Risks during the pull back operation of horizontal directional drilling
Apr 02, 2008
The success of the pull back operation of horizontal directional drilling can be increased by performance of a risk analysis. The problems, which might occur during a pull back operation, are varying from high pulling forces to uncompleted pull back operations and from slight coating damages to deteriorated pipelines. The nature of these problems is related to the pipeline-soil interaction. The pipeline soil interaction can be defined as the combined behaviour of the pipeline and the surrounding soil in terms of stresses and deformations. Consideration of all aspects of the combined pipeline soil behaviour formed the bases of a global risk analysis of the pull back operation. Besides high normal forces on the bore hole wall in case of small bending radii or irregular shaped bore holes, occurrence of obstacles and bore hole instability are identified as major risks during the pull back operation. The risk analysis resulted in a series of risk reducing measures for future horizontal directional drillings.
- Stiffness and in minor extent strength of the pipeline
- Stiffness and Strength of the soil
- Occurrence of obstacles in the soil
- Shape of the borehole in axial and radial direction
- Effective weight of the pipeline
- Borehole stability
M = EI / R
M = Bending moment [kNm]
E = Modulus of elasticity of pipe material [kN/m2]
I = Moment of inertia [m4]
R = Bending radius [m]
Qr = 0.322 [λ2 / De] M
Qr: the soil reaction pressure [kN/m2]
De: the outside diameter [m]
M: the bending moment [kNm]
λ: the characteristic length [1/m]
R = C ⋅ √De ⋅ dn
R: is the minimum bending radius [m]
De: is the outside pipe diameter [m]
dn: is the nominal wall thickness [m]
C: is a soil dependent constant [-]
For soft soils, the C constant has higher values, for soils with higher stiffness (and strength) the C constant is considerable lower .
geff = g - gopw
gopw = π ⋅ re2 ⋅ γfl
re: Outer radius of the pipeline [m]
gopw: Upward force of the pipeline [kN/m]
g: Weight of the ballasted pipeline [kN/m]
γfl: Unit weight of the drilling fluid
- Initial unit weight of the drilling fluid
- Type of soil through which the drilling is carried out
- Number of reaming operations
- Flow characteristics during the last reaming operation
- Time since the last reaming operation
- Flow characteristics during the pull back operation
- Roundness of the obstacle
- Dimensions of the obstacle
- Deformability of the soil in which the obstacle is embedded.
- A smaller diameter of the bore hole at the top or at the bottom (smaller reamer sizes during the first reaming operations)
- A deviation of the axial shape of the bore hole with respect to the drilling line and therefore a deviation of the pulling in line for the pipeline
- A relative high hydraulic head in an aquifer (groundwater pressure higher than the static pressure of the drilling fluid).
- An instable drilling fluid due to flocculation of bentoniet suspension in salt or brackish groundwater
- A borehole in loose packed sand, in which small shear stresses may lead to collapse of the surrounding soil.
- A borehole in a granular soil type with an uniform grain size distribution, so that interlocking, which is required for arching in granular soils, can not occur.
- A high pulling force caused by wrong ballasting with respect to the weight of the drilling fluid.
- A high pulling force caused by a small design bending radius. Higher normal forces on the bore hole wall and higher radial displacements of the pipeline during the pull back operation are also caused by a small design bending radius. High radial displacements may lead to breakage of drill pipes.
- Damage of the coating of the pipeline caused by unawareness of obstacles of natural origin due to misinterpretation of the results of the soil investigation.
- Damage of the coating of the pipeline caused by occurrence of obstacles of man made origin.
- Damage of the coating of the pipeline caused by wrong choice of the coating of the pipeline in relation to the forces in between the pipeline and the obstacles.
- A high pulling force caused by an irregular shaped bore hole with small 3D-bending radii.
- A high pulling force caused by bore hole instability due to soil and groundwater conditions.
- A high pulling force caused by bore hole instability due to erroneous chosen drilling fluid.
- Measurement of the weight of the drilling fluid during the drilling stages and application of additional reaming operations.
- Usage of new formulas for the calculation of the design bending radii.
- Detailed soil investigations with attention for the occurrence of obstacles.
- Historical research on the occurrence of man made obstacles.
- Usage of a strong coating to prevent damage due to contact with obstacles.
- Measurement of the shape of the bore hole before starting the pull back operation.
- Detailed soil investigations with attention for the occurrence of bore hole instability.
- Measurement of the chloride concentration of the groundwater in the soil layers through which the horizontal directional drilling is carried out.
 Brink H.J., Kruse H.M.G., Luebers, H., Hergarden. H.J.A.M., and Spiekhout J., 2007, Design guidelines for the bending radius of large diameter steel pipes for HDD construction, submitted for publication in 3R international.
 Bles, T.J., Hergarden. H.J.A.M, and Kruse H.M.G 2007, Soil Related Risks during the Pull-back-phase of a Horizontal Directional Drilling (HDD), Proc. Oldenburger Rohrleitungsforum.
 Bisschop, F., 1995 transport processes in borehole and pipeline (In Dutch), Drilling of tunnels and pipelines research group, Delft.
 Cheng, E. and Polak, M. A., 2005 Modelling installation loads for pipes in horizontal directional drilling, Proc. No dig conference Rotterdam
 Viehofer, T., Linthof, T. and Bezuijen, A. 2005, Stability of a borehole during horizontal directional drilling, Proc. No dig conference Rotterdam
 Meijers, P. and De Kock, R.A.J. 1993, A calculation method for earth pressures on directionally drilled pipelines, Pipeline conference 1993, Belgium.
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