Trenchless International January featured an edited extract of James Thomson’s chapter covering the use of microtunnelling. Here we present part two, exploring the installation of marine outfalls using HDD.

Horizontal directional drilling (HDD) evolved in the 1970s from a combination of technology from the directional drilling techniques used for drilling oil wells and the techniques used for boring road crossings. The HDD process is quite different from microtunnelling, which evolved from tunnelling technology. HDD is based on drilling technologies and the equipment and the skills needed to operate the rigs have little in common with tunnelling.

Unlike tunnelling and microtunnelling methods, HDD does not have any capability of installing a pipeline to the close accuracy required for a gravity sewer. Rigs are positioned at the surface so the path of the bore is normally a vertical curve. The vertical curve can be designed and followed with an accuracy of around 1 per cent of the drive length.

HDD is widely used for installing crossings for all kinds of pipelines and cables under rivers. In the 1990s the HDD contractors modified their methods of working to install outfalls.

HDD and outfalls

The HDD process has been adapted for applications such as outfalls, intakes, and landfalls for pipes and cables. To date the HDD technique has found the greatest use in the installation of outfalls with diameters of less than 1,000 mm and with lengths as great as 2 km. There are two approaches used in the construction of these types of installation:

The drill is sited on the land side and the drilling operation. As the drill head exits through the seabed so sea-going vessels are required for the reaming and product pipe pull back operations. One particular advantage is that if polyethylene is used as the product pipe, it can be floated in the water and even brought to site by towing in a long length behind a tug. However where the seabed floor is sand or soft clays it may be difficult to maintain the bore open where it exits into the sea. This can create problems for installing the outfall pipe. Drill rigs and the support modules can also be sited at sea on a suitable jack up platform. Drilling takes place through a moon pool and the drill pipe and wash-over pipe can be supported from the platform by sheaves. Succeeding stages of reaming and pull back of product pipe are done from the landward side. The problems of exiting into the sea bed are avoided. In both of the approaches, the installation of any outfall structure or diffusers is carried out using sea going craft and equipment in the traditional manner.

Case studies

Castro Urdiales, Spain

As part of the European community requirements on wastewater discharges, the town of Santander in Northern Spain installed a new wastewater treatment plant and two 900 mm outfalls to carry treated effluent out into the Bay of Biscay.

The contractor opted to use HDD techniques for these two lines of 615 m and 210 m respectively. A key consideration was that the outfall started on top of a rocky cliff some 50 m above the shore level. The difficulties of the location can be seen in the aerial view of the site.

The total drop from inlet to outlet was 70 m.

The geotechnical investigation showed that the bore would be located in limestone, some of which would be fractured. Because of the nature of the limestone, the final hole was opened up to 1,320 mm OD.

The product pipe installed was a 900 mm polyethylene pipe. The pipe was taken out to sea and pulled back to the land with the aid of offshore craft.

Mear/Veg Rock Outfall, Isle of Man, UK

This installation is typical of a number that have been installed around the coast of the UK. The Isle of Man is located off the west coast of England in the Irish Sea. A new outfall was required to carry treated effluent from a new treatment plant out to sea. An 800 mm polyethylene outfall over a length of 400 m was required. The location of the outfall is in a very attractive natural coastline and environmentally sensitive area.

The geology along the coastline consists of metamorphosed mudstones and silt/sandstones. By definition these are quartz rich, and are hard and abrasive. Because of the size of the outfall and the nature of the rock, the contractor opted to use a 100 tonne rig and 4 stages of hole enlargement: 437 mm, 650 mm, 850 mm and 1,000 mm. A 437 tungsten carbide insert tri-cone bit was used.

After the pilot hole exited on the seabed, a soft plug was created with swelling polymers and a cement pill at the end of the hole. Succeeding reaming stages stopped short of the end of the pilot exit point. The various sizes of reamers were then reintroduced one after the other to complete the bore. When the reaming operation broke through the seabed floor the bentonite drained away and from that point large volumes of fresh water were used, which was more environmentally acceptable. To assist the divers in locating the end of the hole, the last reamer was left in place and air was also pumped into the line. Once the head was located, an excavator on the barge dug down through the debris and the reamer was pulled back to the surface.

The polyethylene product pipe was welded up on land and pulled out to sea ready for pull back. The pipe was fitted with a pulling head at one end and a flange plate with valve and a pulling eye on the other. The valve allowed the buoyancy of the pipe to be regulated to assist the pull back. Pull back was by the rig on the landward side.

The pipe at the exit point was protected by a concrete mattress. Prefabricated diffuser sections were then lowered and aligned so that they could be bolted up to the end of the pipe and finally, the area around the diffusers was suitably backfilled to provide protection. All this work was accomplished with sea-going craft and divers.

