HPT-JV (Porr, Skanska, A.Hak): NG Feeder 9 - Humber Gas Pipeline Replacement Tunnel

National Grid’s Feeder 9 pipeline transmits 23% of the UK’s natural gas. The section crossing the environmentally sensitive Humber estuary is near the end of its life. A JV of Skanska, Porr and A-Hak, with Atkins as tunnel designer, were commissioned to replace the asset for NG. Traditional pipeline methods don’t provide the asset protection required, so a 5km long, 4.1m diameter precast concrete segmental tunnel was driven from a launching site near the village of Goxhill in Lincolnshire, to the reception shaft at Paull in Yorkshire. Both sites are rural, providing several logistical challenges.

The ‘fit and forget' nature of this tunnel led to stringent performance requirements from the client; including the preclusion of any inspection and maintenance post pipe insertion and a novel ‘flooded tunnel’ pipe insertion methodology, proposed by the JV at tender stage. Specific focus was given to the design at the start and end of the tunnel, where minimal overburden and low stiffness ground presented particular challenges. The project experienced some programme creep due to the challenging ground conditions experienced across the alignment but was ultimately successful with delivery of a high-quality tunnel. The JV employed tunneling practices that ensured excellence in delivery, evidenced by the high build quality for the tunnel and an exemplary health and safety record. Some specific measures employed by the JV include: • Flooding the tunnel post construction to allow the replacement Feeder 9 pipeline to be floated into the tunnel, significantly reducing the amount of in tunnel works required for installation and realising substantial programme benefits; • Use of a 3D virtual reality model of the tunnel and shafts, which was used for planning the works, engagement with stakeholders and health & safety reviews; • Modification of the reference design by raising the alignment and introducing a wedge-shaped launch pit allowed for a reduction of lifting operations by 95%. The use of MSVs (instead of on rail locomotives) removed some significant constraints on the route and gradient of the tunnel; and • A risk-based ground investigation programme that sought to minimise investigative works required in the difficult terrain estuary and intertidal zone sections of the alignment. The project utilised talent and expertise from around the world; with JV parent companies from the UK, Germany and Holland; peer reviews from projects in Australia and America; and technical specialists from various European countries. The team is especially proud of the female engineers that held senior roles, including the Project Director, Temporary Works Designer, Environmental Manager and TBM Shift Engineers. Engagement with the local community was important for the project team. The project was delivered in a low-income rural region. The JV organized open days, a TBM naming competition for schools and frequent donations to local charities through a safety observation scheme. The JV went to great lengths to upskill the workforce, many of whom were from the local area. This included apprenticeships, upskilling of labourers to plant operators and specialised training for activities like compressed air works. The project will leave a legacy of construction knowledge and expertise in the North East. Measures employed by the JV to mitigate potential environmental impacts included: • Use of a 5D+ BIM model, allowing carbon data to be assessed across the project; • Increased efficiency in the management of the tunnel arisings lorry fleet through GIS modelling; and • Reuse of excavation arisings on site for landscaping and shaft filling. Spoil removed from site was used for quarry reclamation in the local area. Skanska used the project as a carbon reduction case study for PAS 2080 certification, becoming the first UK Contractor to secure this accreditation.

This award seeks to recognise excellence in delivery of a tunnelling project or tunnelling-related element of a project up to the value of $500M that was completed between July 2019 and July 2020 Judges will be looking to reward the project that best meets or exceeds the design brief and/or client expectation but that also have sought to advance industry best practice and have worked in a truly collaborative way.