In Lincolnshire, engineers are boring a tunnel beneath the River Humber to accommodate a new gas pipeline.
Project: River Humber Gas Pipeline Replacement Project
Client: National Grid
Contract value: £100m
Region: Yorkshire & the Humber
Main contractor: HPT JV – a Skanska / Porr / A.Hak joint venture
Start date: August 2016
Completion date: March 2020
The north Lincolnshire coast and the Humber Estuary is no stranger to engineering brilliance, both natural and man-made.
The 2.2 km-long Humber Bridge linking north Lincolnshire to East Riding of Yorkshire was the world’s longest single-span suspension bridge when it opened in 1981, and the estuary’s coastline is dotted with major port workings at Hull, Immingham and Grimsby.
There are various areas of scientific and natural interest too, including the world-famous Spurn Head spit that reaches like a witch’s finger from the northern bank of the mouth of the Humber toward the south.
Even beneath the water surface the Humber bristles with technology. Cables and pipelines cross backwards and forwards across from the north and south bank, linking the East Riding of Yorkshire with Lincolnshire and the rest of eastern England.
Now engineers are gearing up for the next crossing.
In the shadow of the Humber Bridge, a team is working on a major 5 km-long, 4 m-diameter tunnel running from the village of Goxhill on the southern side of the estuary to Paull on the northern coast. It will accommodate a new main gas pipeline as part of the 7,660 km network that makes up the National Transmission System.
The ebb and flow of the tide through the Humber Estuary has eroded the riverbed that covers the existing pipeline, leaving parts of it at risk of exposure.
The NTS is owned and operated by National Grid, which is responsible for maintaining the network and balancing gas supply and demand across the country, with this scheme crucial to the UK’s uninterrupted flow of natural gas.
“We realised from the first procurement event that we didn’t want to manage the risk between all the different elements”
Phil Croft, National Grid
At Goxhill National Grid, senior project manager Phil Croft is overseeing the work being undertaken by Humber Pipeline Tunnel (HPT), a joint venture between Skanska, Austrian tunnelling specialist PORR and Dutch pipeline company A.Hak. The JV model was preferred from the start.
“It is a very well-balanced joint venture,” Mr Croft says. “They are all experts in their own areas and offer real expertise: Skanska for the ground and civil engineering, PORR for the tunnelling and A.Hak for the pipeline itself.
“We realised from the first procurement event that we didn’t want to manage the risk between all the different elements. This is the best way to deliver the scheme.”
“This is a huge deal for us. The pipeline carries around 20 per cent of the UK’s gas through it”
Phil Croft, National Grid
Replacing the existing gas pipeline was first mooted at National Grid back in 2012, but it wasn’t until late 2015 that the project was launched under a six-month procurement process.
The internal team at National Grid was busy working through the needs case for the project, with every option looked at and analysed. It looked at all the options for replacing the existing pipeline and even whether a new pipeline was needed at all.
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“This is a huge deal for us,” Mr Croft says. “The pipeline carries around 20 per cent of the UK’s gas through it. The last thing we want to do is jeopardise that supply if there is no need.
“We looked at all the feasible options – bringing the pipeline offshore around the estuary and a longer onshore pipeline around Goole – and this came out as the best. It has the smallest environmental impact and is the least costly.”
By April 2016, the £100m contract had been awarded to the HPT JV. It will drive the 5 km-long tunnel beneath the Humber estuary along a route that crosses the existing gas pipeline three times before tying back in to that pipeline at Goxhill and Paull.
The agreed design will see the team eventually install the pipeline through the tunnel as a single string, with the pipe sections welded together on site before being fed through the entire 5 km length. It will, according to the project team, be the world’s longest gas pipeline in a tunnel and inserted in a single string.
The new gas pipeline is set to be installed in a single string, fed through the eye of the 5 km tunnel from the site at Goxhill.
But before the pipeline gets anywhere near the tunnel, bore sections have to be welded together at a specially constructed laydown and work area to the rear of the site offices, an area labelled the ‘pipeline field’ by the project team.
This 800 x 60 m piece of land has been fully prepared with compacted sub-base to a depth of 750 mm so that the large sections of steel pipes can be unloaded and welded together before being fed through the tunnel.
A series of beams and track bogeys will support the pipes as they are laid out for connection. Individual pipes will be welded to form the eight sections of pipeline that measure between 600 and 650 m long. These longer sections will then be welded together to form the one pipeline string and pushed through the tunnel using a thruster rig and a series of powered and passive rollers.
The pipe sections are manufactured in Germany by specialist producer Europipe using 19 mm-thick steel. From here the pipes are shipped over to Leith in Scotland to have a 75 mm-thick protective concrete coating applied to them before being transported down to Goxhill and worked into the final pipeline string.
