The need to better
understand pipeline stability in the unique environment of Western Australia’s
North West Shelf has lead to the establishment of a research facility in Perth
that is attracting worldwide interest.
In February this
year, the O-Tube – a closed-loop channel of water that can simulate
hydrodynamics near seabed generated by tropical cyclones and their effects on
pipeline stability – was used commercially for the first time by researchers
and engineers from the University of Western Australia (UWA) and Atteris, on
behalf of Woodside Energy.
The team has been
running a series of tests over a four- to five-month period to assess pipeline
stability on mobile seabeds. The north of Australia is subject to frequent
severe tropical cyclones, and the continental shelf comprises unique seabed
materials, including carbonate marine sediments. The O-Tube facility provides
an opportunity to better understand the physics behind the behaviour of subsea
pipelines on these seabed conditions under extreme hydrodynamic loadings.
In 2005, Woodside
engaged Atteris to undertake pipeline stability studies. Theoretical
assessments comprised studies of pipeline behaviour as well as seabed behaviour
under extreme storm events such as tropical cyclones. This approach was unique
because conventional pipeline on-bottom stability design methods ignore seabed
instability occurring during the build-up, peak and ramp-down of a storm. This
work culminated in the concept of building a brand new laboratory testing
facility in which the three-way pipeline on-bottom stability processes –
fluid-pipe, fluid-soil and pipe-soil – could be physically tested.
UWA designed,
supervised the construction of, and commissioned the O-Tube testing facility.
UWA has a long track record of delivering specialist numerical and physical
model testing services to the offshore hydrocarbon industry, and is regarded
worldwide as a state-of-the-art research centre for the subsea pipeline
engineering industry.
The O-Tube
comprises a closed-loop channel of water driven by an axial flow pump, with a 1
m wide and 1.4 m high test section. This test section allows large diameter
pipelines to be modeled at scales of 1:5 to 1:6, with smaller pipelines such as
flowlines capable of being modelled at prototype scale. The O-Tube is unique
because it can generate a combination of steady and oscillatory flow to produce
realistic on-bottom flow conditions.
The pipeline designers
have been particularly interested in studying the effects of fluid-soil
interaction on pipeline stability. Fluid-soil interaction includes processes
such as free field scour, local scour, pore pressure build-up and soil
liquefaction. These processes are difficult to predict analytically and can
contribute significantly to the overall stability of a pipeline.
The O-Tube is being
used to assess the stability of new and existing subsea pipelines and
flowlines. These assessments will aim to reduce the level of conservatism
associated with current design approaches and minimise costs relating to
potentially unnecessary stabilisation measures. The work that has gone into
this research program is unique on a worldwide scale, and has attracted the
attention of multi-national companies and bodies, including Det Norske Veritas.
The physical model
tests performed by a team of engineers and academics from Woodside Energy, UWA
and Atteris will pave new ground in the development of a comprehensive subsea
pipeline stability assessment method, inclusive of fluid-soil interactions.
The delivery of the
O-Tube has been possible thanks to financial contributions from Woodside
Energy, Chevron Australia, UWA and the Australian Research Council.
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