Modelling rogue waves to mitigate risk offshore

CSIRO Scientist Dr Murray Rudman; Computer image of a semi-submersible platform in the ocean; Computer image of a rogue wave smashing into a semi-submersible platform weighing around 32,000 tonnes.

CSIRO Scientist Dr Murray Rudman; Computer image of a semi-submersible platform in the ocean; Computer image of a rogue wave smashing into a semi-submersible platform weighing around 32,000 tonnes.

Although once considered a myth, rogue waves represent a considerable risk for offshore oil and gas production platforms. CSIRO’s Dr Murray Rudman spoke to Gas Today about computer modelling undertaken to compare the effect of these waves on different offshore platforms.

Rogue waves – massive waves which can be more than 20 m in height – pose a considerable risk to offshore structures in the open ocean. Measurement on offshore platforms and satellite imaging has shown that these waves are in fact real and much more frequent than originally believed, says CSIRO scientist Dr Murray Rudman.

“Unlike tsunamis, which occur as a result of earthquakes or undersea avalanches and are dangerous only at near-shore and onshore locations, rogue waves are not associated with rare events and occur in deep water,” Dr Rudman explains.

Making waves

“Waves are extremely difficult to predict, especially when they begin to break,” Dr Rudman says. “Also, the movement of the wave and the platform influence each other in complex ways. We use a mathematical technique which was originally developed in astrophysics to model stars forming and galaxies exploding.”

Dr Rudman’s ComputationalModelling group has been developing simulation techniques that predict the coupled motion between fluid and structures – in this case, the ocean and semi-submersible oil or gas rigs. This computer simulation, combined with fluid-flow mathematics, can assist in the sensible and safe design of platforms.

New experiments

Traditionally experiments of this type are performed using wave tanks, which can be costly and time-consuming. Measurements made also need to be ‘scaled-up’, which can be difficult to do with confidence, says Dr Rudman.

Computer modelling is relatively cheaper and quicker, and can incorporate a large number of ‘virtual sensors’ to measure things that can be difficult to assess in a wave tank experiment. Non-linear and breaking waves can be measured using computer modelling, and the simulation can be run at ‘full-scale’, meaning that researchers can have more confidence in the measurements they predict.

Testing moorings

To date, Dr Rudman’s team have considered semi-submersible floating platforms with several different mooring systems, including a tension leg platform configuration and a taut spread moorings configuration. The two systems have also been coupled in order to consider how different mooring line cables affect platform movement when rogue waves hit.

Dr Rudman says that the two configurations show significantly different vertical and horizontal movement characteristics. The taut leg platform has no lateral restoring force and will move significantly in the direction of the wave, says Dr Rudman, whereas the taut spread moorings system has greater vertical movement, potentially to the point of submerging the platform deck below the surface.

Although preliminary, results to date show that a combination of steel vertical cables and polyester spread moorings seems to give a good trade-off between the two behaviours, says Dr Rudman.

Extending the research

Can the computer program be applied to different types of offshore platforms, for example, Shell’s proposed floating LNG platforms? It can, says Dr Rudman; “From a computational point of view, it is simply a matter of replacing the semi-submersible with an FPSO and modifying the mooring configurations.”

Although the initial research was taken as proof of concept for the method in the oil and gas domain, Dr Rudman’s team are keen to engage in the sector. “There is tremendous scope for application here that is untapped,” he says.

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