Horizontal Directional Drilling, known as HDD, has been around for many years. During this period, it has become one of the most effective methods of installing pipelines under rivers, in swampy areas and or through terrain where the conventional open cut method is not feasible.
HDD is a trenchless construction method utilizing a Horizontal Directional Drilling rig with accessory equipment to drill at an angle normally of 20 degree or less to install pipelines (steel, composite, plastic), fibre optic and electric cables and water and wastewater pipelines. HDD Technology is used in many situations including the following:
i. Lake crossings
ii. Wetland crossings
iii. Canal and Water crossings
iv. Valley crossings
v. Pipeline crossings
vi. Sensitive Wildlife Habitat
vii. Road and Railway crossings
viii. Community dense areas
ix. Vandalized Areas
x. Onshore to offshore installations (outfalls/shore approaches)
From an investors point of view, the execution of HDD needs to be efficient, effective, and a cost saving solution for pipeline installations in challenging conditions. A proper survey analysis, design and engineering, effective planning and budgeting coupled with past experiences from proven professionals is the key to success in the HDD industry.
The execution of HDD installation involves five main steps:
i. Survey, Design & Engineering
ii. Pre-Site Planning
iii. Pilot Hole Drilling
iv. Reaming (Expansion of pilot hole)
v. Pull Back (Installation) of prefabricated pipe length
Survey:
For a proper HDD execution, it is imperative to conduct a survey analysis of the area where the expected pipeline is to be installed be HDD. The survey analysis includes the following:
Geotechnical Investigation: is performed by geotechnical engineers/geologists to obtain information of the physical properties of soil earth works sub surface. The investigation is additionally used to test thermal resistivity of soils. Subsurface exploration involves soil sampling and laboratory tests of the soil samples retrieved. Knowing the subsurface soil conditions is crucial in determining the required drilling tools and cuttings tools necessary to execute the HDD installation successfully.
Topographic Investigation: is performed to understand the shape and features of land surfaces. This is necessary in planning the entry location where the drilling spread will mobilize to and also the exit platform for where the pipeline will be fabricated and prepared in anticipation for the pipeline installation.
Bathymetric Investigation: is performed with depth sounding to understand the underwater depth of the lake, swamp, river or ocean where the pipeline is to be installed. With the bathymetric survey, we receive information about potential obstacles or obstructions in our drilling path which allows us to plan for the best drilling path to install the pipeline to the appropriate safe working depth.
Once the survey team has finished these necessary applications and any additional survey analysis, the information is packaged in AutoCAD format. Once we have the information in this format, we can plan our design and carry out an engineering analysis for the crossing.
Design:
The design of the drilling path is critical as we need to account for the safe working radius of the pipeline. Our engineer will acquire the necessary pipeline data sheet when creating the drill profile. Once the drill profile has been created, we will then perform and engineering analysis.
Engineering:
Within the engineering preparation, the following analysis will be conducted to determine that the installation of the pipeline on pullback will be successful.
Pull force Analysis is the calculation of the total pull force of the pipeline. The pull force is calculated at segments of the pipeline from initial pulling of the pipeline prior to installation to the final segment of pulling as the pipeline would be ultimately installed.
Stress Analysis
The pull force analysis has to take into consideration a stress analysis also. The stress analysis will confirm that the proposed drilling trajectory is feasible for the pipeline installation. Stress analysis is a combination of Tensile Stress, Bending Stress and External Hoop Stress. We utilize our engineering team to calculate the combination of three stress figures with the pipeline data sheet to determine the appropriate safe installation parameters. These parameters are utilized to confirm that the drilling path can be approved for the execution phase of the project.
Upon completion of the survey and engineering analysis along with the approval of the client for execution of the said drilling parameters, pre-site planning commences. Within the pre-site planning the following is accounted for:
A. Procurement of the following:
Drilling & reaming tools.
Drilling Fluid: Bentonite & Polymers
Any miscellaneous items &/or equipment for any specialized method within the installation execution.
B. Preparation of all drilling equipment & support equipment for mobilization:
Ensuring that all equipment has been pre-mobilized for execution preventing unnecessary delays upon mobilization.
C. Engagement of core & expert staff in preparation for mobilization and commencement of execution
After pre-site planning, mobilization to site commences. Within the mobilization period, anchor piling and equipment set-up takes place. Within a short timeframe of the mobilization, the execution team will be ready to commence pilot hole drilling.
Pilot hole drilling is executed in two options:
Drilling with a jetting assembly: Design for soft formations
Drilling with a mud motor: Design for hard formation
The drilling tools that are required to drill, are subject to the ground formation. There are a range of tools that can be specific to soft formation to hard formation. Based on the geotechnical analysis, the appropriate drilling tools will be mobilized to site.
