NDLI: Variation of Heave Added Mass and Damping Near Seabed

Content Provider	The American Society of Mechanical Engineers (ASME) Digital Collection
Author	Wadhwa, Hemlata Krishnamoorthy, Balaji Krish, P. Thiagarajan
Copyright Year	2010
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.
Sponsorship	Ocean, Offshore and Arctic Engineering Division
Starting Page	271
Ending Page	277
Page Count	7
File Format	PDF
ISBN	9780791849095
DOI	10.1115/OMAE2010-20456
e-ISBN	9780791838730
Volume Number	29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 1
Conference Proceedings	ASME 2010 29th International Conference on Ocean, Offshore and Arctic Engineering
Language	English
Publisher Date	2010-06-06
Publisher Place	Shanghai, China
Access Restriction	Subscribed
Subject Keyword	Damping Actuators Cranes Fourier transforms Transducers Submersibles Oscillations Fluid-dynamic forces Wire Seabed Displacement Stress Cavities Algorithms Weight (mass) Disks Ocean engineering Resonance Tension
Content Type	Text
Resource Type	Article

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