Gasholders, or gasometers, were built from the late 19th century and through the housing boom in the inter-war period. They held the ‘town gas’ manufactured from coal and helped even out daily variations in gas demand. Latterly, they were used to compensate for seasonal variations. Now that most UK gas comes direct from under the North Sea and via connectors, having a local storage capacity is no longer necessary. In addition, residential gas mains replacement has also ensured improved capability of supply to meet current demand.
Just because they have become redundant does not mean that they were falling apart; many have far exceeded their operational lifetime. For example, the listed Hendon, Sunderland gasholder, built in 1888, was still in operation in 1998, and only purged of gas in February 2019. “The engineering construction work and design that went into these over the years was nothing short of remarkable, given when they were constructed,” marvels Mark Johnson, Northern Gas Networks’ senior projects manager, capital projects. NGN is just one of a number of gas distribution networks, and it is part-way through a 13-year project to take all of its gasholders down.
A key issue is that gas distribution networks are responsible for the maintenance and upkeep of assets like gasholders. That means that they have the not-inconsiderable task of painting the steel and iron frames that are 55-60m in diameter and 50m high in most cases, to protect them from corrosion. They also have to repair mechanical moving parts in the tank, pumps and other balance of plant items. Since 2013, Johnson has been involved in taking down 23; the utility now has another 22 planned to go in the next five years. With no delays, one can be taken down in seven months, two in nine months and three in about a year.
Safe and efficient decommissioning and dismantlement requires an intimate understanding of the history and structure of each one, explains Johnson, a 30-year veteran of the gas industry.
The oldest aboveground gasholders, built in Victorian times, are marked by a complete cast-iron lattice ring holding the gas chambers, which rise vertically as they fill with the gas, which is lighter than air. Newer ones built in mid-last century, rotate as they rise, guided by spiralling steelwork sections. Most below ground gasholders consist of a brick-built bund, or tank, bottomed off with concrete. The bottoms of these feature a dumpling-shaped structure sat in the middle of the floor that supports the central roof column, which rises out of the ground.
Whether underground or aboveground, gasholders operate in a similar way. They consist of a group - perhaps three to five – open-ended vessels nested inside each other which sit within a deep pool of water (perhaps 10m deep), either within a foundation (belowground) or resting on the ground like a garden swimming pool (aboveground). Gas is pumped into the gasholder from a fill and return pipe installed in the innermost shell. When the innermost tank reaches the top of its travel, it clips on to its neighbour via a hook arrangement, all the way out to the external radius. Whatever the height of the gasholder, the bottom of the outermost shell is submerged in the pool.
The only way that gas could escape is in between one shell and another. But the gasholder is so designed that a contoured rim of the top of each shell (after the first) picks up a channel of water deep enough that its pressure head exceeds the pressure of the gas in the holder, so forms an effective seal to keeps the methane in.
A jet booster house or fan booster house pumps the gas at relatively low pressure – 26-30mBar – from the upstream supply network into the gasholder the gasholder via a fill and return pipe fitted with a U-bend like a sink. If the level of water in the gasholder rises, it will go down the fill and return pipe, and must be cleared via a siphon. (Other gasholders are fitted with overflows that drain the excess water directly into a local drain, thanks to a discharge licence.) Another set of pipes removes the gas and sends it on to the downstream network.
In considering decommissioning, the first issue is that they are full of water. So the gas distribution network has to work closely with the local water authority to agree a discharge consent for the volume of water (with stipulated levels of pollutants), as well as an Environment Agency deployment licence. On its way down the drain, the water passes through a 4-5-screen filtration and dewatering system. Weekly samples are taken to make sure that the water quality meets consent requirements. Issues include deviation from neutral pH, the level of suspended solids, chemical oxygen demand, and the presence of oil, grease or saturated methane, which have to be removed.
For recent projects, Northern Gas Networks has been using a BakerCorp skid-based filtration system using 4-in diameter flexible hose. The water is first sent into a holding tank to settle out suspended solids to settle, followed by a filter bag and a methane stripper.
Terms of the discharge consent includes stopping water releases in extreme weather. This helps ensure that the municipality’s wastewater treatment system won’t be overwhelmed. The filtration systems feature a flow meter or telemetry to automatically cut off the drainage pumps in case of torrential rains or blockages, for example.
On an average gasholder project, nearly a month - 20-25 days - is spent pumping 24/7 to drain the water down to the residual water line, which in an aboveground tank is a metre from ground level (on a two-foot thick reinforced concrete pad).
Next, the team cuts its way through the shells to the centre. To guard against the risk of accumulated pockets of methane remaining behind, it must use only cold cutting processes – shears – to make a hole, rather than an oxyacetylene torch, for example.
Sitting on the remaining water is a layer of oil, accumulated drip by drip over decades of maintenance of the metal-to-metal moving parts of the gasholder. This oily water is pumped into tankers and sent to specialised processors that recover the oil from the water using centrifuges. Then, the rest of the water is pumped through filtration and down the drain.
Beneath that is a layer of what is known as gasholder sludge – organic materials such as leaves that fell between shells and settled at the bottom of the tank. It is tested, placed in dewatering boxes dosed with a polymer flocculant that expels water from the material. The water is filtered and treated as normal. What is left, dry filter cake, is classified as hazardous waste, so is disposed of.
