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. Since then, 18 projects have laid a total of more than 9km of pipeline in Europe and the US (status June 2012).
The now established process is characterized 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.
Planning aspects
In contrast to the HDD method, where the opposite relation applies, in applications using the Direct Pipe method the maximum drilling length increases with the pipe diameter. Figure 1 reflects a fairly conservative estimation of potential drive lengths. The experience of the past few years has shown that drives of up to 1,400m (Ø=48in) are possible. In the future, the application scope can be expected to expand continually.
Figure 2 shows that the minimum pipe diameter is currently 28in (OD=711mm). The AVN machines deployed for this very small diameter cannot be equipped with a power unit for lack of space. This leads to a drilling length restriction of approximately 300m for pipeline diameters below 40in (OD= 1,016mm).
Geology
Detailed analysis of the geological characteristics provides important reference material for selecting a suitable laying method during the planning phase of a project. Planners can also assess whether the desired pipeline diameter can be laid directly in the existing geology, or if laying in stages is required. In case of homogeneous conditions, this assessment is always easier than in conditions with a mix of geologies. The important influencing parameters are, for example, the sieve corn analysis and the consistency of granular soils as well as the compressive strength, tensile strength and the abrasivity of the rock.
Because the Direct Pipe method involves thrusting the pipeline into the ground in sync with drilling the bore hole, it substantially reduces the risk to the terrain compared with the HDD method. This means that the pipeline can be laid directly, even in unstable geologies, such as rough grained or similarly graded sand or gravel, without the bore hole collapsing. With HDD, this typically necessitates the use of a casing. If the entire drilling route does not traverse stable geology, the pipe thrusting method alternatively uses a larger concrete pipe as protection. Direct Pipe offers an alternative for these cases.
Figure 2 illustrates the geological application scopes of the various Direct Pipe machines. One explanation for the levels shown is the increasing torque that becomes available as the machine diameter increases.
As described in part one of this series, stone and rock chips are crushed in the crushing chamber.
The reason for the levels shown in rock is the application scope of the cutting tools on the smaller AVN machines. They are designed or usable for a maximum specific compressive strength. The larger the cutterhead, the more space there is for larger disc cutters.
The contact pressure required to loosen chips from the rock formation needs to be transferrable to the disc cutter bearing without causing damage to it. As a general rule, it can be said that as the hardness (compressive strength) of the rock increases, the diameter of the machine must also increase to ensure economic drilling.
Cutting tool change
In the course of choosing the method or machine, it is important to assess as accurately as possible if and how often the tools mounted on the cutterhead will need to be changed due to wear during the drive.
In Direct Pipe the cutting tools are changed above ground for all pipeline diameters of less than 56in (OD=1,400mm). To allow this to happen, the machine is pulled out of the bore hole along with the pipeline. After replacing the tools, the machine is pushed back into the same bore hole along with the pipeline. In the meantime, the bore hole is filled with high-viscosity bentonite. In contrast to thrusting smaller concrete pipes, the Direct Pipe machine is directly welded onto the pipeline. All machine parts are joined captively to withstand pulling force.
As of a pipeline diameter of 56in, access to the rear of the cutterhead is possible below ground. The AVN1200T deployed for this application has a door (‘T’ stands for the German word for door = ‘Türe’). The special backloading system makes it possible to change worn tools in layers that do not contain groundwater. If it is necessary to change in groundwater conditions, the machine has to be pulled out of the bore hole because the use of an airlock lock is not permissible for these small diameters.
Figure 2 shows the guide values for economic deployment of the various machines in loose stone and rock with compressive strength of up to 200MPa, independently of the length of the section. If longer distances with even harder, or very abrasive rock are expected, it makes sense to deploy a larger machine with an airlock to be able to handle frequent disc cutter changes underground without any difficulty.
Thrust force
The Pipe Thruster pushes the machine and the pipeline into the bore hole. In addition to the contact pressure required to act on the cutterhead, the friction between the lubrication bentonite or the bore hole wall and the pipeline needs to be considered. The radial overcut of approximately 50mm is completely filled with lubrication bentonite. Although the pipeline ideally floats in this, the friction grows with increasing laid length. However, experience shows that the thrust forces required for Direct Pipe are relatively low compared with standard pipe jacking. A representative example of the thrust force required by the pipe thruster is given in Figure 3.
An evaluation of nine of the 12 projects implemented thus far with a diameter of 48in (OD=1,220mm) reveals the following range of friction values or thrust forces. In the case of three drives in clay, the friction was so low that a thrust force of just 0.11-0.33t/m of 48in pipe (=0.03-0.09t/ m²) needed to be applied. The length of the PE coated gas pipelines was between 370m and 590m. Historic values in sand and gravel with PP coated gas pipelines of lengths between 370m and 1,400m are 0.35-0.45t/m of 48in pipe (=0.06- 0.15t/m²).
A Dutch institute has now developed a mathematical model that allows the required thrust forces to be computed. The model is based on a FEM. It was combined with historic data from various projects.
Projects
Up to mid-year 2012, a total of 9km of pipeline had been laid in 18 projects. Five drives are described in more detail in the following case studies.
