Horizontal Directional Drilling

1 Overview: Horizontal Directional Drilling

In this article, EPCM explains the principle of horizontal directional drilling as it relates to pipeline installation.  It will also highlight some technical requirements that should be fulfilled for the successful execution of horizontal directional drilling.

It should be noted that horizontal directional drilling is not only used for pipelines installation but is commonly used for installing cables below features (obstacles).

HDD can be conveniently used for installing pipelines made of different materials, including steel pipes and HDPE pipes. However, this article covers pipelines made of carbon steel materials.

2 Introduction to Horizontal Directional Drilling

Various features (obstacles) are encountered during pipeline installation. These obstacles include roads, railways, water bodies, sensitive areas, buried pipelines, and cables.

They require special crossing technology when an open cut cannot be performed. Therefore, it is imperative to select an appropriate crossing method during the design phase of the project.

Horizontal directional drilling (HDD) is one of the most suitable means of crossing such obstacles.

Horizontal Directional Drilling application has expanded to cover shore crossing for pipeline transiting from offshore to onshore.

Horizontal directional drilling is a trenchless method of installing a pipeline underground below identified features (obstacles) without disruption to any of the identified features. It entails drilling a hole at a specified depth below the features (obstacles) and pulling in the pipeline into the drilled hole.

Crossing length is dependent on the drilling rig capacity. However, there has been significant improvement in technology, which has led to the successful crossing of large water bodies and greater depth.

HDD has been utilised for crossing length of over 1km utilising one rig; however, the advent of drill and intersect method has led to the successful execution of crossing length longer than 2km.

Drill and Intersect method entail drilling the pilot hole from each side of the proposed drill path and intersecting the two pilot holes. This method requires high drilling precision to ensure that the pilot hole intersects at the designed point.

3 Terms Used in Horizontal Directional  Drilling

In this section, I have defined/explained the meaning of some of the terms used in HDD

3.1 Horizontal Direction Drilling

Horizontal directional drilling is a trenchless method of installing pipelines underground below identified features (obstacles) without disruption to any of the identified features

3.2 Drill Rig

This is the major equipment of a typical horizontal directional drilling setup. It is positioned on a flat surface and may be appropriately anchored to the ground to ensure the machine stays in position during drilling and pipeline pulling operations. The rig is equipped with a power unit that drives a travel mechanism back and forth on a structural framework. Within the rig assembly is a component attached to the drill pipes, this component performs rotational motion during drilling activities.

3.3 Control Cabin/Room

The Cabin is placed almost immediately beside the drill rig. The drilling machine operator controls drilling activities from here. It also houses the HDD steering system and other display devices for monitoring and controlling drilling activities.

3.4 Drilling Mud

Drilling Mud is a mixture of natural and synthetic chemical compounds that may be oil-based or water-based. Appropriate additives are added to the drilling fluid, depending on the drilling requirements. It provides lubrication and cooling effect for the drill bit, stabilises the hole, prevents the hole from collapsing, suspends, and transport the cuttings from the drilled hole.

Drilling fluids are typically made from Bentonite, water, and other additives. The type of additive to the drilling mud is a function of the soil type. When drilling coarse or sandy soil, the drilling fluid should prevent water loss. This can be achieved by using a water/filtrate control additive such as polyanionic cellulose.

3.5 Mud Mixer

The Mixer is an assembly of drilling mud hopper and centrifugal pumps. They are used for bentonite mixing, which is the drilling fluid utilised for HDD drilling activities.

3.6 Drill Pipes

Drill pipes are essential components of horizontal directional drilling activity. They are usually furnished with threaded ends, internal threads at the rear, and external threads in front. With the aid of the threaded ends, the pipes are easily joined as the drilled length increases. The drilling machine has a rotating mechanism with external thread that connects to the drill pipe at the rear, rotates and drives the pipes into the ground.

3.7 Drill Bit

This is the primary boring tool attached to the drill pipes and used to drill the pilot hole. The type of soil to be drilled determines the type of drill bit, e.g., rocky soil requires special drill bits. After the pilot drill, the drill bit is removed and replaced with a reamer.

