Laggan/Tormore opens gas corridor in frontier North Atlantic region

Laggan-Tormore export routes.

The selection of a long distance subsea tieback to a shore-based gas processing facility was the conclusion of an extensive conceptual/pre-project studies program managed by TEP UK and its partners from 2005-2008. Three generic concepts were considered:

• Deepwater hub: based around a floating deep draft semisubmersible moored in 600 m water depth above the gas reservoirs with subsea wells connected to the floating process facility by risers
• Shallow-water hub: based around a 35-km (21.7-mi) subsea tie-back to a new production platform with jacket installed in 150 m (492 ft) water depth
• Onshore hub: based around a 143-km (89-mi) subsea tieback to a new gas processing facility situated on Shetland close to the existing Sullom Voe oil terminal operated by BP.

The onshore hub concept was eventually chosen as it represented the safest option (minimizing offshore work) and best economics (lowest capex and opex). It also offered the greatest flexibility for expansion as the process facilities were onshore. The path to eventual concept selection was complex and involved close liaison between the UK's Department for Environment and Climate Change (DECC)) and the West of Shetland Task Force (BP, ExxonMobil, Chevron, DONG, and TOTAL) in order that established industry players should have an opportunity to invest in the new infrastructure. Approval of the Laggan-Tormore Field Development Program (FDP) was linked to TEP UK ensuring that the design was compatible with a potential late-life installation of subsea gas compression as well as offering pre-installed tees for third party fields to join the system after first gas.

Laggan-Tormore represents one of the longest subsea to beach tiebacks in the world, equal with Norway's Snøhvit development. Of note is that Laggan-Tormore has a modest reserve base compared to its Norwegian counterparts Ormen Lange and Snøhvit. From an economic standpoint this means the success of the development is heavily dependent on delivering the project on schedule – first gas mid-2014 – and within budget.

SPS scope of work

The SPS work package has responsibility for delivery of the following key items of equipment:

• Design, procurement, and installation of two off six-slot integrated template manifolds and protection structures at the Laggan and Tormore sites
• Design, procurement, and installation of one off satellite well protection structure and subsea guide base for tieback of the suspended Tormore discovery well
• Design and procurement of nine off 5 x 2-in. 10K vertical subsea trees, tubing hangers and associated 18-3/4-in. wellhead systems
• Design and procurement of onshore control system equipment and associatedOffshore control modules for manifolds and subsea trees
• Design and procurement of equipment associated with the umbilical electrical and fiber optic connection to theOffshore template manifolds
• Design and procurement of a workover system including lower riser package and emergency disconnect package, umbilical, and workover control system
• Design and procurement of two off 18-in. tie-in hubs (for the pigging loop), eight off 10-in. tie-in hubs and associated universal connection (UCON) tie-in system and tooling.

The contract for design and procurement of the subsea production system was awarded to FMC Technologies in Kongsberg, Norway. Key components of the system were sourced from an international supply chain including Scotland, USA, China, Singapore, the Netherlands, Italy, and Norway. The various components have been brought together for an extensive System Integration Testing (SIT) program at Tønsberg, Norway. Installation of the two template manifolds will be performed by Heerema Marine Contractors (HMC) this summer using theThialf deepwater construction vessel.

While not deep by international standards, the water depth of 600 m and tieback distance of 143 km both present a significant technical challenge in the west of Shetland (WoS) environment. During the conceptual studies, discussions took place with contractors familiar with the WoS subsea environment and the Laggan-Tormore template manifold concept evolved from a consideration of environmental factors. The strong water currents close to the seabed make handling of equipment difficult in 600 m water depth and led to the adoption of a template concept with wells drilled close together, eliminating the need for individual flowlines to connect wells back to a central cluster. Deviated wells will be drilled from the Laggan and Tormore six-slot templates and the production combined into two 10-in. headers within the manifold at each location.

A key feature of the subsea production system is the flexibility offered by the manifold arrangement to produce any well into either of the two 18-in. production import lines. Within the subsea manifold, individual wells can flow into either of the two 10-in. headers which are connected to the 18-in. pipelines via 10-in. rigid steel spools.

Drilling of the nine development wells (five on Laggan, four on Tormore) should start in September using the dynamically positioned rigWest Phoenix, once the subsea template manifolds have been installed on the seabed. These structures each weigh 850 metric tons complete with the integrated manifold. Each structure is 40 m (131 ft) long, 35 m (115 ft) wide and 22 m (72 ft) high, and all will be installed by the Thialf.

