IBP1070_09

METHODOLOGY FOR DEFINITION OF BENDING RADIUS AND PULLBACK FORCE IN HDD OPERATIONS Danilo Machado L. da Silva1, Marcos V. Rodrigues2, Asle Venås3 Antonio Roberto de Medeiros4

Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009, held between September, 22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings.

Abstract

Bending is a primary loading experienced by pipelines during installation and operation. Significant bending in the presence of tension is experienced during installation by the S-lay method, as the pipe conforms to the curvature of the stinger and beyond in the overbend region. Bending in the presence of external pressure is experienced in the sagbend of all major installation methods (e.g., reeling, J-lay, S-lay) as well as in free-spans on the sea floor. Bending is also experienced by pipelines during installation by horizontal directional drilling. HDD procedures are increasingly being utilized around the world not only for crossings of rivers and other obstacles but also for shore approach of offshore pipelines. During installation the pipeline experience a combination of tensile, bending, and compressive stresses. The magnitude of these stresses is a function of the approach angle, bending radius, pipe diameter, length of the borehole, and the soil properties at the site. The objective of this paper is to present an overview of some aspects related to bending of the product pipe during HDD operations, which is closely related to the borehole path as the pipeline conforms to the curvature of the hole. An overview of the aspects related to tensile forces is also presented. The combined effect of bending and tensile forces during the pullback operation is discussed.

1. Introduction

Originally used in the 1970s, directional crossings are a combination of conventional road boring and directional drilling of oil wells. Horizontal directional drilling (HDD) is an alternative construction method in the trenchless industry and it has experienced rapid growth in the construction industry over the past few decades. The horizontal- directional-drilling process represents a significant improvement over traditional open cut method for installing pipelines beneath obstructions, such as rivers, highways, railroads, islands and others. HDD is also increasingly being utilized for shore approach of offshore pipelines mainly because it has less environmental impact in certain cases vs. alternative methods. Installation of a pipeline by HDD is generally accomplished in three stages. The first stage involves drilling a small-diameter pilot hole along a designed directional path. The second stage consists of enlarging (reaming) the pilot hole to a diameter that will support the pipeline and the third stage consists of pulling the pipeline back into the enlarged hole. Despite its growth and popularity, a number of issues related to HDD installations remain poorly understood. These issues are even more significant in shore approach HDD installations. Shore crossing installation using HDD are much more complex and challenging than typical surface to surface installations. The increased challenges and complexity arise from an inability to readily access the exit location, less geotechnical information associated with ocean floor sediments, elevation differences between entry and exit locations, complexity of coordinating diving operations, tidal and storm influences, bore instability and drilling fluid management, product pipe installation strategies, and buoyancy control. An extensive theoretical research is needed in order to develop rational analysis procedures for the combined behavior of soil, product pipe and drilling fluids during HDD installations method. Such procedures are necessary to ______1 D.Sc, Engineer – Det Norske Veritas, DNV – Rio de Janeiro, Brazil 2 D.Sc, Senior Engineer – Det Norske Veritas, DNV – Rio de Janeiro, Brazil 3 Segment Director Offshore Pipelines – Det Norske Veritas, DNV – Oslo, Norway 4 HDD Coordinator – Subsea 7 – Brazil Rio Pipeline Conference and Exposition 2009 establish rational design guidelines for HDD. In general, current design methodology relies on the experience and judgment of contractors, manufacturers and engineers.

