The Northern Underwater Gas Gathering Export and Treatment System (Nuggets) is located in blocks 3 / 18c, 3 / 19a, 3 / 19b, 3 / 20a, 3124a, 3 / 24c and 3125a in the UK sector of the northern North Sea, south of Dunbar in water depths of 120m.
The field is operated by TotalFinaElf, which holds 100% of the equity holding.
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It lies in an Eocene production horizon and recoverable reserves are estimated at 500 billion cubic feet (bcf) of sales gas.
Wells discovery
The field was discovered in July 1973, when the submersible Ocean Traveller, operated by Total, drilled well 3 / 19A-1B in 168m of water.
The well was drilled to a depth of 2,468m and the discovery tested 12.4 million cubic feet a day (Mcfd) of gas.
Another well in the block, drilled in 1989, using the semisubmersible Benreoch, tested up to 23.4Mcfd gas at 2,180m.
These two wells formed the Nugget 1 accumulation.
The first oil was discovered in 1988 by Shell during drilling in the ninth-round block 3 / 19B. The semisubmersible Stadrill, while drilling in 109m of water, predominantly found gas with some oil in the 4,100m well. This was the Nugget 2 accumulation.
In 1991, Total drilled 3 / 19a-4 to the west of Shell’s find and tested gas at 25.5Mcfd, through a 48 / 64in choke, using the Sedco 706. This discovery was named Nugget 3.
In 1974, when the Total-operated wildcat 3 / 25a-2, drilled by the Neptune VII semisubmersible, tested 14.9Mcfd of gas: this was called Nugget 4.
A subsequent well (3 / 24a-3), drilled to the east in 1990, also encountered gas and proved the extension of Nugget 4 into the 3 / 24a block.
This geology suggests that the field may extend into seven blocks.
Nuggets development
The Nuggets field development plan consisted of subsea wells to be tied back via a subsea pipeline to Alwyn North Facilities and be controlled from the Dunbar platform.
The field’s four gas-bearing accumulations, namely Nuggets N1, N2, N3 and N4, are located about 400km east of Aberdeen and around 20km south of Alwyn satellite field Dunbar.
N1 comprises two wells, while the other three each have one well. The first three accumulations began producing in November 2001.
The group of four subsea wells were initially tied back 54km to the Alwyn North installations and controlled from the Dunbar platform.
Later, in October 2003, the N4 gas field came onstream and was tied back to a 13km subsea pipeline to N3.
The five wells had a total production of about six million cubic metres of gas per day in 2007.
Contract details
TotalFinaElf Exploration awarded the EPIC contract for phases 1 and 3 of the development, covering subsea facilities and pipelines, to a joint venture (JV) between Coflexip Stena Offshore Limited (CSOL) and ABB Offshore Systems.
The base case scope of work for Nuggets phase 1 comprised a new manifold at Nuggets, tied back by a 40km, 12in-diameter pipeline with a 3in MEG piggyback to the North Alwyn platform.
Two subsea wells were installed: one adjacent to the manifold and one 3km away, connected by 6in and 2in pipelines respectively.
Controls were provided from the Dunbar platform via a 20km umbilical, which stretches to the manifold then onwards to the two wells.
The contracts, including two options for additional facilities, were awarded to the consortium.
Nuggets phase 3 comprises a second manifold with an adjacent well, umbilical and a 14.5km, 12in pipeline with a 4in MEG piggyback line tied back to the phase 1 manifold.
The Nuggets phase 2 option included a further well, tied back 4.4km to the phase 3 manifold by a 6in pipeline, with a 2in MEG piggyback line and umbilical.
CSOL was contracted to design, procure and fabricate the pipeline systems and perform the offshore installation and tie-in of the pipelines, umbilicals and manifolds.
Installation of the system was undertaken in mid-2001.
ABB provided the horizontal subsea trees and the control system. The umbilicals and manifolds were supplied by Duco.
The control system and umbilical installed in the initial phase have the capacity to support the full field development, including expansions, resulting in an overall subsea step-out of 43km.
Pipeline
Natural gas processed on the Alwyn North platform is transferred to the St Fergus terminal in Scotland by a pipeline via Alwyn and the Frigg UK pipeline.
The gas is transferred through the Frigg Transportation system.
