Direct Steerable Pipe Thrusting (DSTP) is the common name for the installation of a steel pipeline that is installed into a bore using a pipe thruster, steerable and typically installed between shallow launch and reception portals along a designed bore path that includes curves. Herrenknecht Corporation was the first manufacturer to bring this technology to the market with its Direct Pipe® system, which is a subset of DSPT. The first installation occurred in 2007. Since that time, the application of DSPT has increased with 162 completed installations worldwide prior to 2021 all with the Direct Pipe System. (Herrenknecht, 2021).
DSPT is often described as a combination of horizontal directional drilling (HDD) and Microtunnelling (MT) as DSPT has operational and behavioural characteristics that are similar to microtunnelling while allowing the installation of pipelines with geometric characteristics that are similar to HDD installations. However, understanding the features of DSPT and how they compare to microtunnelling and HDD is critical to determine and analyse the mechanisms that govern the development of thrust forces, the risk of inadvertent returns, and the behaviour of the pipe within the borehole. The following compares and contrasts the DSPT method with microtunnelling and HDD to establish a basis for understanding the technology as it relates to these two more established trenchless technologies and their associated design and construction best practices
DSPT comparison to Microtunnelling
The bulleted items below describe the primary similarities and differences between DSPT and Microtunnelling. These items are not exhaustive but represent the primary characteristics that govern the behaviour of the installation method. Similarities between DSPT and microtunnelling:
- A microtunnelling machine is used to excavate the geologic material along the designed bore path for both methods.
- Both microtunnelling and DSPT use pressurised slurry to remove excavated material from the borehole and counterbalance the groundwater pressure.
- The operator controls the slurry pressure to counterbalance inflows of groundwater, typically operating with a pressure differential between the slurry and groundwater that is +/-0.1 bar, resulting in a very low probability of slurry escape from the face. (Lang, 2017).
- Both microtunnelling and DSPT excavate a borehole that is larger than the machine and pipe outer diameter which establishes an annulus into which lubrication is injected to lower frictional resistance. The rotating cutting head on the microtunnelling machine produces an overcut which establishes the annulus.
Differences between DSPT and microtunnelling:
- Microtunnelling propels the machine and pipe segments, which are typically 1 to 6 m in length (3 to 20 ft) with a jacking frame that pushes one pipe section at a time and is practically limited by the stroke of the jacks, whereas DSPT propels the machine and the pipe with a pipe thruster, allowing much longer pipe sections to be installed without stopping to make pipe, slurry, and other utility line connections.
- Microtunnelling can be used to install a variety of jacking pipe materials including concrete, RPMP (fibreglass reinforced polymer mortar pipe), polymer concrete, steel, vitrified clay, and a number of other jacking pipe materials, whereas DSPT exclusively uses steel pipe as a casing or final product pipe.
- Microtunnelling typically occurs from a launch shaft to a reception shaft that is excavated to the desired elevation of the pipeline, whereas DSPT initiates from a shallow launch portal and the machine is launched at an angle to achieve the desired depth along a curved bore geometry.
- With microtunnelling, the radial overcut between the fully excavated borehole and the pipe typically ranges from 0.5 to 1.5 in (12 mm to38 mm), whereas the radial overcut between the fully excavated borehole and the pipe for DSPT is typically significantly larger, measuring 5 in (127 mm) or greater. (Robison and Chen, 2017).
- Curves can be introduced into a microtunnelling operation with the use of specialty jacking joints (Althuser, 2021).. Curves introduced into microtunnelling designs are typically horizontal curves, whereas DSPT alignments can incorporate vertical, horizontal and compound curves.
DSPT comparison to HDD The similarities and differences between DSPT and HDD below are again not exhaustive but represent the primary comparison characteristics considered to govern the behaviour of the installation method. Similarities between DSTP and HDD: • Both HDD and DSPT are capable of installing a bore geometry that includes vertical, horizontal, and compound curves with limitations that are governed by product pipe stresses. • Both HDD and DSPT install long lengths of pipe that are assembled on the ground surface prior to installation. If layout is available, longer assembled pipe segments are generally preferable to minimise the downtime during installation associated with welding shorter sections together.
