The trenchless technique horizontal directional drilling · PDF file4th Pipeline Technology Conference 2009 The trenchless technique horizontal directional drilling Soil related risks

4th Pipeline Technology Conference 2009

The trenchless technique horizontal directional drilling

Soil related risks and risk mitigation

Dr. H.M.G. Kruse Deltares national institute, unit geo-engineering

(E-mail:[email protected]) Abstract The horizontal directional drilling technique is very popular nowadays and the majority of the cables and pipelines installed with this technique succeed, but in several cases the technique still lacks sufficient control of risks coming from the underground. Therefore, a soil related-risks analysis for the prevention of unwanted events during the drilling stages or after the installation activities is recommended. The identification of the risks is the most important stage in the risk analysis. The identification of the risks requires sufficient knowledge of the processes during the drilling stages. Once all the risks are identified in a qualitative way the importance of risks can be determined. Very often calculations are necessary to classify the risk in a quantitative or semi –quantitative way. After classifying the risks, mitigation measures can be established for the significant risks. Since an overview of all possible risks in horizontal directional drilling would be extensive, this paper focuses on the most occurring soil related risks in horizontal directional drilling: (1) High pulling forces or incomplete pull back caused by local bore hole instability, (2) High pulling forces or incomplete pull back caused by frictional forces in the borehole and (3) Seepage through the borehole to the surface. 1. Introduction The limited amount of space in our densely populated area is constantly under pressure. A growing population combined with increased business activity has meant that new residential areas and industrial sites are being built at a wide variety of locations. The underground infrastructure is an important part in our modern world. Socio-Economic developments in usually short time periods and in limited space require an adequate policy and flexible attitude of both the authorities and the construction sector. Trenchless techniques for the installation of cables and pipelines facilitate the building in short time period and limited space operations and are therefore a necessary tool in our modern world. Since the early nineties of last century the trenchless techniques were applied more often [1]. Especially the horizontal directional drilling technique became very popular. Horizontal directional drilling (HDD) is one of the fastest growing techniques for trenchless installation of pipelines. On one hand they provide a logical alternative when cables, pipelines or small tunnels need to cross roads, railways, dikes, wetlands, rivers and other structures that have to remain intact. On the other hand HDD techniques minimize the impact of installation activities in densely populated and economical sensitive areas.


Figure 1 Horizontal directional provides installation of pipelines below economical sensitive areas In a very short time period, horizontal directional drilling has become a normal technique for the installation of cables and pipelines [1]. The benefits are very clear, while the horizontal directional drilling technique:

• requires a relatively short installation period; • relatively low installation costs; • minimizes negative impact of installation on structures such as roads railways and

dykes. • no interruption of the traffic on roads railways and waterways; • minimizes the negative impact of future activities on the surface above the pipeline or

cable. Although the horizontal directional drilling technique is very popular nowadays and the majority of the cable and pipeline installations succeed, the technique still lacks sufficient control of risks coming from the underground [2]. Performance of a risk analysis can help to control the risks. 2. Horizontal directional drilling Horizontal directional drilling has been used on a large scale since the eighties. Since the seventies Deltares has been involved in the development and execution of trenchless technologies. Years of research have resulted in one of the first design codes for Horizontal directional drilling, as well as a computer program called Mdrill. In horizontal directional drilling three installation stages are considered (figure 2):

• Pilot drilling • Reaming the initial pilot borehole • Pulling back the pipeline in the reamed borehole

The initial borehole is called a pilot hole. The borehole is drilled using drilling fluid, which is brought into the borehole by the rotating drilling head. Due to the relative high pressure at the drilling head the drilling fluid facilitates the transport of cut soil pieces towards the surface The diameter of this pilot hole is enlarged using a reamer. Depending on the required final borehole diameter, the borehole can be enlarged in several steps using reamers of increasing diameters. Finally, the pipeline is pulled into the created borehole. In all the three drilling stages drilling fluid is present in the borehole


Figure 2 Horizontal directional drilling stages (Source: Bayer [1]) In horizontal directional drilling three themes are important for a successful horizontal directional drilling:

• The drilling equipment; • The drilling fluid; • The soil layers ( including groundwater) through which the drilling is carried out.