A pilot hole will be advanced along a predetermined path from the entry point to the exit location and vice-versa. In front of this pilot drill string a drilling assembly is attached. Drilling mud (bentonite) which is environmentally friendly, comprising a suspension of water and drilling fluids, will be pumped down the drill string to carry out the cuttings and to stabilize the bore.
Placed right behind the Bottom Hole Assembly (BHA), inside the non -magnetic part of the string, a steering tool is mounted. The pilot hole will be monitored and surveyed along the entire bore hole length and will be recorded onto a profile plot drawing. Although the probe is inserted inside non-magnetic housing, readings can be subject to interference from down hole tools, drill pipes and other artificial as well as natural magnetic fields.
To get the best result we use on sections of the drill path additionally an independent "Gyro Steering System" system for verification of the survey records.
Directional control is accomplished by rotating the drill string to adjust a bend axis housing on the down hole assembly thus creating a steering trend in the direction and plane of the bent housing. The pilot bore is completed when the bottom hole assembly appears at the exit location as shown above.
During pilot hole drilling, the following measurements will be recorded and reported in a logbook:
i. Pull and push forces
ii. Number of drill pipe
iii. Rotary speed and torque of the drill string
iv. Drilling mud pressure
v. Pump volume
vi. Time
vii. Density and plastic viscosity of drilling mud profile drawing
viii. Drilling Mud
Drilling mud such as fluid bentonite clay, which has been approved by the Client, is injected into the bore during cutting and reaming to stabilize the hole and remove soil cuttings. Drilling mud can be made from clay or polymers. Bentonite is especially good as it is environmentally friendly and not hazardous to human health. Portable water is required for the mixture.
The purpose of reaming is to enlarge the pilot hole to the desired diameter, at least 30% above the diameter of the product pipe and ultimately ensure safe passage of the line pipe during pullback.
When the pilot drill is finished, a reamer and/or a fly-cutter will be connected to the drill string to enlarge the bore. The reaming or cleaning process starts by pulling and rotating the drill string to the offshore rig.
The reaming diameters are dependent on the size of the final pipeline diameter to be installed. During the reaming operations, drilling mud (bentonite) is pumped through the drill string and out through the nozzles of the reamer to clean cuttings from the hole and to lubricate the drill string. Please note that the drilling mud used in the HDD works from the pilot drill to the pull back of the product pipe is recycled using the HDD recycling units.
During the reaming operation, the final decision will be made regarding the number of ream steps and the final borehole diameter based on the product pipe configuration, soil information and experiences gathered during the ongoing operation. Usually a borehole diameter is selected, which is 1.3 to 2.0 times larger than the diameter of the product pipe to be installed.
The product pipes are welded to the desired length. Non-destructive testing, Holiday detection, field joint coating, hydrotesting of the HDD spool, the product pipe is placed on Over-bend with rollers fixed on them. The over-bend is designed to give the product pipe an easy entry at the designed entry point angle. This helps in eliminating stress on the pipe during pull-back. In areas where we have swampy terrains, ditches will be made for the straight part of the pipe to float in and connect to the over-bend of the product pipe on the over-bend, a Pull-head is welded to the entry end of the product pipe for pull into the bore. In areas where this cannot be achieved, the straight path of the pipe is placed on rollers on timber mats, spaced adequately to reduce stress on each individual roller. Prior to placement of the product pipe on the over-bend, a Pull-head is welded to the entry end of the product pipe for pull into the bore.
Prior to pull-back preparations, the bore hole contains a continuous drill string from the pipe side to the rig. The pullback assembly is determined on-site, based on the ream operations and then attached to the drill string. A swivel connects the down hole assembly to the product pipe to prevent torsion stress from being transmitted to the product pipe.
For pull back usually a slightly smaller diameter reamer is used than during the last enlargement stage and acts as a centralising device. The tool is equipped with nozzles with determined number and diameter to guarantee the drilling mud flow during pull back. The product pipe will be gradually installed in the reamed hole by pullback of the entry rig.
Pulling forces and all other parameters are permanently monitored and documented.
After completion of the installation of the Pipe, the pulling head located on entry site is cut off.
Demobilization
On completion of product pipe installation, the down hole assembly is removed from the pipeline, the rig and ancillary equipment will be cleaned and disassembled, and demobilisation will commence.
As-Built Survey
At the completion of the work, our surveyor shall prepare a survey As-built and submit to the Client.
As-Built Drawings
The AFC drawings shall be upgraded and re-issued by the site engineering team as "as built drawings". They shall include but not necessary be limited to:
i. Piping and instrument diagram, if applicable.
ii. Pipelines route map
iii. Ground profile of pipelines, if applicable
iv. Pipelines alignment sheets
v. Detailed drawing of crossings
vi. Arrangement drawings of installations
vii. Scraper launchers and receivers plot plans, piping arrangement and isometric drawings where applicable.
viii. Foundation plans and reinforced concrete details, civil drawings relevant to scraper traps, etc. where applicable.
x. Final list of test points installation.
