4 - Memorial University of Newfoundland DAI

al frequcncic. occurred when member(a) in the k-brace waslwere runwed.
Thcy concluded that these changes were largeenough to be detected reliably,
and that the method should give positive indication of atruetursl failure.
A slate-af.the.srt discussion (which fosuaaed on pible limitation8 of
tho lechnique in an offahotr cnvimnment) is presolted in Reference [lo]. For
example, Bcgg showed that the higher fnquenciea involved only a small mm-
poncnt; so modes involving underwater memben cannot be measured, much
loss identified, from positions above the water line. Begg and Maekenzie [I11
showcd lhnt under normal drmmdmcea, vibration monitoring pmvides a
hnck-up asaurnnw of thc ability of the platform to wilhstand major wave
Ionding. As a mult, to obtain an assurance of the passible degree of arcu-
mcy, it is wry II-sary that the dynamiccharacteristienof the platform be
mmplctcly underataod, ao a good datum is obtaincd for later mmpsriaon..
Ilcnw, it is necessary lo cdeulnte the sensitivity d the platform at mrious
localions in order to provide a baais for identifying the damage fmm changes
in lle vibrdion spectr~
A short study was carried out by Lock and Jon- [I21 to investigate the
fcaibility of using vibration monitoring for assessing the structural intwity
(stillness) of oil production platforms, especially thme constructed of steel.
Toaclrievc this abjective,sxperimental work wss done on a space framemodel
mnstruetcd fmm 3.8 m and 1.9 cm 16 gauge stesl tubes. The madel was 3.8
m long with 0.9 m wido aides and weighed 46.0 kgf. Based on the results
obtained, they suggested lhat monitoring the vibration of an oil production
platform would provide information about it. condition. Notwithstanding,
tile major impediment lo the successof the methcd would be eatimatinb the
clcgreo of change in the natural frequency of any particular mode, since mode