North Sea Nuggets field to come on

July 16, 2001
By yearend 2001, TotalFinaElf Exploration UK PLC's Nuggets natural gas field in the Northern North Sea will come on stream following installation of tie backs to the company's Alwyn North platform.
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By yearend 2001, TotalFinaElf Exploration UK PLC's Nuggets natural gas field in the Northern North Sea will come on stream following installation of tie backs to the company's Alwyn North platform.

The company had increased the processing capacity of its Alwyn North platform by debottlenecking the gas treatment plant. This increased capacity enabled the TotalFinaElf to consider developing and operating the Nuggets field, south of the platform, via a tie-back to Alwyn North.

After a tendering process, ABB Offshore Systems Ltd. (ABB) and Coflexip Stena Offshore Ltd. (CSOL) won the subsea production and pipeline transportation systems, not on an individual basis as initially tendered but combined into a single contract on an engineering, procurement, installation, and construction basis.

The work, scheduled for completion during last quarter 2001, will be performed by the two companies in a consortium with ABB in the lead. ABB and CSOL have completed other projects around the world using a similar contracting model.

Nuggets

The Nuggets field lies to the east of the Shetland Islands in the Northern North Sea in water depths of 110-125 m (Fig. 1). The gas-bearing accumulations that make up the field have been designated as N1 (North Nuggets), N2 (West Nuggets), N3 (Southwest Nuggets), and N4 (South Nuggets).

TotalFinaElf had proposed that these accumulations be tied-back to the Alwyn North platform via subsea pipelines. During this project, CSOL and ABB are tying-back accumulations from N1, N2, and N3, the production rate of all is estimated to reach 165 MMcfd of gas.

The base case scope of work for Nuggets N1 consists of a new manifold tied back by a 39.5 km, 12-in. pipeline with a 3-in. monoethylene glycol (MEG) piggyback to the Alwyn North platform. Two subsea wells will be installed, one next to the manifold and one 3.3 km away connected by a 6-in. and a 2-in. flexible pipeline laid and trenched separately.

Nuggets N3 consists of a second manifold, a further 14.5 km south of N1, served by a 12-in. pipeline with a 4-in. MEG piggyback line, an adjacent well and, similar to N1, also includes a further well (N2). This well will be tied back 4.4 km to the N3 manifold by a 6-in. rigid pipeline with a 2-in. MEG piggybacked line.

Control of the Nuggets development system will be from the nearby Dunbar platform where a new, dedicated control system is being installed.

The 20-km main umbilical to the N1 manifold and further umbilical of 14.5 km to N3 can accommodate the full field development potential. All four wells (Nuggets A, B, C, and D) will effectively require approximately 42 km of seabed umbilicals.

CSOL undertook the design, procurement, and fabrication of the pipeline system as well as all of the offshore construction work. ABB undertook system engineering, design, procurement, manufacture, testing, and commissioning of the subsea systems (which includes the provision of the wellhead systems), horizontal subsea trees, the topsides and subsea production control systems, the umbilicals (to be supplied by DUCO Ltd., a wholly owned subsidiary of Coflexip Stena Offshore Group) and the manifolds.

Subsea pipelines

Timing is critical to bringing Nuggets onstream.

With a target of first gas scheduled for mid-October 2001, installation of the subsea pipelines was scheduled to begin by Apr. 1, 2001; as it turned out, much of the installation work was completed by Apr. 1. This meant that any pipe-laying vessel would be out at sea very early in the season, a time when severe weather can often restrict operational ability.

Use of CSO's reelship CSO Apache, however, can minimize time on site, as outlined presently.

For the size of the project and the sheer length of rigid pipeline required for the Nuggets tie-back (58.4 km in total), the CSO Apache installed the pipelines in four trips to the Nuggets field in order to transport the required length of rigid pipelines.

The pipelines were pre-fabricated into 1,000-m lengths onshore at CSO's Spoolbase in Evanton (near Inverness, Scotland), spooled onto the CSO Apache, and transported out to Nuggets where they were laid in short weather windows.

The CSO Apache was then able to return to collect the proceeding stage of pipeline for its next trip. Total time onsite and therefore cost of operations were minimized for each trip (36 hr laying time/trip), and the possibility of encountering poor weather while onsite was significantly reduced.