Differences between DSPT and HDD:
- HDD uses pressurised drilling mud to aid in borehole excavation, remove soil from the excavation, and cool the drilling tools. Continuous circulation of drilling mud in the bore is critical to HDD success. The DSPT process includes injection of lubrication into the borehole annulus as the pipe is propelled forward; however, the lubrication is not actively pressurised and does not flow in the annulus, support the borehole, or transport excavated spoils from the borehole.
- One primary consideration for HDD design and construction is the risk of inadvertent drilling fluid returns if the drilling mud pressure is higher than the confining strength of the formation. With DSPT, the spoils are removed with the slurry system that is largely contained within the microtunnelling machine and trailing pipe, eliminating the need for flow and pressurisation within the bore annulus.
- HDD can be used to install several different pipeline materials. DSPT can only be used to install steel.
- HDD uses multiple passes with different tooling to construct a fluid supported bore into which the product pipe is pulled. The bore is regularly unsupported except by drilling fluid, with only a small drill pipe section present in the bore. DSPT immediately supports the excavated bore with the machine and thrusting pipe.
- The cutting torque used to excavate the bore during HDD is applied at the entry location, through the drilling steel and to the downhole tooling. The cutting torque used to excavate during DSTP is applied directly at the cutting face by the machine.
Inadvertent Returns – Hydrofracture
When designing an HDD, it is necessary to determine an appropriate bore depth to confine the drilling fluids within the borehole. If the borehole is not significantly deep, or the geotechnical formation has low strength, the pressurised drilling fluid may readily escape to the ground surface when the bore pressure exceeds the soil confining strength (hydrofracture), or the fluid travels along preferential flow paths such as existing utilities, foundations, or fractures. The generic instance of drilling fluid escaping to the surface during HDD installation is referred to as ‘inadvertent returns’.
The potential for inadvertent returns or hydrofracture is a significant risk on a project, especially when traversing beneath environmentally sensitive areas. These concerns are shared by Owners, Contractors and Environmental Agencies. Governmental Agencies (such as the US Army Corps of Engineers) have established guidelines for the evaluation of drilling fluid pressures and the potential for inadvertent returns (Staheli, et. al, 1998, Latorre, 2002). These guidelines are used to design HDD installations and to justify environmental permit applications when crossing sensitive features, including levees and rivers.
Figure 5 illustrates the concept of inadvertent returns or hydrofracture. Inadvertent returns occur when the annular pressure exceeds the strength of the overlying soils, resulting in drilling fluid escape. When this occurs, there are both environmental concerns and operational concerns. Clean-up of inadvertent returns can be considerably costly if large amounts of drilling fluid escape.
However, the greatest impact to HDD is that the drilling fluid no longer circulates within the borehole and excavated material remains in the bore. During design, engineers predict the minimum pressure that is necessary to create a borehole in the geotechnical conditions at the site and then calculate the limiting pressure of the formation. These values are compared to determine the safety factor against inadvertent returns.
Inadvertent Returns on DSPT Projects
A major advantage of using DSPT is the excavation of the borehole without the use of pressurised drilling mud, allowing pipeline installations at shallower depths than would be required to confine the minimum drilling fluid pressures associated with HDD (Lang, 2017). However, permitting agencies remain concerned about loss of drilling mud into environmentally sensitive areas and have required calculations for inadvertent returns for DSPT projects in the past (Robison and Wilson 2016).
The design guidance provided by the US Army Corps of Engineers (Staheli, et. al., 1998) has been applied to DSPT projects to determine the risk of the borehole lubrication escaping to the ground surface (Robison and Wilson, 2016; Robison and Sparks, 2015). However, an evaluation of annular pressures and how they manifest in DSPT compared to HDD is necessary to determine if methods developed for the HDD industry are applicable to the DSPT technology.
