The offshore environment is dynamic. Cyclones lead to significant wind and wave loading on offshore facilities, posing significant challenges for the engineers that design these structures. However, there is more than just the processing facility to worry about – hundreds of metres beneath the sea surface lie a network of pipelines and subsea structures that form the means for transporting gas to where it is processed. As the facility is turned on and off, these pipelines expand and contract, causing the pipelines to move axially (along its length) over the seabed. This motion, known as axial ‘walking’, can stress members connecting the pipelines to other seabed infrastructure, which in turn can impact the integrity of the network. To avoid this, solutions are needed to mitigate axial walking.

The National Geotechnical Centrifuge Facility

The National Geotechnical Centrifuge Facility (NGCF) is a world leading geotechnical centrifuge modelling facility. Hosted by the Centre for Offshore Foundation Systems (COFS) at the University of Western Australia (UWA), the NGCF combines proven technical expertise and three state-of-the-art geotechnical centrifuges. Their centrifuges spin an average of 270 days a year, making the centre one of the busiest in the world.

NGCF Director Associate Professor Conleth O’Loughlin leads a team of centrifuge modelling specialists, working with local and international academics and professionals on a wide range of real world geotechnical problems. Extensive partnerships with industry, forged across 30 years of applied research, gives the team first-hand perspective of the problems faced in practice.

A pipe-clamping mattress

One approach to mitigate pipeline walking is to place a rock berm over the pipeline, in order to increase friction between the pipeline and the seabed. This typically requires the use of expensive vessels capable of transporting and placing rock in deep water, and with much of the rock ending up on the seabed instead of acting on the pipeline, may not be an optimal solution.

As an alternate to rock berms, Shell developed the Pipe-Clamp Mattress (PCM) concept to mitigate pipeline walking at their Malampaya project in the Philippines. Detailed at the Offshore Technology Conference in 2017, the technology went from ideation to installation in a short period of time, while still meeting the strict design requirements of the project. In simple terms, the PCM involvement installation of a ‘nutcracker-shaped’ clamp onto the pipeline, which supports a heavy concrete mattress. The mattress acts to securely attach the clamp to the pipelines, with the full weight of the system acting directly on the pipeline, thereby providing the maximum increase in frictional resistance.

Following its successful deployment on the Malampaya project, where the PCMs stopped the pipeline from walking, Shell patented the concept and are now commercialising it in partnership with subsea company, Subcon.

When it came to the problem of axial walking, we were aware of the pipe-clamp technology, but were also aware there was little data to inform how it would behave over a wider range of application, and this could be a barrier to other operators selecting it for their developments.

Assoc. Prof. Conleth O’Loughlin, NGCF Director

Accordingly, from mid 2019 to early 2020, the NGCF team busied itself with the task of modelling the complex response of a PCM placed onto a subsea pipeline.

Collaboration between Operators

First initiated through the Shell Professor in Offshore Engineering at UWA, initial discussions with the NGCF team centred on how to perform centrifuge modelling so as to capture the key mechanisms associated with installation of the PCM onto the pipeline, and how to quantify the extent to which the PCM would mitigate axial walking.

Initially planned to explore PCMs response in very soft clay from the Gulf of Mexico, a focus area was the potential for large settlements to lead to unclamping of the PCM from the pipeline, and a subsequent loss of resistance. The model developed by the NGCF team allowed this to be explored, by replicating that mechanism incorporated into the actual design. Tests were then performed to simulate expansion and contraction of the pipeline over 6 years of pipeline operation, which showed that whilst the pipeline and PCM may settle into the seabed, the level of axial restraint remained high. Preliminary findings from these tests are to be presented, in conjunction with Shell, at the 4th International Symposium on Frontiers in Offshore Geotechnics (ISFOG), to be held in Texas in November 2021.

In parallel with the tests being performed on Gulf of Mexico clay, UWA was approached by both BP and Woodside for similar testing – but in different soil conditions, and incorporating subtle differences in PCM design and test programme. In both cases, the testing represented real world applications for PCM use.

The soil conditions for the BP and Woodside projects are very different. In each case however, testing focused on investigating the potential for PCM embedment and unclamping, as well as quantifying the magnitude of axial restraint provided by an individual PCM – thereby reducing the risk and uncertainty of this new technology, and also informing the optimum number of PCMs required for each project.

The PCM advantage

Whilst traditionally risk averse when it comes to adopting novel technology, Operators recognise that measures need to be cost effective and safe – but without compromising people, the environment or the engineering outcome.