2. Drill-Path Design

As in any other installation method, in HDD operations the pipeline is under combined tension and bending. The pipe conforms to the curvature of the hole, which makes the magnitude of the stress a function of the HDD path design. This means that it is a function of the approach angle, bending radius, product pipe diameter, length of the borehole, and the soil properties at the site. The combined tension and bending ovalizes the pipe cross section reducing its resistance to external pressure. In case of a pipe in contact with a curved surface, the applied tension is reacted by a distributed force acting on the surface of the pipe. This contact force tends to increase the ovalization induced by bending (Kyriakides, 2007). The designed drill path should meet all the location and depth control points while keeping the drill length as short as possible. Drill paths are made up of a series of straight lines and curves, which are typically sag bends, over bends, or side bends depending on their axial plane. It is not uncommon for HDD drill paths to have compound bends even though they are generally avoided if possible. Combined small radius in vertical and horizontal plans, dog legs, and average 3D curvature deviated of the predicted profile is a common problem to avoid while drilling the pilot hole. The location and configuration of a drilled profile are defined by its entry and exit points, entry and exit angles, radius of curvature, and points of curvature and tangency. When designing the drill path it is desired to keep the number of bends to the minimum required. This reduces pullback loads and extends drill-rod life. The best bore path starts with a straight tangent section at the prescribed entry angle to gain the depth required for steering control and the depth of cover. At the required depth the drill head is steered upward with a curve, then transitions to a horizontal segment, and again turns upward with another curve before transitioning to another straight tangent section at the desire exit angle. Therefore, the key parameter in HDD path design is the bending radius. In a few cases HDD profiles can be defined with only two curvatures and two straight sections. This paper presents the most common used equations for definition of the bending radius for HDD. These equations are derived from established practice rather than from theoretical analysis. Of course, the bending radius has a significant influence over the pullback operation, in a way that small radius and dog legs increase the pullback forces. An overview of aspects related to the definition of the bending radius and the pullback forces in HDD operations is presented. Several relevant aspects for the success of HDD operations, mainly related to bending radius and pullback procedures, are presented and discussed.

3. Bending Radius

An important parameter in the design of a crossing by means of horizontal directional drilling is the bending radius or radius of curvature. The bending radius is determined by the bending characteristic of the product pipe, increasing with the diameter. It is usual in designing HDD paths to consider a bending radius equal to 1000 times the nominal diameter of the pipe to be installed. Another general “rule-of-thumb” for the bending radius is 100ft/1in diameter for steel line pipe, which is equivalent to 1200 times the nominal diameter of the pipe. These connections between pipe diameters and bending radius are derived from established practice for steel pipe rather than from theoretical analysis. Typically, the minimum radius determined using a stress-limiting criterion would be substantially less than 1000 times the nominal diameter. For this reason, bending stress limits rarely govern geometric path design but are applied with other stress-limiting criteria, in determining the minimum allowable bending radius. The following equations, from the Strength of Materials, can be used to determine the minimum bending radius for a steel pipe.

M σbend · I 1 M E·I E· D σbend = rpipe → M = ; = → rbend = ; σbend = (1) I rpipe rbend E·I M 2·rbend

Where, σ is the bending stress, M is the bending moment, I is the moment of inertia, E is the modules of elasticity, rpipe is radius of the pipe, rbend is the bending radius and D is the pipe outside diameter. The following equation is commonly used for calculating the allowable bending radius for steel pipe (Willoughby, 2005).

3·E· rpipe rmin = (2) 2·Sa

2 Rio Pipeline Conference and Exposition 2009

Where rmin is the recommended smallest radius of curvature that can be used without overstressing a straight pipe, Sa is the allowable stress (0.9·SMYS ). The bending radius becomes a very critical issue for large-diameter pipes. Serious problems have been faced with large-diameter HDDs that had been designed with a relatively-small bending radius. There is not a consensus about the approach to obtain the best design bending radius for large-diameter pipes. In general, the design radius is ranged from 1000D to 2000D. The DCA Technical Guidelines (2001) recommends the following equation for calculation of bending radius.

