generic hdd crossing method statement

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APPENDIX 3.4 GENERIC HDD CROSSING METHOD STATEMENT WILLINGTON C GAS PIPELINE ENVIRONMENTAL STATEMENT VERSION 1 View from Twyford Village along the River Trent looking upstream towards Willington. Willington C Gas Pipeline Environmental Statement Appendix 3.4 Generic HDD Crossing Method Statement Version 1, July 2013 Approved for Submission Application Reference Number – EN060001 Document Reference – WCGP 014.2.3.4 Required document as set out in Section 5(2)(a) of the Infrastructure Planning (Applications: Prescribed Forms and Procedure) Regulations 2009

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Page 1: Generic HDD Crossing Method Statement

APPENDIX 3.4 GENERIC HDD CROSSING METHOD STATEMENT WILLINGTON C GAS PIPELINE ENVIRONMENTAL STATEMENT

VERSION 1

View from Twyford Village along the River Trent looking upstream towards Willington.

Willington C Gas Pipeline Environmental Statement

Appendix 3.4 Generic HDD Crossing Method Statement Version 1, July 2013 – Approved for SubmissionApplication Reference Number – EN060001 Document Reference – WCGP 014.2.3.4

Required document as set out in Section 5(2)(a) of the Infrastructure Planning (Applications: Prescribed Forms and Procedure) Regulations 2009

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Generic HDD Crossing Method Statement

Prepared by :-

A K Knight

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Contents

1 Introduction .................................................................................................1

2 Design of Crossing.....................................................................................1

2.1 Geotechnical Assessment.............................................................................1 2.2 Drill Profile.....................................................................................................1 2.3 Pulling Calculations. ......................................................................................1 2.4 Tooling Schedule...........................................................................................1 2.5 Mud Return Design .......................................................................................2

3 Mobilisation and Site Preparation.............................................................2

3.1 Access...........................................................................................................2 3.2 Existing Services ...........................................................................................2 3.3 Drill Sites .......................................................................................................2 3.4 Water Requirements .....................................................................................3 3.5 Drill Profile and Setting Out ...........................................................................3 3.6 Anchorage.....................................................................................................3 3.7 Rig Up ...........................................................................................................3

4 Pilot Drilling Operation...............................................................................3

4.1 Guidance System..........................................................................................4 4.2 Drilling Fluid...................................................................................................4

5 Reaming Operation(s) ................................................................................5

6 Pipe-String Preparation..............................................................................6

7 Pull Back Operation....................................................................................6

8 Pipeline Completion ...................................................................................6

9 Typical HDD Site Layouts & Photographs ...............................................6

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1 Introduction

The purpose of this document is to provide guidance on how the HDD is likely to be

undertaken. Until such time as a specialist contractor is contacted, the contents of this

report should be treated as Provisional because there are several options that could be

employed depending on the type and size of the drilling machine used and the preference

of the contractor for other issues.

2 Design of Crossing

Prior to commencing site works, a drill profile, tooling schedules and pulling calculations

will be prepared by the drilling contractor and/or drill designer for client approval.

2.1 Geotechnical Assessment

Prior to any design work on the HDD crossing, an analysis of any available geotechnical

information for the area of the drill will be undertaken. Bore Holes will be required on

either side of the crossing to a depth of at least 20m below the river bed level. The bore

hole cores will be subjected to a laboratory analysis to provide soils type and strength

information to the HDD contractor and/or HDD designer.

2.2 Drill Profile

Based on the information provided by the client, a drill profile will be produced showing the

local geological formations and the proposed drill dimensions including the entry and exit

points and angles the overall length of the crossing, the radii to be used when changing

direction and the levels of cover maintained throughout the crossing.

2.3 Pulling Calculations.

Utilising the information within the drill profile and industry standard good working practice

it is possible to estimate the major forces acting upon the product pipe and drilling

equipment during the installation of the pipe, once the forces have been analysed the

buoyancy system for the pullback can be designed so that the minimum forces possible

are induced within the pipe as it is installed within the pre drilled hole

2.4 Tooling Schedule

The Tooling Schedule is based on the drill profile and the geotechnical information

available, the schedule describes the types and sizes of tooling which should be the most

efficient to produce the desired bore. Once the tooling has been confirmed, a construction

programme can be produced as well as an estimate of the cost involved in the project.

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2.5 Mud Return Design

Depending on the location and length of the HDD, various methods are available for returning the drill mud from the receive pit to the drill launch site. The main methods are to transport the fluid by road tanker, lay an above ground mud return line or drill a separate smaller HDD and install a mud return line within the additional drilled hole. The cost effectiveness and practicalities of each potential method will need to be assessed as part of the overall HDD design.

3 Mobilisation and Site Preparation

The following activities will be completed prior to the arrival of the HDD equipment on site.