A particular advantage of PCMs is the expectation they will have a lower environmental footprint than more conventional solutions. This reflects the fact that PCMs are inherently efficient, with the full weight bearing on the pipeline rather than the soil around the pipeline, and that they can be installed with a larger range of vessels.

For pipeline designers, the team’s research shows that PCMs are reliable, reducing risk and uncertainty in offshore pipeline design. Site-specific testing of the type undertaken by the NGCF allows designers to estimate confidently how many PCMs are needed for an individual project, unlocking cost savings through minimising fabrication, transportation and installation costs.

Modelling capabilities

Although difficult to replicate the complexities of a dynamic underwater environment at small scale, testing that compares physical modelling with numerical and analytical models can provide confidence in data validation.

The NGCF team include experimentalists, mechanical, electronic and mechatronics engineers, data acquisition specialists and instrumentation technicians, all of whom work closely with academics, post-doctoral researchers and PhD students. This combination of skills, expertise and world-class facilities allow the team to rapidly respond to industry driven problems.

Centrifuge modelling provides a means of mirroring the actual site, making the results more accurate and applicable. In their testing of PCMs, the team designed and fabricated 1:30 reduced scale models that were installed in seabed soils recovered from the offshore sites, all while the centrifuge was spinning at 30 times Earth’s gravity. Spinning a soil sample at this level creates stress conditions in the soil that replicate those seen at full scale – which is an essential aspect of physical modelling in geotechnics, and why most of the experimental modelling at COFS in undertaken at the NGCF.

At 30g, a one day test is modelling 2.5yrs at field scale. So industry get to see their design come to life and see whether it works over a long period.

Dr Colm O’Beirne, NGCF Research Engineer

Robotic actuators are used in the centrifuge to force the pipeline to ‘walk’, in order to directly measure the magnitude of resistance that can be generated between the seabed and the pipeline, both before and after the PCM is placed, and over time. The NGCF team use actuators developed in-house, controlled by their purpose built motion control software, enabling the model to be moved while the centrifuge is continuously spinning. Large volumes of data are captured, along with live video streams that are fed back to a central control room. The NGCF actuators, motion control and data acquisition systems are internationally recognised and sold to other geotechnical centrifuge centres around the world.

The centrifuge is an amazing tool to compliment what we do as geotechnical engineers. When you combine this capability with numerical studies or analytical engineering assessments, overall your confidence in the final design solution can climb.

Prof. Phil Watson, Shell Professor in Offshore Engineering, TIDE Director

Next steps

By June 2020, the NGCF team had completed testing of the PCM concept in three real-world situations, each with different seabed properties and varying design requirements.

A key aspect of these PCM projects was the willingness from the operators that the data would be shared, as that would allow the industry more broadly to embrace the PCM technology.

Assoc. Prof. Conleth O’Loughlin, NGCF Director

Analysis of the data from all three projects, complemented by other ongoing research activities related to the study of pipeline walking, is to be published in partnership with all three Operators – as a step towards the development of industry design guidance, thereby allowing widespread adoption of the technology to mitigate axial walking of subsea pipelines.

The PCM work is a great example of how operators can benefit through collaboration between themselves. There’s a role for us as researchers, to make that happen.

Prof. Phil Watson, Shell Professor in Offshore Engineering, TIDE Director

A new TIDE

Leveraging long standing industry collaborations, 2021 will see the launch of the ARC Research Hub for Transforming energy Infrastructure through Digital Engineering (TIDE) at UWA.

Hosted by the UWA Oceans Graduate School, and in collaboration with researchers from the National Institute for Applied Statistics Research Australia at the University of Wollongong, TIDE is funded by the Australian Research Council. Industry partners in the programme include Woodside, Shell and Inpex (Operators), plus representatives from certification groups Lloyd’s Register Group and Bureau Veritas, as well as Fugro and Wood.

Led by Professor Phil Watson, TIDE will fuse data science techniques with engineering, leveraging industry acquired and experimental data, in order to transform the management of critical energy infrastructure (such as pipelines, structures and vessels) – making this process cheaper and yet more reliable.

Our strength stems from adding value to real world projects, and has been built over 30 plus years working with the offshore industry in Perth and around the world.

Prof. Phil Watson, Shell Professor in Offshore Engineering, TIDE Director

Professor Phil Watson leads the Shell Chair in Offshore Engineering research team at The University of Western Australia, which is sponsored by Shell Australia.