3.8 Reamer

Reamers are drilling tools used to widen or increase the diameter of the pilot hole to a diameter greater than the pipeline size. They are larger than the drill bit and are equipped with cutting teethes. High-pressure drilling fluid flows through ports provided on the reamer to lubricate the reamer and soften the soil.

The type of reamer selected is dependent on the soil type as well as the hole configuration desired. Below are three types of reamers commonly used:

3.8.1 Fly Cutter Reamer

This type of reamer has an open body with good cutting and mixing capabilities. Fly cutter reamers are mostly used for dense soil applications.

3.8.2 Blade Reamer

They are mostly used for good soil such as sandy soil, loamy soil, tightly packed clay etc.

3.8.3 Barrel Reamer

This is the most common reaming tool that can be used for various types of soil. Their body is round with tapered nose equipped with cutting teethes and fluid ports.

3.9 Pulling Head

The pulling head is a fabricated attachment to the pipeline that is hooked to the drill pipes during pipeline pullback. It is welded to the pipeline and cut out after pullback is complete.

4 Horizontal Directional Drilling: Predesign and Design Activities

Before the commencement of actual drilling activities, there are activities performed to guaranty a successful Horizontal Directional Drilling.

4.1 Predesign Activities

The output data of these activities are used as inputs to the design. Activities performed include pipeline route selection, surface and topographical survey, subsurface investigations.

4.1.1 Pipeline Route Selection

Pipeline route selection is one of the essential activities in HDD design. It should be noted that the section of the pipeline installed using HDD is not routed separately from the entire pipeline. However, the HDD section routing shall be done with attention given to the space required for drilling activities, features (obstacles) to be crossed, etc. Usually, a desktop study is performed to develop various route options. These routes are further narrowed down to one after site investigation.

4.1.2 Surface and Topographical Surface.

Surface surveys are carried out to identify and locate all visible obstacles and utilities within a defined width along the proposed route. Also, a topographical survey is performed to determine the elevation along the route. The elevation is a crucial component of the survey works because the minimum required pipeline depth can reduce tremendously in the undulating ground. The minimum required depth is defined concerning the point with minimum elevation.

All collected data are presented in appropriate drawings and documentation, such as crossing plans, crossing lists, profile drawings, etc.  As stated in ASME B31.4 Section 434.13.5 (a)  crossing plan and profile drawings should show all pipelines, utilities, cables, and structures that cross the drill path, are parallel to and within 100 ft. (30 m) of the drill path, and that are within 100 ft. (30 m) of the drilling operation, including mud pits and bore pits.

Below is a list of some of the data that should be collected during the survey activities

• Elevation along the route referenced to a specified datum
• Surface Features including roads, railways, sidewalks, utility poles, power transmission lines, electricity transmission towers, culverts, manholes, buildings etc.
• Test Pit Location
• Water Bodies

4.1.3 Subsurface Investigations

After the completion of surface investigations, the subsurface investigation should be performed. These activities can be categorised into the following:

4.1.3.1 Underground Utilities Survey

It is mandatory to perform an underground survey to detect all underground infrastructures including buried pipelines, buried electric cables along the proposed pipeline route. The survey should be performed to capture utilities parallel or crossing the drill path. The depth of the utilities should be captured as well as the coordinates. As stated in ASME B1.4 section 434.13.5 (a)  crossing plan and profile drawings should show all pipelines, utilities, cables, and structures that cross the drill path are parallel to and within 100 ft. (30 m) of the drill path, and that are within 100 ft. (30 m) of the drilling operation, including mud pits and bore pits. The survey method employed should be capable of detecting all buried utilities. Some of the survey methods are:

4.1.3.1.1 Metal (Pipe Locator):

The metal (pipe locator) is used to detect underground metallic structures such as pipes. They are mostly employed to give a preliminary assessment of buried metals within 4m depth, however more sophisticated versions of these devices can be used for depths greater than 4m. It should be noted that this device cannot detect plastic pipes and HDPE pipes that are not installed with tracing wire. Some of these devices detect metallic objects by the interference of magnetic fields generated by the device when it is operated. More sophisticated devices utilise the application of electric current on the identified utility, the current travels through the metallic object, which is further detected by a surface metal detector. This method is more accurate than the previously described method.