Once the tank is cleared out, physical demolition is fairly straightforward: a 490 excavator fitted with demolition shears. First, it creates a bigger opening to get in, drives to the centre, and then takes off the interior roof. Then it starts the process of cutting the inner tank, working clockwise, until the internal lifts have all been cut up. Then, all that is left is the external tank wall and the floor. These outer walls, built thicker to resist the force of the water, may be too much for the shears, particularly if riveted. Since the site is now open to the elements, and there is no risk of methane, workers can use hot cutting torches to systematically deconstruct these sections.
All of the scrap metal is processed on site, and taken away in 25t bulker wagons. In fact, there is so much metal that one of the contractual terms with the demolition company is an agreed rate per tonne of the scrap offered to the operator.
Those contracts are let through framework agreements, and demolition contractors’ bids are assessed systematically. “We don’t go for the cheapest; we go for the people that we think will do the best and safest job,” states Mark Johnson. Other contracts are let for the dewatering operation, oil and sludge removal and on-site welfare facilities.
During the dismantling of an underground, column-guided gas holder at Minton Lane, North Shields, Tyne and Wear, Mark Johnson’s team had a surprise during the dewatering phase. In these gasholders, the roof framework would support the roof using a kind of umbrella structure: a sparse metal frame. A desk-based survey had found that the gasholder had been bombed in WWII; this was not uncommon, as the gasholders were easy targets for German bombers. Their destruction was also a strategic priority to the Third Reich, who hoped to weaken morale at home. So the team was curious to see whether the original roof had been taken away, or whether there were two rooves, one stacked on top of the other. What they found was that there was a double-roof situation, under which was a kerosene-impregnated oak umbrella arrangement. That posed two challenges: first, the sheer weight of it, and second, the risk that it might collapse during demolition, breaking up into the soup of oil and sludge at the bottom of the gasholder, and massively complicating clean-up. Instead, the group brought in two tower cranes; one held a man-basket with workers cutting through roof segments hooked to the second crane. “Once both roofs were off, it was clear that with Victorian engineering, there was no way that the support structure would have collapsed. It was so robust, it was breathtakingly unreal,” comments Johnson. Since then, two other Northern Gas Networks gasholder structures have also been found to have full or partial wooden umbrellas.
BOX: Dealing with ground contamination
Even once the main structure of a gasholder has been dismantled, the job of remediation is not necessarily over, particularly if chemicals have leached from the gasholder or surrounding historical town gas manufacturing structures.
According to Neil Whalley, environmental strategy manager at Northern Gas Networks, the potential ground contamination conditions on former gasworks and gasholder stations in the UK are well-established; it cites for example 2014’s ‘The History and Operation of Gasworks (Manufactured Gas Plants) in Britain’ by Russell Thomas, Parsons Brinckerhoff (now WSP) technical director (www.is.gd/fodeyu).
Says Whalley: “Ground contaminants typically comprise historical by-products of town gas manufacturing and purification process and include phenols, benzene, toluene, ethyl benzene, xylenes, polycyclic aromatic hydrocarbons, and ammonium, cyanide and sulphur-based compounds.”
To assess ground contamination, the team takes a ‘tiered approach’, starting with a desk-based literature review drawing on site archive information, published local environmental data regarding site setting and environmental sensitivity, review of historical mapping, plus a site walkover inspection. That research is analysed to develop a preliminary conceptual site model, upon which is developed a preliminary environmental risk assessment.
In cases where potentially significant contamination risks to environmental receptors have been identified, a quantitative site assessment is carried out. “Typically this involves an intrusive investigation of ground conditions on site, involving the collection of soil and water samples for laboratory analysis. The findings from such an investigation are subject to detailed interpretation, including comparison against published UK criteria for soil and water quality to enable refinement of the site environmental risk assessment, potentially including detailed quantitative environmental modelling,” he adds.
That work then leads in to remediation works as necessary to permanently reduce any significant contamination. That might include excavation or pumping, and installation of an engineered cap.
For example, in 2017-2019 Northern Gas Networks pumped out historical coal tar from the base of an infilled underground former gasholder tank at Redheugh Gas Holder Station, with Sweco UK and Geo2 Remediation (not pictured).
The team determined that removing dense non-aqueous phase liquid coal tar was a more sustainable and cost-effective means of clean-up than either bulk excavation or pumping and treating groundwater.
Four self-contained remediation systems were deployed, each comprising a 100mm diameter, 9m deep recovery well and remediation equipment. ¬The remediation systems were a bottom loading pneumatic pump that recovers DNAPL and contaminated water from the well into intermediate bulk containers stored within constructed bunded areas. In 17 months, the remediation system recovered approximately 5,150litres of DNAPL coal tar, according to an awards document published by Ground Engineering (the project won the magazine’s 2019 Sustainability Award).
As no site electricity was available, the team took the decision to power each pneumatic pump by a 100W photoelectric solar panel, via a battery and individual receiver compressor. The solution was said to have saved 55t of CO2equivalent, £3,600 in energy costs and more than £22,000 in equipment hire and fuel costs over the 17-month project compared to using petrol-powered pumps.