Time for a new design recipe for offshore engineering

The Institution of Engineers Singapore (IES) has presented Oxford University’s Department of Engineering Science and the National University of Singapore with an IES Prestigious Engineering Achievement Award 2015 for 'Current blockage in offshore engineering - Morison revised on the 65th anniversary of the famous equation'.

This Award marks a new design approach for offshore platforms exposed to waves and current. This is a major revision of the standard industry method dating from 1950 when Morison, a graduate student at Berkeley, had his master’s project published. His equation then became the basis of design specifications for most offshore platforms constructed over the last 65 years.

IES Award WinnersThe IES award winners, (left to right): Professor Choo, Professor Taylor and Dr Santo.
Main photo courtesy of Wikimedia.
This new research into 'current blockage in offshore engineering - Morison revised on the 65th anniversary of the famous equation' has been undertaken by Professor Paul Taylor, from Oxford’s Department of Engineering Science, Professor Yoo Sang Choo, from the National University of Singapore (NUS), and Dr Harrif Santo, then a PhD student from NUS who is now working as a Post- Doctoral Research Assistant with Professor Taylor at the Department of Engineering Science.

Collaboration between University of Oxford and the National University of Singapore

Professor Taylor said: “Our research shows that, for structures exposed to large waves and ocean currents, the standard design recipe is probably too conservative. Potentially, the engineering community has been wasting money which could have been better spent in improving safety elsewhere. Morison’s simple equation does not differentiate between local flow due to waves and ocean currents, and assumes that the presence of a second and more cylinders has no effect on the forces exerted on each. Even in the original Morison paper, caveats were included '...these preliminary results are applicable only to single piles without bracing and are likely to be modified somewhat where .. one (is) within the influence of the other or where multiple piles are connected by submerged bracing ...'. Such an improved model has been developed over the last few years collaboratively between Oxford and the National University of Singapore”.

Singapore, a small island country with no offshore oil or gas, is supporting research in offshore engineering. Why? The answer is simple: two of the major companies worldwide constructing offshore structures and equipment for offshore operations are based in Singapore. These companies even supply rigs and equipment for installation in the North Sea.

Professor Taylor said: “Hurricanes Katrina and Sandy affected many in the USA, and the tragic footage of the Indian Ocean and Japanese tsunami waves filled our TV screens. My work on current blockage is a just a small part of my water wave research. We have developed a mathematical model for the following aphorism: ‘On a windy day the force on a single tree in a forest is much smaller than the force on the same tree in isolation'. Each of the individual structural elements in a complex offshore platform is like one of these trees, yet the Morison equation assumes that all the other elements are absent: there is no forest. A revised methodology based on a theory for propellers has been developed for the loading from large waves and a relatively small steady current. The theory has been validated against small laboratory tests, simulations using computational fluid dynamics, and large-scale wave tank testing”.

Getting new approach introduced

Professor Taylor added: “The rather impressive stainless steel model that we have tested in a large wave flume in Glasgow (shown in the inset photograph with the three researchers), shows the complex arrangement of structural elements in a typical offshore structure, albeit at 1:70 scale. One might reasonably expect the flow around any individual structural component to be modified by the presence of the rest of the structure, and this is exactly what we observed. We are presently working on getting our new approach introduced into a revision of the offshore structures design standards issued by the American Petroleum Institute and the ISO-committee”.

Where else can this work be applied? Obviously, it is relevant to both existing and new oil platforms. A newer application is to renewable energy. As offshore wind turbines get larger and are installed further out in deeper water, where the winds are stronger, turbine towers are starting to be supported on large truss-type platforms, for which this new design approach is appropriate. Examples include the ten turbines being installed offshore Aberdeen and a similar array already operating off Ostend in Belgium.

Professor Taylor said: “So, despite it reaching retirement age, we aren't retiring the Morison equation. Instead, we are simply updating it – 65 years late!”

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