In this article, the second of a two-part series, our HDD expert Charles Stockton takes a closer look at the impact HDD technology has had on shore crossings in pipeline installations.
How is using HDD in water environments different to standard use of HDD?
The main difference in the HDD operation is that the pipe-side is managed over water by a marine spread.
It is essential to correctly specify the vessel requirements and establish what duties it will be performing and in what conditions; this may include seabed preparations, dive support, lifting and recovery of downhole tooling, winching, alignment and hook up of pipe string and placement of clump weights/mattresses for temporary stabilisation, as well as flooding and gauging of the pipeline.
Another technique that has evolved is to drill and leave the bore closed just prior to exit.
This then allows for the bore to be opened by forward reaming, thereby limiting offshore operations and ensuring drill fluids are returned to entry for recycling rather than being lost to the ocean floor.
The final section of the bore can then be reamed out using biodegradable fluids to limit any potential environmental impacts of the break through to the seabed.
Have you been involved in any projects which employed HDD at water crossings?
Stockton Drilling Services has been involved with the majority of complex shore crossings constructed in Australia over the past 15 years, including:
- Minerva Shore Crossings (two) in Victoria for BHP Billiton
- Gorgon Shore Crossings (nine) in Western Australia for Chevron
- Kupe Shore Crossings in New Zealand for Technip/Origin Energy
- Victorian Desalination Pilot Plant Shore Crossings (two) in Victoria
- Narrows Shore Crossings (four) in Queensland for APLNG/QGC
- Gladstone Harbour Shore Crossing Design for Arrow Energy
- Anglesea WRP Shore Crossing replacement in Victoria for Barwon Water
Two projects that are interesting to note are the Gorgon shore crossings, which were constructed on a Class A Nature Reserve and won the national Environmental Engineering Excellence Award at the 23rd annual Australian Engineering Excellence Awards in Canberra.
In presenting the award, Ian Pedersen, Chair of the National Engineering Excellence Awards Judging Panel said, “The uncompromising environmental commitment to this project suggests engineering construction techniques can be ecologically sensitive, allowing us to maintain our natural environment for the future.”
The second project which clearly indicates how the development of new construction techniques have allowed for improved design is the Anglesea WRP replacement, located 25m above sea level on the Anglesea coastal cliffs.
The previous outfall consisted of a 30m deep drop structure which transfers flows from the treatment plant level to the base of the cliffs.
A 185m outfall pipe then discharged flows from the base of the drop structure to the ocean.
The outfall was constructed in 1995. Since construction cliff erosion had exposed the section of the outfall pipe that runs through the base of the cliff, from the drop structure to the beach.
The current rate of erosion is estimated at four metres every ten years.
In May 2006, a rock fall crushed a section of the exposed pipe and emergency repairs were required.
The pipe was repaired and a concrete block was formed around the exposed pipe.
Since this work, the cliff eroded further, exposing the pipe again, undermining the concrete block and placing the pipe at risk of failure again.
The instability of the cliff and risk of further collapse meant it was too dangerous to carry out temporary repairs to the broken pipe.
As a result, Barwon Water initiated a project which required the design and construction of a new outfall pipeline and associated works.
In August 2017, Barwon Water awarded the design and construction of an emergency outfall replacement to Dunstans Construction Group.
Stockton Drilling Services was engaged by Dunstans to provide engineering support and construction supervision.
A geotechnical desktop assessment was undertaken to allow for the design of alternative preliminary drilling profiles and to define the scope for further geotechnical investigations.
The project team then undertook bathymetric surveys and seabed sampling of the works corridor to establish suitable exit point locations.
Seabed profile, water depth, currents, geology and environmental impacts were then evaluated to determine the lowest risk and optimised length and location for the drill exit and diffuser installation.
Considering the peak flows from the Water Reclamation Plant and installation forces, it was determined that a 450mm diameter HDPE pipeline would be required.
The pipeline would extend 700m from within the plant boundary to approximately 500m offshore to a water depth of 15m, where a 16m long diffuser would be installed.
The pipeline was installed within three weeks of mobilising to site.
About Charles Stockton
UK-born Charles Stockton has been a part of the HDD sector in Australasia since 2003.
He is the Managing Director of Stockton Drilling Services, a leading engineering consultancy specialising in HDD and other trenchless pipeline installation methods.