During pipelay, a midline Tee was welded into the pipeline at the stern of the CSO Apache and installed in the N1-to-Alwyn North pipeline next to the Dunbar platform. This will facilitate a potential future spur to Dunbar.

Installation of the 12-in. gas pipeline and 3-in. MEG pipeline from the N1 manifold to the Alwyn North platform also involves three crossings. These are a 10-in. pipeline, two 6-in. pipelines, and two umbilicals, and one 24-in. pipeline, all owned and operated by TotalFinaElf.

Once the pipelines were laid but not trenched, it was necessary to protect them from potential entanglement and damage from fishing activity. As a result, three guard vessels were employed through the Scottish Fishing Federation to sail above the development and act as a warning to passing vessels until the pipelines had been trenched.

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The next stage of pipeline installation saw the trenching of the rigid pipelines shortly after the final pipes were laid. The CSO Multipass Plough (MPP) performed this with support from the Normand Pioneer (Fig. 2).

The pipelines were then tested to ensure their integrity, including flooding, gauging, and 24-hr strength hydrotesting, all carried out diverless from the Skandi Inspector.

Before departing from the site, CSOL installed mattress protection onto the untrenched pipeline ends and rock dump protection on the crossings. These will also serve to mitigate upheaval buckle of the pipes.

Fitting into the next stage in the schedule, the drilling rig is now in place to begin drilling on well NGB. As the rig moves onto the final well, NGC, sometime in August or September, CSOL will return to begin installation of the manifolds and umbilicals and perform all tie-ins and pre-commissioning.

Subsea equipment

Of utmost importance for TotalFinaElf when planning this tie-back project was that only the most robust, reliable, and established equipment be used throughout the field. The use of such field-proven equipment would ensure that installation times could be accurately planned, almost guaranteed, potential problems minimized, and the final schedule adhered to.

In order to satisfy TotalFinaElf's requirements for Nuggets N1, N2, and N3, ABB contracted to design, manufacture, and supply two manifolds, five horizontal christmas trees, wellhead systems, and the control systems for the entire field including the design and supply of more than 40 km of umbilicals, manufactured by DUCO.

ABB's capabilities in all areas of supply have been proven on many worldwide subsea EPIC projects, with specialist ABB business units working together in a consortium. This consortium consists of ABB Vetco Gray for wellhead and christmas tree systems, ABB Offshore Systems (UK) for system engineering and production controls, and ABB Offshore Systems (Norway) for the manifolds and structures.

The two manifolds (about 100 tonnes) will be capable of hosting three christmas trees, two production inlets for the planned wells, and one spare inlet for potential future expansion of the field by TotalFinaElf.

Although in shallow water, the manifolds have been designed for both diver and ROV intervention and are specifically designed to allow retrieval and reinstallation of all subcomponents. The manifold structure itself will be permanently piled to the seabed with overhead protection to the internal equipment.

The recovery of equipment packages is broken into differing levels enabling either the complete manifold to be recovered, including the production control and distribution packages, or any of the individual modules or components to be recovered individually.

The production trees are based on ABB standard 10,000-psi horizontal tree configurations. The individual tree incorporates a production choke with retrievable inserts, subsea control module, flow measurement devices, and all facilities for the primary control of the produced gas and for a wide range of intervention and maintenance operations.

All critical components for the process flow control and instrumentation are also being supplied as part of an integrated ABB solution.

The horizontal tree (HT) will be run, without the use of guidelines, to the MS700 wellhead system which provides for full metal-to-metal sealing, without use of elastomers in its sealing mechanisms.

High integrity of all valves and seals is being ensured by the performance of high and low pressure and gas testing in accordance with PSL4 or API6A. The HT tree will be used in a drill-through application to gain the advantages and savings in the drilling and completion program.

The downhole completion will include two downhole pressure and temperature transmitter gauges and one surface-controlled subsurface safety valve routed through the tubing hanger tree head interface. The tubing hanger and tree head have the facilities for future expansion for downhole chemical injection.

A dedicated production control system for the Nuggets development will be constructed and installed on the Dunbar Platform while gas production is tied back to the Alwyn system.

Day-to-day running will be from the Alwyn North facilities via an updated master control system, connected to the Dunbar production control system through an existing redundant fibreoptic link. Electro-hydraulic control of the subsea system is by a master control station and hydraulic power unit connected to the subsea system via an electro hydraulic umbilical.

The control system, while initially running only four wells, is designed to accommodate the full potential development of up to six wells.

