Experimental and numerical investigation of wave induced forces and motions of partially submerged bodies near a fixed structure in irregular waves. (1st September 2018)

• Record Type: Journal Article
• Authors: Sayeed, Tanvir; Colbourne, Bruce; Molyneux, David
• Is Part Of: Ocean engineering. Volume 163(2018)
• Page Start: 451
• Page End: 475
• Publication Date: 2018-09-01
• Subjects: Iceberg -- Bergy-bit -- Irregular waves -- Hydrodynamic interaction -- Offshore structure -- Wave-body interaction; Ocean engineering -- Periodicals; Ocean engineering; Periodicals; 620.4162
• DOI: 10.1016/j.oceaneng.2018.06.020
• Languages: English
• ISSNs: 0029-8018

Abstract

Abstract: This paper is the third of a series covering investigations of the wave induced force and motion behavior of ice masses near a fixed structure. The objective of this phase of the study was to determine whether the effects observed in regular waves (Sayeed et al. (2017b, 2018)) still held in irregular seas. In the present case, wave characteristics measured in front of the fixed structure showed that the significant wave heights follow a standing wave pattern generated by the superposition of the incident and reflected peak frequency wave. Thus it may be inferred that the significant surge and heave forces and motions will be increased or decreased at the node and antinode locations in front of the structure as previously observed for regular waves. Force spectra generated from regular wave force data (Sayeed et al. 2017b) and wave measurements from the present study showed that beyond the nodal location at D λ p = 0.25, the significant surge force goes down and significant heave force goes up as the body gets close to the structure. Direct measurements of irregular wave forces on a fixed berg model at different proximity to the fixed structure demonstrate the change in significant wave forces based on the ratio of separation distance to peak wavelength. When the berg body is positioned closer to the fixed structure, the normalized significant forces in the horizontal direction decrease, and the normalized significant forces in the vertical direction increase. Significant differences were found between the directly measured force spectra and those produced by linear superposition of regular wave force transfer functions. This is attributed to non-linear effects arising from the iceberg profile and low freeboard. Free floating motion experiments showed smaller differences in the surge and heave velocity spectra due to the presence of the structure. This is because there is insufficient motion data at any specific location to capture the motion responses for all the frequencies in the spectrum. The proximity effect on wave loads and wave induced motions in the vertical direction is found to be more pronounced than the same in the horizontal direction. Numerical simulations using RANS based commercial CFD code Flow3D © are compared to the experimental results. Selected test cases from both force and motion experiments are simulated in irregular waves generated from selected experimental wave spectra. Numerical results for irregular waves demonstrate the ability to simulate random waves and resulting forces and motions with trends comparable to the experimental results. Highlights: Experimental and numerical studies were conducted to investigate the wave induced forces and motion behaviour of ice masses near a fixed structure in irregular waves. Wave data measured in front of the fixed structure showed that the significant wave heights follow a standing wave pattern. The separation distance to peak frequency wavelength ratio dictates the corresponding horizontal and vertical wave forces. The proximity effect on wave loads/motions in the vertical direction is found to be more dominant than the same in the horizontal direction. Commercial CFD software Flow3D © showed the capability to simulate random waves and force and motion results are also reasonable showing the expected trends.