Since the first two themes are strongly related to the third one (the choice of the right drilling equipment and engineering of the drilling fluid is mainly determined by soil and groundwater), the theme soil (including groundwater) is the most important in horizontal directional drilling. Knowledge of the mechanical en chemicals characteristics of the underground and the groundwater and the influence of these characteristics on the drilling processes is of major importance for a risk analysis for a horizontal directional drilling 3. Soil related risk analysis The risk is usually defined as the probability of an unwanted event multiplied with the consequences of this event. In the analysis of the risks the following steps can be distinguished [10]:

1) Data gathering 2) Risk identification 3) Risk classification 4) Risk mitigation by taking control measures 5) Evaluation of the residual risk profile


The second step is the most important one in the risk analysis. The identification of the risks requires sufficient knowledge of the processes during the drilling stages. Once all the risks are identified in a qualitative way the importance of risks can be determined. Very often the calculations are necessary to classify the risk in a quantitative or semi –quantitative way. After classifying the risks, mitigation measures can be established for the significant risks. Since an overview of all possible risks in horizontal directional drilling would be extensive, this paper focuses on the most occurring soil related risks in horizontal directional drilling:

1) High pulling forces or incomplete pull back caused by local bore hole instability; 2) High pulling forces or incomplete pull back caused by frictional forces in the borehole; 3) Seepage through the borehole to the surface.

The bore hole instability related problems can occur during the three drilling stages. Of course, collapse of the borehole is most dramatic during the pull back operation. Pulling forces may increase beyond the drilling rig capacity or beyond the yield strength of the pipe material. The following causes for borehole instability can be distinguished:

• Due to loose packed granular soil layers; • Due to the presence of very permeable gravel layers; • Due to high ground water pressure; • Due to groundwater flow; • Due to extreme low strength of soil layers; • Due to chemical reaction of drilling fluid with ground water; • Due to chemical reaction of drilling fluid with soil particles.

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 larger part of the magnitude of the shear force in axial direction during the pull back operation and lead to a deformation of the soil. The magnitude of the normal forces on the bore hole wall increases in the curved sections of the drilling path The following causes for high frictional forces can be distinguished:

• Due to a stiff pipeline in relation to the strength of the soil and the bending radius of the drilling path;

• Due to heterogeneous soil, which lead to steering problems, which in turn lead to an irregular borehole and an irregular drilling path;

• Due to erroneous ballasting of the pipeline in relation to the unit weight of the drilling fluid in the borehole.

In case a horizontal directional drilling is carried out through layers in which a higher ground water pressure is present the risk of seepage exists. In this case the role of the drilling fluid is extremely important since the occurrence of seepage is controlled by the static pressure of the drilling fluid. The following causes for occurrence of seepage can be distinguished:

• Due to a high hydraulic head in an aquifer, which exceeds the static downward pressure of the drilling fluid;

• Reduction of the density of the drilling fluid during the drilling stages; • Excavations at the entry or exit point for the tie-in weld; • Due to chemical reaction of drilling fluid with ground water; • External causes of high groundwater pressure such as pipeline damage.

Knowledge of the above mentioned processes which determine the magnitude of the 3 types of risks is required for an adequate classification and the establishment of mitigation measures. The subsequent chapters deal with the background of the mentioned processes.


4. Bore hole instability 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 length in the bore hole will certainly lead to a stuck pipeline and thus an incomplete pulling operation. Centrifuge tests on borehole stability during horizontal directional drilling were carried out by Viehofer et. al. [4]. 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 [3]. The centrifuge test showed that borehole stability can be achieved with only small excess drilling fluid pressures in the borehole due to the occurrence of arching. In the centrifuge test and in finite element calculations was found that a significant reduction of the pressure of the drilling fluid leads to bore hole instability (figure 3).