4.1.3.1.2  Seismic Survey

The device utilises the principle of reflection of sounds. The sound wave generated by the device is recorded by microphones (geophones) connected along a predetermined and prepared path. The time required for the wave to reflect after striking the buried infrastructure is recorded. This is used to calculate the depth of the object.

4.1.3.1.3  Ground-Penetrating Radar

The device works on the principle of radio wave reflection. Radio waves are sent into the ground and reflected to the surface; a receiver records the reflected waves. The data are further analysed to calculate the actual depth of the infrastructure.

4.1.3.1.4  Excavation

Excavation may be performed to detect unground infrastructures that are shallow. Depending on the utility, manual excavation or vacuum excavation may be utilised.

4.1.3.2 Geotechnical Investigations

Geotechnical investigations are performed to determine minimum soil conditions along the proposed route. These investigations are performed by drilling boreholes at appropriate locations and collecting soil samples for classification and further laboratory analysis. Seismic survey and ground-penetrating radar may as well be utilised to collect geotechnical information.

The actual number of boreholes and spacing is project-dependent as well as the length of the HDD alignment. As stated in GP 59-01-22, ExxonMobil requires that spacing between boreholes should be at least every 500–750 ft. (150–230 m) along the HDD alignment, a minimum of three borings shall be required at each HDD site. If geotechnical results indicate significant variation in soil strata, additional boreholes should be drilled.

The depth of each borehole should be deeper than the depth of the proposed HDD profile. As stipulated in ExxonMobil GP 59-01-22, the borehole should penetrate through an elevation below the depth of the proposed HDD hole profile by at least 30 ft. (9.1 m). This is necessary to provide information for HDD design modifications as well as pilot hole deviations during construction. Drilled borehole should be minimum 20 to 30 ft. below the proposed hole depth (ASCE MOP 108-2014).  Boreholes should be at perpendicular offset from the HDD profile centreline. Perpendicular offset distance from centreline required by ExxonMobil in GP 59-01-22 is 15–30 ft. (4.6–9.1 m).  Also, soil sampling in a borehole shall extend 20–40 ft. (6.1–12.2 m) below the HDD profile, and under any obstacle, by 40 ft. (12.2 m).

4.1.3.3 Hazardous Material investigations

This is performed in conjunction with geotechnical investigations. It is executed by taking samples of underground water, performing a visual examination, and smell to check for preliminary pollution. The water sample should be further analysed to determine the presence of hazardous materials. It should be noted that underground water if contaminated, can be harmful to drilling personnel and also affect the drilling mud.

4.2 Design Activities

4.2.1 Site Layout Design

Layout drawings showing the entire drilling site, facilities utilised for drilling activities at the exit point should be prepared. The layout in minimum should include areas reserved for:

• Entry point
• Exit point
• HDD rig
• Control cabin
• The access road to site and drilling locations
• Dry material storage such as Bentonite
• Drilling fluid mixing
• Mud pumps
• Drill pipes storage
• Store

The site should be planned such that there is no or minimal interruption to drilling activities. The available space is planned such that lifting equipment (crane) can position between the drill pipe stacking area and the drill rig. It is essential to minimise crane travel after picking up a drill pipe. The crane should be able to swing and position the drill pipe on the rig.

The layout area required to execute HDD varies depending on the drilling rig size, which is a function of the pipeline size. However, an area of 70m by 70m has been proven to be sufficient.

