HDD aids clean coastline

The North Coast Wastewater Treatment Scheme involves laying 24km of pumping mains, the construction of eight major sewage pumping stations, and the refurbishment of five others.

Martin Watson of Allen Watson reports on how horizontal directional drilling (HDD) played a key role

The scheme to clean up the costline in Northern Ireland is designed to rationalise the existing wastewater networks, which have historically developed separately.

The scheme involves laying 24km of pumping mains and gravity sewers, varying in diameter from 125mm to 1,200mm, along with the construction of eight major new sewage pumping stations and refurbishment of five existing pumping stations. The new WwTW will provide a 59,079m3/day treatment capacity.

Part of the construction work on the North Coast Waste Water Treatment Scheme scheme required pumping mains to be installed beneath the River Bann at two strategic locations. One crossing point was located near to the village of Riversdale and another was close to the village of Ballycairn. Careful examination of the required pipeline route and the crossings showed that horizontal directional drilling (HDD) offered the best solution for the required installations.

After issuing open tender documents for the required crossings, specialist trenchless HDD and boring contractor Allen Watson, of Horsham, West Sussex, submitted a technical proposal, winning the main contractors confidence, and was awarded the contract to complete the HDD work.

The River Bann crossings requirement was precise. At each location, twin pipelines (two parallel bores each containing single PE pipe) were to be installed at specific depths below the river bed. At the first location, Riversdale, the two pipes required were 400mm diameter, butt welded, each to be installed over lengths of 200m.

The second crossing, near Ballycairn, was of similar design but in this instance the pipes were 560mm diameter, butt-welded, PE pipes each to be installed over a distance of 220m. In the event, each work site brought its own individual and somewhat unexpected challenges.

Riversdale
Initial investigation showed the Riversdale bores would pass through boulder clay with some expected stone inclusions. The information obtained covering ground conditions did not lead Allen Watson's engineers to expect any particular concerns.
The two separate bores were designed to be installed with starting and end positions just 2m apart, at the launch and target sides of the river. The design allowed for these bores to be steered forming a separation of around 5m under the middle of the river, simply as an added safety factor for the pipes once installed.

Allen Watson decided to utilise its Prime Drilling PD50/33, 64 tonne pullback capacity drilling rig for this operation. Designed to produce 33,000Nm of torque, the track mounted rig utilises 5m drill pipe. In this instance, a drill string comprising 90mm diameter drill pipe was used for the pilot, reaming and pullback operations.

Once the pilot bore commenced, using a 250mm diameter jet head, from what was a standard launch site, it was found that the expected boulder clay contained a much higher proportion of stone than had been expected. This made the ground react more like a broken rock formation than a clay formation.

Despite this, the crew completed the pilot bore with a jetting head, although this took three days to complete instead of the scheduled one day. Guidance through the pilot bore was provided by a wireline system.

Having completed the first pilot bore, the machine was moved to the second position and the adjacent pilot started and completed in a similar manner. The reason for this manoeuvre was to allow the first pilot bore to serve as the drilling mud return line for its neighbouring bore during the reaming and pullback stages of the installations.

Reaming at the Riversdale site was completed in two stages. The first ream was to a diameter of 460mm. Initial reaming was done with as standard fly-cutter reamer unit.

The aggressive ground, however, quickly wore the teeth on this reamer. The reamer was then backed out of the bore, removed and replaced with an 460mm diameter hole-opener unit which successfully completed the reaming operation.

Using a similarly designed rock reamer a further reaming step, to 610mm diameter was then completed. The 610mm diameter reamer was then again pulled through the bore as a cleaning run, which was followed by the pipe pullback again using the same reamer as the lead. On completion of the pipe installation on the second pilot bore, a string of drill pipe was passed back through the installed PE pipe and utilised as the drilling fluid return line on the adjacent.

The second ream and pipe pullback
operation was completed using the same down hole tooling and completed without problems as that ultimately used on the first bore.

The reaming operation for both bores took just four days to complete. Throughout the boring, nine tonnes of bentonite was utilised with, minimal requirement for additional additives. Pullback forces required for the pipe pullback did not exceed seven tonnes due to good management and cleaning of the bore prior to pullback.

Ballycairn
The 220m long installation of twin 560mm diameter, PE pipe at the Ballycairn site followed a similar process to that used at Riversdale.

The difference at this site was that ground conditions comprised shallower boulder clay/glacial till horizon, which overlaid strong basalt rock. And the plan was to bore through the clay layer above the rock horizon. Topography at the Ballycairn site was, however, somewhat different with steep banks on one side of the river. This meant the drilling rig - the same one used at Riversdale - had to be set up with a very steep entry angle given a height of 12-15m above the river level.

Once drilling commenced, the strong, basalt rock head was found to be much shallower than indicated. The pilot bore ran along the shallow interface. And, unable to penetrate into the stronger material below using this equipment, it was decided a safe depth for the pipeline could not be achieved. For the safety of the pipeline from vessels at anchor, channel dredging and any shifting formations, a decision was made to bore much deeper into the rock horizon.

The jetting equipment was replaced with a 130mm diameter mud motor with 165mm diameter TCI drill bit. With this arrangement, both pilot bores were then successfully completed without further difficulty in just two days for each bore, again using wireline guidance.

Given the new rock bore circumstances and the smaller pilot drill, reaming of each bore had to be completed in three stages. This included 310mm and 508mm diameter ream using hole-opener rock reamers followed by the final stage with a 760mm diameter ream again using a hole-opener.

Mud return and management arrangements were the same as those used at Riversdale although the change to mud motor pilot bores meant that 14 tonnes of drilling mud was used over the course of the two bores using the same BoreGel mix.

After cleaning the bores with the 760mm diameter hole opener, the pipe pullback was achieved with the same reamer unit as the lead. Pullback forces on the drill rig did not exceed nine tonnes. And this was again due to good fluid mix and bore management. Prior to pullback, at each location, all PE pipes were pressure tested and proven prior to installing.

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