Oil, gas and energy

How has HDD changed the way projects tackle shore crossings?

Working in the nearshore environment can be very challenging and inhospitable for both land-based and water-based construction equipment. Neither a marine nor land-based construction method is ideally suited to construction in this shallow, tidal, high energy zone; it is too shallow for marine vessels, and too deep and exposed for land-based work.

This is the zone that neither the offshore contractor nor the onshore contractor are ideally placed to manage – it is not their normal working environment and it is this challenge that makes shore crossings very interesting to design and construct.

Up to 15 years ago, these crossings would have required large-scale open battered excavations onshore and the process was very susceptible to adverse weather conditions and tidal variations, presenting numerous challenges for managing worker safety and potential damage to the environment including oil spills and erosion.

What are the benefits and what are some challenges?

By using HDD to construct the shore crossing, proponents can eliminate the requirement for works to be constructed in the nearshore environment. Using HDD allows for the crossing length and depth to be increased, positioning the rig back on level land and allowing the exit to be beyond the surf zone.

The rig will be placed well behind the dune system, which can then be preserved to prevent erosion and maintain existing flora and fauna within the coastal corridor. The exit point will be selected to provide appropriate conditions for positioning subsea structures or providing a suitable transition to offshore pipeline.

The exit location must provide sufficient water depth to allow safe vessel access and anchoring as well as diving operations. The pipelines can be prefabricated onshore and thrust through the bore from entry to exit or, alternatively, the HDD rig can be used to pull back the pipeline from offshore if the pipe is fabricated by a lay barge or towed offshore from a spool base/launching area.

One of the main challenges for designing a HDD shore crossing is selecting an appropriate exit location and then obtaining reliable cost effective geotechnical offshore information. For shorter crossings it may be possible to interpolate onshore and nearshore boreholes but for longer crossings it is extremely important to develop an understanding at the exit topography and geology.

This is critical for developing the HDD methodology; determining whether the hole will be forward reamed or back reamed and if the pipe will be thrust or pulled into the bore. Even though the majority of the works can be conducted from onshore, marine vessels and divers still need to be deployed during critical stages of the operation.

Having these windows well identified and then having contingency planning for delays is essential. Clearly setting out what conditions the marine vessels can operate in, including considering weather, and determining how this risk will be costed will be important to prevent cost escalation and potential disputes between parties.

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 will now be managed over water by a marine spread. It is essential to correctly specify the vessel requirements and establish what duties it will perform 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, allowing the bore to be opened by forward reaming thereby limiting offshore operations and ensuring drill fluids are returned to entry for recycling. 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.

Has Stockton Drilling Services 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 (2) in Victoria for BHP Billiton
• Gorgon Shore Crossings (9) in Western Australia for Chevron
• Kupe Shore Crossings in New Zealand for Technip/Origin Energy
• Victorian Desalination Pilot Plant Shore Crossings (2) in Victoria
• Narrows Shore Crossings (4) in Queensland for APLNG /QGC
• Gladstone Harbour Shore Crossing Design for Arrow Energy
• Anglesea WRP Shore Crossing replacement in Victoria for Barwon Water.

The Anglesea WRP replacement project, located 25 m above sea level on coastal cliffs, clearly shows how the development of new construction techniques have allowed for improved design. The previous outfall consisted of a 30 m deep drop structure which transfers flows from the treatment plant level to the base of the cliffs.

A 185 m outfall pipe then discharged flows from the base of the drop structure to the ocean. As a result of cliff erosion and pipe damage, Barwon Water initiated a project which required the design and construction of a new outfall pipeline and associated works; the pipeline was installed within three weeks of mobilising to site.

This article was featured in the June edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.

For more information visit the Stockton Drilling Services website.

If you have news you would like featured in Trenchless Australasia contact Assistant Editor Nick Lovering at nlovering@gs-press.com.au