In pipe and pipeline construction, safe passageways through sensitive areas are a matter of course and only rarely a major challenge. But in every project the question is which of the available construction methods is the most economical or simply makes the most sense from an engineering point of view.
Apart from characteristics such as the pipeline diameter and drive length, the choice of a suitable method mainly depends on other project-speci¬fic parameters, geological or geometrical conditions. The combination of a large pipeline diameter and unstable geology can lead to increased risk in terms of deadline and cost reliability in case of horizontal directional drilling (HDD). After considering the economic and ecological bene¬ ts and drawbacks, the client may not accept one or several methods and may therefore rule them out at tender stage.
The direct pipe method opens up new application options by combining the bene¬ ts of accepted technologies to create a new method. For example, it uses microtunnelling machines, which have proven their value in pipe jacking for decades, for excavation work. Pushing the pipeline is handled by the Pipe Thruster, which grips the pipeline circumferentially and pushes it into the ground. This means that the pre-fabricated pipeline can be laid in the ground simultaneously with the excavation process, thus permanently supporting the bore hole.
The direct pipe method, which was developed in the scope of a research project sponsored by the German Federal Ministry of Education and Reseach (BMBF), was successfully deployed for the first time in 2007 for a Rhine crossing in Worms, Germany. Since then, the individual process components have been continuously improved and adapted to reflect increasing requirements. To date, 18 projects have laid a total of more than 9km of pipeline in Europe and the US (status June 2012). The pipeline diameters vary between OD 30in (762mm) and OD 56in (1,422mm) with a maximum drive length of 1,400m.
The now established process is characterised by the fact that it is suitable for direct laying of larger diameter product pipes. In specific project framework conditions, direct pipe offers benefits compared with older established laying methods, and is thus a useful alternative in many cases.
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Comparable with the jacking frame used for standard pipe jacking with short concrete pipes, the Pipe Thruster acts as the thrust unit in direct pipe. It hydraulically grips the pre-fabricated and outlaid pipeline and pushes it into the ground in strokes of 5m. Coated product pipes can be pushed directly. The required bore hole is excavated by the direct pipe machine, which is based on an AVN micromachine (AVN is Automatischer Vortrieb Nass, which is the German abbreviation for automatic slurry supported tunnelling/drilling process). The machine is deployed at the front end of the pipeline and pushed into the ground together with the pipe. The excavated material removed by the slowly rotating cutterhead at the tunnel face is mixed inside the machine with the drilling fluid (bentonite suspension) and then fed through a discharge line through the entire pipeline to the separation plant on the surface (through a separate discharge line). After treatment, the material is pumped back into the circuit via a feed line. The drilling fluid thus not only discharges the excavated material but also supports the tunnel face.
The overcut created by the cutterhead, approximately 50mm radius, is filled with high-viscosity lubricant (bentonite suspension). This reduces the friction between the bore hole wall and the laid pipe.
Direct Pipe machine
The slurry supported direct pipe machine works roughly in a similar way to a conventional slurry supported microtunnelling machine (AVN), but differs in terms of length. To allow for a curved motion of the machine and the trailing pipeline in the culvert path, the machine is equipped with two to three backup pipes. The fact that all of the joints in the individual backup pipes are articulated and tight connected to resist pulling force, ensures optimum steering capability of the machine. An additional benefit is the fact that the machine can be pulled out with the pipeline, again with the Pipe Thruster, in a case of emergency. In the scope of project planning work, the machine’s cutterhead is adapted to the geological conditions of the project. If boulders or rock are anticipated en route, disc cutters can be deployed in addition to the cutting knives. Installing a cone crusher behind the cutterhead helps to substantially extend the AVN machine’s range of applications in geologies with coarse gravel, cobbles and boulders. The crusher basically uses the same principle as a coffee grinder. Stones are ground until they pass through the round openings in the inner cone, the last stage of the screen-type grain size limitation. This prevents blockages in the slurry line, or at least reduces them to a minimum. The conservative upper grain limit resulting from these two crushing mechanisms is 30 to 40 per cent of the cutterhead diameter.
For a pipe diameter of OD 40in (1,016mm), direct pipe machines have an integrated power pack. The unit creates the hydraulic pressure required to turn the rotating cutterhead and power the steering cylinders. It is located in the backup pipe behind the cutterhead and the steering cylinders. The advantage of generating energy locally in this way is that longer drive lengths can be achieved. However, due to the lack of space, smaller machines for pipeline OD of 28in to 38in (711mm to 965mm) cannot be equipped with a power pack. The hydraulic losses resulting from a therewith necessary hydraulics supply from the control container thus restrict the drive length to approximately 300m.
