Technical paper: fluidic drag estimation in HDD
The following is an excerpt from a larger research paper that proposes a new method for evaluating hydrokinetic pressure and fluidic drag changes during pipe installation operations using horizontal directional drilling (HDD). The method is based on applying the solution of eccentric annular flow using the finite volume method (FVM) to the HDD case.
Using MATLAB programming language, a computer code has been developed. Through an iterative procedure, the code determines the hydrokinetic pressure that satisfies the continuity equation within the bore, and returns the corresponding calculated fluidic drag.
For verification, pull-back data of two actual HDD steel pipe installations completed in Alberta, Canada, have been used, and the estimated forces are compared against recorded ones in the field. The differences between the estimated and measured end pull-back forces are 40 per cent and 7 per cent for the two installations, while the corresponding values for the Pipeline Research Council International (PRCI) method are significantly higher, 238 per cent and 85 per cent, respectively.
As one of the fastest growing trenchless techniques for pipe installation, HDD has gained attention from both industry and academia over the past few decades. This growth in popularity can be attributed to HDD’s ability to provide a practical solution to factors such as the costs associated with traffic disturbance in congested urban areas, and strict environmental regulations regarding utility placement across rivers and wetlands.
The technique causes minimal damage to the surrounding environment and disruption to surface traffic, making it a promising solution for pipe placement in high-risk areas. A HDD project typically includes three main phases: first, a steerable and tractable driller creates a pilot bore; second, the pilot hole is enlarged to a diameter greater than the product pipe; and third, the product pipe is pulled into the enlarged bore.
In spite of HDD’s growth in popularity, the interaction of the installed pipe with the surrounding soil and slurry medium during pipe installation phase is not well understood, leading to conservative pipe design.
Due to a lack of relevant investigations, current HDD design references rely on studies completed in other industries, such as oilwell directional drilling and utility cable installation industries.
Subsequently, in design references such as ASTM F1962 and PRCI, unique HDD characteristics are sometimes ignored. One area requiring investigation is the evaluation of hydrokinetic pressure and fluidic drag during the pipe placement phase, which is the focus of this study. Hydrokinetic pressure is the minimum in-bore pressure at the location of the drilling fluid discharge from the drillstring into the bore in addition to the hydrostatic pressure.
This incremental pressure must be maintained during pull-back operations to exhaust the slurry from the bore to the surface at a desired flow rate. This helps maintain bore stability, lubricates the bore wall, and transports residual cuttings created in the reaming or swabbing phase.
Fluidic drag is the incremental force developed on the pipe’s leading end during installation due to slurry interaction with the in-bore portion of the pipe. Similar to the frictional drag component of the pullback force, this component also resists pipe installation.
In oil well drilling operations, the drilling fluid is pumped from a ground mounted rig into the drillstring. After being discharged from the nozzle and getting mixed with cuttings, the resulting slurry flows back from the well bottom to the ground surface through the annular space between the drillstring and the well.
In terms of the slurry flow pattern, this process resembles the pilot bore drilling phase, where the total slurry volume within annular space flows in a direction opposite to the pipe movement throughout the operation.
During the pull-back phase, however, since the total bore length has already been reamed to its final diameter, the slurry exits to the ground surface through the product pipe
and/or drill rod annulus depending on the product pipe’s leading head location. Similarly, lightweight utility cable installation via the blown-cable technique, which is the basis of the current ASTM F1962 fluidic drag evaluation, observes annular flow in a singular direction during installation.
Adopting the equation used to determine the drag force exerted on a utility cable’s outer surface, ASTM F1962 recommends using the following simple equation for estimating the fluidic drag:
where rb (m) and rp (m) are the radii of the bore and pipe, respectively, and ΔP (Pa) is the hydrokinetic pressure; ASTM F1962 estimates the hydrokinetic pressure to be
70 kPa without any qualification. This equation does not provide any information on drag change with the installation’s progress, and the effects of slurry rheology are also implicitly considered in the pressure value. PRCI does not provide an estimate for the hydrokinetic pressure.
However, it proposes determining the fluidic drag by multiplying the pipe’s outer surface area in contact with the slurry by a fluid drag coefficient as follows:
where L1 (m) is the in-bore length of the product pipe and μmud (Pa) is the fluid-drag coefficient with a value of 350 Pa adopted from the Dutch standard NEN 3650, Requirements for pipeline systems. Contrary to ASTM F1962, the estimated drag by PRCI evolves with the installation progress; however, the effects of slurry rheology and annulus size are ignored.
To estimate the fluidic drag component of pull-back force, Polak and Lasheen implemented the solution of Navier-Stokes’ equation in cylindrical coordinates for the flow of incompressible Newtonian fluids. In the proposed solution, the pipe pull-back rate and the annulus geometry were considered.
The main shortcoming of their work is the assumption that the entire drilling fluid volume returns to the surface through the space between pipe the and the borehole wall in the opposite direction of pipe pull; this is applicable to oil well drilling operations, but not to HDD.
Baumert et al. reviewed the existing methods for hydrokinetic pressure evaluation during the HDD installation phase and provided recommendations to improve the current design practice. Supposing the slurry to behave as a Bingham plastic fluid, they concluded that a slot-flow approximation could be adopted for pressure drop modeling; however, some improvements, such as introducing a factor for pipe eccentricity inclusion and using low shear rate mud rheological parameters, were suggested to improve accuracy.
The adopted equations for pressure drop estimation in this study were originally from the oil-well drilling industry, and the authors implemented them without making any adjustment to account for different flow patterns observed in HDD pipe installation.
In an attempt to model the fluidic drag component of the pull-back force more realistically, Dyvestyn also implemented the slot flow approximation but considered the slurry flow direction change during the pull-back (pipe installation) phase. He assumed that in an installation with the pipe leading head between the pipe entry and crossover points, the total slurry volume flows to the surface through the annular space between the product pipe and the bore. Then, once the crossover point has been reached, the slurry flow direction switches, and the slurry moves in front of the product pipe towards the rig.
To determine the crossover point location, the hydrokinetic pressure required for exhausting the slurry via the product pipe and the drill rod annuli was calculated in terms of in-bore pipe length. After equating these pressures and solving for the in-bore pipe length, the crossover point was calculated.
The major shortcoming of this study was to ignore the fact that, in a typical installation and over a considerable length, the slurry returns to the surface through the both the product pipe and the drill rod annular space. Furthermore, the pull-back rate effects were not considered.
This paper introduces a new method for determining the fluidic drag based on using the finite volume method (FVM) for solving the governing partial differential equation of fluid motion in an eccentric annulus. This method, unlike the ASTM and PRCI methods, considers the effect of different parameters, including the geometry of the product pipe and the drill rod annuli, pull-back rate, pipe eccentricity, and slurry rheology, making it a promising tool for accurate pipe slurry interaction modeling.
The new method enables the HDD designers to simulate and follow the hydrokinetic pressure and fluidic drag changes with an installation’s progress. Furthermore, it equips the HDD contractors and field crew with a tool to estimate the pattern of the gradual change of drilling fluid return to the surface with installation progress, which is an important feature of the new method from the drilling fluid circulation management point-of-view.
The full version of this paper was published in the September 2016 edition of the Journal of Pipeline Engineering (JPE). To request a copy of the full article, or to subscribe to the journal, visit the JPE website
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