Last autumn, the turnkey work for inflow and outflow lines of a desalination plant in Algeria saw subcontractor Radco successfully perform a micro-bore pipe-jack offshore that also called for replacing some cutterhead discs in tough conditions after hard ground was met below the sea bed.

 Options to access and replace the worn discs settled on foam injection around part of the micro-tunnel boring machine (MTBM) to reduce inflow further, as it was already being pumped out during normal operations.

The micro-tunnelling machine did not have hyperbaric equipment and the skilled crew worked on the laborious task in the confined space to switch out some of the discs.

The challenge on the Engineering, Procurement and Construction (EPC) contract arose after hard ground was unexpectedly met in a portion of the MTBM drive, which was pipe-jacking a steel pipeline into place for the desalination plant’s seawater inlet line.

Radco began the tunnel boring in early September and by early October had completed the task, despite the challenges, T&T was told.

THE PROJECT

The El Marsa desalination project is being constructed in Algeria on the Mediterranean coast to produce 60,000m3/day of treated potable water for drinking and further uses. To achieve the production output, the plant takes in far more water – 134,000 m3/day – to the treatment process.

The developer is Algerian energy company AEC.

The inflow of water to the plant is to be conveyed from the Mediterranean Sea via the inlet pipeline – in this case a GRP pipeline, which was fitted inside the pipe-jacked steel pipeline, after tunnelling was completed. The tunnel is 240m long.

The outflow of unwanted water from the desalination treatment process is conveyed by a HDPE outlet pipeline for discharge to the sea. This 800mm OD flexible outlet pipeline was placed on the sea bed and is kept in place by ballast and concrete holding blocks.

Contractor on the project is Cosider Canalisation and Radco has an EPC subcontract for the maritime works, consisting mainly of constructing the inlet and outlet pipelines. Tunnel boring for the intake line has been the key challenge in the works.

Radco’s subcontract began in mid-February 2022 and, with the contractor having already chosen the pipes due to availability of local products, the solution was then developed that enabled pipe-jack tunnelling with the MTBM to begin later in the year.

PREPARING FOR TUNNELLING

For the MTBM, Radco used a new Herrenknecht AVN1200 slurry system. But transport problems with a strike at Antwerp port, in Belgium, caused a delay of a few months for the MTBM to arrive at the port of Algiers.

The tunnelling work would see the MTBM launched from a shaft (L x W x D = 12m x 8m x 5m) to bore offshore, and the jacking force was 700kN, says Radco. The inlet drive tunnel has an incline of 33mm/m.

Originally, the project owner had a design for the intake pipeline to be 360m in length but Radco, in its EPC contract, persuaded them that it could be shorter by a third. It says the 240m-long pipeline would give the flow rate and velocity required, and helped the project economics.

The EPC contract was to be eight months duration with one month of tunnelling, to install the 240m-long, 1490mm OD steel pipeline, jacked into place behind the MTBM. The pipeline was formed of 6m-long sections of 21mm wall thickness. The pipe section joints were welded together. Bentonite lubricant was used for the pipe-jacking.

Upon completion of the pipe-jack tunnel, a 1200mm OD GRP would be inserted along the steel pipeline, for seawater conveyance during operations.

TUNNELLING

The subcontractor had requested that site investigation offshore be undertaken – at least boreholes – but they were not undertaken by the contractor, it says.

The MTBM was launched on 6 September.

Geology in the area was found to be mixed sand and clayey marl with a surprise of the intake tunnel alignment passing through two fault zones and then, near the end of the drive, meeting hard rock of more than 200MPa UCS, says Radco.

When the MTBM was operating and the manlock door was open, the door was set on a trigger to close if required, such as when one of the three pumps in the machine failed. The inflows were drained via two pipelines running from the machine to the service shaft.

Hard rock was encountered approximately 207m into the drive and damaged the cutterhead tools. At that location, with the inlet tunnel on an inclined slope, running offshore, the MTBM was approximately 11.5m below the sea bed. The machine was only slightly more than 30m away from the end of its drive.

Various options were considered and given the availability of equipment and local materials, and the construction schedule, says Radco CEO Bahman Nazari.

With bentonite used for lubrication of the MTBM in the pipe-jacking operation, the machine was equipped with injection ports at two areas, at the tail skin and in the telescopic section, he says. The choice was made to inject two-component PU foam into part of the ground around the machine to limit inflows and enable crews to work.

Radco injected the PU foam through the ports into the ground around the middle of the MTBM, pumping at maximum pressure. A ring of conditioned ground was formed around the machine, about 2m behind the cutterhead, says Nazari.

Some water inflow was heard and seen behind the cutterhead where the crew then worked in the confined space, trying to remove the worn and damaged discs. The discs were found to be completely jammed in place and there was damage to their housing and wedges, says Nazari. Eventually, they had to be removed with welding torches and air cutting equipment, he adds, and then the new discs were installed.

Almost two weeks were spent on the operation, crews working around the clock in the MTBM and supporting from shore, to changeover the cutterhead discs on the machine while constantly ensuring the injected ring of foam maintained the reduced inflows.

The MTBM resumed boring the pipe-jacked tunnel and completed the drive on 8 October, says Nazari.

By the end of the project, the MTBM was recovered from a depth of 12.5m on 29 October by the Radco team.

The GRP carrier pipe has been installed for the inlet, as has the outlet HDPE pipeline. A jack frame and two support rollers were installed on either side of the GRP pipe – also in 6m-long sections – to guide them through the steel pipeline.

The desalination plant recently commenced operation.