For some 30 years now Horizontal Directional Drilling (HDD) technology has been earning a reputation as a solution to various, largely utility based, underground pipeline/casing installation projects. There have been numerous road, rail and waterway crossings, sea outfalls and the like installed using HDD, which has been used to limit disruption to transport services, minimise environmental impact and allow the hustle and bustle of every day life to continue largely uninterrupted.
Most of these works have been for the construction of water and gas pipelines and cable installations as well as, more recently, on-line and grade sewer construction. However, just now and then, the technology comes to the aid of some quite unusual situations, many of which appear to be environment related in one way or another. Scotland-based HDD specialist Longbore was involved in one such, ultimately award winning, project.
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As part of Wales’ Glyncastle Minewater Remediation Scheme, designed to address pollution problems associated with the old, and long closed, Glyncastle coal mine, Atkins Limited, acting as consultant engineers for the Coal Authority, invited Longbore to submit a design for a directionally drilled bore to enable the interception and capture of contaminated mine water in the Neath Valley in South Wales.
The former Glyncastle colliery had been mined since 1875. When it was abandoned in 1965, the mouth of the Tyn-y-Cwm adit was sealed up and, following a collapse in the adit in 1994, water levels within the mine rose causing an accumulating of dissolved iron of up to 60mg/l within the mine water. The contaminated mine water began to escape from the ground at a number of diffuse locations around the old adit, most notably from the collapsed crown hole above the adit in Resolven, a village located some 14 miles north of Port Talbot, South Wales. Emanations also occurred at an airshaft in the adjacent Clydach Valley, causing staining of the Clydach Brook and the River Neath with orange ochre. This situation also prevented salmon from spawning in these water courses.
Atkins proposed that, if the contaminated mine water could be captured, it would be possible to treat it via a series of settlement lagoons and reed beds before the flows were re-released back into the River Neath.
Drilled solution
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By GlobalDataThe diffuse nature of the mine water discharges can be likened to an overflowing bath. After careful consideration of the problem, the solution proposed by Longbore was to utilise horizontal directional drilling techniques to drill into the lowest point of the mine workings from below, effectively ‘pulling the plug out of the bath’ and allowing the mine water to exit the mine workings in a controllable manner. Doing this from a location below the level of the adit would allow the mine water to flow out under its own head of pressure. Conventional vertical wells drilled into the adit from above would require abstraction pumps and associated power supplies, which would bring with it ongoing maintenance and therefore cost implications.
To complicate matters, although the water levels had to be lowered to below the existing discharge points, if they dropped too far and air was allowed into the mine shafts, oxidation of the dissolved iron would take place within the mine and ferrous deposits would begin to accrete in the discharge pipelines and valves that were to be installed to capture and control the flows. Additionally, this situation could promote the presence of mine gases within the adit. A system to control the mine water flow would therefore have to be installed. However in order to accomplish this, two significant issues had to be overcome.
The first major problem would be drilling into a mine tunnel system containing a water level some 17m above the proposed drilling rig elevation. A head of pressure of over 170kPA had been estimated given the information available on mine water levels. If this water pressure was not contained during the drilling process, the entire mine would discharge with catastrophic results for the project and the local environment. Longbore’s solution was to cement a surface casing into the bedrock at the bore launch site, onto which was bolted a series of gate valves and rubber snubbing units designed to allow the various drilling and hole opening assemblies to be run in and out of the well while maintaining circulation and pressure control over the drilling fluids and the mine water once the pilot bore holed out.
Bore guidance was also critical to success of the project. Although the location of the adit beyond the collapse had been confirmed with a vertical bore, the width of the adit was unknown, so the target point for the drill had to be as close to the vertical bore as possible. As directional drilling is a three dimensional process, the bore also had to punch into the adit floor at the correct azimuth and inclination to allow sufficient pipe to be pushed into the adit to clear the anticipated sludge levels on the floor of the mine tunnel.
As the target point for the bore was 30m below ground level, a real time wireline magnetic guidance system was specified for the project in preference to a surface based locating system. No records were available relating to the abandonment of the mine and if steel rail tracks or pit props had been left in the mine, they would interfere with the quality of the magnetic survey. A downhole magnetic target was therefore mobilised to use as a back up guidance option, although in the event it was not required.
