Consultants STATS key role in pipeline project

STATS has recently completed work in preparation for the construction of a pipeline more than one mile in length to supply gas to a new GBP700m Combined Cycle Gas Turbine power station in Kent.

STATS acted as specialist consultants to Stockton Drilling, who were commissioned by E.ON UK.

to investigate, design and construct the pipeline, which will transfer high pressure gas from the National Grid Grain LNG (liquefied natural gas) terminal to a new power plant which will be constructed on the site of the existing Grain oil fired power station.

Although the straight line distance between the Grain LNG and the power station site is only 2000 metres, the two sites lie on opposite sides of the former Anglo Iranian oil refinery, which was constructed in the 1950's and decommissioned in the 1980's.

Previous studies had identified a number of preferred route corridors, all of which presented very significant technological, ecological, environmental and geotechnical constraints which the design build contractor would have to overcome.

These included heavily contaminated ground with highly sensitive receptors, numerous deep buried foundations, active and very highly sensitive LNG facilities and tight physical constraints with limited stringing areas.

Extensive unrecorded active and disused underground services and near-surface buried obstructions presented further challenges in addition to numerous planning conditions relating to environmental issues, archaeology, dust, noise and traffic.

As specialist consultants to Stockton Drilling, STATS were initially required to carry out a wide range of site surveys, including geotechnical and environmental investigations, geophysical surveys and feasibility studies.

The results of these surveys identified a relatively simple geological profile comprising made ground, over very soft alluvium overlying River Terrace Deposits over London Clay.

However, the ground was locally heavily contaminated, primarily with asbestos and hydrocarbons, and although decommissioning had removed most of the above ground structures, STATS found that the poor ground conditions had resulted in piled foundations to practically every structure, even lightweight box culvert pipe bridges.

The geophysical surveys also identified numerous buried services and unidentified obstructions, some of which were potentially UXB/UXO (unexploded bomb/ordnance).

Using this information, STATS worked closely with Stockton Drilling's designers to determine the feasibility and risks associated with constructing the pipeline.

A number of options were considered including micro tunnels, auger bores, horizontal directional drilling and open cut.

The preferred design comprised an 1188m HDD (horizontal directional drilling) from Grain power station to an exit point on the south side of the LNG line and to the west of the A228 Grain Road.

The remaining 600m was designed as conventional open cut.

Although this design appeared to offer the optimum combination of technique and cost, there were several areas of uncertainty that had to be resolved prior to construction.

One the most critical issues was that the HDD bore had to enter at a steep angle to clear the underside of a piled flood defence bund before threading through a very narrow window between the piled foundations to a high voltage transmission tower and the piled foundations to a former tank farm.

The designers could not avoid the HDD passing beneath the first tank base, but there was no information on the toe depth of the piles.

STATS geophysicists were therefore called in to prove the pile lengths.

The first step was to use time domain electro-magnetic mapping (EM) techniques to image the position of the piles through the concrete base of the former tank.

Small areas of the tank base were then broken out to expose the tops of the piles, after which a series of boreholes were formed adjacent to the piles and around the perimeter of the tank base.

The STATS Geophysical team then employed the parallel seismic technique, in which the depth of a pile can be determined from the change in velocity of a compressional wave induced at the top of the pile.

The wave travels rapidly through the pile and then slows in the surrounding medium.

A hydrophone is lowered into a borehole constructed adjacent to the pile and compression waves are induced at surface.

The interval travel time of the pulse is determined at typically 500mm increments and an evaluation of the change in velocity with depth should allow a precise determination of pile toe depths.

The technique proved very successful with pile toe depths clearly identified.

Based on this information, the design was finalised and agreed with the client.

When horizontal directional drilling commenced there were tense moments as the pilot bore passed within three metres of the predicted pile toe depths.

No obstructions were encountered, but the presence of the piles was detected as small disturbances in the earth's magnetic field by the highly sensitive instruments used to steer the drill.

At the planned depth of 28m below ground level, the pilot bore was then steered on a 1200m radius horizontal curve to pass under, and then turn parallel to, the main LNG line from Thamesport to Grain LNG.

The LNG line rests on piled foundations and is subject to very stringent settlement criteria and is continuously monitored for any evidence of movement that could affect the integrity of the line.

If any movement had been detected, HDD operations would have been immediately shut down and the whole pipeline design would have been in jeopardy.

The HDD was successfully steered around all the below ground obstructions and no disturbance to the LNG line was detected.

The pilot bore emerged within .1m of the planned exit point.

The only problems experienced were drill flush breakout through unrecorded boreholes, believed to be associated with the original oil refinery construction.

These breakouts were quickly contained and remediated with no impact on the ecology.

The HDD was reamed out during the following weeks and the product pipe was pulled in two strings without incident.

The 600m open cut section was also tightly constrained to a narrow corridor along a former access road between the LNG line, services and the massively reinforced foundations to refinery structures.

During breaking out operations, STATS personnel detected asbestos fibres and ACM in the made ground and unexpectedly, as fibre reinforcing for cement between culvert sections.

In accordance with contingency plans the STATS asbestos team then introduced stringent asbestos control measures with continuous surveillance and strict control measures for material storage and disposal.

Localised dewatering of the trench produced ground waters contaminated with petroleum hydrocarbons, chlorinated solvents, volatile and semi-volatile organic compounds.

These were expected from the results of the STATS ground investigation and in accordance with the Environmental Management Plan, which was prepared by STATS, these contaminated waters were stored for subsequent off-site disposal.

The open cut section was successfully completed with the construction of a 300mm thick blast resistant slab cast over the top of the pipe.

As part of the requirement to discharge planning conditions imposed by Medway Council, STATS also carried out noise, traffic, dust, ecological and topographic surveys.

Gearoid O'Connell, Director at Stockton Drilling, said: "The project presented a wide variety of challenges so it was important for us to work with specialist consultants that have experience and expertise across a wide range of disciplines, with which to address both the anticipated and unforeseen issues that arose.

The STATS team was able to provide a broad spectrum of specialist skills and made a very significant contribution to the successful delivery of the project".

Paul Stearns, a Director at STATS, added: "Although this was one of the shortest pipelines that we have been involved with in recent years, it proved to be one of the most complex projects that we have undertaken, requiring the expertise of specialist teams from across virtually the whole company.

"Stockton Drilling has forged a formidable team of consultants that all work closely together right from the very start of the project and stay involved through to completion.

This gives our staff a clear understanding of where their contribution can be most effective and allows us to use our extensive knowledge and experience of the pipeline industry to best advantage in supporting our client.".

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