Pilot Project Worms 2007
A 464m 48in steel pipe was laid under the Rhine (48in casing pipe for various lines with an OD=1,219mm). The reasons for choosing this method were the partly unstable geology on the one hand and the cramped conditions on the other. It was impossible to lay the pipe in a single section on either bank of the river, thus making HDD impossible, or at least extremely risky.
The fact that the machine was thrust into the small target pit in Worms harbour with a thrust force of just 80t and in just 13 days shows the enormous speed on the one hand, and that the new technology has minimal thrust force requirements on the other. Microtunnelling with concrete pipes would have taken longer and thus also cost more.
The Netherlands
The first gas product pipe laying task took place in the Netherlands. In 2010 and 2011, a total of six projects were completed. The drives of between 360m and 1,400m below small rivers or rail tracks were part of a 500km north-south route. The pipeline with its diameter of 48in will transport gas throughout Holland in the future.
The 540m crossing below the very deep Hartelkanaal in Rotterdam used by vessels at Rotterdam’s Europoort harbor in the summer of 2010 was one of the most unusual projects due to the required passageway depth of 30m below the surface. The cramped space available, which meant that the crossing had to be planned to be as short as possible, led to very sharp launch and exit angles of 10 and 12 degrees (approximately three to five degrees had been typical thus far).
In a geology of sand and silt, the entire pipeline was laid successively in 10 pipe sections of 54m each within some two weeks.
The Netherlands Society for Trenchless Tunneling awarded the client and the construction company the No- Dig Prize 2010 for the successful use of this alternative laying method.
Laying a 1,400m 48in gas pipeline is currently the record holder in terms of drive length. The pipeline was laid out in three pipe sections of 500m each. With two pipe changes, the thrust duration from launch to arrival of the machine at the target was just 16 days (Figure 4). The maximum advance in a 24-hour period was 232m.
Those 16 days include setting up a second Pipe Thruster, after advancing approximately 900m. The feed force of 500t was insufficient at approximately the 900m point. After installing the second machine, the remaining pipe was pushed into the target pit within just four days by the two Pipe Thrusters.
This is equivalent (with an effective working period of 11 days) to an advance performance of around 125m per day.
With a pipeline of this large diameter, this kind of laying speed would definitely have been impossible to achieve using any other method.
Florida gas pipelines
Direct Pipe successfully celebrated its first outing in the US in August 2010. The three gas pipeline projects in Florida used diameters of 30in and 36in (OD=762mm and 914mm), compared to the 48in pipelines laid in Germany and the Netherlands. The drive lengths were between 119m and 226m.
One special feature in the first American Direct Pipe project was its curved route. In contrast to the previous routes, the pipeline had to be thrust under Highway 70 not just with a vertical curve (R=914m) but also with a horizontal curve (R=1,828m). The gyroscope-based navigation system and the electronic hydrostatic hose balance kept the machine safely and precisely on the required curved trajectory.
After just three days of drilling (12- hour shifts), the on-site team had laid the 215m gas pipeline in a single section and precisely on target.
In early 2011, the third project in the US involved retracting the Direct Pipe machine and 36in pipeline using the pipe thruster for the first time. After a drive of 102m the machine was pulled above ground in just one day; the cutterhead was changed and the machine then pushed back into the bore hole. The cutterhead change was made necessary by an unforeseen rock horizon that was impossible to pass without disc cutters. The remaining 124m pipeline was then laid within three days.
WasteWater pipeline in England
The Direct Pipe method was assessed as being low-risk for an 860m-long crossing below the River Ribble in England. Due to the anticipated coarse gravel and boulder strata, use of the HDD method was assessed to be too difficult and risky.
The route of the two waste water pipes to be laid led through rapidly changing layers of various soil layers. Long sections in clay with various sand and gravel content necessitated the use of a centrifuge.
From a shared launch pit, the two parallel 56in pipelines (OD=1,422mm) were laid at a distance of just 3.5m apart. The two pipes acted as a casing for two waste water lines (DN900) later installed in them.
In contrast to most of the projects realised previously, whose target pits were near to the surface, the machine had to be recovered from a 15m deep target shaft in this case. The navigation system deployed (gyroscope and water levelling system) allowed for precise targeting of the reception seal installed in the shaft.
Conclusions and Future
As the first 18 projects have shown, Direct Pipe achieves fast laying speeds. This makes the method an alternative to HDD and microtunnelling in terms of both technology and economics.
The drives completed thus far suggest the feasibility of increasingly long distances, given the right geology, that are only restricted by the smaller defined diameters of the product pipes and thus the limited accessibility of the machine for cutting tool changes.
Improved laying safety in difficult terrain – compared with HDD – and the economic benefits compared with standard microtunnelling and or pipe jacking, make the method extremely competitive.
The safety aspect of being able to recover the product pipe and the machine, and that of ‘minimally invasive’ passageway below habitats or obstacles, are decisive selection criteria for purchasers and official sponsors.
The benefits and applications described in this series of articles will lead to the Direct Pipe method continuing to assert itself compared with alternative methods such as HDD, pipe jacking and segmental lining.
