Variation of Heave Added Mass and Damping Near Seabed

Abstract

During installation of subsea structures such as mud mats, the tension in crane wires can experience spikes when the structure is near the seabed. It is hypothesized that such spikes may be caused by the structure undergoing resonant oscillations, which in turn may be due to changes in added mass and damping near the seabed. Such motions can cause hardship for operators as they interfere with precise positioning during installation. With increasing exploration and production in deep and remote fields, the size and weight of subsea equipments are continuously increasing. Installation operations such as lifting and lowering, positioning of the object require good knowledge of the hydrodynamic coefficients. Following on ideas used in Norwegian offshore, the mud mat is modeled as a circular disk. Experiments are conducted on an oscillating solid disk of diameter and thickness 200 mm and 2 mm respectively. The heave oscillations are forced by a programmable actuator, at amplitudes varying from 1–56 mm and frequencies from 1.0–1.8 Hz. The elevation ‘h’ of the disk from the mean seabed is varied from 0.2–2 times the disk radius. The forces on the disk are measured using a submersible high-sensitivity load cell. The motions of the disk are restricted to axial (heave) direction, and are measured with a displacement transducer. The measured forces and displacement are analyzed using a Fourier Transform algorithm to separate the added mass and damping effects. The authors have found similar trends in the hydrodynamic behavior of a disk approaching the seabed to what was found when the disk approached the free surface in Wadhwa & Thiagarajan [1]. The added mass and damping coefficients were found to increase with increasing KC, as well as with increasing proximity to the seabed. Another noticeable feature of the experiments was the cavity formation underneath the oscillating structure. The width of the cavity was about 3–4 times the radius of the disk and depth was about one third/fourth of the radius of the disk. The size of the cavity and the increase in hydrodynamic forces near the seabed suggest the importance of knowledge of hydrodynamic behavior near the seabed.

Study of the force coefficients on plates using an open source numerical wave tank

“…al, 2007), so that the coefficient C b = (KC)C d /(4) has been used extensively instead in the literature (e.g., Lake et al, 2000;Vu et al, 2004;He et. al, 2007;Tao and Dray, 2008;Wadhwa et al, 2010;Garrido-Mendoza et al, 2013). The expression for C b can be recovered by non-dimensionalizing B with 2 m a for circular disks.…”

“…A more recent experimental study by Wadhwa et al 2010 was concerned with the effect of the proximity of a solid boundary (seabed) on heaving circular disks. They concluded that a distance of about one diameter from the seabed corresponds to unbounded-fluid cases for the range of KC under consideration.…”

“…(2013) used the measurements of Wadhwa et al (2010) as a benchmark for numerical simulations based on the CFD package OpenFOAM. Therefore, both data and simulations from these two teams can be used to evaluate the performance of our numerical wave tank.…”

“…The conventions adopted for the display of experimental data and simulations are identical to those inFigure 12. The measurements ofWadhwa et al (2010), or the processing thereof to determine A', are questionable since the KC → 0 limit for A' is 1 by definition in an unbounded fluid (i.e., practically, for deeply submerged disks far away from solid boundaries). All other results for deeply submerged disks are generally following the same trend as KC varies; among them, experimental data are more scattered and not available at KC less than 0.2.…”

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

“…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.…”