Figure 3.. Bore hole instability in FEM calculations due to absence of mud pressure (After Viehofer et.al. [4]) When the ratio of the soil cover and borehole diameter is low [3], there is a change that the borehole will collapse, which results in soil deformation at surface. This phenomenon can often be seen near entry and exit point where the depth of the borehole is small.

Figure 4 Ratio of soil cover and borehole diameter is important for the development of arching

H

D

β

borehole

Dborehole

Harching


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 (especially in granular soils); • 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. If possible, the design-drilling path should avoid situations, in which borehole stability is difficult to maintain. For example, drilling paths through layers of coarse gravel, loose sand and very soft clay/peat should be avoided or mitigation measures should be taken. 5. High frictional forces during the pull back operation Calculation of the pulling force During pulling back, the product pipeline in the bore hole will be subjected to tensile forces resulting from: - friction between pipe and drilling fluid - friction between pipe and the soil and the bore hole wall The friction in between the drilling fluid and the pipeline does not vary along the drilling path. The friction in between the product pipe and the bore hole wall depends upon the distribution of the forces perpendicular to the bore hole wall. Since this distribution of forces depends upon the curvature of the drilling path differentiation between straight sections of the drilling path and curved sections of the path must be made [5]. Straight section In order to reduce the normal forces on the borehole wall, the pipeline is often ballasted during the pull back operation. The distribution of the normal forces on the bore hole wall in the straight section of the drilling path is determined by the effective weight of the pipeline. The effective weight is defined as follows:

opweff ggg −=

With:

fleopw rg γπ ⋅⋅= 2

And

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 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 [6]. Curved section In curved sections of the borehole, the pipe is subjected to elastic bending. From elementary beam theory it is known that if a pipeline is bent into a perfect circle the bending moment is:

REIM =

Where EI is the bending stiffness of the pipeline and R the circle radius. The bending moment can only exist if the pipeline is able to mobilize reaction forces. The forces of the moment must be provided by the soil. Using Hetényi’s theory [7], it is possible to determine the profile of the soil reaction (see also figure 5).

Figure 5. Soil reaction at the end of an elastic bend according Hetenyi [3] The soil reaction of the pipeline located in a curve is calculated as follows:

with:

where: Qr = maximum soil reaction near the end of the bend [N/mm2] kv = vertical modulus of subgrade reaction [N/mm3] y = maximum displacement [mm] EI = bending stiffness of the pipe [Nmm2] R = radius of the bend [mm] The described calculation method is valid when the pipeline is pulled in entirely in the curved section of the create borehole. When the head of the pipeline is located in a curved section or near a curved section, the distribution of the soil reaction forces is different. Deltares developed an FEM based pull back model based on Abaqus software in order to investigate the behaviour of the pipeline in the curved sections on. FEM Simulations are performed in order to investigate the behavior of the head of the pipeline during the pull back operation in the upward circular bend.

RDEIykQ

ovr .

..322.0.2λ

==

4.4.EIDk ov=λ


In the model simulations the curved section of the horizontal directional drilling path extended with a straight section towards the surface with a variable length of 0, 10, 20, 40, 60, 80, 100 m (figure 6).

Figure 6: Simulation results of the soil reaction forces for a pipeline lying in the upward circular bend of a drilling path and at certain distances beyond the bend. For a straight section length of zero, when the head of the pipeline is located in the curved section, it can be seen that the a peak soil reaction force is observed. For increasing lengths of the straight section the peak soil reaction force becomes less. For a straight section length of 100 m, the forces and corresponding displacements (bore hole wall penetration in figure 7) at the end of the circular bend have the same magnitude and shape as those at the beginning of the circular bend (on the left side). The force at the head of the pipeline has become zero, which indicates that longer straight sections will not influence the distribution of the soil reaction forces. It can be concluded that the soil reaction forces are much higher when the head of the pipeline is located in the bend compared to when the pipeline head has passed through the bend.