The size and weight of the six-slot integrated template manifold is driven by two key factors: the seabed soils and the requirement to ensure the structures do not present a danger to theOffshore fishing fleet. The geotechnical properties of the soils at the Laggan and Tormore sites required large suction cans (6 m or 19 ft diameter and 12 m or 39 ft high) to reach consolidated soil and achieve the bearing capacity required to control the structure settlement. Once in place they anchor the templates to the seabed through friction at the interface with the steel of the suction cans.

Fishing intensity studies have revealed that the area around Laggan and Tormore is fished by demersal (on bottom) trawlers. As a result the SPS project team built a scale model of the template manifold and performed trawling tests at the Force Technology facilities in Denmark using a full trawl and door model at various speeds to demonstrate that the trawl net and associated equipment would not snag on the subsea structures.

The WoS harsh environment has also driven the design of the template and installation method since the project tender phase. The solution retained is a single lift of template and manifold from the deck of the heavy lift vessel (HLV); reducing to a minimum the number of lifts and associated weather windows. The template/manifold have been designed and configured to make maximum use of the weather window, allowing the lift to be performed at a significant wave height of 3 m (9.8 ft). The size of the template is just at the limit of the HLV crane clearance for a dual crane lift from its deck.

Production from the development wells will be controlled via subsea trees which provide the chokes, isolation valves, multiphase flow meters, chemical injection, and control modules to safely produce the gas 600 m below the Atlantic Ocean. The vertical subsea trees are being built at the FMC Dunfermline facilities in Scotland and will be installed subsea from theWest Phoenix.

In addition to the integrated template manifolds and subsea trees, FMC has been responsible for qualifying a new tie-in system to connect the rigid 10-in. spools between the manifold and the inline structures on the 18-in. production lines as well as the 18-in. pigging loop and pig receivers. The UCON, which employs FMC field-proven collet finger connector technology, has been developed to improve the operability of the existing diverless connection system for deepwater development through use of smaller and lighter weight tools that can be transported by an ROV from a subsea working station. This UCON tie-in system was particularly attractive for the WoS environment. All necessary tools will be lowered to the seabed in a subsea work station allowing the ROV to perform various types of operations, and reducing significantly the number of ROV dives and recovery.

The pipelines work package within the Laggan-Tormore project has responsibility for the following key items:

• Design and procurement of all line pipe (i.e. 2-in. 143-km duplex service line, 8-in. 143-km carbon steel MEG injection pipeline, 2 x 18-in. 143-km carbon steel import flowlines, and 30-in. 234-km carbon steel gas export pipeline)
• Design, procurement and installation of the 143-km electro-hydraulic control umbilical
• Design and installation of all pipelines both onshore and offshore.

The various diameters of line pipe required for the project have been manufactured under purchase orders signed between TEP UK and three pipe mills:

• Corus UK (now part of Tata Steel Europe): 18-in. and 30-in. SAW line pipe and bends
• Vallourec & Mannesmann (Germany): 8-in. SMLS line pipe and bends
• Salzgitter Mannesmann (France): 2-in. SMLS line pipe.

Pipelay restrictions

The weather is a major challenge for pipeline installation contractors operating in the waters west of the Shetland Islands. The extreme weather defined in terms of waves, wind, and subsea currents leads to a narrow six-month installation window that effectively runs from April to September.

Installation of the 18-in. and 30-in. pipelines was contracted to Allseas UK which has experience of the WoS environment through previous work on the WoSPS, EoSPS and Clair pipeline projects. Installation of the 8-in. and 2-in. pipelines was awarded to Subsea7 which had previous experience with the Foinaven and Schiehallion FPSO facilities.

Allseas' DP vesselAudacia was due to complete the 18-in. offshore pipeline installation campaign last summer but this work will run into the 2012 season. Audacia is involved in installation of the two 141-km offshore lengths of 18-in. import pipeline from the land fall at Orka Voe out to the Laggan and Tormore production sites, including six hot tapped tees (HTTs), four inline tees (ILTs) and two flowline end terminations (FLETs).

This summer, Subsea7 will lay the 8.5-in. offshore pipeline. The scope of work involves installation of 141 km of 8-in. MEG pipeline and associated piggybacked 2-in. service line from the land fall at Orka Voe out to the Laggan and Tormore production sites. Similar to the 18-in. pipeline, the scope program includes installation of three HTTs, two ILTs, and one FLET.

