APPENDIX 2.1  HORIZONTAL DIRECTIONAL DRILLING FOR SUBMARINE OUTFALL AT SOK KWU WAN

 

1. OVERVIEW

 

Originally used in the 1970s, directional crossings are a combination of conventional road boring and directional drilling of oil wells.  Crossings have been installed for pipelines carrying oil, natural gas, petrochemicals, water, sewerage and other products.  Ducts have been installed to carry electric and fiber optic cables.  Besides crossing under rivers and waterways, installations have been made crossing under highways, railroads, airport runways, shore approaches, islands, areas congested with buildings, pipeline corridors and future water channels.

 

The longest crossing to date has been about 6,000 ft. (1,828m).  Pipe diameters of up to 48 in. (1,219mm) have been installed.  Although directional drilling was originally used primarily in the U.S. Gulf Coast through alluvial soils, more and more crossings are being undertaken through gravel, cobble and hard rock.

 

Directional crossings have the least environmental impact of any alternative methods.  The technology also offers maximum depth of cover under the obstacle thereby, affording maximum protection and minimizing maintenance costs.  Owing to the nature of the works involved, it is anticipated that specialist contractors will be required to undertake the HDD works.

 

Reference is made to the installation of the shore approach crossing at Tasmania, Australia by HDD.  Reasons for proposing the HDD method include environmental, site safety, risk and construction duration considerations.  The method allows minimization of dredging of marine deposits and hence brings forth environmental benefits.  Site safety is also enhanced as most of the work is carried out on the working platform above water level.  Risk is minimized as no intermediate working platforms and stage drilling is required.  Construction time is shortened since time required for the construction works is relatively short compared with the traditional dredging method.  By undertaking works at considerable depths below the seabed, it is apparent that the HDD technique is not prone to induce adverse ecological impacts. 

 

 

2. PROFILE SURVEY

 

The area should be surveyed and detailed drawings prepared.  The eventual accuracy of the drill profile and alignment is dependent on the accuracy of the survey information.  Once the crossing profile has been taken and the geotechnical investigation complete, a determination of the depth of cover under the crossing is made.  It is recommended that the minimum depth of cover be 10m under the seabed.  With this depth, it is envisaged that armoured rock will only be necessary where the outfall is constructed by dredging method, including the diffuser section.  It would be required at the near shore section where rock is encountered.

 

An entry angle between 8 and 20 degrees to the horizontal can be used.  It is preferable that straight tangent sections are drilled before the introduction of a long radius curve.  The radius of the curve is determined by the bending characteristic of the product pipeline, increasing with the diameter.  The curve usually brings the profile to the elevation providing the design cover of the pipeline under the seabed.  Long horizontal runs can be made at this elevation before curving up towards the exit point.  Exit angle should be kept between 5 and 12 degrees to facilitate handling of the product pipeline during pullback.

 

The accuracy of the drill profile is largely dependent on variations in the earth's magnetic field.  A reasonable drill target at the pilot hole exit location is 1m left or right, and within ±5m in length depending the actual seabed level.

 

3. SETTING UP

 

Rig Side

 

A steel working platform of size 30m wide by 10m long is to be erected close to the shore along the alignment of the pipeline.  The working platform will facilitate the setting up of the horizontal directional drilling system, fluid mixing system, a return pit, a "settling and containment pit." and lifting equipment.

 

The drilling diameter will have to be approximately 75 - 100 mm larger than the external diameter of the HDPE outfall pipe.  The drilling operation requires large volume of water for the mixing of the drilling slurry.  A nearby source of water is necessary.

 

Pipe Side                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

 

A barge shall be employed for installation of the HDPE pipes.  The HDPE pipeline shall be pulled back into position by the HDD system on the rig side.  During the pullback operation, the 225mm diameter HDPE pipe sections are heat welded on the barge to form the submarine outfall pipeline.

 

Figure HDD is an illustration of the construction of submarine outfall using the Horizontal Directional Drilling method.

 

4. DRILLING FLUID

 

The drilling fluid is a mixture of water and specialized additives used in the drilling process.  The drilling fluid stabilizes the hole against collapse, cools the drill head and transmitter, lubricates the drill string and the pipeline being pulled back and suspends drill path cuttings into a slurry which flows out of the drill path as the pipeline is pulled into the bore path.

 

The marine clay will prevent water from flowing into the formation, but have a strong tendency to become sticky and swell when mixed with water.  Polymers and surfactants will be used as the drilling fluid.  Polymers reduce swelling of the soil and lubricate the drill path to reduce friction on the drill stem and product.

 

The slurry is pumped downhole and circulates back to the surface and collected in "return pits."  These pits typically have a volume of at least 50 cu m.  Depending on the nature of the project, the slurry is pumped from the return pits to a "settling and containment pit."  These pits vary in size depending on pumping rates and contain the slurry for recycling or disposal.

 

Slurry that has been circulated downhole and collected in the containment pit is then passed through machinery that separates the cuttings from the slurry.  This process involves a series of shaking sieves and various size hydroclones.

 

5. TECHNIQUE

 

As the bedrock may be shallow near the shore, steel casing shall be installed using Odex method at a prescribed angle from horizontal till marine deposits are encountered.  A pilot hole with a reamer attached are then drilled and continues under and across the obstacle along a design profile made up of straight tangents and long radius arcs.

 

Mud flow is created by pumping a combination of water and drilling fluid through the drill rod and head (or backreamer).  The drilling fluid then mixes with soil in the drill path and creates a flowing slurry back out of the path as the product pipe is pulled into place.

 

The directional control is brought about by a small bend in the drill string just behind the cutting head.  The pilot drill string is not rotated except to orient the bend.  If the bend is oriented to the right, the drill path then proceeds in a smooth radius bend to the right.

 

The drill head is equipped with a transmitter that sends signals to a receiver during the bore.  To determine location of the drill head, the receiver uses signal strength from the transmitter (sonde) in the drill head to indicate its depth and position.  The depth and position information is displayed on the locator screen.

 

Silt curtains shall be installed around the exit area.  When the pilot drill is about to break ground, pumping of slurry shall be stopped.  The drilling is then resumed till the pilot drill breaks ground.  Divers will be employed to remove the pilot drill, and connect the reamer to the pipeline pullhead via a swivel.  The swivel prevents any translation of the reamer's rotation into the pipeline string allowing for a smooth pull into the drilled hole.  The drilling rig then begins the pullback operation, rotating and pulling on the drill string and once again circulating the drilling slurry.  The pullback continues until the reamer and pipeline break ground at the drilling rig.

 

The water quality of the surrounding waters may be affected by the flushing fluid discharged.  A silt curtain is proposed to enclose the working area at the diffuser to contain the discharged fluid.  The flushing fluid adopted in a previous project contains no dangerous contaminants and is environmentally friendly.  The product is extensively used in Europe as well as America for HDD.

 

6. RISK OF FAILURE

 

The risk of failure is low with the areas of risk identified are as follow:

 

(i)                  The pilot/reamer may get jammed during piloting or during the reaming process.  The worst scenario is to abandon the pilot/reamer and form a new path.  Jamming of the pilot/reamer is unlikely to occur as no cobbles or boulders are expected to be encountered in the marine deposits.

 

(ii)                The pipeline could be jammed during the pulling process.  The worst scenario is to abandon the pipeline and restart the drilling and pulling process once again.  The jamming of the pipeline is unlikely to happen as bentonite fluid will be used to lubricate the path and keep the hole open at all time, and that the reamer leading the pipeline will ensure the path is clear for the pipeline.