D < 400mm → rbend = 1000 D 3 400mm < D < 700mm → rbend = 1400 D (3) 3 D > 700mm → rbend = 1250 D

These rules are empirical and do not take into account the influence of the wall thickness on the bending stiffness of the pipe. A greater wall thickness results in a stiffer pipe which is less easy to bend when pulled in, and causes a higher soil reaction pressure. Exceeding the allowable soil pressure may lead to failure of the pipe or the pullback equipment, or to the pipe becoming stuck. The bending moment and the resulting distribution of normal forces on the borehole wall leads to a soil reaction pressure. It should be noticed that, especially at the head of the pipeline, soil deformation behavior can be plastic. Plastic soil behavior leads to non recoverable deformation, which in turn may lead to a displacement of the stiff pipeline beside the pre-reamed borehole. Figure 1 shows the displacement of the pipeline to a position below the pre-reamed bore hole in the upward bend of the HDD. It can occur at the moment of change of the drill pipes at the rig, when the pulling force drops allowing the pipe to bend into the borehole wall. These aspects were considered in derivations of the following equation for the minimum bending radius for the pull back operation of large-diameter HDD (Brink et. al., 2007).

rbend = C · D · t (4)

Where C is a soil dependent constant and t is the pipe wall thickness. For soft soils, the C constant has higher values, for soils with higher stiffness (and strength) the C constant is considerable lower. Brink et. al. (2007) present some typical values for constant C, as shown in Table 1.

Table 1. Soil dependent Constant.

Sand C Clay C Densely-packed sand 8500 Medium-stiff clay 11500 Medium-packed sand 9400 Soft clay and peat 12500 Loosely-packed sand 10200

Figure 1. Problem related to pipe-soil interaction during pullback operations.

The following tables show values for bending radius calculated using the mentioned practices. A medium-packed sand was considered in Equation 4, Table 3. It should be noticed that very different values of bending radius can be found, especially for large-diameter pipes, as shown in Figure 2.

Table 2. Bending Radius (m); 12”– 22” (API-X60).

D (in) Eq. 1. Eq. 2 1000 D 1200 D 12 91.26 136.89 308.00 369.60 16 114.52 171.79 406.40 487.68 20 143.16 214.73 508.00 609.60 22 157.47 236.21 558.80 670.56

3 Rio Pipeline Conference and Exposition 2009 Table 3. Bending Radius (m): 28”– 30” (API-X65); 42”– 48” (API-X70).

D (in) Eq. 1. Eq. 2 Eq. 3 1000 D 1200 D t (in) Eq. 4 t (in) Eq. 4 28 185.2 277.8 749.7 711.2 853.4 0.625 1047.9 1.000 1263.4 32 211.7 317.5 916.0 812.8 975.4 0.750 1217.1 1.125 1432.7 42 257.7 386.5 1377.3 1066.8 1280.2 1.000 1594.6 1.250 1730.0 48 294.5 441.7 1682.8 1219.2 1463.0 1.125 1803.0 1.250 1849.4

2000 1000 D 1200 D 1800 Eq. 3 Radius (m) Eq. 4 (< t) 1600 Eq. 4 (> t)

1400

1200

1000

800

600 25 30 35 40 45 50 D (in) Figure 2. Bending Radius.

4. Pullback Force

Prior to the installation of a horizontal directional drilled pipe it is important to estimate the pipe pullback force during installation so that: (1) the pipe is not overstresses during HDD installation, (2) a drill rig with sufficient pullback capacity is used to and (3) an economical design is implemented. The required pullback force on HDD is a result of interaction between a pipe, a soil, and a drilling fluid. The mechanisms that generate frictional forces are due to contact forces between the pipe and the soil. The contact forces consist of contributions from two major components: the effective weight of the pipe, and directional changes which consists of contribution from flexural stiffness of the pipe as it conforms the curves in the borehole. The magnitudes of these contact forces depend on the effective weight of the pipe, geometry of the borehole, clearance between the pipe and the soil, and flexural stiffness of the pipe. Steel pipes have a high pulling load capability and can handle considerable pullback loads. The allowable pulling loads for steel pipes are a function of the steel material grade, pipe diameter and wall thickness, and safety factors. Equation 5 is a commonly used formula for determining the allowable pullback forces for steel pipes (Willoughby, 2005).

