms or through a movable facility like the MOgPU or an FPSO, likely based at Belanak.
Conoco's recent exploration efforts have confirmed additional oil and gas with Block 19. Movable assets, such as the MOgPU provide Conoco the flexibility necessary to optimally develop these reserves and meet growing natural gas demand in the region, Tururea concluded.
