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SNØHVIT LESSONS LEARNED Report on SUT London Evening Meeting 30 April 2009 by J Sanchez Monge Cranfield University MSc student
Diagram of the Snøhvit development Picture: StatoilHydro/Even Edland
There are many challenges of exploiting reservoirs with solutions, such as ultra-long tie-backs of subsea fields. One of the main challenges is the way in which we will control our production system over such long distances. Clearly, this topic attracted the attention of the subsea community attending the London evening meeting chaired by Phil Hawthorn of J P Kenny, as we all enjoyed the presentation of Stuart Holley. Stuart was VetcoGray’s lead software engineer on Snøhvit project from the execution through the installation and commissioning. During his presentation he went into detail on the challenges associated with the control systems. He focused on the technical and logistical achievements in executing the ultra-long tie-back, subsea-tobeach project. Snøhvit is the first offshore development in the Barents Sea. The fields that integrate the development are Snøhvit, Askeladd and Albatross. The subsea infrastructure stands on the seabed in water depths of 250–340m, and all the gas produced is sent to land through a 144km pipeline. The processing facilities are situated in Melkøya Island outside Hammerfest in northern Norway, where gas is converted into LNG for further commercialisation. Once the scene was set, he went into detail on the control system. The main umbilical length is 14km and interconnects the control room in Melkøya with a central distribution unit (CDU), from where all the services are distributed to the Snøhvit and Albatross fields and provisions are taken for a future 34km infield umbilical to Askeladd. Power and communication are provided through a 3KV, three-phase power line and fibre optics to the CDU, where using a power and communication distribution module (PCDM), a step-down in voltage to 660V and distribution to the different templates
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take place using infield umbilicals. We learnt that Snøhvit project has the record for the longest umbilical (144km) in operation and 220km in design. In his presentation, Stuart also covered the implementation of the backup intervention control system (BUICS) designed for use in the event of main umbilical failure or unavailability. It consists of a deck-mounted subsea control system that incorporates satellite communication with the control room. In case of any problem with the main umbilical, the system can be deployed to interconnect and supply hydraulic and electric power, as well as control communication using the satellite links. Main milestones and commissioning details were introduced by Stuart in the second part of his presentation. He indicated that since the award of the contract in 2002, it took five years to get the first gas in 2007, including two years of intensive commissioning. During commissioning, they experienced failures in
some high-voltage subsea connectors that were changed, but no further report of failure is known. At the end of his presentation Stuart described the main challenges that the project faced, which were retention of key personnel, new technologies, integration of plant and subsea control system, realistic simulation of ultra-long umbilical and the remote location of the project. The presentation ended by sharing the lessons learned from Snøhvit, including the importance of key personnel, extended factory acceptance test (EFAT) experience with commissioning team involved, close team work with operator, flexibility of subsea routing and the benefits of remote access to the control system. As expected, a number of questions were raised by the audience just before we all shared our thoughts with one or two glasses of wine and cheese.
Subsea Sensors for New and Retrofit Applications In what has become almost an annual event, a team of MSc students from Cranfield University’s MSc in Offshore and Ocean Technology option in subsea engineering, presented the findings of their group project. This year it concerned a detailed review of the full range of sensors and associated technologies that are available for use within subsea infrastructure.
Going forward, more complex subsea production systems, using separators and rotating equipment, are being planned. In order to achieve good reliability and availability, such systems will require far more complex instrumentation. These technologies are already proven for surface applications, but will require marinisation for subsea deployment.
Subsea production systems and their associated instrumentation are evolving significantly. Ten years ago, instrumentation was often limited to pressure and temperature sensors, with communications provided by ‘twisted pair’ copper cables. Today, a wider range of sensors is available, with communications provided by fibre-optic cables.
As part of its core technology programme, BG Group, under the guidance of Alex Hunt, its engineering technology manager, commissioned the student group to review the instrumentation available for replacement, retrofit and new subsea applications. Alex was chairman for the evening and, after welcoming the audience, gave a