SMYS · fl E· D F = ⎡ − ⎤ · A (5) ⎣ fs 2·rbend ⎦

Where F is the maximum allowable pullback force, A is the pipe cross-sectional area, SMYS is the specified minimum yield strength of the steel, fl is the maximum load factor and fs is the safety factor. Several approaches for calculation of pullback forces can be found in literature. A consistent theoretical model for calculating installation loads imposed on pipes during the pulling operation in horizontal directional drilling is presented by Cheng and Polak (2007). This model considers fluidic drag, solid friction and effective pipe weight. The model also emphasizes the pullback load contributions from directional changes. It should be noticed that the drilling process leads to deviations of the design path during the pilot phase. These deviations are corrected decreasing the deviation to the designed path. These corrections however, may lead to small distance irregularities in the drilling line. Although the reaming operations may have a smoothing effect on the drilling line, the existence of irregularities and small 3D bending radius along the axis of the reamed borehole cannot be neglected. Deviations to the design drilling line, which remain after the reaming operations, lead to higher pulling forces during the pull back operations. Among the most used methods for estimates the pullback force in HDD operations is the PCRI Method presented as follow.

4.1 PRCI Method The PRCI (Pipeline Research Council International) design procedure was developed by Huey et. al. (1996) for calculating installations loads on pipe, including pulling forces, and analyzing combined stresses in steel pipe during HDD installations and operations. The estimated pulling load is calculated as a series of straight and curved segments, 4 Rio Pipeline Conference and Exposition 2009 Figure 3. The forces acting on each segment are calculated sequentially from pipe side to rig side to determine the tensile load at the end of each segment. The total tensile load at the end of the pullback is the sum of the individual forces required to pull the pipe through each of the straight and curved segments in the bore hole.

Figure 3. PRCI Method: Straight and Curved Section.

The tension T2 is calculated as follow for straight and curved sections respectively:

T2 = T1 + | fric | + DRAG ± Ws·L·Sin θ (6)

T2 = T1 + 2·| fric* | + DRAG ± Ws·Larc·Sin θ (7)

Where T2 is the tension at the rig side of the straight (or curved) section required to overcome drag and friction, T1 is the tension at the pipe side of the straight (or curved) section, fric = Ws·L·Cos(θ)·μsoil is the friction between pipe and soil, fric* = N·μsoil , DRAG = π·D·L·μmud is the fluidic drag between pipe and slurry, Ws is the submerged weight per unit length of the pipeline, L is the length of a straight section, Larc is the arc length of a curved section, N is the normal contact force at center of a curved section, θ is the angle of the axis of the straight hole section relative to horizontal (θ = (θ1 + θ2) / 2 in curved sections), μmud is the fluid drag coefficient, μsoil is the average coefficient of friction between pipe and soil. The +/- term is (-) if T2 is downhole, (+) if T2 is uphole, and (0) if the hole is horizontal. Since the development of the PRCI method, calculated pulling loads have been compared against actual field pulling loads on HDD installations. Puckett (2003), conducted analyzes of the calculated versus actual pulling loads using documentation from completed HDD installations. This analysis focused on a more accurate parameter for the fluid-drag coefficient, the input parameter with the greatest uncertainty. Adedapo and Knight (2003) investigate the applicability of this method for estimating HDD force. In this study the PCRI predicted pipe load is also compared with loads from field installations. It should be pointed out that these simplified methods for pullback force calculation usually provide a poor quality estimative of actual pulling forces. In general, it occurs because these equations do not properly considerer the pipe-soil interaction, which is a very important aspect in HDD operations (Kruse, 2007). In a poorly planed pullback operation the consequences can be: • High pullback force caused by wrong ballasting with respect to the weight of drilling fluid; • High pullback force caused by a small design bending radius; • High pullback force caused by an irregular shaped borehole with small 3D-bending radius; • High pullback force caused by borehole instability due to soil conditions; • High pullback force caused by borehole instability due to erroneous chosen drilling fluid; • Damage of pipeline coating caused by unexpected obstacles due to misinterpretation of soil investigation; • Damage of pipeline coating caused by wrong choice of the coating in relation to contact forces.

