National Grid is breaking new ground with its Feeder 9 Humber Crossing Replacement Project. It will involve a full year of tunnelling under the River Humber and deliver the biggest pipeline river crossing anywhere in the world. Senior Project Manager Phil Croft tells us more about this important reinforcement of the gas network.
“National Grid is proud to innovate and this section of feeder will become the longest run of pipeline under a river anywhere in the world"
Phil Croft, Feeder 9 Replacement Senior Project Manager.
The tunnel itself will need to be at least 3.65m in diameter in order to achieve the 5km drive we need and meet all the latest safety requirements.
Source: Feeder 9 Replacement team.
The Feeder 9 pipeline is buried deep in the River Humber, a large tidal estuary on the east coast of northern England. It’s one of about 30 pipelines on the National Transmission System (NTS) and facilitates the transportation of gas imported from Europe to other parts of the UK.
Born in the 80s
The Feeder 9 crossing was built in 1984. Back then, the construction method was to dig a channel in the Humber estuary and lay the pipeline in it. Nowadays, environmental legislation prevents us from repeating the technique.
The currents of the estuary were then allowed to backfill the channel with silt, sand and other natural materials to protect the pipeline. But over time, tidal patterns in the estuary have changed so much that the materials that were protecting the pipeline have moved, almost uncovering it in places.
So for several years we’ve been doing deep-water and sonic surveys to monitor the river bed and condition of the pipeline. We’ve placed temporary protection measures over the pipeline in the form of frond mattresses, which have plastic weeds attached to them. They attract gravel and silt back to that part of the pipeline and bring some natural protection.
But laying them isn’t as easy as it sounds. There’s zero visibility at the bottom of the Humber and the enormous tidal currents mean divers can only work for about an hour at a time in the ‘slack’ tide. It was never going to be a long-term fix.
Back in 2012, the need surfaced to put a permanent solution in place – and that’s where the Feeder 9 Humber Crossing Replacement Project was born.
The project got under way and we looked at various options to replace the pipeline. The most viable – and the one we’re going with – is to tunnel under the estuary, almost at the same point as the existing crossing, and then insert 5km of new pipeline into the tunnel. From an environmental and cost perspective, it is the optimum solution.
National Grid is proud to innovate and this section of feeder will become the longest run of pipeline under a river anywhere in the world.
The pipeline itself needs to operate at high pressure – 70 bar – which is the standard for the NTS. So we’ve worked out that we need a pipe that’s 1.05m in diameter. The tunnel itself will need to be at least 3.65m in diameter in order to achieve the 5km drive we need and meet all the latest safety requirements.
Before the work can start, we need planning approval – called DCO (Development Consent Order) – through a process with the Planning Inspectorate and ultimately approval from the Secretary of State. The process is due to finish in September 2016 and once we’ve received the green light, we’ll set in motion our exciting construction plans.
Our first task will be to establish a construction site of around 34 hectares at the south end of the Humber. This is where most of the activity will take place.
Next, we’ll sink a deep shaft into the ground, from where we’ll launch a large tunnel boring machine (TBM). We’ll also dig out a similar pit at the north end to receive it.
The TBM will be lowered in and begin tunnelling in late summer 2017. It will take a year to excavate the full length of the tunnel.
It’s a complicated picture working underground. As the TBM pushes into the ground, the slurry it clears needs to be pumped out. So you have the TBM driving forward, then behind it are electrical cables feeding power to the machine, pipes pumping out slurry and a locomotive system that brings in sections of the tunnel as you progress. A year later, you come out the other end.
During the dig, we’ll also be building the pipeline on the surface. We’ll need to build 5km of pipeline, in seven strings of approximately 750m, and insert it into the tunnel. We’re still fine-tuning the best technique to do this.
The end date to decommission the current pipeline is September 2019. The programme for building the tunnel and laying the new pipeline stretches out until May 2019, our target date for getting the new pipe commissioned. We’ll continue to survey the existing pipe and check the mattresses are still doing their job right up until then.
Sustainability and savings
One final thing I’d like to mention is how an early piece of work has already saved £8m on the potential project cost and cut our carbon impact considerably. With a tunnel of 3.65m diameter and a pipe of 1.05m diameter, there’s a huge void to fill so that the pipeline stays protected through its life.
The traditional way to do this is to fill it with grout, but we’d need tonnes and tonnes of it. This would not only be expensive, but would use a huge amount of carbon both to produce and deliver.
With sustainability at the heart of our project management processes, we thought outside the box. We took the decision, based on learning from other projects, to flood the tunnel with sea water instead.
You may be thinking that sea water would corrode the pipe, but we’ll attach things called sacrificial anodes to the pipeline, which remove most of the oxygen in the sea water and create a safe environment for the pipeline and its ongoing protection system.
It’s my job as Senior Project Manager to enable decisions like this, spending efficiently at every stage, choosing the right options and ensuring we build the project correctly. It’s exciting to lead such an innovative piece of work. By managing the risks in our network, we provide a safe, reliable and sustainable gas infrastructure for all our customers in the UK.