Woodside Petroleum’s Pluto LNG Project will include an LNG production train at the Burrup LNG Park, with forecast production of
4.3 MMt/a. The onshore LNG train will process gas from the offshore Pluto and Xena fields, which are estimated to hold a dry recoverable volume of 4.6 Tcf of gas and 0.5 Tcf of contingent resources. First gas from the field is expected in late 2010 and first LNG in early 2011 – only six years after the discovery of gas in the Pluto and Xena fields.
The offshore components of the Pluto project have been completed and consist of five subsea wells in 820 m of water, tied back by two 20 inch diameter, 27 km flowlines, a 4 inch diameter monoethylene glycol (MEG) pipeline and a control umbilical to a platform in 85 m of water. A 36 inch diameter, 180 km trunkline with a 6 inch diameter MEG piggyback line then connects the platform to the shore crossing at Holden Point in the Mermaid Sound.
The pipelines are fabricated from Grade 450 carbon steel with the exception of 424 m in each flowline, which is constructed from 316L clad stainless steel. The 316L clad portion of the flowlines is installed on either side of tee valve assemblies within the 20 inch flowlines, and is required to ensure that any gas entering the flowlines through the tees has time to mix with the production fluids and cool before entering the carbon steel section of the line.
The Pluto project had a very fast-track development strategy. Woodside’s offshore trunkline and flowline E&C delivery manager Hosi Sabavala said “For the project to be developed and constructed within the envisaged timeframe, many activities that are conventionally run sequentially were pursued in parallel.”
Engineering the Pluto pipelines
During the detailed design stage, a team of 50 engineers from JP Kenny worked on the Pluto project. The role of the engineering support team was to manage design changes immediately prior to and during construction, focus on the fabrication of design-critical components, and assess the impacts of deviations from design, installation and construction procedures.
JP Kenny Lead Pipeline Engineer Hammam Zeitoun said “The engineering team ensured that construction-related design changes did not compromise through-life operational system integrity and minimised the impact of these changes on operational system costs.
“Pluto was generally a very challenging project. For the trunkline, the main challenge was the severe metocean conditions along the route, which resulted in the difficulty of stabilising the pipeline and maintaining its integrity under the action of waves and currents.”
Preliminary design calculations indicated that a 30 km section of the
180 km, 36 inch diameter trunkline could move an unacceptable 160 m laterally off the design route in extreme storms, and that, in addition to the applied concrete coating, around 2,000 concrete gravity anchors – each weighing 45 t – would be required to stabilise the line.
Mr Zeitoun explained that stabilising this section with gravity anchors would have cost millions, and also imposed integrity risks during construction and operation, associated with the possibility of the anchors sinking in the seabed and crushing the piggybacked MEG Line.
The underlying seabed was also hard, which made it difficult to trench and bury the pipeline in certain regions, meaning that unconventional and innovative engineering solutions were required.
For the flowlines, the main challenge was the seabed topography and the steep slope along the route.
“The steep slope, combined with the high temperature in the line, resulted in a high risk that the flowlines may “˜walk’ down the slope after every operation shutdown and restart cycle. Walking is a phenomenon that results in pipelines creeping toward one end after every operational shutdown and restart cycle, which can risk the integrity of the tie-ins at the pipeline end,” said Mr Zeitoun.
The flowlines and MEG line had to be routed in a very narrow corridor, termed the “˜choke’, of approximately
100 m between seabed canyons. A very robust engineering scheme had to be developed to ensure that the flowlines would not laterally buckle in the narrow corridor as a result of the high product temperature in the line.
The risks and costs associated with the trunkline stability design were addressed by developing and applying industry-first, state-of-the-art engineering techniques to the Pluto trunkline stabilisation design, which cost $2 million in engineering and research. “This resulted in eliminating all gravity anchors and trenching requirements, saving millions of dollars,” said Mr Zeitoun.
The applied techniques, involved setting up and running a research and development program, aiming at a more accurate assessment of the hydrodynamic loads and wave kinematics on the bundled trunkline and MEG line system, and the behaviour of the pipeline under hydrodynamic loading.