Figure 7: Bore hole wall penetration as a results of high soil reaction forces


Depending on the ground conditions, the high soil reaction force in the curved section may lead to damage of the coating of pipelines and may lead to penetration of the borehole wall (figure 7), which in turn leads to high pulling forces and may lead to a stuck pipeline or to damaged pull back equipment. The complex drilling process leads to deviations of the design drilling path during the pilot phase. Deviations and anticipating steering actions lead to corrections, which often decrease the deviation to the design drilling path. These corrections however, may lead to the so-called small distance irregularities in the drilling path. Although it should be noticed that the reaming operations may have a smoothing effect on the drilling path, the existence of irregularities and curved sections with small 3D bending radii along the axis of the reamed bore hole can not be neglected. Since the curved sections lead to higher soil reaction forces and hence to higher pull back forces shape of the reamed hole is of major importance. The subsequent figure 10 shows a series of curve related soil reaction forces at the end of the pull back operation of a horizontal directional drilling in the Netherlands with an irregular drilling path.

Figure 8. The soil reaction forces at a completed pull back operation for an irregular drilling path. 6. Seepage When drilling through water bearing sand layers (aquifers), with a higher water pressure then the phreatic groundwater level in the aquitard, there is a possibility of seepage through the borehole during the performance of the directional drilling or after the pipeline installation [11]. Occurrence of seepage depends on the height of the piezometric head of the groundwater in these layers. Seepages should be avoided, while it can lead to a groundwater flow through the borehole resulting in a collapse of the borehole or in subsidence of the surface due to erosion of soil. Seepage through the borehole is difficult to stop once the process started. The risk of seepage should be checked for the following stages of horizontal directional drilling:

• During drilling. The weight of the drilling mud should be sufficient to resist the groundwater pressure in all soil layers through which the drilling is carried out (figure 9);

• After drilling when the pulled pipe is connected with the field pipeline (tie-in weld). When the connection takes place in a dry pit, the height of the column of drilling mud will be decreased and therefore the weight of the column (figure 9). The risk on seepage will increase.

In case of a chemical reaction between the groundwater and the drilling fluid the characteristics of the drilling fluid can change. For example in salt or brackish water a


bentonite based drilling fluid is susceptible to flocculation. Flocculation leads to sinking of the solids so that the density of drilling fluid decreases. If the density of the drilling fluid becomes lower than the upward ground water pressure seepage will occur.

Figure 9 Seepage through the borehole during drilling and after drilling

(Source: Hergarden [11] Once the installation of the pipeline including the tie-in weld is completed, local high groundwater pressures can be caused by damaged water pipelines. In case the installed water pipeline is damaged in a relative short period after the construction, the drilling fluid is still in a liquid condition, so that seepage may occur. Deltares developed a method to calculate the increase in groundwater pressure due to a damaged water pipeline in an aquifer [9]. Although this method was originally developed for the evaluation of the stability of river dikes and other constructions below which water pipelines were installed using a trenchless technique, it can be applied for the evaluation of seepage in horizontal directional drilling. The increased groundwater pressure in the aquifer can be calculated using the subsequent equation:

2 v

R r Rh erfcr c tϕ ⋅ −

=


where: R = pipeline radius [m] h = hydraulic head groundwater [m] Ф = water pressure in the pipeline [N/mm2] r = radial distance from the pipeline [mm] cv = consolidation coefficient [m2/s] t = time after damage [s] For a water pipeline with an internal pressure of 7 bar installed in a sand layer (aquifer) below confining clay layers (acquitard) the increase in groundwater pressures in case of leakage is calculated. The calculation results are shown in figure 10.

