Operational modal analysis on the SA Agulhas II

• Main Author: Van Zijl, Christof Moolman
• Other Authors: Bekker, Annie, Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
• Format: Thesis
• Language: English
• Published: Stellenbosch : Stellenbosch University. 2020
• Subjects: Modal analysis; Stability of ships; Icebreakers (Ships) > Hydrodynamics; Operational research; SA Agulhas II (Ship) > Stability; Accelerometers; Vibration (Marine engineering); UCTD; Antarc*; Antarctica; Antarktika
• Online Access: http://hdl.handle.net/10019.1/108057

Thesis (MEng)--Stellenbosch University, 2020. ENGLISH ABSTRACT: Full-scale operational measurements were conducted on a polar vessel during an expedition to Antarctica. Accelerometers distributed along the length of the vessel were used to capture vertical, lateral and torsional hull global motions. A parametric study on a stochastic subspace identification technique was conducted to investigate the effect of user-specified parameters on modal parameters. With low block sizes weakly excited modes were not identified and increasing block size up to fifty was found to reduce both random and bias errors. Measurement duration below twenty minutes were found to produce relatively large bias and random errors, however there was little improvement with increasing measurement duration beyond twenty minutes. Extreme model orders were found to produce scatter on estimates and a suitable range of model orders were found to be between 200 and 300. A modal model consisting of five modes was identified for the SA Agulhas II. Modes were automatically tracked across four different cases of operational data. Vertical bending modes were tracked consistently across all four cases, but lateral and torsional modes were not. Energy in modal responses were highest for vertical bending modes and significant energy was found to be concentrated around vertical bending modes during an open water storm case. An analysis of the time-varying behaviour of modes in the four cases revealed highly non-stationary modal responses during an open water storm and two ice cases. The transients were found to correspond with impulsive wave slamming events in open water and ship-ice interactions during ice breaking. AFRIKAANSE OPSOMMING: ’n Volskaalse meting is uitgevoer op ’n poolnavorsingskip tydens ’n ekspedisie na Antarktika. ’n Stel versnellingsmeters is gebruik om die vertikale, laterale en torsionele bewegings van die romp van die skip te bepaal. ’n Parametriese studie is uitgevoer op die stochastiese subspasie identifikasie tegniek om die ...