The integrity risks associated with the flowlines routeing were addressed by installing the lines in the choke within very tight routeing tolerances. Mr Zeitoun explained that extensive finite element analyses were performed to develop a highly reliable scheme utilising “˜displacement initiators’ at each end of the choke. Displacement initiators are structures crossed by the pipeline to initiate buckling at specific locations.
With respect to flowlines walking, advanced finite element analyses were performed to predict the walking behaviour of the lines, and mitigations schemes were developed to intervene should the walking of the flowlines reach unacceptable values.
Pipeline construction
In 2007, Allseas Construction Contractors was contracted to install the pipelines.
Due to the fast-track nature of the project, it was necessary to select and secure installation vessels before the final design details of the pipelines were locked in. Due to the tight timeframe, a combined lump sum, day rate and reimbursable contract was developed for the pipeline installation, which enabled Woodside to secure installation assets early and gave the flexibility to modify scope as the project’s detailed design progressed.
The pipeline installation campaign commenced in January 2009, with the survey of the 20 inch diameter flowline routes and the installation of over 50 seabed transponders to enable the pipelines to be accurately installed within the tight tolerances dictated by the seabed topography and lateral buckling design.
Allseas’ pipelay vessel Audacia arrived in the field at the end of February 2009 and, aided by TS Marine’s construction vessel Havila Harmony, installed
22 structures along the flowline routes. The structures, known as displacement initiation structures (DIS), are used to control the lateral displacement of the flowlines as they expand and contract during operation due to changes in temperature and internal pressure. Woodside’s Mr Sabavala explained that each DIS is 36 m long and consists of
20 inch diameter polyethylene coated pipe supported by a mud-mat structure to keep it stable on the muddy seabed. The 20 inch flowlines are laid over the DIS and they slide freely over the structures during expansion, thus releasing stresses in the pipelines.
The Audacia completed installation of the flowlines in May 2009 and successfully installed the termination assemblies on the ends of each line at the drill centre, to enable diverless tie-in of the spools to the drills centre, before commencing installation of the trunkline and MEG pipeline.
The Audacia initiated the trunkline and MEG pipeline in 20 m of water, approximately 24 km from the shore and laid out toward the platform location, leaving the remaining 24 km to be installed by a vessel capable of operation in restricted water depths.
Within the Mermaid Sound, the Allseas Tog Mor flat-bottom barge was used to lay the pipelines. The scope of works for the Tog Mor was highly challenging due to the incorporation of the shore crossing and the fact the pipeline route crossed the busiest shipping channel in Mermaid Sound.
The Tog Mor was required to lay the pipelines across the North West Shelf Venture (NWSV) navigation channel, used by up to 420 LNG vessels a year as they transit to and from the NWSV LNG loading facility. An installation methodology was developed that allowed the trunkline and MEG supply pipeline to be installed without stopping operations within the navigation channel at any time.
The Tog Mor uses a ten-point mooring system and lays approximately 500 m of pipeline a day. It was therefore necessary to develop a system that ensured the Tog Mor and her anchor spread remained clear of the navigation channel. This was achieved by installing a string of 36 inch diameter trunkline approximately 850m long, and a string of 6 inch diameter pipeline approximately 1,500 m long on one side of the channel, and then pulling the strings across the seabed and through the channel. Once pulled across the channel, either end of the pipeline strings were lifted above the surface of the water and connected to the main body of the pipelines either side of the channel.
The Tog Mor commenced operations in early May 2009 and completed installation of the nearshore trunkline and MEG supply pipeline in early September. The Audacia completed installation of the offshore trunkline and MEG supply pipeline in early October after being in the field for more than seven months.
Over 1,400 personnel were directly involved with the pipeline installation scope of work, with more than 500 personnel working offshore at one time during the height of the campaign on over
22 different vessels. The largest vessel in the fleet, the pipelay vessel Audacia accommodated 270 personnel and was able to install more than 2 km of trunkline and MEG supply pipeline bundle a day.
Mr Sabavala said “Every vessel, from the Audacia to the smallest crew boat, played its own key role in the installation campaign and ultimately it was the teamwork of all involved that made the project a success.”
The shore approach
An evaluation of a range of different LNG plant sites and corresponding pipeline routes resulted in Holden Point on the Burrup Peninsula to be selected as the preferred site to receive and process the natural gas.
