[Article Sharing] Pipeline On-Bottom Stability

Breaking new ground in pipeline on-bottom stability design | The Australian Pipeliner

Source: http://pipeliner.com.au/news/breaking_new_ground_in_pipeline_on-bottom_stability_design/063943/

Test section of the O-Tube facility

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.