Risks during the pull back operation of horizontal directional drilling

1. Introduction

The success of a horizontal directional drilling (HDD) is largely dependent on the success of the pull back operation, when the product pipe is installed in the created borehole. The cost of damaged pipelines and the costs for additional measures during and after the pull back operation can be considerable. In order to reduce the unexpected costs related to problems during the pull back operation, a detailed risk analysis should be carried out. The results of the risk analysis can be used to take a series of risk reducing measures.

2. Background

Recently the Gasunie constructed a gas-pipeline which connects the English gas distribution network to the Dutch gas distribution network. Part of the gas pipeline is constructed in the North of the Netherlands in between the North Sea Coast and the Gasunie distribution station. The larger part of the 48” gas pipeline is constructed in a trench, but several motorways, dikes and rivers are crossed using the horizontal directional drilling method (figure 2.1).

During the pullback operations at several HDD’s some problems arose. The problems were varying from high pulling forces to uncompleted pull back operations due to a jammed pipeline and from slight coating damages to deteriorated pipelines and deteriorated drilling pipes. The nature of the mentioned problems is related to the pipelinesoil interaction.

3. Pipeline-Soil Interaction during the Pullback Operation

The pipeline soil interaction is defined as the combined behaviour of the pipeline and the surrounding soil in terms of stresses and deformations. During the pull back operation the moving pipeline contacts the wall of the borehole and pushes with a certain forces perpendicular to the wall of borehole. These perpendicular forces (normal forces) determine the magnitude of the shear force in axial direction during the pull back operation and lead to a deformation of the soil. The distribution and the magnitude of the normal forces on the bore hole wall are of major importance in the pipeline soil interaction.

The pipeline soil interaction is mainly determined by the following aspects:

• 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

The subsequent paragraph deals with the different aspects and their influence on the pipeline-soil interaction.

3.1 Stiffness of the pipeline

The stiffness of the pipeline and in minor extent the strength of the pipeline determine the reaction of pipeline on bending in the created bore hole. The bending moment in a horizontal directional drilling bend is defined as follows:

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]

The magnitude of the moment is determined by the stiffness (EI) of the pipeline and the bending radius. A higher stiffness or a smaller bending radius lead to a higher bending moment and hence lead to higher normal forces on the bore hole wall. Since the Stiffness of the pipe depends upon the pipe material and the moment of inertia, both the diameter and the wall thickness of the pipeline are significant parameters.

In case the stresses in the pipeline due to bending and the pulling force during the pull back operation is further increasing, the yield stress of the pipe material may be exceeded. In this undesirable situation, the normal stress distribution on the bore hole wall deviates from a normal situation.

3.2 Stiffness and Strength of the soil

The bending moment and the resulting distribution of normal forces on the bore hole wall leads to a soil reaction pressure. Using the theory described by Hetényi [1], the following expression for the maximum soil reaction can be derived:

Q_r = 0.322 [λ² / D_e] M

With:
Q_r: the soil reaction pressure [kN/m²]
D_e: the outside diameter [m]
M: the bending moment [kNm]
λ: the characteristic length [1/m] 

The above mentioned soil reaction pressure is based on elastic soil behaviour. It should be noticed that, especially at the head of the pipeline (at the connection with the barrel reamer-drill pipe configuration), soil deformation behaviour can be plastic. Plastic soil behaviour leads to non recoverable deformation, which in turn may lead to a displacement of the stiff pipeline beside the pre reamed bore hole. In the subsequent figure 3.1, the displacement of the pipeline to a position below the pre reamed bore hole in the upward bend of the HDD is shown. At the moment of change of the drill pipes at the rig, the pulling force drops, which allows the pipe to bend into the bore hole wall. Due to the significant higher stiffness of steel pipelines with respect to synthetic pipelines, the soil reaction pressure induced by bending is significant higher for steel pipelines.

The above described deformation-mechanism was the reason for Brink et. al. [2] to derive an expression for the minimum bending radius for the pull back operation of steel pipelines in a type of soil:

R = C ⋅ √D_e ⋅ d_n

In which:
R: is the minimum bending radius [m]
D_e: is the outside pipe diameter [m]
d_n: is the nominal wall thickness [m]
C: is a soil dependent constant [-]

The minimum bending radius can be used for a design with a minimal displacement of the steel pipeline beside the prereamed bore hole.

For soft soils, the C constant has higher values, for soils with higher stiffness (and strength) the C constant is considerable lower [2].

3.3 Effective weight of the pipeline

In order to reduce the normal forces on the borehole wall the pipeline is often ballasted during the pull back operation. In the part of the HDD without significant bends, the distribution of the normal forces on the bore hole wall is determined by the effective weight of the pipeline. The effective weight is defined as follows:

g_eff = g - g_opw

With:
g_opw = π ⋅ r_e² ⋅ γ_fl

And

r_e: Outer radius of the pipeline [m]
g_opw: Upward force of the pipeline [kN/m]
g: Weight of the ballasted pipeline [kN/m]
γ_fl: Unit weight of the drilling fluid

From the above described equations, it can be deduced that the unit weight of the drilling fluid plays an important role in ballasting the pipeline during the pull back operation. It should be noticed that the unit weight of the drilling fluid depends upon various factors:

• 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

Possibly, in certain soil types a vertical gradient of the unit weight of the drilling fluid in the bore hole may exist. Experimental studies of the transport of soil particles in a bore hole show the existence of a so called ‘gelled bed’ in the bore hole [4].

