UK National Grid will award £150 million construction of the new 5km undercrossing of the Humber Estuary next week. TunnelTalk understands that the successful bidder is Skanska, although the project owner would not confirm this ahead of a planned official announcement on Monday (April 11).

Alignment of the 5.03km Humber Estuary tunnel

Project scope will include TBM excavation of a 5.03km-long x 3.63 (minimum) – 4m (maximum) i.d. segmentally lined tunnel between above-ground gas installations at Paull on the east side of the Humber Estuary, and Goxhill on the west side. The TBM will be launched and serviced from a launch pit to be constructed on the Goxhill site, which will also accommodate settlement and slurry tanks, facilities for pipe-stringing, and storage for excavated material.

A 30m deep TBM reception shaft will be excavated on the Paull side, where a further 400m of pipeline will be laid onshore to connect the tunnel to the National Grid gas facility at that location. The tunnel is designed by the owner’s design consultant, Capita, to sit 7m below the estuary, at a maximum depth of 35m.

Once the tunnel is completed, a 1.22m diameter steel gas transmission pipeline will be inserted, before then being flooded. The new infrastructure, once completed and connected, will replace the existing Feeder 9 pipeline that was laid using the dredging method in 1984. Condition surveys carried out by National Grid in 2012 revealed that this critical pipeline has become exposed at the crown in four locations as a result of strong tidal flows.

A National Grid spokeswoman told TunnelTalk: “The project is still subject to planning consent, which we hope to have in September this year (2016). No tunnelling work will start until 2017, assuming we receive the planning permission.” A construction period of 12 months is specified for the tunnelling work, and 35 months for full completion of the project.

Detailed design and front end engineering for a critical Humber Estuary gas pipeline replacement project in the UK is awarded to Capita. Following a consultation process and examination of five possible 1km wide route corridors, project owner National Grid specifies a 4.4km x 3-3.5m diameter segmentally lined TBM-bored tunnel crossing in preference to dredging and horizontal drilling construction techniques.

Fig 1. Preferred corridor for 4.4km segmentally lined tunnel

The link in question is identified by National Grid as the single most important one on the UK's National Transmission System, carrying between 70-100 million m3 per day of imported and stored gas from the Easington facility - more than any other single connection on the entire grid. If the connection was to be taken out of service millions of homes in the south of England would be affected and domestic supply would have to be curtailed 'across a significant proportion of the year', according to National Grid.

Design of the replacement pipeline crossing will incorporate construction of launch and reception shafts at least 30m deep on either side of the estuary, as well as options for onshore excavation methods for a further 1.6km of pipeline to complete the link between above-ground installations near Paull, near Kingston-upon-Hull in Yorkshire on the north bank of the estuary, and near Goxhill on the south side (Fig 1). The pipeline diameter is 1,050mm. Capita's remit includes identifying a more exact route, depending on geological and environmental considerations, within the preferred 1km wide corridor. Preliminary investigations reveal a geology composed mainly of clay, silt and sand.

Critical tunnel required to cross the Humber Estuary

The planned tunnel and new pipeline will replace an existing pipeline known as Feeder 9 which was laid close to the proposed route of the tunnel in 1984 in a trench in the boulder clay excavated by a cutter dredging vessel. The trench was allowed to backfill naturally by the tidal flow changes of the river, but the riverbed in the Humber Estuary is extremely changeable and strong tidal currents of up to 4 knots and seabed erosion since the 1990s have forced remedial engineering measures to prevent continued exposure of the pipeline.

The 2010 annual condition survey revealed four sections (15m, 21m, 15m and 41m) of pipeline to be exposed at the crown, and National Grid has had to cover these with gravel filled bags fitted with floating fronds to mimic seaweed and attract silt. The remedial measures, completed in December 2011, have solved the problem, but are considered short term (10 years).

A report drafted by National Grid in 2012 concluded that the "need to pursue a replacement pipeline is essential. Solely waiting to see what happens with the completed remedial work is not acceptable due to the national importance of this pipeline for the gas supply industry."

Planning permissions are yet to be granted, but National Grid expects the contractor procurement process to begin in 2016 for a construction start in 2017 and completion by 2021.