The pipeline will be floated through the flooded tunnel (see ‘tunnel field’ box) with the concrete coating and a protective nose cone set on the leading end of the string helping the pipeline reach neutral buoyancy. Some loose pea gravel aggregate may be introduced to the tunnel invert to help roll the nose cone and pipeline through during an operation that is expected to take five weeks.
“Most tunnels are driven slightly uphill whenever possible, but this tunnel is actually U-shaped,” Mr Croft says. “That means there will be a tendency for the pipeline to try and run away downhill as it is installed. The thruster rig and powered rollers will help act as a brake.”
Before then, though, the project team has had more than its fair share of preliminary works to get through.
As well as setting up the main site office and compound at Goxhill – no small matter such is the scale of the work here – and a smaller satellite office at Paull, the team has also had to develop a rigorous one-way access system to help keep the impact of construction traffic on the narrow country lanes to a minimum.
Numerous passing places have been installed alongside those lanes to enable local traffic and the huge pipe-carrying trucks and material delivery vehicles to co-exist safely on the roads.
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“We take our obligation to be a good neighbour very seriously,” Mr Croft says. “We are going to be here for the next three-and-a-half years, so we need to work with the local community to try and make sure we [do that] as closely and safely as we can. We want to keep the impact of the pipeline’s construction as low as possible.”
As well as the pipe laydown and assembly area, the team has labelled the ‘pipeline field’ (see box, above) and the tunnel launch area (the ‘tunnel field’ (see box, below)), a processing system has been installed for dealing with the more than 100,000 tonnes of inert arisings expected from the tunnelling.
Since the team started on site it has been busy delivering the launch box that will allow the TBM – named Mary after Yorkshire-educated Mary Fergusson, the first woman elected to be a Fellow of the Institution of Civil Engineers – to begin its journey from south to north.
This ‘Tunnel Field’ is essentially a 200 m-long secant and steel sheet pile walled box, 7 m wide at the tunnel head and narrowing to 5 m at the open end adjoining the Pipeline Field. Its concrete base is ramped upwards toward the open end.
This ramp improves access to the tunnel head and will ensure the pipeline is drawn through to the tunnel safely and accurately.
The 1.2 m-diameter secant piled walls are located at the tunnel headwall and have been installed to depths of 30 m before being dug out to 15 m. The sheet piles were installed to depths of 22 m and 11 m. A series of hydraulic props prevents the overturning of the piled walls and a reinforced concrete base-slab seals the box from the surrounding groundwater table, helping to reduce impact on the areas hydrology (see ‘Saline walls’ box).
The TBM is a slurry pressure balance machine and features a 4 m-diameter cutting head that will grind through the chalk using a bentonite slurry as a lubricant. The 160 m-long train will travel behind the cutting edge, placing the six, 225 mm-thick, 1.2 m-wide trapezoidal tunnel lining segments, before the annulus between the outer diameter of the lining and the chalk is grouted to prevent any water leakage.
Because the tunnel is set to be flooded on completion using cleaned fresh water, the team is installing two sacrificial corrosion protection systems, a primary linear anode system that will be set at 9 o’clock and 3 o’clock in the tunnel section and an impressed current system.
“We need to protect the pipeline. It is an asset that will remain insitu for the next 40 years. We want the steel to stay in perfect condition,” Mr Croft adds.
Here a series of sorting sifts and settlement tanks will process the stone and liquid-suspended solids that will be brought to the surface as the tunnel boring machine grinds through the Flamborough Chalk beds beneath the estuary, pressing water from the sludgy liquid arisings to produce chalk ‘cakes’ that are virtually dry.
For the time being, though, all efforts are focused on getting the TBM lined up and ready to take its first real push into a process that will see it beneath the Humber at depths of 35 m below low tide level.
There are many complications yet to be faced along the project delivery path, and although the Humber Gas Pipeline Replacement Project is in its infancy, it has all the makings of a career-defining project for those that spend time working on it.
The Flamborough Chalk bedrock the team is tunnelling through is actually a protected freshwater aquifer that is used to supply thousands of homes and businesses.
The flow of water within an aquifer is sensitive to pressure changes and when adjacent to a body of saline water, such as the Humber Estuary, it can make the aquifer susceptible to saline contamination.
A saline intrusion within an aquifer can take years to return to its original position. So, in a move to ensure the pipeline project does not interfere with the saline/fresh water aquifer balance, the team has introduced a system where water drawn out from production wells during the work is pumped back into the aquifer closer to the saline/freshwater interface.
These recharge wells artificially increase the level of the water table locally, creating a ‘wall’ against saline intrusion and protecting the aquifer.