DSPT mechanically excavates the borehole with a microtunnelling machine and removes the spoils from within the machine. There is no minimum pressure in the annulus governed by a fluid pressure gradient required to transport cuttings. Because the fundamental spoils removal mechanism is different for DSPT than for HDD, it is necessary to identify the key elements of the DSPT that impact the potential for inadvertent returns to: a) determine what elements of traditional inadvertent return risk models are applicable to DSPT b)further refine design guidance. The primary area of focus is the evaluation of annular lubrication versus engineered drilling fluid, and the thixotropic properties of bentonite lubrication.
Annular lubrication versus Drilling Fluid In horizontal directional drilling, drilling fluid is pumped to the drill bit to provide, among other uses, fluid assisted excavation. The drilling fluid typically consists of a mixture of water, bentonite, and polymers. The drilling fluid is mixed to a target viscosity that is selected based upon the geotechnical conditions. Arguably the most important function of the drilling fluid is removal of the excavated material from the borehole. The drilling fluid is continuously pumped throughout the borehole, providing borehole stabilisation and a means to remove excavated material. To this end, the drilling fluid must be pumped to form a pressure gradient large enough to lift the soil-laden drilling fluid from the point of excavation to the entry location. There is concern for inadvertent returns when the confining stresses of the soil or rock formation may not be sufficient to contain the pressure. Designers typically estimate the minimum drilling fluid pressures necessary to create the borehole, compare them to the confining stresses and apply a reasonable safety factor to determine an appropriate depth of the pipeline to minimise the potential for inadvertent returns during the HDD installation.
In many cases, the inadvertent return analysis establishes the minimum depth of the pipeline, often resulting in significantly deep installations. The lubrication that is injected into the annular space on a DSPT installation provides a different function to facilitate DSPT operations. With DSPT, the annulus is defined as the space between the full excavated diameter and the outer diameter of the steel pipe (Smith and Toelke, 2018), as shown in Figure 6.
The lubrication that is injected is a bentonite or bentonite/polymer mixture that is mixed to a significantly high viscosity, much higher than the drilling mud used in HDD. The highly viscous bentonite lubrication is pumped into the annular space from the lubrication ring located at the tail end of the microtunnelling equipment, providing full circumferential lubrication around the pipe.
The amount of lubrication pumped is based on a volumetric analysis, the target pumping rate will fill the annulus as the machine moves forward, ensuring that the annular space is completely filled with lubricant to the extent possible. Once injected the lubrication is not pressurised and is not subject to flow in the annulus. A launch seal is mounted at the entry portal that prevents the flow of lubrication up the annulus and into the launch portal. Figure 7 shows a typical entry seal that is used to contain the bentonite lubrication in the annulus.
Analyses of annular pressure have been completed suggesting that the bentonite lubrication induces a pressure in the annulus that is indicative of the elevation difference between the entry and the point of injection multiplied by the unit weight of the lubrication, resulting in a pressurised annulus capable of initiating an inadvertent return (Robison and Wilson, 2016, Robison and Sparks, 2015). However, it is important to consider the properties of the highly viscous bentonite used for lubrication, specifically the thixotropic nature of the lubrication. In layman’s terms, thixotropic means that the material changes from a fluid to a solid in the absence of shear force. In other words, the thixotropic material has a gel strength when stationary but flows under pressure. Both HDD and DSTP use bentonite-based fluids in the annulus with thixotropic properties (engineered drilling fluid for HDD and lubricant for DSTP). But while HDD clearly acts as a liquid when pressurised to circulate through the bore, DSTP does not have an active pressure gradient to induce fluidic properties in the annulus and a higher viscosity.
Although the lubrication is exposed to shearing in the annulus, it manifests as sliding friction on steel pipe as the surface roughness of the pipe is relatively smooth, allowing shearing to occur in very close proximity to the surface of the pipe (Reilly and Orr, 2016 and Staheli, 2006). The sliding friction mechanism is illustrated in Figure 8.