4.2.2 Drilled Path

Drilled path design shall take into consideration all identified obstacles, geotechnical survey reports. The drilled path for water bodies is more complicated compared to other obstacles (features). Drilled path design for river crossings shall take into consideration

• River migration. Water bodies are dynamic, meaning that river location changes with time. River width may increase or decrease. Data relating to river migration should be reviewed else the pipeline may be exposed in the future.
• Wetland at the river banks. The length of the wetland bounding the river on either side should be investigated and if required, added to the crossing length.
• River Scouring. Some instances buried pipelines in water bodies are exposed after a few years of installation. Pipelines installed below rivers with a high level of scouring should be installed deeper, taking into consideration the rate of scouring.
• Sand Mining (Dredging Activities). Pipelines installed below rivers where sand is mined should be installed deeper. A typical example of this is a pipeline installed across Shipping Lane close to a seaport that is continuously dredged. After some time the depth of cover for the pipeline at some sections may not be sufficient to protect the pipeline against anchor drops.

4.2.3 Entry Angle

This is the angle between the drill bit/drill pipe assembly and the horizontal plane at the location where the drill bit penetrates the ground surface. Entry angle is usually between 8^o and 20^o. However, this is mostly dependent on the pipeline diameter and the drilling rig capabilities. Large pipeline diameter may require entry angles lesser than 8^o because they require a larger bending radius.

4.2.4 Exit Angle

This is the angle between the drill bit/drill pipe assembly and the horizontal plane at the location where the drill bit exits the ground surface. The exit angle should be designed such that the pipeline can be easily pulled into the drilled hole as well as facilitate easy support for the pipeline during pullback. The exit angle ranges from 5^o to 12^o. However, this is dependent on the pipeline diameter. It is better to specify a small exit angle to limit pulling loads.

4.2.5 Depth of Penetration

The depth of penetration is dictated by the location of obstacles and geological features.

The possibility of hole frac-out, the surface settlement should also be evaluated in setting the depth of penetration.  The minimum depth should provide a margin for error during preconstruction (surveying) and construction activities. Where complex soil type will be encountered the depth of penetration may be increased to soil location that is more drilling friendly. A minimum clearance of 15 ft. beneath the obstacle should be maintained (Directional Crossing Contractors Association 1995, Hair and Hair 1988).

4.2.6 Radius of Curvature

The radius of curvature is dictated the pipeline diameter, the thickness of the pipeline, and the bending stress imposed on the pipeline as the pipeline is pulled into the curved drilled hole.  A radius of curvature of 1200D (1200 x Pipeline Diameter) is generally used for large diameter pipelines while a smaller radius is utilised for smaller diameter pipelines. The radius can be further optimised after necessary analysis and calculations have been performed to justify a reduction in radius.

4.2.7 Drilling Accuracy and Tolerances

It should be noted that there might be deviations from the planned/designed drilled path; however, the deviation should be within defined tolerances. Also, the drilling accuracy is well dependent on the type of machine, downhole tool, and the ability of the steering engineer to control directional changes.

During the drilling operation, care shall be taken to ensure there is no significant deviation from the designed drilled path. Below is a summary of some requirements stipulated by ExxonMobil in GP 59-01-22. These may be taken as guidelines during pilot hole drilling.

• The pilot hole shall be drilled along the axis shown in the construction drawings.
• Entry point location shall be the exact point as specified in project drawings and as staked in the field.
• The pilot hole shall exit the ground surface within 5 ft. (1.5 m) left or right of the staked alignment, and plus 15 ft. (4.6 m) (longer) or minus 10 ft. (3.1 m) (shorter) of the exit stake.
• The pilot hole should not exceed 3 ft. (0.9 m) above and 10 ft. (3.1 m) below the designed drill path elevation. Restrictions imposed by Right of Way (ROW), utility crossings and other underground infrastructures shall take precedence over vertical deflection tolerances.
• The pilot hole shall remain within 5 ft. (1.5 m) left or right of the designed drilled path except where the designed drill path passes within 5 ft. (1.5 m) of the ROW limits or any below-grade structure. In no case shall the horizontal deflection exceed the limits of the permanent ROW as shown on the construction drawings. In all cases, restrictions imposed by Right of Way (ROW), utility crossings, and other underground infrastructures shall take precedence over horizontal deflection tolerances.
• Increase in entry angle up to 1 degree (steeper), but no decrease in angle is allowed.
• Increase in exit angle up to 1 degree (steeper). Decrease in angle up to 2 degrees (flatter)
• The pilot hole shall be drilled to the radius of curvature specified in the design documentation.