Completing the bore
Having had the design approved, Longbore was sub-contracted to complete the required drilling work by Interserve Project Services Limited, the Coal Authority’s framework contractor, on a fixed price lump sum contract.
Longbore decided to utilise its American Augers DD-6 HDD rig, and its associated equipment to complete the bore. A comprehensive Health, Safety & Environmental plan was established prior to work commencing although no operating accidents, near misses or environmental incidents were incurred during the course of the project.
The vertical Site Investigation bore was pumped to provide a water supply for the drilling operation, and to attempt to reduce the mine water to a manageable level.
Drilling commenced using a 170mm diameter steel, Milled Tooth tri-cone drill bit fitted to a downhole drilling motor. This assembly was attached to an adjustable bent housing set to achieve the required curve build rates and directional control.
Ground conditions at the site comprised Silesian Sandstones, locally known as ‘Ironstones’. The drilling operation achieved an average Rate of Progression, (ROP), of some 13m/hr to the initial casing setting depth of 43m. Tungsten Carbide Insert drilling assemblies were available on site in case the prevailing rock conditions proved to be harder than anticipated, although in the event they were not required. The pilot hole was then opened in two passes to first 305mm diameter and then to 510mm diameter.
During this stage of the drilling operation, prior to the installation of the pressure control device, dense drilling fluids were utilised to provide hydraulic control over the mine water in the event that contact between the mine and the bore was experienced. As a contingency, barite weighting agents were available on site to increase the density of the drilling fluid further if required.
The 355mm diameter SDR-17 surface casing pipe was electro-fused together, with a cement basket being mounted on the leading joint, which was then run into the borehole. A tremie pipe was then run into the annulus between the pipe and the bore and the surface casing was cemented into place and allowed to cure, so providing a pressure tight seal.
The pressure control device and diverter system was then attached to the wellhead and the 170mm diameter drilling assembly was used to drill out a cement shoe.
Drilling then re-commenced toward the adit floor and progressed to the target at an ROP of 6.4m/hr. Great care had to be taken to ensure accurate surveying. Ultimately the drill head entered the adit at a drilled distance of 71.7m, at an accuracy of 0.1m to the left of target and 0.5m long of the target. The pilot hole assembly was removed from the bore under pressure, and the hole opening assembly was used to open the pilot bore to 305mm diameter.
The 200mm diameter SDR-11 Butt Fusion Welded drainage duct was prefabricated in a single length and Longbore utilised its patented Pipe Pushing Technology to insert the duct into the enlarged bore, again controlling mine water egress using the snubbing unit. An inflatable packer was used to isolate the annulus between the drainage duct and the surface casing. Cement was then pumped into the void to provide a physical seal.
On completion of the first bore, the pipeline was valved and gauged and placed in operation to drain the mine water whilst the drilling rig was moved and a second back-up pipe installation was installed using the same methodology.
Success
The completed scheme has successfully enabled the contaminated mine waters to be captured, so eliminating emanations from the diffuse discharge points that had led to the original pollution problems. The new controlled discharge pipeline has enabled controlled flow to be transferred to a sustainable treatment system that is currently reducing the mine water iron content to <1 mg/l. In addition, the newly constructed wetlands have provided a biodiverse habitat for flora and fauna as well as a new local leisure amenity.
According to John Ritchie, managing director of Longbore: “To our knowledge, this is the first time well head pressure control equipment been used in conjunction with a Horizontal Directional Drilling project in the United Kingdom. It use has provided a potential solution to diffuse groundwater control and contaminated land remediation for the numerous abandoned mines in all parts of the country.”
In recognition of the innovation and success of the Glyncastle Minewater Remediation Scheme, Longbore were awarded a United Kingdom Society of Trenchless Technology (UKSTT) New Installation – Small Project Award and an International Clean Up Innovation Award.
The mine water pollution at the Crown Hole in Resolven The mine water pollution at the Crown Hole in Resolven The mine water outflows colour the local water courses rust brown due to the high iron content, at the Air Shaft in the Clydach Valley The mine water outflows An overview of the drilling site set up for the first of the two bores undertaken at the former Glyncastle colliery An overview of the drilling site Drilling into the Adit. Note the wellhead for pressure control of the mine water during the drilling operation Drilling into the Adit The drilling site for the second bore for the back-up pipeline The drilling site for the second bore Fig 1 – A section through the Resolven bore site showing the path of the main bore in relation to the mine workings Fig 1