3.1 Access

Access is required to the entry and exit sites of a suitable quality for all-terrain mobile

cranes, 40’ articulated trucks, and ongoing use by delivery vehicles.

Therefore any necessary preparatory works, ground stabilisation, hard standings and road

improvement will be completed prior to the arrival of the HDD equipment.

3.2 Existing Services

All existing underground services shall be located, marked, and protected as necessary.

Any services that may conflict with the proposed installations or site operations will be

exposed to prove their exact location and appropriate measures taken to protect them

from damage and to ensure the safety of site operations.

3.3 Drill Sites

A suitable working area is required at both sides of the drilled crossing, these will be

striped of topsoil and fenced off with either stock proof fencing or security fencing as

required. Stone / Hardcore is then laid to provide a level, firm area approximately 40m x

40m for the HDD rig and ancillary equipment, and at the pipe side of the crossings, an

area approximately 30m x 25m is required to encompass the exit pit and mud lagoons.

The plan area of the lagoons will vary depending on the dimensions of the crossing, but

will be a minimum of 5 m x 10 m. The depth of the lagoons will be generally be 1.5m but

may also vary according to local topography. In any event excavations shall be battered

to a safe angle of repose and excavations fenced off with ‘Heras’ type fencing.

As well as for mud storage excavations will be required at the entry and exit points of the

crossing. The entry and exit pits shall be 3 m x 3 m x 1.5 m deep. These pits shall be

safely battered and securely fenced with ‘Heras’ type fencing.

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Welfare facilities will be established at both sites to include at least 1 canteen and drying

room and toilet at each site as well as a site office, normally on the rigsite

3.4 Water Requirements

A non-saline supply of water is required for mixing drilling mud and general site usage,

during drilling and reaming operations the volume of water required is an estimated

average of 30m3/hr with an estimated maximum demand of 60m3/hr provided circulation is

maintained. Therefore depending on the type of water source and the distance of the

source from the rig site some preparatory works may be required to produce the initial

volume of water required for pilot drilling.

3.5 Drill Profile and Setting Out

According to the construction drill profile the entry and exit points will be marked. Also the

centre line of the pipeline shall be marked at 10m centres where feasible.

3.6 Anchorage

Depending on the size and type of drill rig used, an anchorage assembly will be designed

and constructed. The anchorage is used to securely hold the drill rig in position throughout

the drilling works including the installation of the product pipe. It also ensures that the

entry angle and position are as designed and constant throughout the works. Generally for

larger crossings a reinforced concrete anchor block is used however, sheet piles or other

designs have been used effectively in previous projects.

3.7 Rig Up

Upon delivery of the drilling rig and ancillary equipment to the previously prepared work

area, the plant items shall be situated in their working positions and all the necessary

connections made. The drill rig is set up, anchored and adjusted to give the required

ground entry angle

4 Pilot Drilling Operation

During the pilot drilling, a drilling head is driven forward along a predefined drill profile by

the drilling rig erected above ground. The push force generated at the drilling rig is

transmitted by the drilling rods to the drilling head. At the beginning of the pilot drilling, the

first part of the down hole assembly (DHA) consisting of drill bit, bent sub and non-

magnetic drilling rods is pushed into the ground at the previously determined entry angle

by the drill rig.

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Drilling fluid is then pumped at high pressures through the hollow drill pipes to the DHA

where it jets out from the end disturbing the soil, however, this jetting method is only

feasible in very soft formations.

In firmer soils, drilling fluid flows into a mud motor, which houses auger like flights, thereby

producing rotation within the DHA. This in turn rotates the drilling bit used for breaking the

formation.

In both cases, the drilling fluid exits the DHA at high pressure suspending the loosened

material and transporting it to the surface by flowing back through the annulus.

Each drilled rod is followed by another one from the drilling rig and the drilling process

continues in cycles until the drilling head punches out at the target destination.

4.1 Guidance System

The respective position of the drilling head underground is determined with the aid of a

sensor located directly behind the drilling head, which measures an induced magnetic field

produced by an electric coil or beacon in conjunction with the existing earth’s magnetic

and gravitational field.

Data collected from the downhole instruments provide the azimuth and inclination of the

hole and rotational position of the drill. This data combined with accurate record keeping

on the length of drill pipe inserted into the hole allow simple mathematical computations to

be made to record the progress and position of the pilot drill.

The DHA contains an angled section (the bent sub) to give the drill a bias in one direction.

By rotating the drill string, the tool face of the downhole drilling bit is turned, altering the

drilling bias and so changing the drilling direction. In this way the direction of the drill can

be controlled.

Once the pilot hole has been completed, it is normal to de-mobilise the guidance system

an the subsequent reaming operations will naturally follow the original pilot hole through

which there is always a drill string until the product pipe is installed.