4.2.8 Installation Stresses

The only difference in installation stress analysis in HDD is the resulting high tensile stress, bending stress, and external fluid pressure imposed on the pipeline by the drilling fluid.

Appropriate pipe material must be selected to withstand the loads imposed on the pipeline.

In performing the analysis, the HDD profile is broken down into several sections. These profiles are grouped into curved sections or straight sections.

Though there are specialised software and in-house spreadsheets for performing HDD calculations, this section gives preliminary guidelines on how pipeline stresses may be estimated. Calculated stresses should be within allowable limits as defined in appropriate codes and standards.

4.2.8.1 Pipeline Weight Calculation

The weight of pipeline can be estimated using the formulae below. I have assumed that the pipe is coated with 3LPE (3- Layer Poly Ethylene).

4.2.8.1.1 Weight of Bare Steel Pipe

4.2.8.1.2 Weight of Epoxy Coating

4.2.8.1.3 Weight of PE Coating

4.2.8.1.4 Total Weight of Empty Pipe

4.2.8.1.5 Buoyancy Force (Weight of Bentonite Displaced)

This is the force exerted on the submerged pipeline in the drilled hole by the drilling fluid. It can be estimated using the equation below

4.2.8.1.6 Submerged Weight of Pipe

This is the resulting weight of the submerged pipeline, which may be negative or positive. This is estimated using the formula below.

4.2.8.2 Pull Force

This is the force required to pull the pipeline from the exit point to the entry point. This force must overcome the frictional force, drag force and the force required to overcome the elevation changes along the drilled path. The total pull force is therefore equal to the summation of the forces required to pull the pipeline in each section of the profile (curved and straight sections).

Pull force required for straight sections is estimated using the below equation.

Pull force required to pull the pipeline through curved sections of the hole can be estimated using the equation below.

4.2.8.2.1  Frictional Force

The frictional force has two components. They are the forces acting between the pipe and the rollers and the force acting between the pipe downhole and the soil. These two frictional forces can be estimated using the formula below.

Frictional force acting between pipeline and rollers

The frictional force acting between pipeline straight section and soil

Frictional force acting between pipeline curved section and soil

4.2.8.2.2  Drag Force

Drag force results from the fluidic drag between the drilling fluid (Bentonite) and the downhole pipeline.

Drag force can be estimated using the formula below

4.2.8.2.3 Force Due to Change in Elevation

The elevation change either support or counters pulling in of pipeline. When the pipeline is pulled downhole the elevation change supports the pulling in when pulled uphole elevation changes obstructs pulling in, for horizontal sections the force is zero.

The force due to change in elevation can be estimated using the formula below:

4.2.8.3 Tensile Stress

The tensile stress for each section and the entire pipeline can be estimated using

4.2.8.4 Bending Stress

The bending stress is estimated using the equation below

4.2.8.5 Hoop Stress

The hoop stress resulting from external pressure generated by drilling fluid on the empty pipeline can be estimated using.

4.2.8.6 Combined Stress

Unity Check for Tensile and Bending Stress

This should satisfy the equation below

Unity Check for Tensile, Bending and Hoop Stress

This should satisfy the equation below

4.2.9 Operation Stresses

Operation stress is the same as in a conventional pipeline. However, the elastic bending stresses should be taken into consideration.

5 Horizontal Directional Drilling: Inspection and Testing

All pipes should be appropriately tested as per written specifications approved by the client. See below requirements for testing/examination to be performed before pipeline pullback.

5.1 Non-Destructive Examination

After pipeline welding, all pipeline welds shall be inspected by qualified personnel. As stated in ASME 31.8 section 841.1.9 (j) (5) (a) non-destructive examination of 100% of all circumferential welds before installation

5.2 Holiday Test

Holiday test shall be performed on pipelines to detect defective external coatings and appropriate repairs shall be performed on defective coating.

