Experimental Assessment of Hydrodynamic Coefficients of Disks Oscillating Near a Free Surface

Abstract

The use of different types of subsea equipment is continuously increasing in offshore field development. Installation operations such as lifting and lowering of these equipments require knowledge of the hydrodynamic coefficients of the object. An accurate prediction of these coefficients on typical subsea structures is a challenging task. The main coefficients in this context relate to added mass, damping and slamming effects. Formulations have been presented by various authors in literature for evaluating these coefficients for simple shapes. Some of them have found widespread application in the industry. The authors have considered a solid circular disk as a base case for initiating study on subsea module hydrodynamics. Experiments were conducted on an oscillating solid disk of diameter 200 mm and thickness 2 mm near the free surface. Forced oscillations were conducted at amplitudes varying from 3mm–36mm and frequencies 0.9–1.5 Hz respectively. The forces on the disk were measured using a submersible high-sensitivity load cell. The motions of the disk were restricted to axial (heave) direction, and were measured with a displacement transducer. The measured forces and displacement were analyzed using a Fast Fourier Transform algorithm to separate the added mass and damping effects. From the rate of change of added mass with depth of submergence, slamming forces were identified. The measured coefficients were compared with similar published data by Vu et al [1] and Tao & Dray [2]. The paper presents various formulations for added mass, damping and slamming obtained from literature and currently in use in the industry. These formulations are compared with measured values of the coefficients and suggestions are made on the importance of these formulations for flat subsea structures.

Hydrodynamic coefficients and pressure loads on heave plates for semi-submersible floating offshore wind turbines: A comparative analysis using large scale models

“…However, apart from comments from Ref. [9] indicating that some corrugations were placed in order to minimize these kinds of effects, no other mention has been found in the literature.…”

“…To put this figure in perspective, it must be noted that, for example, Ref. [9] in their specific work regarding free surface influence in hydrodynamic coefficients, considered only depthradius ratios lower than 0.5. With this depth-radius ratio (1.55), it is expected that viscous damping will be dominant with respect to radiation damping.…”

“…[5,9] where a small prismatic tank was used, and that of Ref. [19] who employed a slender basin (a towing tank), akin to the one used in the present research.…”

“…Philip et al [7] and Nallayarasu and Bairathi [8] numerically and experimentally assessed the effects of including a heave plate in a spar type floater. Wadhwa and Thiagarajan [9] and Wadhwa et al [10] analyzed the dependence of the hydrodynamic response with the motion amplitude for a fixed oscillation frequency, finding a critical value above which response stops increasing. This analysis was done both close to the free surface and close to the seabed.…”

Vortex shedding modes around oscillating non-uniform double heave plates

“…2,10 On the other hand, many of the experimental based studies were conducted via an exciting actuator in which the effects of weight and buoyancy should be considered. [11][12][13][14][15][16] Moreover, some researches have been conducted based on the experimental and numerical approaches. 2,17 The use of damping plates in a uniform arrangement can generally increase the total hydrodynamic performance based on the findings from the numerical simulations and laboratory tests.…”