Mobilizing subsea foundations

Susan Gourvenec

April 22, 2014

The Prelude FLNG subsea architecture, supported by traditional mudmats. image source: Shell.

Mobile foundation technology could offer an economic solution to some stranded offshore hydrocarbon reserves. Professor Susan Gourvenec looks at the issues.

Latest statistics show Australia’s total petroleum output has an annual export sales value of more than A$25 billion (US$22.8 billion) and annual taxation payments by the industry of more than A$8 billion (US$7.3 billion).

One of the challenges the industry faces is unlocking the huge hydrocarbon reserves offshore Australia, and elsewhere, that are currently unreachable.

Australian research and development into mobile foundation technology is one piece of the jigsaw.

Existing foundations

Subsea developments involve a number of wells across a field connected by a network of pipelines, so-called infield flowlines. A variety of pipeline infrastructure supports the flowlines, such as pipeline end terminations (PLETs), pipeline end manifolds (PLEMs) and in-line tee structures (ILTs).

This seabed infrastructure is typically supported on shallow mat-type foundations, so-called mudmats, that are ideally small enough to be installed by the same vessel that lays the subsea pipelines, which transport oil or gas from wells to processing facilities.

Mudmats are typically 5-15m in edge length, but can be more than 40m in some cases. The purpose of the mudmats is to transfer the weight of the structure it supports over a sufficiently large area of the seabed so as to avoid unacceptable settlements and resist the lateral loads applied by the connected pipelines.

Offshore pipelines undergo cycles of thermal expansion and contraction over the lifetime of a field through cycles of start-up and shutdown. In service pipelines expand as hot oil or gas passes through them and contract when they cool due to a shutdown. The thermal expansion of the pipelines transmits lateral loads to the mudmats supporting the structures attached to the pipelines.

Increasingly, as developments move into deeper water, where seabed soils can be extremely soft, and as conditions such as operating temperature become more onerous, these foundations can be too large or heavy to be installed by the pipelaying vessel.

The requirement of a specialized heavy-lift installation vessel to be brought to site (to install these structures) adds considerable expense – pushing projects further up the cost curve.

Schematic of a possible mobile subsea foundation. Image source: Centre for Offshore Foundation Systems, University of Western Australia.

Mobile foundations offer a solution

The term “mobile foundation” may seem an oxymoron—we typically expect foundations, and the structures they support, to remain stationary. It would be alarming if our homes or office buildings started to wander, but we accept that ocean-going vessels experience stability and mobility concurrently.

The concept of a mobile foundation is similar to a sleigh or ski on snow. It’s a foundation that is engineered to slide or glide across the seabed to absorb some of the applied loads, rather than simply being engineered to be big enough to resist the loads and remain stationary.

A mobile subsea foundation might look much like a conventional mudmat foundation, at least at the seabed—comprising a rectangular plate resting on the seabed. Traditional mudmats are typically equipped with short skirts that penetrate the seabed, whereas mobile foundations would be best designed to rest on the surface of the seabed to minimize the sliding resistance of the foundation. This encourages sliding rather than overturning.

A critical design feature of mobile foundations is that the predominant mode of displacement is translation, since overturning could lead to overstressing of connectors between the pipes and structure. A mobile foundation could be provided with sloping sides to assist in the desired translational motion should it settle into the seabed.

Where can mobile foundations be used?

Mobile foundations have particular potential in deep water or remote oil and gas fields that have been identified offshore Australia and elsewhere. A pipeline heats up as hot oil or gas passes through it. This, in turn, causes thermal expansion along the length of the pipe and increased loading to the foundations supporting the connected pipeline infrastructure. To resist these loads can require a massive foundation.

Alternatively, a mechanical arrangement can be introduced to relieve the loads, with the pipeline termination being mounted on sliding rails. However, this solution involves a moving part that must remain operational during the life of the project, and the foundation footprint is still large because it must encompass the slider arrangement.

As an alternative, a compact mobile foundation can be designed to undergo controlled and limited sliding across the seabed to absorb some of the loads caused by this thermal expansion of the connected pipelines.

While some parallels exist between the mode of operation of a mobile foundation and a sleigh or ski, mobile foundations would not be intended for long distances of travel.

The movement would be limited to just a few meters at most, as a pipeline is turned on, heats up, and expands, and back again during shutdown, when the pipeline cools down and contracts.

The movement is periodic over the lifetime of the facility, each time the pipeline is started-up or shutdown, but would be sufficient to relieve loads both on the foundation and within the pipeline.

Mobile foundations will add to a portfolio of seabed infrastructure designed to allow movement. Other examples include the steel catenary risers that form trenches where they touch down on the seabed—which actually assist by easing this fatigue hotspot. Other examples are the installation of drag or plate anchors and lateral buckling of subsea pipelines.

Mobile foundations are a logical evolution of the now widely-accepted practice of allowing seabed pipelines to move in a controlled manner to accommodate operational loading.

Leading the way

Australia is well positioned to lead technology development of mobile foundations. Not only is it recognized worldwide as a centre of excellence for seabed engineering, but Australia has substantial stranded gas reserves, which coupled with Australia’s geographic remoteness provides complementary drivers. The industry is interested in a scientific basis for understanding the behavior of mobile foundations, but to date virtually no information is publically available.

World-first experimental testing of mobile foundations at prototype scale is now being carried out at the Centre for Offshore Foundation Systems (COFS) at the University of Western Australia (UWA), supported by a threeyear Australian Research Council grant, to create a design framework for industry for mobile foundations.

UWA is ideally placed to carry out the research, as host of the National Geotechnical Centrifuge Centre, which houses the busiest geotechnical centrifuge in the world. Geotechnical centrifuge modelling enables small scale models of geotechnical systems to be tested at accelerated gravity levels (up to 400x Earth’s gravity), allowing field-scale soil stresses to be accurately replicated. Stress similitude is critical to capture the true response of geotechnical systems, while field scale tests of offshore geotechnical systems are prohibitive.

A first campaign of geotechnical centrifuge modelling of a prototype mobile foundation has been undertaken at UWA and observations of the foundation under realistic field conditions are promising.

The next stage is to develop a theoretically- based framework to describe the soil mechanics response of the seabed during the periodic cycles of foundation sliding, in order to predict the foundation response. Ultimately the method will be incorporated into a design tool for use by industry.

The Centre for Offshore Foundation Systems has a long-established relationship with industry and development of a design tool for mobile foundations will be carried out with input from operators, consultants, and contractors with an interest in the technology. The motivation for the project was driven by industry needs and the output of the research will be used by industry, so we will work with our industry colleagues to develop a usable and practical solution.

Conclusion

Costs associated with mobilizing a second installation vessel for subsea foundations are particularly significant for the bottom line of smaller developments and when vessels have to be brought in from far afield.

Mobile foundation technology offers an economic solution to a present challenge to offshore development, and will contribute to unlocking the huge hydrocarbon reserves located offshore Australia and elsewhere that are currently “stranded”.

Susan Gourvenec is a Professor at the Centre for Offshore Foundation Systems at the University of Western Australia. Gourvenec has more than 15 years of geotechnical engineering experience, with particular interest in offshore geotechnics. She is a consultant offshore geotechnical engineer to industry and member of the ISO and API Committees for Offshore Geotechnics. Mobile foundation technology could offer an economic solution to some stranded offshore hydrocarbon reserves. Professor Susan Gourvenec looks at the issues. The Prelude FLNG subsea architecture, supported by traditional mudmats. Image from Shell. Schematic of a possible mobile subsea foundation. Image from the Centre for Offshore Foundation Systems, University of Western Australia.