The energy and sewerage sectors draw widely upon the equipment, experience and capabilities of the tunnelling industry to create diverse projects across the utility sector.
Energy support projects include power cable tunnels that run far and deep, and call for multiple stages of construction and years to develop; others, instead, might call for single, shorter tunnels to house key natural gas pipelines at challenging points of the network.
Among the large power-cable tunnel projects underway in the UK are contractor Hochtief-Murphy JV’s preparations to start main boring on Phase 2 of the National Grid’s (NG) major infrastructure expansion in London. Hochtief is working with Implenia on another cable tunnel in Sweden, where excavations are advancing on the latest of Svenska Kraftnat’s (SVK) growing electricity grid in Stockholm.
Recent UK tunnelling challenges from the gas sector included the new tunnel to replace a strategic natural gas pipeline at risk in the Humber Estuary, UK. The task for National Grid called first for Skanska to execute a TBM drive and then a world-beating pipeline jack.
More widely, though, the gas industry is looking at if, and how, its network might be affected when looking to introduce hydrogen supplies nationally to help address climate change concerns. According to a recent study, involving Costain, the existing network can be used, adapted and enlarged to carry a blended mix with hydrogen, and this should allow for ongoing work for the tunnelling sector.
In serving the sewerage sector, tunnelling often faces different challenges in scale and nature with smaller pipelines running shallow in limited stretches through complex urban settings. Many methods of excavation or tunnel solutions come to be called upon, as Barhale has used recently, to negotiate those multiple barriers.
London Power Cables
Preparations are underway for major tunnelling to start on another large utility tunnel in London. Main tunnelling on the second phase of National Grid’s power tunnel project is due to start in March-April 2021 with a TBM launch, and three more TBMs to follow on from around mid-year, T&T has been told.
London Power Tunnels, Phase 2, involves construction of a total of 32.5km of new cable tunnel running across the south of the capital, from Wimbledon in the west to Crayford in the east. Contractor on the project is a JV of Hochtief and Murphy.
West End Tunnelling
Tunnel boring on the project is to start in the west end of the cable tunnel, where the first drive is to be performed with a refurbished Lovat TBM. The shield is to bore 5.7km in open mode through London Clay, constructing a 3m ID expanded precast concrete tunnel lining. The TBM will be launched at the Kings Avenue shaft (12.5m ID, 34m deep to invert level) to drive to the Wimbledon shaft (15m ID, 34m deep).
The remainder of the Phase 2 cable tunnels are to be constructed by three new Herrenknecht EPBMs, with one of the shields performing two drives. The TBMs are to operate in closed mode, installing 3m ID universal precast concrete linings.
Central Section Tunnelling
Two of the EPBMs will be launched from New Cross, where two shafts are being constructed (15m ID, 35m deep). They will bore in opposite directions: one TBM is to drive west, boring 6.2km through chalk, Thanet Sands, Lambeth Group and London Clay to reach the Kings Avenue shaft; the other TBM is to drive east to Eltham shaft (12.5m ID, 43m deep), constructing an 11kmlong section of the cable tunnel mostly through the same geology, approximately half in chalk. Most variability in the geology is in the Kidbrooke area, where a shaft is also to be constructed (10.5m ID, 20m deep).
East End Tunnelling
The third of the Herrenknecht EPBMs will perform two separate drives in the east end of the cable tunnel, being launched on both occasions from the Hurst shaft (15m ID, 44m deep). The shield will drive 6.7km west mostly through Thanet Sands to Eltham shaft, and 2.5km east in chalk to Crayford shaft (9m ID, 16m deep), respectively.
The construction contract was awarded in late 2019. Following the UK-wide Covid lockdown in late March 2020, National Grid stated that because the project constitutes critical national infrastructure it was able to quickly perform a risk assessment and then implement new safety and worker distancing arrangements to allow early construction activities to resume in April 2020.
Stockholm Power Cables
In Stockholm, TBM boring has been underway for some months on the Anneberg-Skanstull project, where SVK is adding a further cable tunnel to the electricity grid serving Stockholm.
Contractor Hochtief-Implenia JV is using a 5.03m-diameter Herrenknecht main beam gripper to bore the 13.4km-long tunnel, driving at depths of 50m–100m below the ground surface. The project is the first time grid company SVK has had a cable tunnel constructed by TBM.
Geology along the route comprises greywacke, granite and gneiss. The route is hilly along parts of the alignment, from Anneberg in the north end of Stockholm toward Skanstull, near the centre of the capital, crossing sea inlets on the way where the tunnel is at least 30m below the sea bed. Located in an archipelago, the coastal city is known by many locals as the ‘Venice of the North.’ The water table is generally close to the surface and so the deep new tunnel is entirely below groundwater level. The city has environmental laws to limit changes to groundwater levels and the ingress limits vary along the tunnel route. If needed during the lifetime of the tunnel, groundwater replenishment will be undertaken.
Given the hydrogeological conditions, pre-grouting work was planned from the outset to limit seepage. The TBM was fitted out with grouting equipment and would stop every 15m for the task. No continuous lining was planned, only spot- or pattern-bolting with shotcrete, as required.
TBM excavation began in February 2020, slightly ahead of schedule. Driving south from the Anneberg substation, the hard rock shield has advanced approximately 1.4km, SVK project manager Rolf Aarflot tells T&T. He says that rock quality ‘is according to expectations’ and ‘weakness zones are monitored to adapt the works and grouting accordingly,’ but more pregrouting has been required than expected, and in more spots, along the advance so far.
