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Lookup NU author(s): Sudheesh Ramadasan,
Professor Longbin Tao,
Dr Arun Dev
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An excellent vortex suppression device, in the form of an antinode fairing is presented towards reducing the VIV of structural members exposed to flow. An analytical procedure is also developed to optimise the length and location of fairings. Two fairing sections are tested on cylindrical legs of a Jackup model and the results are compared with the mathematical predictions. A simplified mathematical model is developed based on the principle of conservation of energy, which could be used to decide the optimum location and length of vortex suppression devices for the cylinders undergoing VIV in a uniform flow. Based on the mathematical studies, modal antinodes are found as the optimum locations for the installation of the fairings and accordingly two optimised leg fairing designs have been developed for practical applications. Those are based on NACA0018 profiles with L/B ratios around 2.40 and 3.00. The fairing designs are tested on scaled down Jackup models with cylindrical legs experiencing VIV in uniform flow. The tests results are then compared with the mathematical predictions. The mathematical studies reveal that the optimum locations for the installation of fairings are the antinodes of the excited mode as the major excitation and damping contributions arise from the vicinity of same. It is found that the depth of the fairing along the cylinder length can be optimised through appropriate selection of a limiting response amplitude ratio and locating the fairings in the way of the antinodes. The tests with scaled down models demonstrated that both the fairing designs could reduce VIV amplitudes significantly, by more than 80%. The weather waning stability of both the fairing designs were also found to be satisfactory. From the test results, it is inferred that fairings contribute to the reduction of VIV in three ways; reduction of excitation force, disruption of vortex synchronisation and the increase in fluid damping. Fairings are also found not to have significant bearing on the drag coefficient of the structure. The fairing design with the lesser L/B ratio of 2.40 is found to be an optimal solution for vortex suppression, when the structure is not sensitive to drag forces. However, fairings are found to initiate galloping responses at higher flow speeds above the lock-in range and hence it is recommended that the same shall be of the detachable type for cylinders exposed to large flow speed variations. The mathematical approach presented will enable practicing engineers to design and optimise antinode fairings for structures experiencing VIV. The two fairing section offsets presented may be readily used by Industry for practical applications.
Author(s): Ramadasan S, Tao L, Dev AK
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: Offshore Technology Conference Asia 2018
Year of Conference: 2018
Online publication date: 20/03/2018
Acceptance date: 01/03/2018