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HDD construction methodology for river crossings

Maxibor prepares an HDD Construction Methodology document to provide a comprehensive overview of the processes involved in the physical delivery of major projects and demonstrate how key activities and issues specific to the project are to be managed.

The key to a successful horizontal directional drilling (HDD) project is clear communication to asset owner and principal contractor stakeholders of what you are going to do and how you will do it.

This is particularly so for more challenging projects such as river crossings. 

The extent of contents of an HDD Construction Methodology highlights the diverse range of activities undertaken for an HDD installation.

Typically, the document includes:

• Scope 

• The stages of the drilling process

• Drill path design

• Drilling fluids 

• Downhole survey 

• Integrated construction management 

• Construction schedule 

• Key contacts 

• Design and engineering analysis
o HDD alignment design and hydrofracture analysis
o Pipe stress analysis and selection
o Downhole tooling and equipment selections

• Site preparation
o Survey control
o Site and equipment set up

• Site specific HDD construction procedure
o Pilot hole construction
o Reaming and hole conditioning
o Pipe welding and testing
o Product pipe installation and testing
o Demobilisation and reinstatement
o As-built data and deliverable submission

• Contingency measures
o Loss of drilling fluid
o Hole collapse
o Fluid pit overflow
o Drill pipe failure
o Side tracking

In this article, edited extracts of two of the more important areas of a typical HDD Construction Methodology document that apply to a river crossing project have been selected to provide insight as to how Maxibor communicates to project stakeholders.

Drilling fluids 

The foremost purpose of drill fluid is to provide a medium to carry the drilled cuttings from the face of the bore along the annulus formed between the drill pipe and borehole to the surface.

Other functions include promoting borehole stability, reducing infiltration through borehole membrane, reducing drilling fluid loss to the formation through the build-up of a borehole wall filter cake and lubricating and cooling the drill string, drill bit and downhole instruments and product pipe during installation.

To achieve these functions, a carefully engineered drilling fluid design is required.

The fluid design takes  into account the geological formation being cut, annular fluid pressures and pump rates, properties of drill fluid additives, make-up water properties and recycling methods and equipment available.

Effective circulation of drill fluid requires a recycling system with a capacity to match the expected pump/flow rates required to clean the borehole. 

Typically, a freshwater bentonite-based system is used. These systems offer a high degree of cuttings suspension, as well as friction reducing lubrication. 

This enables large granular materials to be removed, as can be anticipated where rock is present.

The drilling fluid can be modified with clay-inhibiting additives to mitigate swelling and bit-balling in sections of reactive clays.

Long chain polymers additives can also be added to the drilling fluid to build up a ‘filter cake’ on the borehole wall to prevent fluid losses and improve borehole stability. 

The density and pressure of the drilling fluid should be monitored and recorded at frequent intervals and, should the density rise above preset limits, drilling fluid should be diluted in the bore using freshwater and additional additives.

The dilution volume will account for additions of fresh mud volume to reduce low gravity solids as well as some losses to the formation. 

Where the drill fluid emerges at the HDD exit or entry location throughout the stages of drilling, fluid can be pumped directly from the point of return through the fluid recycling systems and then onto the drill rig.

This ensures a non-stop efficient drilling operation which is especially important in hard rock drilling operations.

HDD alignment design and hydrofracture analysis

The river crossing alignments are designed to meet the requirements of the product pipeline while in service.

HDD entry and exit angles and minimum radius of curvature of the proposed borehole ensure that the HDPE carrier can be installed without exceeding material limits. 

The alignment designs take into consideration safe depths of cover required to resist fluid escape to surface, particularly during drilling of the pilot hole, when borehole pressures are at their highest. 

A close review of the soil properties and the interaction of the pilot hole is undertaken to determine the Minimum Required and Maximum Allowable pressures as per the soil properties. 

In practice, the drilling may vary the flow rates up or down to suit the stratum being drilled at the time.

As such, fracture plots represent a good average across the bore and show areas where frac outs are likely so that appropriate control measures can be positioned ahead of the drilling operation. 

A Hydrofracture Analysis is completed at each crossing location to identify high risk areas and to iterate the bore profile accordingly to find the optimum solution for geological stability, profile alignment, depth of cover, radius of curvature and frac resistance. 

To ensure that pilot hole construction is conducted in a safe manner, downhole pressures are monitored using a downhole annular pressure probe which feeds real time data directly to the driller’s cab and can be compared to the frac prediction pressure calculations in real time.

Because it is capable of giving a rapid indication of an increase in downhole pressure, the measurement of pressure while drilling is an effective technique to reduce the risk drill mud breakout during the drilling process.

It can also help ensure that a drilled hole is kept clean and free of blockages, thus reducing operational risks. 

For more information visit the Maxibor website.

This article was featured in the July 2021 edition of The Australian Pipeliner. To view the magazine on your PC, Mac, tablet or mobile device, click here.

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