Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 526

Vibration Monitoring
Structural vibration monitoring offers a low-cost method of assessing structural
integrity. The cost is low because monitoring is done exclusively above water at
deck level by a small crew using lightweight portable equipment.
Platform natural frequencies and mode shapes are typically measured using
sensitive accelerometers. These are mounted horizontally and detect the small
sway movements of the platform. These movements occur primarily at the
wave period but the platform s natural frequencies are usually clearly identifi-

able. By measuring movements at different locations on the deck, it is possible
to distinguish between sway and torsional natural frequencies.
Similarly, by measuring movements at different elevations on the jacket, it is
theoretically possible to distinguish between the first sway natural frequency
and higher-order sway natural frequencies, although in practice higher-order
natural frequencies tend not to be strongly excited.
The effect of damage of a single member on the overall natural frequencies
will depend on the level of member redundancy of the jacket, and also on the
contribution the specific member makes to the dynamic stiffness of the platform
at that particular natural frequency.
The offshore platform natural frequencies depend, of course, on deck mass
as well as jacket stiffness. Other factors may have a secondary effect, including
variations in the effective mass of entrained water and nonlinearity of founda-
tion stiffness. Further, the mathematical calculation of natural frequencies is a
statistical process and each estimate of natural frequency has an associated
error. Natural frequency data from continuously monitored platforms can be
used to show day-by-day variations.
The above discussion indicates that natural frequencies are adequately
stable and so sensitive to damage that they can be used for detection of
changes in stiffness of the order of

'

1% change in natural frequency).
Damage of a bracing can be detected on platforms with low-redundancy mem-
ber configurations, whereas several or many member failures may occur on
higher-redundancy structures before a change is detected. For example, a
loss of a diagonal on a K-braced structure results in a frequency change of
9.5% to 11.5%, whereas a similar loss on an X-braced structure results in a
change of only 1% to 2%. The former is detectable and indicative of a signif-
icant loss of overall stiffness.
Topside accelerometers, shown in Figure 7.32 , are cabled back to a central
data-collection station using conventional methods. A significant change in
recent years has been the development of cableless underwater sensor packages.
These systems are battery powered, and data are transmitted by hydro-acoustic
telemetry.
Nowadays, vibration monitoring is widely used because it is so much less
expensive than follow-up and because monitoring the performance of the nat-
ural structure allows the required action to be taken in a reasonable time.

± 3% ( ±