Jeremy Beckman - Editor, Europe
Statoil’s Tyrihans is the largest-scale new subsea development this year in the Norwegian sector. The project came onstream this July, on schedule, almost three and a half years after gaining government approval. It is the first tieback to the high-pressure/high-temperature Kristin complex in the Norwegian Sea, also making extensive use of the Aasgard export and injection facilities.
Among the stand-out features of this project, either completed or to come, are a novel installation method for the subsea production structures; new techniques for subsea raw seawater pumping and power distribution; and a direct electrical heating system for the long-distance production pipeline.
Well design issues
Tyrihans, discovered in the early 1980s, comprises two structures in Norwegian Sea blocks 6407/1 and 6406/3, in water depths of around 285 m (935 ft). Tyrihans South, an oil field with a gas cap, extends into production license 091, while Tyrihans North, a gas-condensate field with a thin oil zone, is on the Haltenbank. Statoil estimates recoverable volumes in the reservoirs at 186 MMbbl of oil/condensate and 34.8 bcm of gas.
The two fields are being developed via 12 subsea wells on five, four-slot seabed templates, with the well stream exported through a 43-km (26.7-mi) multi-phase pipeline to the Kristin semisubmersible platform. According to Rune Skotvold, Statoil’s drilling superintendent for Tyrihans, “we made an early start on detailed well planning, and have been ahead of the game throughout. This has meant that the wells have been delivered ahead of schedule and to a good quality standard.”
Early involvement in the project by the drilling and petroleum technology teams allowed well equipment to be devised specifically for the field’s requirements. The teams defined improvements that needed to be made, and realized them in harness with Statoil’s research center and equipment suppliers. Eileen Andersen Buan, the project’s operations vice president, said the collaborative effort had lifted the expected recovery factor above the level foreseen in the original development plan.
As the Tyrihans reservoirs are elongated and cover a large area, seven of the wells will be multi-laterals – a record for a Statoil-operated project – drilled to a subsurface depth of 3,700 m (12,139 ft). Implementing one side track in each well should also save the partners around NOK 1.5 billion ($267.7 million) in drilling costs.
Some will be smart wells, including a downhole instrumentation and control system which will allow remote operations of downhole valves to lift oil output and decrease water production. The system also has been designed so that if one multi-lateral experiences a gas surge, one branch of the well can be shut down while another remains in production. In addition, pressure can be regulated. Aside from optimizing oil and gas throughput, this approach reduce intervention costs.
The wells will comprise eight oil producers, one gas producer, two gas injectors, and one water injector. The semisubmersibleTransocean Arctic started its NOK 2-billion ($357-million) drilling program in April 2008. Three wells were online when production started in July, followed by a fourth in August. All were completed around one month ahead of schedule and according to the planned design, said Sktovold, although varying degrees of difficulty had been encountered during drilling.
Following this early cluster, the rig was withdrawn for classification purposes: It was due to resume work on the fifth well last month. According to field analysts ScanBoss, the final scheduled well – a gas producer – probably will be drilled in 2015.
Subsea workscope
FMC was awarded the $225.4-million engineering, procurement, and construction contract for the subsea production equipment in 2006. This comprises:
- 13 EXHT 10,000 psi trees, including one spare and reportedly one all-electric tree, rated for water depths of up to 300 m (984 ft). These are manufactured in Dunfermline, UK
- Five template structures with manifolds, each weighing 250 tons, fabrication of which was subcontracted to Grenland in Norway
- One 45-km (28-mi) umbilical, subcontracted to Nexans
- Topsides control and 16 subsea control modules, manufactured at Kongsberg in Norway
- Two workover systems for well interventions
- System integration, testing, installation assistance, service, and maintenance.
According to FMC, the project is technologically challenging, as a new ISO standard has imposed stricter safety integrity levels on all the equipment.
The production templates were installed in 2007 using a novel technique. Initially, they were taken to Trondheim fjord by a sheerlegs crane and deposited on the seafloor in 50 m (164 ft) water depth. Later on, they were retrieved by Subsea 7’s construction vesselBotnica and towed to location on Tyrihans, where they were suspended in water through the vessel’s moonpool. This saved the expense of hiring a large crane vessel.
The required modifications to the Kristin platform, managed by Aker Reinertsen, were taken into account before the platform was towed to the Kristin field in 2005. According to ScanBoss, the extra equipment comprised 630 tons of additional topsides, the main item being a 330-ton manifold module. Parts of this module were fabricated in Murmansk, and the remainder at Reinertsen’s yard in Orkanger, Norway. Saipem’s crane barge S7000 lifted the completed module onto the platform in May 2008.
Insulation measures
During the development planning stage, Statoil changed its original concept from pure pressure reduction to gas and water injection for pressure support. Combining this approach with intelligent completions, the company aims to recover larger volumes of oil and gas.
Injection gas/gas lift from Aasgard is supplied through a 43.1-km (26.8-mi), 10-in. (25.4-cm) carbon steel flowline between the Aasgard B platform and the Tyrihans D template, with connections to templates A, B, and C. The Tyrihans well stream heads through the 43-km (27-mi), 16/18-in. (40.6/45.7-cm) rigid production pipeline, connected on the seabed via a flexible riser to the Kristin platform. Both lines were installed in 2007 by theAcergy Piper and Acergy Falcon, the lay-scope also including four hot-tap tees, three tee assemblies, and two 16/18-in. reducers. Ramboll in Denmark performed engineering for the pipelines and their respective routes.
The multi-phase production pipeline comprises an outer carbon pipe with a 316L stainless steel inner cladding – a first for a Statoil project. Butting in Germany supplied the line pipe. This pipeline has been insulated, alongside a direct electrical heating (DEH) system, to maintain the temperature above the hydrate formation temperature during normal production in the event of shut down.
The DEH system, supplied by Nexans, comprises an armored feeder cable and a piggyback cable. The latter has been installed on top of the entire length of the production pipeline, while the armored feeder cable was laid parallel to the pipeline for the first 1,000 m (3,280 ft) at the Kristin end.
Next summer, Statoil should start injection of raw (untreated) sea water, the first such application in the Norwegian sector. Seawater will be injected on a daily basis into the intermediate water “saddle” by a two-pump subsea station installed on the saddle to maintain reservoir pressure in Tyrihans North and South. The injection rate will be around 88,000 b/d, with a pump pressure of 206 bar (20.6 MPa).
Water injection has other purposes on this project, including stabilizing the oil zone in Tyrihans South and delaying gas breakthrough; equalizing the pressure differences between the two reservoirs by injecting water into the saddle between them; and overcoming limited gas injection capacity.
Raw seawater injection is to be controlled via the main umbilical from the Kristin platform. Electricity for the pumps will be supplied through a dedicated power cable, also from the platform. Power will be transmitted at 2.5 MW and stepped down on the field.
Aker Subsea supplied the pumping system early on in the development. FMC provided the 230-ton subsea water injection station, comprising a manifold housed on a template. This and the pumps were installed by theSkandi Acergy in 2008. Construction and installation of the umbilical and the power cable was handled by Acergy and Subsea 7.
Following separation onboard the platform, Tyrihans’ gas is sent via the Aasgard Transport pipeline to the Kaarsto processing complex. Tyrihans’ oil and condensate are diverted through an existing pipeline to the Aasgard C FSO for export via tanker.
Plateau production from Tyrihans is expected to reach 13,000 cm/d (81,768 b/d) of liquids and 13 MMcm/d (459 MMcf/d) of gas.