A telescopic station is located between the backup pipe with the power pack and the conical transition piece. It is possible to thrust the machine forward after a longer period of standstill with the three or four hydraulic cylinders of the telescopic station, without thrusting the pipeline and thus needing to overcome the friction acting on the entire surface area of the pipe. Additionally, there is more control over excavating obstacles, such as a large stone, if the required contact pressure on the cutterhead only needs to be applied by the cylinders in the telescopic station. In contrast to the telescopic station deployed in pipe jacking with concrete pipes, the telescopic cylinders on the direct pipe machine can act in both directions. If it becomes necessary to withdraw the machine with the pipeline, the fiction between the ground and the machine can be handled by the telescopic cylinders and the friction between the ground and the pipeline by the Pipe Thruster. This can be useful if the machine is locked into the ground. The telescopic station thus acts as a safety tool which can be deployed under certain conditions.
The transition piece onto which the pipeline is welded has a conical shape. It reduces the larger diameter of the machine to the smaller pipeline diameter. The conical transition piece has a lubricating ring from which the main volume of bentonite lubricant is pumped into the overcut. The built in rear bulkhead hermetically seals off the machine from the pipeline and the slurry lines it contains. This separates the machine components from the pipeline, which can improve operational safety in case of leakages.
Push unit Pipe thruster
The Pipe Thruster was originally designed as an auxiliary tool for the pipe pull-in with the HDD method (installed on the pipe-side). The Pipe Thruster’s applications are as follows:
¦ Pushing or pulling pipelines into excavated open bore holes (e.g. created by HDD) or existing tunnels. This has already been implemented in several projects worldwide. The maximum length of a pipe pushed in like this is currently some 4,000m.
¦ Pulling out previously laid steel pipes from the ground. This is also established in the US and Europe.
¦ Direct pipe method for laying pipelines (with coating if needed). Thus far, some 18 direct pipe projects have been successfully completed.
The maximum pipeline diameter that the biggest Pipe Thruster model can clamp is OD 60in (1,524mm). The clamping unit is mainly adapted by changing the clamping inserts to match the pipeline diameter. It was designed to avoid damaging the coating of product pipes, such as gas or oil pipelines. In tests at the Herrenknecht workshop in Germany, it was demonstrated in cooperation with various gas suppliers that no damage is caused to the coating. Polyethylene (PE), polypropylene (PP) and glassfibre reinforced plastic (GRP on PE) coated pipes were tested at the maximum clamping force of the clamping unit and at full thrust force of the two large hydraulic cylinders.
The contact surface between the pipe and the clamping inserts is covered with hot-vulcanised rubber. It is designed to be large enough to minimise the pressure (3.5N/mm²) and shear forces (1.2N/mm²) on the coating.
Setting up on site
Launch and target pits
Using the direct pipe method, the drilling route is typically an arc from the surface of the terrain, underneath the obstacle to be drilled under, to the opposite terrain surface, like in HDD. The benefit here is the simplicity of the required launch and target pits. The Pipe Thruster can either be set up and anchored on the invert of a flat launch pit, or on the terrain surface, or at least fairly close to the surface.
The machine connected to the pipeline is set up at the required entrance angle in front of the launch seal. The overbow of the outlaid pipeline is held in place with lateral booms or either laid on a launch track with rollers. The horizontal and vertical forces to be anchored depend on the entrance angle and the maximum push or pull force to be applied. The forces can, for example, be held by an anchoring frame and sheet piling or foundation piles with a depth sufficient for the geology. Table 2 shows the values for the resulting forces with a corresponding entrance angle.
Just like in microtunnelling with concrete pipes, penetration of groundwater, soil, slurry and lubrication bentonite out of the bore hole into the launch pit must be prevented by means of a launch seal. The overcut of several centimeters is sealed with a neoprene rubber. To be able to compensate for movements in the pipeline, the launch seal for direct pipe comprises two steel components that are supported to allow relative movement by a U-shaped Neoprene rubber. Guide rollers on the floating front part of the construction ensure the required clearance between the pipeline coating and the steelwork of the seal. To ensure the best possible seal, the launch seal is fixed to the pit wall at the selected entrance angle.
The soil coverage over the launch seal should be at least one or two times the machine diameter.
The direct pipe machine can be recovered without sinking a shaft. To allow this to happen, the machine and the pipeline are pushed into a pit excavated near the surface. Here, the machine can be separated from the pipeline, dismantled and transported away. If the machine is to be recovered from a shaft, a reception seal is needed. This prevents water and material from the overcut flowing into the shaft.
