Analysis of Ice-Induced Vibrations and Comparison with Full-Scale Experimental Data
In the present work, a model which estimates the fatigue life for a cylindrical structure interacting with ice is developed. The model is based on the lighthouse of Norströmsgrund, and estimates the fatigue damage for one season. Fatigue is the long-term, cumulative effect of cyclic loads. Cumulative fatigue damage is found by using the Miner-sum together with an SN-curve. Fatigue of the cross-section located 7.5 m above the seabed is investigated. Failure of this particular part is in the model assumed to cause global failure of the lighthouse. The cross-section consists of concrete with reinforcing steel bars.
Different cases are used when estimating the fatigue life. First, an extreme case is considered, assuming continuous cycles the entire season. The next case uses onset criteria for cyclic load conditions. Lastly, the field-observed number of cycles is used. The model produces a total number of cycles ranging from 541008 to 40950144. Field-observed number of cycles is for the considered season 5090. Thus, the model overestimates the number of cycles. Fatigue life is found to be 7 years for the extreme case. The methods using proposed onset criteria give a fatigue life of 31, 54, 61, 70 and 239 years, depending on the method. For the observed number of cycles the time to failure is estimated to be 4911 years. A fatigue life of 7 years is highly unlikely, while 31 to 70 years are considered to be underestimations. This is based on the fact that lighthouse has been operational since the 1970s. Models used in design should always overestimate damages.
The model checks if cyclic load conditions are probable using environmental parameters.
Data from the winter of 2002/2003, measured at the Luleå Airport and the lighthouse of
Norströmsgrund, is utilized. Cyclic loads are associated with steady-state vibrations and structural resonance. The frequency of the ice crushing against the structure is for this condition seen to be close to the lowest natural frequencies of the structure. This phenomenon is termed frequency locked-in crushing, abbreviated FLC. Three different approaches are applied by the model to check for possible FLC conditions. The first uses standardization codes, while the second utilizes dimensionless groups based on ice thickness, ice velocity, structural diameter and structural frequency. Lastly, case-specific methods for Norströmsgrund lighthouse are implemented in the model.
Ice properties, such as the thickness, uni-axial compressive strength, drift speed and drift direction are estimated using standardization codes and established methods.
In addition, a single-degree of freedom model was established. Input values were taken
from previous established FEM models of the lighthouse. The model used the Newmarkbeta
method, and was compared to the in-situ measured lighthouse response. It was seen
that the full-scale response can be fairly represented by the use of simple models.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2013. , 132 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-22099Local ID: ntnudaim:9137OAI: oai:DiVA.org:ntnu-22099DiVA: diva2:646837
Amdahl, Jørgen, ProfessorBjerkås, Morten