The vast majority of the lubrication in the annulus is not subjected to shear forces and retains gel strength. Therefore, using a full column mud pressure to estimate the annular pressure likely does not represent the annular condition and may overestimate the annular pressures. In practice, the thixotropic properties of the bentonite lubrication result in loading conditions that are indicative of a soft clay in the annulus, rather than a full column of fluid and likely do not contribute greatly to a hydrostatic column of pressure in the annulus. The resulting conclusion is the annular pressure due to the static mud column at any given point along a DSPT installation is much less than the unit weight multiplied by the bore elevation difference. Lubrication Injection Pressure and Annular Pressure Sensors
In DSPT, the lubrication is injected into a large annulus created by the difference in diameter between the microtunnelling machine overcut and the steel pipe. Minimum lubrication pressure does not have to overcome the full soil loading on the pipe; however, the lubrication pressure must be greater than the groundwater pressure to allow flow into the annular space. The groundwater pressure dictates the minimum gauge pressure required to pump lubrication into the annulus, rather than the full column mud loading or effective earth pressure. Although an assumed mud column pressure and hydrostatic parameters may be similar where the groundwater table is close to the surface, it is necessary to be aware that the groundwater pressure dictates the minimum lubrication injection pressure at the injection site.
The thixotropic nature of the lubricant combined with the lack of pressure gradient results in a substantially lower potential for inadvertent returns on DSTP when compared to HDD. Furthermore, the industry standard models available to calculate both the static and fluid flow components of the minimum annular pressure for an HDD installation are not applicable to DSTP.
The injection pressure also has a large effect on the frictional resistance of the pipeline. It has been shown that when the annular pressure is equal to or greater than the groundwater pressure, the friction along the pipe wall is markedly reduced due to the change in porewater pressure in the immediate vicinity of the pipe. Some microtunnelling machines have been fitted with annular pressure sensors to confirm the annular pressure (Herrenknect, 2021).
However, this is not a standard feature of microtunnelling machines used for DSPT applications and the body of data is small (Robison and Wilson, 2016). Research presented has shown annular pressure data recorded during a DSPT that illustrated that the actual pressures in the annulus were slightly lower than the full column mud loading. However, the project on which the measurements were taken had groundwater near the ground surface. Since the weight of lubrication that was used in the analysis had a specific gravity of 1.1, the full column mud loading and groundwater pressures differed by approximately 10%. The variation in data was such that 10% differences were not discernible.
Conclusion
Understanding the behaviour of the lubrication in the annulus of a DSPT operation is critical to the effective description and treatment of inadvertent return risk, especially in areas where the groundwater is significantly deep. Potential for inadvertent returns can be analysed with the Delft model, as included in the US Army Corps of Engineers design guidance (Latorre, et.al.).
However, the pressure in the annulus is not effectively modelled using either the static or fluid flow components of the equations often used to estimate HDD bore pressures. The lubrication is not pumped through the annulus to remove spoils and maintains gel strength such that the annular pressure on a DSTP is much lower than the pressure in an HDD annulus. Therefore, it is not appropriate to assume inadvertent returns are a risk that needs to be mitigated for all DSTP projects, but rather a project specific element that should be considered on a case-by-case basis when fluids are being injected into the ground beneath the surface similar to the approach taken during microtunnelling installation.
Understanding the behavior of the lubrication within the annulus of the DSPT will continue to advance as it has significant impacts on the development of thrust loading and is critical to recent record setting projects demonstrating the technologies’ ability to advance upwards of 1,500 m (5,000 ft) and greater in a single drive. Models are currently under development to provide guidance for the determination of thrust loads required to propel the microtunnelling machine and trailing pipe during DSPT which incorporate the principals related to lubrication and annular pressure treatment described above. www.stahelitrenchless.com
