Radiation, Diffraction Analyses on Side by Side Applications

Radiation, Diffraction Analyses on Side by Side Applications

Oude Ophuis, R.B.A.

Huijsmans, R.H.M. (mentor)

Mechanical, Maritime and Materials Engineering

Marine and Transport Technology

Offshore engineering

2016-08-22

Offshore barge mooring is considered a critical operation. The vessels need to be in close proximity, but may under no circumstance collide. Mooring lines and fenders are installed to keep the vessels in place. Experience has shown that lines tend to break occasionally; one of the reasons for this can be the interaction effect of the two vessels and the occurrence of standing waves in the gap. In principle, energy is trapped in the enclosed space between the vessels as a form of resonance. Previous research has shown that the increased vertical flow resulting from the standing waves separates around the bilge, extracting energy out of the upwards motion of the water. To determine the required strength and amount of the mooring lines, the motions and forces on the vessel are usually determined using diffraction analyses. These analyses are based on the principle of linear potential flow in the frequency domain, which allows for computationally quick calculations of the behavior of a vessel. Unfortunately, the creation of vortices is a non-linear effect that is overlooked by potential theory. This results in an unrealistic over-estimation of the water height in-between the vessels (sometimes up to ten times the incoming wave), which could affect the hydrodynamic behavior and thus the results of a mooring analysis. This thesis focuses on the effect of this over-estimation and on how to mitigate this effect in existing software. The Deepwater Construction Vessel Aegir and a cargo barge alongside are modeled in MultiSurf. The resulting patch models are inserted in WAMIT for diffraction analysis. The single body behavior is analyzed, as well as the over-estimated multi-body behavior. To mitigate the over-estimation of the resonant effects, a flexible damping lid is modeled in between the vessels. The lid is mass-less, has no thickness, and is non-permeable. Through so called modal analysis, the response of the water in between the vessel is decomposed into several so called mode shapes, each with a different response. The lid is made flexible by allowing movement only in these mode shapes. The superposition of the forced response of each of these modes provides the total response of the lid. This approach creates additional degrees of freedom to the system that can be damped accordingly, decreasing the energy of the water in the gap. The damping is determined by recreating a model test from literature in WAMIT and tuning the lid to match the measured water level. Large but narrow peaks are found in the various RAOs of both vessels. These are also analytically determined by solving an eigenvalue problem. It is shown that the peaks observed in the motion-, wave force-, water height-, and mean wave drift RAOs can be damped out with the lid. The effect is greatest on the water elevation and wave drift forces. Looking at the response of the vessels in an actual sea state, it is found that the motion and force response are of the same magnitude as those of a single vessel. Due to the non-symmetry of the vessel configuration some small differences are seen, but not as dramatic as in the RAO s. The damped lid shows to have only a marginal effect on these responses. The water elevation and wave drift force responses show large responses compared to those of the single-body. Application of damping shows significant influence on these two responses. This thesis demonstrates that the motions and exciting wave forces obtained through regular diffraction analysis are practically unaffected by the over-estimated gap resonance, and that a damping lid is only necessary when the water elevation in the gap or the drift forces are of importance, such as in a mooring analysis. With the barge shadowed by the Aegir in optimum heading however, the behavior is always determined correctly, and use of a damping lid is not deemed necessary. The model serves as a simple basis for future mooring analyses. In the future more focus should be put on determining damping for other vessel configurations, possibly through model testing or use of computational fluid dynamics.

Gap resonance
Side by Side
mooring
offshore
damping lid
diffraction
radiation
WAMIT

http://resolver.tudelft.nl/uuid:090ef1b0-5453-4d20-9241-40d7e870037d

2021-08-21

Student theses

master thesis

(c) 2016 Oude Ophuis, R.B.A.