Earthworks at Shetland Gas Plant gains CEEQUAL Excellent

The development, known as the Laggan-Tormore development, includes an offshore production system west of Shetland, flowlines to Shetland, a gas processing facility on Shetland, and a gas export pipeline to the east of Shetland tying into the existing Frigg UK Agreement (FUKA) pipeline. The Shetland Gas Processing Plant (GPP) is located adjacent to the existing BP-operated onshore Sullom Voe Terminal. This project deals with the initial earthworks for the construction of the Shetland Gas Plant.

The participation in a CEEQUAL award was not a contract requirement; rather Roadbridge proposed to Total that the project warranted participation in the award scheme. The total civil engineering contract value for the project was £50.6 million and was completed between April 2010 and August 2011.

This project deals with the initial earthworks for the construction of the Shetland Gas Plant. The works comprised of the following activities, as well as the installation of services and drainage:

Construction of a 2.4 km access road
The initial phase of works consisted of the construction of a 2.4km long and 8m wide road from the public road access to the footprint of the site. This includes 2m wide service corridors on each side of the road carrying power, water and telecommunications to the site. These works were carried out in two halves to allow continuous access to the site footprint.

Construction of the peat stores
Initial construction saw the excavation of the stores to a depth of roughly 23m and 35m. The excavated material was crushed and screened for use as fill material. Upon completion of excavating the stores, drainage was installed and the cells were lined. Whilst these works were underway the construction of the earth retaining walls commenced. The construction of the earth retaining walls included the placement of 7000 concrete panels.

The peat stripping required the removal of over 650,000m³ of peat from the footprint of the site and the placement of the peat into purpose built stores on-site. The primary peat store held 450,000m3 and the remaining in the second store. The peat was managed in accordance with the requirements of the facilities associated waste licence.

Terrace construction
The works on site saw the abstraction of 627,340m³ of stone as cut, including the peat stores, and the placement of this stone as fill for the terraces and peat store walls. The excavated rock was crushed and screened for grading prior to placement.

There were certain areas that were key to the overall environmental performance of the site and, by association, were significant in the achievement of the CEEQUAL award, these are summarised below.

Water resources and the water environment
An earthworks project through the winter months in Shetland posed some unique environmental issues, especially for surface water management. An extensive surface water management programme was implemented on site. Roadbridge use a tiered system of controls for surface water management:

• Tier 1: Mechanisms for reducing the volume of water to be managed.
  • Interceptor drains to draw water away from the site

• Tier 2: Mechanisms for reducing the volume of silt available for transfer to surface waters.
  • Weathering off of stockpiles, covering stockpiled material, silt fencing around stockpiled material

• Tier 3: Mechanisms for reducing the sediment load of surface waters (Physical)
  • Sandbags, oyster bags or weirs to create retention within the system and allow settling
  • Oyster bags to create filter beds for water to run through
  • Settlement ponds to take allow material settlement
  • Filtration systems to physical block the transfer of particulates

• Tier 4: Mechanisms for reducing the sediment load of surface waters (Chemical)
  • Addition of a flocculant or coagulant to force the particulate material to settle.

All 4 tiers were utilised during the works. The regulatory authority visited the site weekly and identified no concerns or additional requirements they might have as to the controls used on-site.

Land use
The location was a greenfield site leading to no issues as regards to previous land usage or contaminated land. The primary area of concern was the protection of the surrounding habitat. The dominant vegetation, in the SGP footprint, was blanket bog and marshy grassland. Whilst the site is not designated, the habitat is a sensitive one. The primary control was the reduced footprint of the site and a restriction on plant/site personnel movement. These measures reduced the overall footprint affected by the works and ensured that there was no impact outside of the construction fence line.

The historic environment
There was a listed structure on-site and controls were detailed in the planning consent for the site. The approved mitigation measures were implemented on-site. No impact to the structure was identified during the 3 rd party monthly inspections.

Shetland has a rich past and as such the size of the site made it probable that archaeological features would be unearthed during the works. There was an archaeological watching brief in place throughout excavation works and a number of features were discovered. These were recorded and removed, under licence. Total propose to rebuild the broch.

Ecology and biodiversity
There was a full time ecological watching brief in place during works; this ensured a reduced impact on the local protected faunal communities. Protected species included otters and a large number of nesting birds. Where identified, exclusion zones were erected and maintained as required.

Local community and other stakeholders
Total E & P UK brought in community communications during all phases of the works and where possible incorporated the comments and suggestions received during community meetings. In addition, regular meetings and co-ordinated site visits ensured the on-going goodwill of the local community.