4.2 Pipe-soil Interaction The pipeline soil interaction is the combined behavior of the pipeline and the surrounding soil in terms of stresses and deformations. During the pullback operation the moving pipeline contacts the wall of the borehole and pushes with a certain force perpendicular to the wall of borehole. These forces determine the magnitude of the shear force in axial direction during the pullback operation and lead to a deformation of the soil. The distribution and the magnitude of these forces on the borehole wall are of major importance in the pipeline soil interaction. Kruse et. al (2007) presents a discussion related to the pipeline soil interaction in pullback operations. The pipeline soil interaction is mainly determined by the following aspects: stiffness of the pipeline, stiffness and strength of the soil, shape of the borehole, effective weight of the pipeline and borehole stability. 5 Rio Pipeline Conference and Exposition 2009 The borehole stability is the most important factor for the success of the pullback operation. Borehole stability requires a proper balance among various soil parameters including: soil stress and strength, pore pressure, drilling fluid pressure and drilling mud chemical composition. Borehole instability is influenced by chemical effects (formation of a filter cake) and mechanical effects (soil sloughing and hydraulic fracturing) (Wang and Sterling, 2007). In case of instability of the borehole the pipeline soil interaction changes rigorously. In case of a collapsed borehole a huge soil load exerts on the pipeline. Whereas local borehole instability leads to higher pulling forces, which can be overcome, borehole instability over a larger distance will certainly lead to a stuck pipeline and thus an incomplete pulling operation. One of the most important issues is how to effectively evaluate the stability of the borehole wall, particularly in application of HDD to very loose sand or gravel-sand mixtures. Borehole collapse can lead to drill rods or pipes becoming stuck in the borehole and borehole fracture can result in the release of drilling fluids from the borehole. It should be noted that while borehole stability has been extensively studied in the petroleum exploration industry, the conditions under which borehole stability are investigated are very different from the conditions for horizontal directional drilling. In the former, stability has been studied mainly in vertical boreholes at great depth in rock. In the latter, the stability of horizontal boreholes at relatively shallow depths in loose soils is the issue. Other important aspect of pullback operations is related to the prevention or control of mud loss as a result of soil failure around the borehole. It should be noted that drilling mud plays a key role during the drilling process to return cuttings to the ground surface, stabilize the borehole, and lubricate the pipe being pulled into place. Unfortunately, the pressurized mud will lead to expansion of the borehole, and if the strength of the neighboring soil is exceeded, the mud- loss can occur. Some empirical equations based on the classical plasticity theory are employed to calculate the critical fluid pressures to prevent the borehole from collapse and hydraulic fracture in HDD practice (Staheli et al., 1998). There are basically two possible mechanisms of ground failure that can result in mud loss from a borehole: shear failure and tensile fracture, usually referred to as ‘frac-out’.

5. Combined Stresses

The magnitude of the stresses experienced by the pipeline is a function of the approach angle, bending radius, pipe diameter, length of the borehole, and the soil properties at the site. Proper selection of the radius of curvature and the pipe material will ensure that the stresses do not exceed the pipeline capacity during the installation. The stresses in the steel pipe due to bending are another key factor to consider during the design of a HDD crossing. The allowable bending stress for steel pipe depends on many factors such as the pipe diameter and wall thickness, the steel grade, and any code safety factors. Below are some commonly used equations for calculating the allowable bending stress for steel pipe. These limits were taken from design criteria established for tubular members in offshore structures and are applied to HDD installation because of similarity of the loads on pipe (API-RP-2A-WSD, 2000).