5.3 Hydrotest

Pipelines to be installed by Horizontal Directional Drilling shall be hydrotested before pullback into the drilled hole. The requirement for hydrotesting is also corroborated by ASME 31.8 section 841.1.9 (j) (5) (a) and ASME B31.4 Section 434.13.5 (e)

6 How does Horizontal Directional Drilling Works

Horizontal directional drilling comprises of three stages:

• Pilot Hole Drilling or Pilot Bore
• Reaming
• Pipeline Installation (Pipeline Pullback)

It should be noted that the above activities cannot start until site preparation has been completed. So I will discuss site preparation before the above listed three stages.

6.1 Site Preparation Works

This is the first activity performed before the commencement of drilling.

Activities performed include:

• Site clearing, grading, levelling and other related activities. Perimeter fence to prevent erosion may also be installed.
• Mobilisation of Drilling Rig to Site. HDD equipment utilised for installing large diameter pipelines are massive (Maxi HDD rigs) therefore adequate planning including route studies should be performed
• The Positioning of Drilling Rig. The rig should be installed as per the design documentation. The rig should be in alignment with the proposed hole and entry point, adequately anchored, especially when large pulling forces are required to pull back the pipeline.
• Excavate entry pit. This is used to collect returning drilling fluid. The drilling fluid is also pumped from the pit for proper separation and reuse,
• Position Drill Pipes. The drill pipes should be stacked in a location that is not far from the drill rig. Typically a crane should be able to pick a drill pipe from the stack and drop it for installation onto the rig by performing a swinging operation only.
• Position the control cabin (control room) such that the operator can see the drill rig. This is mostly positioned at few meters away from the drilling rig.
• After preparations are done, the rig is tested to confirm its operational status. This entails performing rotational motion, forward and backward motion of various components.

6.2 Pilot Hole Drilling

This is the first drilling activity performed; below are typical activities performed during the pilot drill. This should commence after all preparatory works have been completed.

• Prepare the drilling fluid as specified by appropriate documentation.
• Start the drilling rig ensuring all components are in good working condition.
• Connect the first drill pipe to the rotating mechanism of the drill rig.
• Connect the drill bit to the front of the first drill pipe
• Commence drilling operation by pushing the drill pipe into the propose entry point.
• After the first drill pipe has been pushed into the ground disconnect drill rig rotating mechanism and installs another drill pipe, repeat this continuously until the drill bit emerges at the exit point.

It should be noted that the HDD operator steers the drill bit into an appropriate location (designed drilled path) during the drilling operation.

During the drilling operation drilling fluid is continuously pumped into the hole to lubricate the drill bit, loosen the soil and stabilise the hole.

Suspended particles are transported back to the entry pit for further separation from drilling fluid.

6.3 Reaming

After the completion of the pilot drill, i.e. the drill bit has emerged at the exit point, the drill bit is disconnected, and the reaming tool is attached to the drill pipe. The reaming tool is run through the pilot hole, gradually increasing its size. Depending on the size of the pipeline to be installed in the drilled hole, there might be more than one cycle of reaming.  The reamed hole usually has an oversize of 1.2 to 1.5 times the diameter of the pipeline. Achieving the size of the hole depends on the soil type, soil stability, drilling depth and drilling mud used. The last reaming activity performed is to run a swab reamer through the hole; this is done to verify that the hole is clear and free of obstructions. It should be noted that the swab reamer diameter should be equal or larger than the pipeline diameter but smaller than the largest reamer utilised for hole widening.

6.4 Pipe Pullback (Pipeline Installation)

Before pipeline pullback, all the pipes are welded together at a designated location. The total length of the pipeline should be a little longer than the length of the drilled hole.

A pulling head is welded to the pipeline (the side of the pipeline facing the exit point). The pulling head is the component that will be hooked to the drill pipe; therefore, it must be of adequate strength. Non-Destructive Testing (X-Ray) is performed on all welded joints to verify their integrity.

All welded joints are coated with 3LPE or FBE as may be required depending on the design conditions. A holiday test is performed on the entire pipeline to detect defective coating, and appropriate repairs are performed accordingly.