3.4 Occurrence of obstacles in the soil

In case the HDD’s are located in the area in which obstacles such as blocks and boulders or man made obstacles can be found, contact between the blocks and boulders or other types of obstacles and the pipeline during the pull back operation may lead to damage of the pipeline or to damage of he coating of the pipeline. The seriousness of the damage depends mainly on the pipeline-soil interaction (normal force on the bore hole wall between boulder and pipe). In straight sections of the drilling line, the normal force) is largely determined by the effective weight of the ballasted pipe, while in the bends this force is largely determined by the bending stiffness of the pipe in relation to the bending radius of the drilling line.

Other aspects which determine the magnitude of the force in between the pipe and the obstacle are:

• Roundness of the obstacle
• Dimensions of the obstacle
• Deformability of the soil in which the obstacle is embedded. 

The sharpness or roundness of the obstacle at the pipeline contact can be described in terms of minimal surface radii of the obstacle. Sharp obstacles with a lower contact area lead to higher contact forces. Of course, the displacement of the obstacle into the soil during contact with the pipeline plays a role in the development of the magnitude of the contact force. This displacement is determined by the obstacle soil interaction, which in turn is determined by the dimensions of the obstacle in combination with the deformability and strength of the surrounding soil.

3.5 Shape of the borehole in axial and radial direction

Obviously, the shape of the pre-reamed bore hole in axial direction (the direction in which the pipeline has to be pulled in) is one of the major aspects in pipeline soil interaction. The complex drilling process leads to deviations of the design drilling line during the pilot phase. Deviations and anticipating steering actions lead to corrections, which often decrease the deviation to the design drilling line. These corrections however, may lead to the so-called small distance irregularities in the drilling line. Although it should be noticed that the reaming operations may have a smoothing effect on the drilling line, the existence of irregularities and small 3D bending radii along the axis of the reamed bore hole can not be neglected.

Deviations to the design drilling line, which remain after the reaming operations, lead to higher pulling forces during the pull back operations (figure 3.3). Cheng and Polak [5] compared calculated pulling forces based on an irregular pre-reamed bore hole with data from field experiments and concluded that the increase in pulling force is considerable.

The shape of the cross section of the bore hole can possibly change during the reaming operations. In a type of soil, which is susceptible to erosion due to flow of drilling fluid, the size of the bore hole can increase easily. In case of heavy reamers or high pulling forces during reaming respectively the bottom of the bore hole or the top of the bore hole are more susceptible to erosion. Due to asymmetrical erosion, a vertical elongated bore hole can be formed. The radial shape of the cross section of the bore hole may affect the pipeline soil interaction in the following ways.

• 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

3.6 Bore hole stability

The stability of the borehole is the most important factor for the success of the pull back operation. In case of instability of the bore hole the pipeline soil interaction changes rigorously. In case of a collapsed bore hole a huge soil load exerts on the pipeline. Whereas local bore hole instability leads to higher pulling forces, which can be overcome, bore hole instability over a larger distance will certainly lead to a jammed pipeline and thus an incomplete pulling operation.

Centrifuge tests on borehole stability during horizontal directional drilling were carried out by Viehofer et. al. [6]. Results of the centrifuge tests showed that a borehole is usually stable in normal soil types due to the mechanism of arching. The arching mechanism of bore holes is described by Meijers and de Kock [7]. From the centrifuge tests results was deduced that just the static mud pressure acts as a load on the pipeline. The centrifuge test showed that borehole stability can be achieved with only very small mud pressures in the borehole due to the occurrence of arching. In the centrifuge test and in finite element calculations was found that reduction of the pressure of the drilling fluid leads to bore hole instability (figure 3.4).

Besides reduction of the pressure of the drilling fluid, bore hole instability can be caused by:

• 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.

4. Risk Analysis

The pipeline soil interaction during the pull back operation has a major influence on the success of the pull back operation. The complex pipeline soil interaction can be divided in a several aspects (processes and phenomena) which are described in the previous paragraphs. The consideration of the described aspects is used for a global risk analysis for future horizontal directional drillings. The following risks were distinguished:

• 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.

5. Risk Management

Based on the defined risks a series of measures to reduce the magnitude of the risks could be established. The following measures can be taken before starting future HDD’s and during the performance of the future HDD’s:

• 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[2].
• 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. 

6. Conclusions

The success of a pullback operation of a horizontal directional drilling is largely dependent upon the pipeline soil interaction. Consideration of all aspects of the combined pipeline soil behaviour formed the bases of a global risk analysis. The shape of the pre reamed bore hole in axial direction, which is the result of design radii and irregularities due to steering corrections, is of major importance for the distribution of normal forces perpendicular to the bore hole wall. Risks on high pulling forces and damage of the coating due to contact in between the pipeline and obstacles are related to the distribution and magnitude of the normal forces. These risks can be reduced by usage of new formulas for the calculation of the design bending radii [2] and measurement of the shape of the bore hole before starting the pull back operation.

The stability of the borehole is the most important factor for the success of the pull back operation. The risk on bore hole instability can be reduced by a detailed soil investigation with attention for the packing and uniformity of the soil type and measurement of the hydraulic head and the chloride concentration of the groundwater in the soil layers through which the drilling is carried out.

7. References

[1] Hetényi, M., 1946 Beams on elastic foundations. Scientific series, Ann Arbor, University of Michigan press.
[2] 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.
[3] 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.
[4] Bisschop, F., 1995 transport processes in borehole and pipeline (In Dutch), Drilling of tunnels and pipelines research group, Delft.
[5] Cheng, E. and Polak, M. A., 2005 Modelling installation loads for pipes in horizontal directional drilling, Proc. No dig conference Rotterdam
[6] Viehofer, T., Linthof, T. and Bezuijen, A. 2005, Stability of a borehole during horizontal directional drilling, Proc. No dig conference Rotterdam
[7] Meijers, P. and De Kock, R.A.J. 1993, A calculation method for earth pressures on directionally drilled pipelines, Pipeline conference 1993, Belgium.