WIND
INDUCED RESONANT CROSS FLOW VIBRATION
By Arne Kvitrud, Sondre Nordheimsgate 9, 4021 Stavanger.
Paper
presented in 1994, but put on Internet 25.9.2002.
The figures
are not presented here.
INTRODUCTION
Several problems have occurred offshore in Norway caused by wind induced
vibrations. Wind induced cross flow has caused severe vibrations in two flare
booms, and fatigue cracking in four flare booms and four jackets.
The guidelines on determination of loads and load effects are
recommending the use of the German DIN 4133 standard, for calculation of cross
flow wind vibrations.
This paper will :
a) describe the vibrations and its characteristics on Statfjord A and
Heimdal, where the largest vibrations occurred. The vibrations cannot be
predicted by vortex shedding theories of individual elements. The vibrations
were caused by vibrations from wake effects and by vibrations of local frames
inside the flare booms.
b) review the work done on evaluating the cracks on other flare booms
and jackets. The work have been based on DIN 4133. It will include evaluations
of vibrations of both individual members and frames.
c) summarize the experience gained from measurements and observations of
damping of individual tubular members, end fixity of members and vibrations of individual
members and frames. Further the paper will describe limitations of the DIN
standard.
IN SERVICE VIBRATIONS
a) During the autumn of 1978
the main tie of the flare boom of Statfjord A experienced severe vibrations.
The severe vibrations occurred during 3 different days. The wind velocity was
about 40 knots when the problems occurred . The amplitudes of the horizontal
vibrations was approximately 100 mm.
The classic theories for calculating when vibrations would occur, were
precise, but the actual vibrations were up to 20 times larger than the theories
predicted.
b) During the winter of 1984-1985 severe vibrations of the flare boom at
Heimdal occurred. The vibrations occurred both as vibrations of individual
members and as vibrations of frames inside the boom.
The vibrations of the individual members occurred at velocities which
were higher than expected from model testing of individual members. The
vibrations were later successfully reproduced in model tests in Trondheim. The
reason for the odd behaviour is concluded to be wake interference between the
elements. The response was also higher than predicted by available standards.
The procedures established in existing standards only consider
vibrations of individual members in a structure. The vibrations of the frames
at Heimdal also highlighted the need to do vortex shedding evaluation of
frames.
c) Based on mountain climbing methods for in service inspection of flare
booms, several cracks have been observed in flare booms during the last years.
On Statfjord B and C ; 45 cracks have been found in the flare boom.
Based on the use of DIN 4133 ; 44 of the cracks have analytically been found to
be caused by a combination of vibrations of individual members and vibrations
of frames.
On Gullfaks B cracks have been found in eight nodes of the flare boom. Calculations
based on DIN 4133, showed that the cracks most probably were caused by
vibrations of individual members. One also found that additional loading was
introduced when the ratio of natural periods of two connected members was 1:2.
In UK sector a similar vibration problem have been reported on
Murchison. The flare boom was designed by the same company that designed the
flare booms on Statfjord, and they are also almost identical.
We have also reviewed all cracks on jacket substructures above sea water
level, reported during the period 1981-92, based on an assumption that they
were caused by wind induced vortex shedding. The conclusion is that at least
four jacket platforms have cracks caused by wind induced fatigue from vortex shedding.
DESIGN PROCEDURES
The use of DIN 4133 has shown to be a useful tool in predicting wind
induced vortex resonant vibrations. The standard have a limitation, however, it
can only handle vibrations of individual members. Experience from vortex
induced cross flow vibrations have shown that it is not sufficient only to
review vibration of individual members. In a real structure the structural
elements interact both with respect to loading (as wakes) and in the response
(as vibrations of frames).
The proposed CEN standard (EUROCODE 1) is based on the DIN standard, but
have some additional useful guidance to designers and will be a step in the
right direction in predicting vortex induced vibrations. Design procedures to
handle arrays of structural elements (as on Statfjord, Gullfaks and Murchison)
is described.
In addition to the DIN or Eurocode standards a design procedure have to
be followed using:
* low values of the damping of individual steel members
* realistic values of end fixity of individual members
* evaluations of possible frame vibrations have to be included