Tie-in provisions

The Laggan-Tormore concept for tie-in of the subsea template manifolds to the two 18-in. production flowlines employs ILTs with tie-back via 10-in. rigid stainless steel spools. The benefit of placing the SPS manifolds offline is that it avoids the main 18-in. production bore passing through the SPS manifold. This would have significant weight consequences and a complex scheduling interface, as it would be difficult to install the 18-in. production lines without the SPS manifolds in place.

Cross-section through the Nexans electro-hydraulic umbilical.

A consequence of designing the LT facilities to accommodate the potential later addition of subsea gas compression as well as third-party gas entrants has been that the design of the ILTs has grown in complexity since the original conceptual study. This complexity has increased with the requirement to provide a round-trip pigging facility to keep the flowlines clear of wax during the early years of production.

As a result, the 18-in. ILTs and FLETs at Laggan and Tormore respectively have become significant piping modules, designed to be installed inline over the stinger of theAudacia, and requiring a substantial protection structure against potential impact from fishing trawl boards.

The 144 km (90 mi) of offshore electro-hydraulic control umbilical will provide the electrical and hydraulic power to control the subsea wells at Laggan and Tormore together with the chemicals required to maintain production. A cross-section of the umbilical is shown which has been manufactured by Nexans in Halden, Norway and supplied under a sub-contract to Subsea7. The key specifications of the umbilical are:

• 2 x HP hydraulic power lines (25Cr duplex stainless steel)
• 2 x LP hydraulic power lines (25Cr duplex stainless steel)
• 4 x twisted power quads, single phase 3 kV rated power supply, 16-sq mm cross-section area
• 3 x fiber optic bundles
• 1 x scale inhibitor (25Cr duplex stainless steel)
• 1 x wax inhibitor (25Cr duplex stainless steel)
• 1 x spare chemical injection (25Cr duplex stainless steel)
• 127 km length, weight = 2,800 metric tons (3,086 tons), sg = 1.8.

The umbilical has been designed with spare capacity to control an additional SPS manifold with up to seven wells situated up to 16 km (9.9 mi) from either the Laggan or Tormore manifold.

A key design challenge for the main 127-km section of the umbilical has been to keep the weight within the capacity of the installation vessel carousel i.e. 3,000 metric ton (3,307 tons). This is in order to avoid a subsea joint which would cause signal attenuation within the fiber optic cables and risk leakage of seawater into the umbilical, and a consequent loss of control capability. Due to the weight restriction the umbilical is not armored and will therefore have to be protected via trenching and rock dumping to minimize the risk of damage. The umbilical installation operation will be a critical part of the overall Subsea7 operation as the umbilical represents a key element in the control of the subsea wells leading to first production in 2014.

Flow assurance issues

The development of any deepwater gas field requires careful attention to flow assurance. For Laggan-Tormore the flow assurance challenges can be summarized as follows:

• Managing the risk of hydrate formation
• Managing the risk of waxing of the 18-in. flowlines
• Multiphase flow and the associated liquid handling issues during start-up, steady state and turndown phases
• High pressure drops in the 143-km flowlines leading to higher abandonment pressures and lower gas recovery. Provision has been made for late life installation of subsea compression.

As discussed above, one of the key design challenges for the umbilical has been to ensure that the total weight is kept within the capacity of the installation vessel carousel (3,000 metric ton) so that the umbilical can be installed as a single length without a subsea joint.

This has called for careful weight control during the design phase and resulted in the addition of a 2-in. duplex steel service line during the basic engineering phase which allows injection of additional chemical products outside of the umbilical.

Since the launch of basic engineering in early 2009, the Laggan-Tormore project has passed the half way mark toward first gas in mid-2014. Onshore the civil engineering works by Roadbridge to prepare the site for installation of the onshore gas plant by Petrofac are largely complete and the temporary accommodation camp is ready to accept the 800-plus construction workers.

The creation of an offshore gas evacuation route serving the Shetland region has acted as a catalyst for renewed exploration drilling with some notable recent discoveries by several operators. The future looks bright for hydrocarbon production, with estimates from DECC that 17% of the remaining UK oil and gas reserves are located in the region.

On plateau, the Laggan-Tormore production hub will contribute 500 MMcf/d of gas and the associated 30-in. Shetland Island Regional Gas Export (SIRGE) system has a planned capacity of 665 MMcf/d, creating a major export route for gas from the region to the mainland at St Fergus.