New developments

Trenchless techniques continue to advance. Notable recent developments are:

Pipe thrusters: This could impact HDD outfall construction. It can be used to install pipelines of up to 1.2 m diameter and lengths of 3,000 m. The device is introduced into the line at the point of pull back and at the exit side. It can simultaneously push the pipe being installed as the rig pulls back.

Direct pipe: This combines microtunnelling and HDD and allows the direct installation of steel pipelines in diameters up to 1.2 m. The bore excavation is undertaken with a microtunneller and the pipe is pushed in behind using the pipe thruster. Unlike HDD it is a single stage operation with full bore excavation and pipe installation simultaneously. Spoil removal is by a slurry circuit. The method has obvious applications to installing steel pipe sea outfalls.

Conclusion

The table compares typical applications for the three methods for different site conditions. This is only a broad guide, however, and every site will have unique characteristics that must be fully evaluated before determining the most effective method of installation. It has become apparent in a number of recent projects that the combination of trenchless with traditional methods and the combination of two or more trenchless methods can achieve the best solutions.

Three main stages of horizontal directional drilling

The pilot bore

A pilot hole is drilled along the prescribed path using a hydraulically driven “mud motor” which drives the cutting head. The cutting head is chosen to meet the anticipated geotechnical conditions. Behind the cutting head is a section of drill string which has a small dogleg configuration. The drill string is not rotated except to orient the dogleg; this provides a steering capability and allows a smooth path to be followed. Behind the dogleg is the survey tool that monitors the drill path position. This electronic package detects the relation of the drill string to the earth’s magnetic field and its inclination. The data is transmitted back to the control where it is processed to give the drill head position.

The contractor often introduces a wash-over pipe, which is a larger diameter pipe that runs over the drill string and provides rigidity and hole stability if it is necessary to withdraw the drill line and drill head for a bit change. The cuttings from the pilot bore are carried back to the launch areas in the drilling mud.

Pre-reaming

Having successfully run the pilot bore, the pre-reaming stage begins. The reamer tool is attached at the exit end and opens out the hole as it is pulled back by the rig. It may be necessary to carry out several reams in order to obtain the required hole size which is typically 50 per cent greater than the pipe diameter to be installed. Large quantities of drilling mud are used in this operation to help maintain the stability of the hole as well as to carry cuttings to the surface. The drill rods are normally attached behind the reamer as it is pulled back to allow for additional reaming or pull back.

Pull-back

The pipe to be installed is attached to the final reamer by means of a swivel to avoid rotation. Again, large quantities of drill mud are used to pull back the drill-string, reamer and pipe to the launch area.

The HDD rig will depend on the length and diameter of pipe to be installed. Typically the launch area will be around 35 m wide by 50 m long behind the entry point. The angle of entry of the bore path lies between 8 and 20 degrees. The rig is surface-mounted and has a number of support units.

Typically, the pipe will be pulled in at the exit side, where the pipe to be pulled must be laid out on the surface. This can mean laying out the whole pipeline length behind the exit point or laying shorter lengths that are then jointed as the pull progresses. This is much less desirable.

Drilling mud and slurry

In HDD, drilling mud or slurry provides the following functions:

• Drives the cutting head
• Provides hydraulic cutting of the soil
• Lubricates the drill head and drill pipe
• Protects and lubricates the product pipe during pull-back
• Stabilises the hole and builds up a ‘filter cake’ barrier which prevents loss into the surrounding soil.

Most slurry used for HDD is based on bentonite, a naturally occurring clay with hydrophilic properties. As large quantities of slurry are used, the returns must be contained to avoid pollution. These returns are recycled or disposed of at an authorised landfill.

Design considerations must account for the stresses imposed on the pipe during its service life and during construction. The profile of the drill path must be designed not to impose unacceptable stresses on the drill pipe and the pipeline. The contractor must also calculate the required pull back based on frictional loads. This pull back loading will determine the rig size.

Drilling mud is used at high pressures. In some circumstances, unless there is a sufficient cover, fracturing in the overburden can occur with loss of mud that causes pollution. For most work, a minimum of 8 m below the bed is recommended, but may need to be greater depending on conditions. For HDD, the majority of pipes installed are either steel or polyethylene as they must have a degree of flexibility to follow the curved drill path. Steel is jointed by welding and coated with fusion-bonded epoxy. In some soil conditions external abrasion can occur during pull back and it may be necessary to provide a suitable sacrificial coating such as a polymer concrete. Polyethylene must be a high strength quality and a minimum SDR of 11 (SDR is the standard dimension ratio, the ratio of the outside diameter to the minimum wall thickness). Jointing is undertaken onsite by fusion bonding.