Tunnelling on the project also has a number of further challenges, such as the ongoing logistical complications arising from noise restrictions. This limits the number of hours the TBM can operate, and consequently impacts pre-grouting activities as well as post grouting to meet leakage limits.
“These challenges are carefully monitored and handled to ensure high quality,” Aarflot adds. In relation to this, SVK monitors groundwater levels along the route before, during and after the TBM has passed.
Additionally, and like so many construction projects internationally, the Anneberg-Skanstull project is affected on an ongoing basis by the extra safety management needs of personnel, as well as the social distancing requirements of the pandemic, which had to be implemented soon after the start of tunnelling. A further challenge, early on, required some maintenance work on the TBM.
“Covid has been a challenge for us all and even more so for the members of staff working in the tunnel,” says Aarflot. “The work has been strictly regulated and monitored by us and our contractor to avoid spreading the virus. The work in the tunnel has been affected but never completely shut down.”
The combination of challenges has meant that overall, tunnelling progress has been slower than expected, Aarflot tells T&T, but he adds there is a margin for unexpected events.
Anneberg-Skanstull tunnel will have six shafts along the route for future use to perform maintenance as well as provide ventilation to manage heat from electrical cables, to be installed after the tunnel is completed. The first shaft to be excavated will be at Stadsgårdskajen, in the south section of the route, and preparations are underway. Four shafts are to be constructed by raise boring (4m diameter) and two by conventional sinking.
Gas and Hydrogen Futures
There are also numerous tunnelling opportunities in the natural gas component of the energy sector, requiring support to new and renewal pipeline projects of different sizes. Sometimes the challenge is greater, such as the recently completed Humber Estuary pipeline replacement project, near Hull, England which called for a record pipeline length to be inserted in a new TBM-bored tunnel.
Faced with risks brought by the shifting river bed exposing the old pipeline, and for which temporary coverings were placed in 2010 to protect the strategic link in the national gas network, National Grid opted for a long-term solution, safe from the tidal waters. A JV of contractors Skanska and PORR with Dutch pipeline and distribution system specialist A Hak was awarded in 2016 the £100m (US$140m) design-and-build contract for the new pipeline-in-a-tunnel.
A 4.34m-diameter Herrenknecht slurry TBM bored the almost 5km long, 3.65m ID tunnel which has a 120-year design life. Subcontractor Joseph Gallagher worked on the TBM launch area civil works. The Mixshield 3650AH completed the tunnel drive in late 2019, constructing a concrete segmentally-lined tunnel below cover varying from 5m-33m. The route mostly passed through chalk and also some glacial deposits.
The new 6,500t pipeline was hydraulically jacked in sections using two Herrenknecht pipe thrusters, the tunnel having been flooded in preparation for the push-through. The 5.4km long, 1,050mm-diameter pipeline has a 40-year design life and is globally, the longest, single-string gas pipeline threaded hydraulically through a tunnel.
In another project in New Zealand, Herrenknecht achieved a further distance record for its Direct Pipe bore-and-push technology: McConnell Dowell pushed a 48” (1.22m) pipeline a distance of 2,120m in Hauraki Gulf – the contractor beating its own record by 190m achieved in 2018 on a Watercare sewerage project near Auckland.
In the UK, the new Humber gas pipeline project was completed and commissioned at the end of 2020 and is prominent among recent tunnelling work for the gas network.
Over the next couple of decades, due to climate change concerns, the energy industry will be looking more to decarbonise its fuels, such as introducing hydrogen nationally.
While it would mean a switchover from natural gas, that may not call for an entirely new and separate pipeline distribution network with associated construction activity, according to a recent study that examined the possibilities of using blended gases in a transition period.
The feasibility study was performed by National Grid, Northern Gas Networks, SGN, Wales & West Utilities and Cadent, with contractor Costain as engineering consultant, drawing upon another, non-tunnelling part of its portfolio – its significant experience in industrial processing in petrochemicals, refineries and ammonia production.
The Energy Network Association (ENA) has identified deblending as a technology to facilitate the UK’s move to a decarbonised gas network. According to the feasibility study, hydrogen could be conveyed in a flowing, blended mix with natural gas through the existing high-pressure network.
Hydrogen would then be de-blended at various tap-off points throughout the network as needed to match demand. As such, the existing network would be sufficient to help use blending to support the gradual switchover from natural gas to hydrogen.
Published in late 2020, the study’s report is entitled ‘Hydrogen Deblending in the GB Network’ (NIA_NGGTo156).
The research was funded under the Network Innovation Allowance (NIA) through which energy operators explore new ways to benefit network customers. The technical and economic peer reviews were performed by the University of Edinburgh and Imperial College London respectively.
The feasibility study states that gas separation technologies are well established but they have not been applied to bulk gas transportation in networks. It recommends more technical research, including construction of a demonstration plant, to enable wide-scale and rapid roll-out of using hydrogen and the switchover.
While hydrogen has less energy density per unit volume, that would not require larger pipelines: construction would depend on design choices for gas network configurations, and operations and demand in different areas, all the while as the grid is enhanced in phases and maintained. Overall, the gradual switchover to hydrogen under this scenario should not unduly impact, or benefit, the volume of construction activity – including tunnelling – on the gas networks of the UK.
While the construction industry should continue to anticipate the support needed for the gas network during the introduction of hydrogen, there will also be other infrastructure needs to handle CO2, arising as a by-product of hydrogen production using natural gas but also from other major fossil fuel users such as power plants. It is expected that CO2 pipelines would likely collect and transport the gas for disposal offshore, where it would be injected into depleted gas fields. However, it is not clear what construction and onshore tunnelling opportunities would materialise from this further side of gas network needs.