0

5

10

15

20

25

0 5 10 15 20 25 30

Horizonta l dista nce be low the aquita rd [m]

t=1 st=5 st=23 st=30 s

Figure 10. Increase in hydraulic head due to a damaged water pipeline The previous figure shows a quick in crease in groundwater pressure in the area where the damage took place. Since the area is relatively small and the time interval is relatively small (due to automatic pressure release in the pipeline in case of damage of the pipeline) the risk on occurrence of seepage is rather small. 7. Risk mitigation Based on the determined risks during the pullback operation a series of measures to reduce the magnitude of the risks can be taken. The following list consist examples of measures have been taken before the start of several horizontal directional drillings and during the performance of the drillings, which have recently been carried out: Measurement of the weight of the drilling fluid during the drilling stages; Drilling fluid engineering; Usage of guidelines for larger design bending radii; Use of casing pipes to stabilize the bore hole in loose packed soil layers; Detailed soil investigations to obtain insight in the variation in soil conditions; Verification of the drilling path by location measurements; Usage of strong coating to prevent damage by high soil reaction forces; Measurement of the chemical composition of the groundwater in the soil layers through

which the horizontal directional drilling is carried out.


8. Conclusion The horizontal directional drilling technique is very popular nowadays and the majority of the cables and pipelines installed with this technique succeed, but in several cases the technique still lacks sufficient control of risks coming from the underground. Therefore, a soil related-risks analysis for the prevention of unwanted events during the drilling stages or after the installation activities is recommended. The identification of the risks is the most important one in the risk analysis. The identification of the risks requires sufficient knowledge of the processes during the drilling stages. Once all the risks are identified in a qualitative way the importance of risks can be determined. Very often the calculations are necessary to classify the risk in a quantitative or semi –quantitative way. After classifying the risks, mitigation measures can be established for the significant risks. Since an overview of all possible risks in horizontal directional drilling would be extensive, this paper focuses on the most occurring soil related risks in horizontal directional drilling:

• High pulling forces or incomplete pull back caused by local bore hole instability; • High pulling forces or incomplete pull back caused by frictional forces in the borehole; • Seepage through the borehole to the surface.

The condition or variation in conditions of the subsoil including groundwater are of major importance for the success of the horizontal directional drilling. A risk analysis is a powerful tool to control soil related risks. 9. Literature [1] Bayer, H.J. HDD pratice handbook (2005), Vulkan-verlag GmbH, Essen Germany [2] Kruse H.M.G. and H.J. Brink.(2007), Soil related risks during the pull back operation

of horizontal directional drilling . Int. No-Dig conf. Rome [3] Meijers, P. and De Kock, R.A.J. (1993), A calculation method for earth pressures on

directionally drilled pipelines, Pipeline conference 1993, Belgium. [4] Viehofer, T., T. Linthof. and A. Bezuijen. (2005), Stability of a borehole during

horizontal directional drilling, Proc. No dig conference Rotterdam [5] Litjens P.P.T. and H.J.A.M. Hergarden (2001). A calculation method to determine

pulling forces in a pipeline during installation with horizontal directional drilling, Von der production zur service Schrift (Schriftenreihe aus dem institut for Rohrleitungsbau Oldenburg)

[6] Bisschop, F., (1995), transport processes in borehole and pipeline (In Dutch), Drilling of tunnels and pipelines research group, Delft.

[7] Hetényi, M., (1946), Beams on elastic foundations. Scientific series, Ann Arbor, University of Michigan press.

[8] Pruiksma J.P and H.M.G. Kruse.(2008) Soil pipeline interaction during the pull back operation of Horizontal directional drilling.. Int. No-Dig conf. Moscow

[9] Boeije R. and H. Sellmeijer (1996) Water Pipelines below River dykes (in Dutch), H2O nr. 16 pp 478-482

[10]] Van Staveren, M.(2007) Uncertainty and ground conditions a risk management approach (2006), Elsevier, the Netherlands

[11] Hergarden, H.J.A.M. (2008) Geotechnical design factors HDD crossings. proc DCA conference, Prien, Germany

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The trenchless technique horizontal directional drilling · PDF file4th Pipeline Technology Conference 2009 The trenchless technique horizontal directional drilling Soil related risks