Before installation, the entire pipeline is hydrotested by pressurising the pipeline as per relevant codes and standards such as ASME B31.4, ASME B31.8, etc.

The pipeline may be positioned on rollers and appropriately aligned to the axis of the exit point. The pipeline should be appropriately positioned to facilitate easy pullback without much resistance at the exit point and curved sections.

The pulling head is connected to the drill pipe, and the pipeline is pulled into the reamed hole. In some cases, a swab reamer is attached to the drill pipe before connecting the pull head. The function of the reamer is to clear all debris in the hole as the pipeline is pulled back.

When the pipeline emerges at the entry point, all drill pipe connections are disassembled and the pulling head cut out.

7 Project Experience: Pipeline Bundle Installation

In this section, I will share my project experience as it relates to pipeline installation using horizontal directional drilling.

7.1 Project Concept

The project entailed building an import berth platform with shallow water of approximate depth 20m. Oil Vessels would berth on the platform to offload their products for onward transportation to various Storage facilities.

Three different pipelines were installed as a bundle from a product reception and metering facility to the Import Berth Platform using Horizontal Directional Drilling. The total drill length is over Eight Hundred Meters (800m).

The pipeline sizes are

16” Multi-Product pipeline

6” Firewater Pipeline

4” Pipeline for Cables

It should be noted the project area (Import Berth Facility) is a bustling lane for ocean vessels. The shipping lane is always dredged to increase its depth; therefore, the pipeline must be installed at greater depth.

The drilling rig was set up onshore at the reception/metering station, and the proposed exit point is offshore just after the import berth platform.

7.2 Project Challenges

• Three Pipelines were installed as a bundle (16”, 6” and 4”) which is different from the conventional pullback of a single pipeline.
• The exit point is not visible because the water depth is over 20m. Therefore, drilling accuracy is of high importance.
• Pipes were welded onshore and towed offshore, aligned with the drilled hole for pulling in activities. Because of the high waves experienced in the project area, adequate review of meteorological data is essential for project planning and execution.

7.3 Project Execution Strategy

• The drill site was prepared as described in previous sections
• The exit point was identified offshore and adequately marked with a buoy.
• A barge with lifting equipment (cranes, excavator) was mobilised close to the drilled hole exit point. The barge has a locally fabricated attachment serving as a stinger that extends into the water. The stinger provides the radius of curvature for the drill pipe onto the barge. On the barge, drill pipes were disassembled, reassembled, reamers were installed on the drill pipe assembly, etc.
• All pipelines (16”, 6” and 4”) were welded onshore close to the shoreline simultaneously with the drilling activities.
• NDT was performed on all pipelines
• Welded joints were coated
• Holiday test was performed on all pipelines
• Pipelines were hydro tested
• A single pulling head was welded to all three pipelines, NDT was performed on welded joints.
• Pipelines were strapped together with spacers installed in-between the pipelines at appropriate locations. The spacers prevent the pipelines from making contact. Note the drilled hole was approximately 26” to accommodate the pipeline bundle.
• Assembled pipelines were lifted on rollers ready for launch into the water.
• Buoyancy drums were installed on the pipeline bundle to provide floatation during towing offshore.
• Two tugboats were mobilised to the site. The boats were used to tow the assembled pipeline bundle offshore and align the pipe into position for a pullback into the drilled hole.
• The pulling head on the pipeline bundle was connected to the drill pipe head just outside the barge. This was done with the aid of the lifting equipment (cranes, excavators) on the barge and the support of the tug boats.
• Pullback commenced, as the pipeline bundle is pulled into the hole the buoyancy drums were removed by cutting the strap holding the drums to the pipeline bundle.
• HDD installation was successfully completed.
• Offshore barge and drilling equipment were demobilised.

8 References

ASME B31.4: Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids

ASME B31.8: Gas Transmission and Distribution Piping Systems

ASCE MOP 108-2014: Pipeline Designed for Horizontal directional Drilling

ExxonMobil GP: Horizontal Directional Drilling