4.2 Drilling Fluid

The drilling fluid is a slurry or “mud” produced by properly mixing, ground and refined

bentonite with other chemicals (depending on the chemical properties of the water used

and the rock types to be drilled) and fresh water. The drilling fluid reduces drilling torque,

imparts lubrication to the pipe and provides annular flushing of the freshly cut borehole

debris and provides support to the bored hole.

Once a pilot hole is complete, drilling fluid will escape from both sides of the bore thus the

mud lagoons on the exit side will fill with used drilling mud. To reduce the expense and

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time involved in continually mixing fresh drilling fluid, the mud which emerges from the

hole is transported by road tanker or mud return line to the drill site where it is cleaned and

reused. On smaller crossings a surface laid return line, normally 6” polyethylene (or wire

armoured) pipe, or tankers can be utilised. Where this is not feasible, a pipe is installed

within an initial pilot bore to act as a conduit for drilling fluid and a second pilot is drilled for

the product line.

5 Reaming Operation(s)

After the main pilot hole has been completed it will be necessary to enlarge the pilot hole

sufficiently to receive the product pipe. This is achieved using pre-reaming passes with

reaming tools. The size of the product pipe will dictate the number of and sizes of any

reaming passes.

Pre-reaming tools are attached to the drill string at the exit point. The drill string is then

withdrawn back through the pilot hole, the reamer being rotated by the drill rig as it is

drawn back towards the rig. Drill pipe is added behind as the reamer progresses, thus

ensuring that a string of drill pipes is always present in the hole. Reaming tools vary in

design dependent upon the type of ground being encountered. As a rule for soft grounds,

barrel reamers are used whereas for medium to hard ground fly cutters are used. For

harder formations, such as rock, hole openers are required.

Drilling fluid is jetted through the reamer to lubricate during cutting and to enable the lining

of the hole to remain stable.

This procedure is repeated until the drilled hole has reached the intended final diameter.

The consecutive cutting tools should always work to the same centerline as the previous

drilled hole. The conventional method of ‘back reaming’, when the reamer is pulled from

the pipe site to the rig site, is occasionally replaced by ‘forward reaming’, when the reamer

is pushed into the ground from the drilling rig. In this instance, the pilot hole serves as a

guide for the reaming tool. Occasionally the drill string is also pulled from the pipe site by

using a bulldozer which is connected to the drill string via a swivel. This method can

provide high axial forces without risking buckling of the drill pipe which would be increased

by the push force between reamer and drilling rig.

The hole is typically cut to a size of 25% to 50% larger than the product pipe it has to

carry. This is to facilitate the passage of soil cuttings as well as the pipe and to allow for

soil expansion during the pulling operation.

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6 Pipe-String Preparation

Prior to the completion of the pre-reaming phases the pipe string will be welded, coated

and tested as required. It will then be positioned on rollers awaiting the pull back

operation. To enable the pipe to be installed with minimal pulling forces, the rollers and

therefore the pipe-string are orientated so as to align with the drill hole exit point and

parallel with the exit slope. Cranes and/or scaffolding may be needed adjacent to the point

of pipe entry as the section of lifted pipe will require support for a length proportional to the

bending radius of the pipe to be installed. This over bend needs to be maintained

throughout the pullback procedure.

7 Pull Back Operation

During the final stage of carrying out a horizontal directional drilling, the “welded“ pipeline

is pulled into the completely expanded drilled hole. During the pull back operation, the

reamer is rotated and pulled back under fluid circulation towards the drilling rig. The

pipeline is connected to the drill string via a swivel and universal joint, therefore the pull

forces are transmitted to the pipe string, but not the torque. The pipe string follows the

reamer through the drilled hole up to the entry pit, in front of the drilling rig, without

rotation. In practice, to accelerate the pull back process, a slightly smaller diameter

reamer is used than during the last expansion stage.

8 Pipeline Completion

Once the swivel and universal joint reaches the pit at the drill launch site, the pipeline can

be disconnected at a position sufficiently deep enough to allow the remainder of the

conventional pipeline to be welded to it. At this time the pipeline can be tested to assess

whether the pipe coating has been significantly damaged during the pull operation.

Results from this test will feed back into the Cathodic Protection design. To complete the

tie-ins at each end of the HDD may necessitate the use of cold, forged or induction pipe

bends depending on the entry and exit pipe angles.

9 Typical HDD Site Layouts & Photographs

Below are two drawings showing typical layouts for an HDD Rig Site and an HDD Pipe

Site. Note that the drilling fluid is returned from the Pipe Site to the Drill Site by means of

road tankers. In the event that a mud return line was being used, there would be an

additional pump between the mud pit (Item 1) at the Pipe Site and the mud pit (Item 14) at

the Drill Site.

Also appended are a number of photographs taken at a typical HDD site.

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HDD Drill Rig commencing Pilot Drill

HDD Drill Site

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Pilot Drill String emerging out of the Reception Pit.

Pipe being pulled into the Launch Pit

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