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A HOLE NEW BALL GAME

TUNELLING - A new trenchless drilling method combines the best of microtunnelling and horizontal directional drilling when dealing with difficult ground. Ian Clarke reports.

Microtunnelling and horizontal directional drilling (HDD) have been widely used for over 30 years.

But there are limitations as both methods are not always best suited for installing large-diameter, long distance non-tensile pipelines in difficult ground.

German tunnelling machinery manufacturer Herrenknecht has introduced a new trenchless method that combines elements of both microtunnelling and HDD. Called Easy Pipe it was developed to provide an economic solution for installing prepared and tested non-tensile pipelines with diameters of between 800mm and 1400mm in difficult ground such as gravel, rubble and rock over lengths of 500m to 1km.

There are two basic ways in which the Easy Pipe method can be used.

The first is where both launch and target points are above ground (or in a shallow pit) with a final curved bore profile similar to a conventional HDD installation, to cross beneath an obstacle. The second is where the launch point is in a shaft with the target point above ground or in a shallow pit at the surface. In this case the bore profile is similar to a conventional pipejacking installation.

This article will focus on the first case, with the launch and target points in near-surface pits.

A conventional microtunnelling set up is used, with jacking pipe sections added to the advancing pipe string as usual, and the cutting head monitored and positioned using pipe jacking methods such as a gyro compass.

The difference between these jacking pipes and conventional ones is their design, which allows them to be used as jacking pipes in the forward direction while allowing them to be retracted from the completed bore to pull in the product pipe.

The sections are bolted together around the circumference of the joint, designed to withstand thrust and pullback forces of up to 6,300kN (630 t) during the initial bore and product pipe pullback.

The jacking pipes also stabilise the bore walls as the bore advances, avoiding the potential for collapse in unstable ground.

After the cutter head has reached the target pit it is separated from the jacking pipe string. The first jacking pipe section is then connected to the product pipe using a specially designed connecting pipe.

The jacking pipe sections are pulled back through the bore by the same jacks that pushed them in - they can work in either direction.

The pipe sections are unbolted in the launch pit and removed until the product pipe is pulled through.

Easy Pipe is comparable with microtunnelling and HDD in terms of drive length, pipe materials, quality standards, ground conditions, drilling fluid, borehole volume, construction risk and construction cost.

For small diameter pipes, longer drive lengths can generally be achieved with HDD technology.

However, microtunnelling and the Easy Pipe method do allow drive lengths to be extended with intermediate jacking stations and, where necessary in tougher or more varied ground, through cutter changes in the borehole, reducing HDD's advantage considerably.

Because the launch shaft can be shallower and overburden thinner than HDD, drive length can be reduced by about 30% when microtunnelling and Easy Pipe methods are used.

Pipe materials used in HDD can also be used with the Easy Pipe technique, including steel, PE and cast iron. However, these materials are either only partially suited or not suitable at all for direct installation using microtunnelling unless the microtunnelled pipe is to be used purely as a carrier pipe (ie keeping the bore open ready for installation of the product pipe).

Easy Pipe improves quality as product pipes can be pre-assembled and tested before final installation, in the same way as HDD.

However, abrasive wear of the product pipe is not as good when using microtunnelling with a carrier pipe, but is comparable to an HDD installed product pipe.

Ground conditions are one of the key factors that led to the development of the Easy Pipe technique, so it is no surprise that, along with microtunneling, it can be deployed in almost all ground formations, including gravel, boulders and rock, if the cutting head, drive units and other equipment are suitably designed. Where HDD is deployed however, ground conditions can be a considerable limiting factor, particularly with regard to largediameter pipelines.

Quality requirements are lower because of the different soil transport design and operating environment.

There is a considerably higher flow rate in the slurry line of the microtunneller than in the annular space between jacking pipes and the bore wall.

Because the borehole does not have to be actively supported against cave-in by the drilling fluid, higher solid concentration is allowed in the fresh fluid which in turn allows the use of simple pumps (centrifugal instead of piston pumps), as compared with HDD systems.

Easy Pipe and microtunnelling also require considerably lower volumes of drilling fluid overall.

The Easy Pipe method has a clear advantage over HDD and microtunnelled (carrier pipe) installations with the diameter of the jacking pipes being optimised to the product pipe diameter, rather than needing oversize bores to safely pull in the product pipe.

In general, construction risk during trenchless installation is directly linked to geological conditions.

Microtunnelling and Easy Pipe installations have the edge over HDD because of their durability in a wider variety of ground. Also, the option to access the cutterhead from within the bore during the jacking using microtunnelling or Easy Pipe can significantly limit risk.

Risk is also reduced as Easy Pipe minimises the potential for the pipe to become jammed by obstacles or sediment in the bore, as is possible with HDD.

It is difficult to fully evaluate the cost benefits of using Easy Pipe because it is such a new technique - Herrenknecht says the first field tests are due to take place by the end of the year. The firm says, however, that initial estimates suggest costs will be lower than conventional microtunnelling with carrier pipe.

Compared with HDD, installation costs depend strongly on ground conditions. In ground formations suited to both Easy Pipe and HDD (sand, clay etc) costs are expected to be similar, but in more difficult ground conditions (gravel, rock etc) it is expected that there will be cost advantages for Easy Pipe.

Microtunnelling and Easy Pipe are expected to require the same amount of investment, higher than for HDD.

Costs for site implementation and removal must also be considered.

While the percentage of machinery required in the microtunnelling and Easy Pipe methods is relatively low, the two methods require additional transport due to the use of jacking pipes, the volume of which depends on the drive length.

Easy Pipe technology is another viable option when looking at conventional microtunnelling and HDD techniques. Technologically, it is more closely related to microtunnelling, but can be classified in the HDD category as regard to its application range. Field tests will be designed to prove the Easy Pipe method under real conditions.

Stage 1 The microtunnelled bore is installed between the launch shaft/pit and the target shaft/pit;

Stage 2 The microtunnel boring head is replaced with a connection pipe between the jacking pipes and the prepared product pipe;

Stage 3 The jacking pipe string is pulled back through the microtunnelled bore, pulling the product pipe into place as it progresses;

Stage 4 The product pipe is fully installed and connected to the main pipeline on either side of the obstacle being crossed.

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