Computational Fluid Dynamics Seen as Complement to FLNG Wind Tunnel Testing
As the proposed number of floating liquefied natural gas (FLNG) projects grows to produce stranded gas reserves in remote offshore locations such as Southeast Asia and Africa, it is critical that the safety risks of FLNG facilities within the concept design phase be fully understood, according to a recent paper by Suba Sivandran, head of oil and gas at BMT Fluid Mechanics, a subsidiary of BMT group.
To address these safety risks, BMT proposes using computational fluid dynamics (CFD) to complement physical modelling through wind tunnel testing and “ensure oil and gas operators have confidence that the design is fit-for-purpose in all operating conditions.”
CFD is a predictive tool for understanding what will happen under a given set of circumstances and conditions. The oil and gas industry has used CFD extensively for a number of years to cope with the daily design challenges associated with fluid flow problems in exploration, production, refining and process equipment.
“However, with each year that goes by, computational power gets stronger and stronger and so more complex, accurate models can be produced on a continuous basis,” said Sivandran.
The tool can quickly answer many “what if?” questions in a controlled environment. These questions include how to decide upon equipment spacing and layout when trying to limit explosion overpressure and fire exposure damage. Other “what if” questions that companies designing an FLNG facility face including how to predict performance of their design when exposed to either internal or external fluid flow.
In the 1960s, fluid dynamics involved only two approaches: theory and experiment.
“The advent of supercomputers and the development of accurate numerical algorithms for solving physical problems have changed the way we study fluid dynamics today,” said Sivandran.
This third approach is CFD.
“BMT Fluid Mechanics uses CFD to model gas, liquid, particular flow and interactions of these flows with solid bodies through force and heat transfer,” said Sivandran.
The versatility of the tool and the unprecedented ability to visualize and quantify the flows has meant that CFD has become an indispensable tool whenever practical analysis and engineering design work involving fluids is required.
The application of CFD for gas explosion studies is common for offshore platforms and is increasingly used onshore in cases where the explosion risk is significant and a better description of the physics is required in order to give a more robust estimate of the risk, Sivandran noted.
“By providing a set of boundary conditions, you can predict how your design will perform, and through sensitivity analysis you arrive at an optimized result. CFD can help maximize process efficiency, lower production costs and meet environmental concerns,” said Sivandran.
Aerodynamics is an important aspect of the safe and reliable design of an offshore installation, Sivandran explained. To achieve safe and reliable design, BMT operates large in-house wind tunnel facilities to help designers assess wind loads, current loads, and the helideck wind environment.
Over the last six years, BMT has carried out testing on seven FLNG designs to assist designers in understanding potential mean forces and moments acting on a FLNG vessel. Wind and current measurements can be combined to determine heeling moments for a stability analysis and wind forces and moments are also necessary inputs to analyses of the mooring and thruster systems.
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