Control Container
All of the process components involved in drilling and laying, such as the machine, the Pipe Thruster, pumps and navigation systems are remotely controlled from the control container. The individual functions can be operated remotely via the control panel. The information is transferred via a databus system to which all components are connected, from and to the PLC (programmable logic controller). This means that, for example, the machine’s steering cylinders can be actuated, or the speed of the slurry pumps controlled. The important functions and measured values are visualised for the machine operator on multiple displays located in the control cabin. One screen displays the navigation system, another the pictures from the cameras built into the machine. To allow the machine operator to look out of the control cabin window and see the Pipe Thruster, the control container is installed next to the launch pit. The hydraulic power supply to the Pipe Thruster is provided by a power unit built into the container.
Navigation system
To allow the machine to drill precisely along the required route, and thus lay the pipeline precisely at the desired location, a suitable surveying system is needed to locate the machine. The horizontal position is determined by a fibre-optic gyroscope. The latest generation gyroscope measures continuously, thus removing the need for the interruptions to the advance, as previously dictated by mechanical gyroscopes. The vertical position is determined by an electronic hydrostatic hose balance, a simple and proven system. The navigation system is accurate to within a few centimeters.
Before drilling begins, the waypoints for the route (start/end of straights or curves) are entered in the surveying software. The machine operator sees a visualisation of where the machine is compared to the target route during drilling. By extending the three steering cylinders appropriately, the steering head can be steered. The machine moves in the changed direction, with the entire pipeline following.
Separation plant
The separation plant separates the excavated material from the drilling mud. Screens and hydro vacuum cyclones separate the individual soil particles from the slurry suspension in various stages. If highly granular geology is anticipated, meaning a soil with a pronounced clay content, it makes sense to consider deploying a centrifuge or a filter press. The separating capability of hydro vacuum cyclones is around 40-60µm. Smaller particles cannot be filtered by a traditional plant.
The size of the separation plant must match both the anticipated geology and the required flow rate. The slurry circuit is comparable with that of microtunnelling AVN machines. One difference is that virtually no idle phases occur in the direct pipe method. If it is possible to lay the pipeline in long pipe sections, drilling continues without interruption for several days, thus permanently feeding excavated material to the separation plant. In comparison to this, a break of about 15 to 30 minutes occurs after advancing 3m in thrusting short concrete pipes. The circuit can then regenerate during the pipe exchange. The separation plant is typically deployed in the middle (half way along) of the laid out pipe sections to reduce the required quantity of feed and discharge lines and pumps.
Use of Bentonite
Bentonite is a clay mineral that has thixotropic properties after mixing with water. This means that it is liquid, or has a low viscosity when in motion, and highly viscous and/or gel-like if no energy is applied. In the direct pipe method, just like in normal pipe jacking, it has three tasks. It is used to support the bore hole (at the face via the slurry suspension and in the annulus via the lubrication bentonite), for transporting the excavated material and for lubricating the annulus.
The slurry suspension is pumped in a circuit from the machine to the separation plant and back again. The roughergrained the transported excavated material is, the higher the bentonite concentration needs to be. In finer-grained geologies it is even possible to completely manage without bentonite additive in the slurry suspension.
The direct pipe machine creates a larger annulus than a slurry machine deployed in pipe jacking with concrete pipes. It is 50mm in radius for the larger diameters, and slightly less for smaller diameters. The lubricating bentonite is pumped through a separate line through the pipeline from the surface to the end of the machine. Here it escapes at a very low, above-atmospheric pressure over openings in the cone-shaped transition piece into the annulus. The bentonite content in the lubrication suspension is higher than that in the slurry suspension, because greater viscosity is required. The rheological properties and the volume of the injected bentonite are accommodated to the geological conditions. The annulus must be completely filled to minimise friction between the bore wall and pipe. Losses into the surrounding geology must be compensated for by increasing flow rate.
Preparation of the pipeline
The pipeline is laid out at the desired launch angle so that it can be easily rolled or moved in the direction of the launch pit when drilling starts. Depending on local conditions and the launch angle, it is supported either by rollers near to the ground or in a raised position. Soil can be heaped up to provide a substrate. If planning envisages an elevated overbow, side booms or cranes can be used to hold the pipeline in position. The feed and discharge lines are routed from the machine through the whole pipeline. The pipeline is equipped during the setup work. All lines are placed on movable support structures and then successively fed into the pipeline. From the end of the pipeline flexible hoses for the slurry circuit and the bentonite lubrication are routed beside the pipeline to the separation plant. The power and data cables are routed from the end of the pipeline via the laid out pipeline to the control container.