All chemicals are also sent subsea through the same umbilical with the exception of MEG, which will be distributed from Alwyn via injection lines piggybacked to the production flowlines.

The hydraulic system is redundant, multi-dropped to all wells, while electrical distribution will take place via three quads. Each quad will be dedicated to a particular reservoir (N1, N2, N3, and N4) and will carry a fully redundant communications and power channel.

The maximum design subsea step out is 49 km, well within the capabilities of ABB's COPS (Comms on Power System), which has demonstrated to be capable of step-outs up to 100 m.

Subsea control of each well will be achieved with dedicated control pods on each tree, controlling tree valves and monitoring tree instrumentation including production and chemical flowmeters.

Additional control pods are provided on each manifold to control the manifold mounted MEG chokes, chemical metering valves (CMV), and flowline isolation valves as well as manifold instrumentation, including pipeline corrosion monitoring.

Chemicals are individually metered to each well by the CMVs from chemical headers on the manifold and via chokes in respect of the MEG system.

Umbilicals; flowlines

The management of this phase of the work is a key factor in the overall success of the project to achieve first gas by the scheduled date.

During this phase, CSOL anticipates that it will have up to three of its vessels working in the field at any one time, in addition to ABB and CSOL's teams working on TotalFinaElf's two platforms, Dunbar and Alwyn North.

At this time, scheduled for August-September 2001, the remainder of the Nuggets field infrastructure will be installed in a sequenced operation, utilizing the CSO Constructor, the CSO Wellservicer, and the Skandi Inspector.

So that the drilling rig can be brought in as soon as possible after pipe installation and TotalFinaElf's tight development schedule can be met, Nuggets well NGB will be tied-back at later with 3.3 km of flexible flowline.

This enables the well to be drilled earlier and the pipe to be installed quickly along with the umbilicals. This timing also avoids the potential clash of the rigid pipelay vessel and the drilling rig onsite.

The flexible flowline has been manufactured at CSO Le Trait, near Rouen, France, and will be installed from the newly converted CSO Constructor, which incorporates an underdeck car ousel that will be used to install all the umbilicals.

The CSO Constructor will be first on site to install the remaining pipeline, the flexible flowline connecting well NGB and the N1 manifold.

At the same time it will begin installation and piling of the two manifolds and installation of the umbilicals, including pull-in of the main umbilical to Dunbar. The Skandi Inspector will also return to Nuggets, with the CSO Flexjet, a lightweight subsea crawler system used for trenching of flexible pipes and umbilicals in soft soil.

The CSO Wellservicer will then carry out all tie-in operations connecting rigid and flexible flowlines and the umbilicals to the trees and manifolds, including installation of mattress protection on all exposed pipelines and umbilicals.

Testing

Once the final tie-ins are completed, a full system leak test will be undertaken from the Alwyn North platform, followed by dewatering and precommissioning of the pipelines performed from the CSO Wellservicer.

Final commissioning and making ready for start-up will be performed by ABB, including full function testing of the control systems, under TotalFinaElf's supervision.

This will be carried out from Dunbar to ensure effective communications and functionality of the system. CSOL will provide the necessary vessel support and monitoring activities to support the final commissioning process.

By early October 2001, the Nuggets development will be in a position to be brought on stream and Nuggets gas processed through the Alwyn North platform.

The authors

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Didier Bertrane has served since January 2000 as Nutter and Otter fields project manager for TotalFinaElf Exploration UK PLC, Aberdeen. He joined Total Austral (Argentina) in 1994 and has handled project management assignments in Argentina, France, Indonesia, and the UK.

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Steve Jackson is a project manager for ABB Offshore Systems Ltd.'s TFE Nuggets development, Aberdeen. He also has held posts with CJB Earl & Wright, a major platform designer. He holds a BSc in civil/structural engineering and has more than 20 years experience in building offshore projects.

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Richard Wylie joined Stena Offshore in 1986 as a senior project engineer and then became a project manager on diving and pipelay projects in 1987. He worked in Norway for 5 years as a project manager on both rigid and flexible pipeline projects before returning to the UK in 1997. He was re cently assigned project manager for EPIC tieback projects such as Flora for Amerada Hess, NW Bell for BP and now Nuggets. Wylie qualified with a BSc (honors) in naval architecture at Southampton University. He is a member of the Royal Institution of Naval Architects and a chartered engineer.