Operational modal analysis on the SA Agulhas II

Van Zijl, Christof Moolman (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

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 effek van die verstelling van parameters op modusse te ondersoek. Teen lae blokgrotes word swak opgewekte modusse nie geïdentifiseer nie, terwyl by die verhoging van blokgrote tot vyftig, is daar gevind dat ewekansige foute, sowel as sydigheid verminder is. ’n Meting onder twintig minute veroorsaak relatief groot ewekansige foute en sydigheid, maar daar is weinig verbetering om ’n meting langer as twintig minute te gebruik. Ekstreme model ordes veroorsaak verstrooiing van die modale skattings en ’n geskikte reeks van model ordes is tussen 200 en 300. ’n Modale model van vyf modusse is vir die SA Agulhas geïdentifiseer. A utomatiese naspeuring van modusse is toegepas op vier verskillende gevalle van operasionele toestande. Die vertikale buigmodusse is goed nagespeur oor die vier gevalle, maar nie die laterale buigmodus en torsionale modus nie. Die energie in die eerste vertikale buigmodus is die hoogste. ’n Groot hoeveelheid energie is gekonsentreer by hierdie modus gedurende die oop water stormgeval. ’n Analise met betreking tot die tydveranderlike gedrag van die modusse in hierdie vier gevalle wys dat die gedrag van die modusse nie-stationêr is tydens die oop water en ys gevalle nie. Die transiënte gedrag stem ooreen met impulsiewe golfslaggebeurtenisse en interaksies tussen die romp van die skip met ys.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/108057
This item appears in the following collections: