Browsing by Author "Soal, Keith Ian"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    System identification and modal tracking on ship structures
    (Stellenbosch : Stellenbosch University, 2018-03) Soal, Keith Ian; Bekker, Annie; Bienert, J.; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH SUMMARY: are currently based mainly on dynamic response feedback. Navigators decide on how to operate the vessel based on how they feel it pitching, heaving, rolling and vibrating. The aim of this thesis is to investigate the idea of using system identification and modal tracking on polar vessels towards the development of a decision aiding system. System identification provides a powerful tool for building mathematical models of dynamic systems. An open source toolbox (openSID) for system identification using Stochastic Subspace Identification (SSI) was developed as a research and learning tool. Full scale measurements were performed on the research vessel Polarstern during an expedition to the Arctic. This is the first comprehensive data set including vibration responses and environmental parameters to span the entire operational profile of a research voyage to the Arctic. System identification successfully identified seven global modes in the bandwidth 2 - 10 Hz. Comparisons between different methods were used to cross validate results. A modal tracking algorithm was developed and relationships between identified modes and system inputs were observed. A novel method is developed to improve the uncertainty and sensitivity of system identification and tracking, based on a data driven statistical model and a Kalman filter. A key objective is to make experimental data maximally informative by using additional system inputs. The model was found to accurately re-create the training data set and was used to make predictions based on future system inputs. The Kalman filter estimates were observed to produce balanced and consistent results. These results demonstrate the potential of an ice force estimation and structural health monitoring system.
  • Thumbnail Image
    Item
    Vibration response of the polar supply and research vessel the S. A. Agulhas II in Antarctica and the Southern ocean
    (Stellenbosch : Stellenbosch University, 2014-12) Soal, Keith Ian; Bekker, Annie; Stellenbosch University. Faculty of Engineering. Department of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Full scale measurements were conducted on the polar supply and research vessel the S.A. Agulhas II during a 78 day voyage from Cape Town to Antarctica in 2013/2014. Investigations were conducted into the effect of vibration on human comfort and the structural dynamic response of the vessel. Vibration measured in the bridge of the vessel is found to have little effect on human comfort for a standing person and is classified as not uncomfortable according to BS ISO 2631-1. Structural fatigue as a result of vibration is found to reach levels where damage is possible in the stern and where damage is probable in the bow during open water navigation, according to ship vibration guidelines by Germanischer Lloyd. Multivariate statistical analyses are performed to investigate the relationships between multiple predictor variables and vibration response. Factor analysis revealed data structure from which specific physical phenomena could be identified. Multivariable linear regression models are developed to predict vibration response and are found to provide more accurate predictions in open water than in ice. The 2-node, 3-node and 4-node normal bending modes of the structure are identified using operational modal analysis while the vessel was moored in the harbour. The natural frequencies, damping ratios and mode shapes are estimated and compared using LMS Operational PolyMAX and ARTeMIS CCSSI. A comparison of operational modal analysis results to the STX Finland finite element model show that the vessel’s modes occur at lower frequencies than numerically predicted. Clear potential is identified to further investigate structural vibration and operational modal analysis algorithm development in future research.