σallow = 0.75·SMYS for D/t ≤ 10340 / SMYS σallow = [0.84 − (1.74·SMYS·D) / (E·t)]·SMYS for 10340 / SMYS < D/t ≤ 20680 / SMYS (8) σallow = [0.72 − (0.58·SMYS·D) / (E·t)]·SMYS for 20680 / SMYS < D/t ≤ 300 / SMYS

Where, σallow is the allowable bending stress. Note that SI units must be used in equations above. The worst-case stress condition for the pipe will typically be located where the most serious combination of tensile, bending, and hoop stresses due to external pressure occurs simultaneously. This is not always obvious in looking at a profile of the drilled hole because the interaction of the three loading conditions is not necessarily intuitive. To be sure that the point with the worst-case condition is isolated, it may be necessary to do a combined stress analysis for several suspect locations. In general, the highest stresses will occur at locations of tight bending radius, high tension (closer to the rig side), and high hydrostatic head (deepest point) (ASCE, 2005). Combined stress analysis usually starts with a check of axial tension and bending according to the following limiting criterion.

σtension σbend + ≤ 1 (9) 0.9·SMYS σallow

This criterion is taken from practices established for design of tubular members in offshore structures with an increase in the allowable tensile proportion to make it consistent with established practices in the HDD industry (API RP 2A-WSD, 2000). The stresses at each stage must be considered both individually and in combination. Stresses come from the spanning between rollers prior to pullback, the hydrostatic testing pressures, pulling forces during installation, bending radius as the pipe enters the ground, the drilling profile curvature, external pressures in the drilled hole, and the

6 Rio Pipeline Conference and Exposition 2009 working pressure. The load and stress analysis for a HDD pipeline is different from similar analyses of conventionally buried pipelines because of the high-tension loads, occasional severe bending, and external fluid pressures endured by the pipeline during the installation process. In some cases the installation loads may be higher than the design service loads. Pipeline properties, such as wall thickness and material grade, and pilot-hole bore path must be selected so the pipeline can be installed and operated without risk of damage. Once determined the individual and combined stresses at each stage of construction and those for the in-service condition, they must be compared with allowable limits. Commonly used limits are provided by ASME B31.8 (2007), Table A842.22 as follows: - Maximum allowable longitudinal stress: 80% SMYS. - Maximum allowable hoop stress: 72% SMYS. - Maximum allowable combined stress: 90% SMYS. Regulatory bodies may impose additional limits to those specified above – it should be identified any such further constraints and ensure the adequacy of the design. Unfortunately, the horizontal directional drilling construction process makes it difficult to control. The entire construction process takes place below ground level and is not accessible for visual inspection. In addition, drilling and reaming actions commonly take place tens or hundreds of meters away from the drill rig. Identifying problems before they occur, in some cases even after they occur, is difficult considering the remote nature of the HDD process. Furthermore, depending on the case even if the problem is detected during or immediately after the installation process, repair requires digging out the entire installation, extracting the pipe or abandoning the original alignment altogether (Allouche, 2002).

6. Final Remarks

Most design methodologies of HDD rely on the experience and judgment of contractors, manufacturers, and engineers and theoretical research on the effect of HDD on the performance of the pipe is limited. It is not unusual have HDD installations that are governed by generally accepted practices with no specific regulations to comply. General construction guidelines (DCCA, PRC, 1995; HDI, 1999; DCA, 2001) are available in the industry and may range from recommended practices to project requirements. The available specifications are mainly concerned with general construction practices. Some questions are not yet satisfactorily solved, for instance, the minimum bending radius that guarantee the HDD pullback success. Of course this bending radius need to be calculated take in account the pipe soil interaction during pullback operation as discussed in this work. Extensive research is still needed in order to develop rational analysis procedures and to increase quality assurance in HDD operations, which is necessary to improve the reliability in HDD installations. It should be highlighted that, besides the pipeline get stuck due the borehole collapse, problems, such as buckling or localized deformation of the pipe cross-section, are discovered only after the problem has occurred and at this point it is often too late to initiate a corrective action and the only recourse is to install a new pipeline. All these aspects are still more significant in HDD for shore crossing where many additional challenges are encountered (Duyvestyn, 2005). As previously mentioned a number of issues related to HDD installations remain poorly understood and an extensive research is needed.

7. References

ADEDAPO, A., KNIGHT, M. A., “Applicability of Methods used to Estimate Pipe-Slurry Fluidic Drag Force During HDD Pipe Installations”, University of Waterloo, Canada, 2003. ALLOUCHE, E. N., “Implementing Quality Control in HDD Projects — A North American Prospective”, Tunnelling and Underground Space technology, n.16 Suppl.1, p. S3-S12, 2002. API RP 2A-WSD, “Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms – Working Stress Design”, Twenty-First Edition, December 2000. ASCE, “Pipeline Design for Installation by Horizontal Directional Drilling”, ASCE – Manuals and Reports on Engineering Practice No 108, 2005. ASME B31.8, “Gas Transmission and Distribution Piping Systems”, ASME, 2007. BRINK, H. J, KRUSE, H. M. G., LÜBBERS, H., HERGARDEM, H. J. A. M., SPIEKHOUT, J. “Design Guidelines for the Bending Radius for Large-Diameter HDD”, Journal of Pipeline Engineering, v.6, n.4, p.263-267, 2007. CHENG, E., POLAK, M. A., “Theoretical Model for Calculating Pulling Loads for Pipes in Horizontal Directional Drilling”, Tunnelling and Underground Space technology, n.22, p. 633-643, 2007.

7 Rio Pipeline Conference and Exposition 2009 DCA, “Horizontal Directional Drilling – Technical Guidelines”, Drilling Contractors Association, 2001. DCCA, “Guidelines for a Successful Directional Crossing Bid Package”, Directional Crossing Contractors Association, 1995. DNV-OS-F101, “Submarine Pipeline Systems”, Offshore Standard, Det Norske Veritas, October 2007. DUYVESTYN, G., “Design and Construction Challenges for HDD Shore Crossings”. Proc. of No-Dig Conference, North American Society for Trenchless Technology - NASTT, Orlando, Florida, 2005. HDI, “Horizontal Directional Drilling Training Manual”, Horizontal Drilling International, Houston, Texas, 1999. HUEY, D. P., HAIR, J. D., MCLEOD, K. B., “Installation loading and stress analysis involved with pipeline installed by horizontal directional drilling”. Proc. of No-Dig Conference, New Orleans, LA., 1996. KYRIAKIDES, S., CORONA, E., “Mechanics of Offshore Pipelines, Vol.1: Buckling and Collapse”, Elsevier, 2007. KRUSE, H. M. G., BRINK, H. J, “Risks During the Pull Back Operation of Horizontal Directional Drilling”, Mediterranean NO DIG 2007 – XXVth International Conference & Exhibition, Roma, September 2007. POLAK, M. A., LASHEEN, A., “Mechanical Modelling for Pipes in Horizontal Directional Drilling”, Tunnelling and Underground Space technology, n.16 Suppl.1, p. S47-S55, 2002. PRC, “Installation of Pipelines by Horizontal Directional Drilling, an Engineering design Guide”, Pipeline Research Committee, 1995. PUCKETT, J. S. “Analysis of Theoretical Versus Actual HDD Pulling Loads”, ASCE "Pipelines 2003" Conference, Baltimore, Maryland, 2003. STAHELI, K., BENNETT, D., O’DONNELL, H.W., HURLEY, T.J., “Installation of Pipelines Beneath Levees using Horizontal Directional Drilling”, Technical Report CPAR-GL-98-1, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS. 1998. WANG, X., STERLING, R. L., “Stability Analysis of a Borehole wall During Horizontal Directional Drilling”, Tunnelling and Underground Space technology, n.22, p. 620-632, 2007. WILLOUGHBY, D. A., “Horizontal Directional Drilling: Utility and Pipeline Applications”, McGraw-Hill, 2005.

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