“While this pipeline route was attractive compared to the other options considered, the shore approach in Mermaid Sound to the Holden Point landfall location was very challenging,” Mr Sabavala said.
This route section is situated in an area with two existing high pressure gas trunklines, several major shipping channels, an existing dredging material spoil ground, and all this in a marine park environment. Detailed evaluation of the shore approach options resulted in a route for the Pluto export pipeline system running along the two existing gas trunklines, along the edge of the dredge material spoil ground and across one shipping lane (the NWSV shipping channel). The pipeline would then cross the shore line just north of the new Pluto LNG Offloading Jetty, which would be built in the same timeframe as the shore crossing.
Detailed seabed surveys concluded that the seabed along the route comprises a combination of fine-to-medium-sized carbonate sediments and calcarenite/limestone rock. At the shore crossing, basalt and granite, both very strong igneous rock types, are present.
The pipeline protection and stabilisation design work resulted in a combination of pre-trenching and sand backfill, and on-seabed rock dump (using a no-cover rock berm design). Along two selected areas, in the very near-shore area alongside the Pluto LNG Offloading Jetty and across the NWSV shipping channel, protection against accidental external impact (vessel sinking/grounding and ship’s anchor drop/drag) required the need to rock dump within a trench.
Construction of the shore approach seabed preparation work commenced in early 2008 at the shore crossing site. The shore crossing and near-shore dredging works were completed in May 2009 in time for the commencement of near-shore pipelay. The shore crossing works experienced planned lengthy interruptions to fit in with the construction schedule of the LNG offloading jetty.
“The early and efficient planning process adopted by the project to co-ordinate the upstream – downstream interface paid off during the execution phase, with smooth and compatible operations on either side of the fence,” said Mr Sabavala.
The dredging of a 7 m deep and
7 m wide trench across the NWSV shipping channel presented a particularly challenging task, with frequent LNG tanker movements providing limited (both in number as well as in length) working windows for the dredging to be performed. Nonetheless, the dredging was executed effectively using a large cutter suction dredge in combination with a large trailing suction hopper dredge without impacting the LNG shipping operations in any way.
Pre-trenching the remaining 35 km of the shore approach route in deep sediment areas proved very cost effective. Despite the tight environmental constraints set by the project in relation to limiting the turbidity plumes during the trailing suction dredging operations, this critical activity was completed
Near-shore pipline installation comprised the construction of some 25 km of pipeline, including a shorepull, wet pull across the NWSV shipping channel, and four above-water tie-ins (two for the
36 inch gas export pipeline and two for the 6 inch MEG pipeline), using a first generation pipelay vessel.
Rock quarrying and transportation of 430,000 t of quarried rock has been completed, as well as the dumping of three quarters of the rock volume. The majority of the pipeline trench will be backfilled with medium coarse local sand. This activity is planned to be executed between February and April 2011.
Strict environmental constraints were set by the project in relation to turbidity levels from the trailing suction hopper dredging operations, to ensure there was no medium-to-long-term impact on live coral reefs. Early trialling with different dredging overflow modes and detailed monitoring of the turbidity levels allowed a very proactive approach during the remainder of the project. As a result no delays were incurred in the pre-trenching operations during events such as coral spawning.
Conclusion
The Pluto pipelines were successfully completed within the challenging tight timeframe, due to the integrated working relationship of Woodside with its pipeline engineering and construction contractors.
Woodside has commenced front-end engineering and design works on a second and third LNG train for the project. Final investment decisions on Pluto Train 2 and Train 3 are targeted by end 2010 and end 2011 respectively.
Shore approach innovations
Innovations by the shore approach front-end engineering and design (FEED) engineering contractors included the following:
- Significant optimisation of the trench depth as part of the secondary stabilisation design; during FEED, a trench depth of 3 m was needed to satisfy the pipeline stability functional requirements; this depth was successfully reduced to 2 m by applying innovative design practices, and allowed to increase the very cost effective pre-trenching scope in lieu of expensive rock dumping;
- Use of sand backfill instead of rock dump to achieve adequate pipeline protection by simply increasing trench depth where possible, resulting in a further significant reduction of the quarry rock volume to be produced and placed; and,
- Use of scaled model testing in an efficient manner to optimise the rock berm designs for stability and protection.