9. VORTEX INDUCED OSCILLATIONS
9.8 Methods for reducing VIO
There exist two ways for reducing the severity of flow-induced oscillations due to vortex shedding, either a change in the structural properties, or change of shape by addition of aerody- namic devices such as strakes, shrouds or spoiling devices which partly prevent resonant vortex shedding from occurring and partly reduces the strength of the vortex-induced forces.
9.8.1.1 Change of structural properties means changing of
natural frequency, mass or damping. An increase in natural fre- quency will cause an increase in the critical flow speed
Thus vcrit may become greater than the maximum design flow
speed, or VR may come outside the range for onset of resonant
vortex shedding. k1 is a safety factor (typically 0.85).
9.8.1.2 An increase in non-structural mass can be used to
increase KS and hence decrease the amplitude of oscillations.
Due attention has however to be paid to the decrease of natural frequency which will follow from an increase of mass.
nn = the number of load cycles in the locking-on period ACF = the maximum cross flow amplitude as derived from
Figures 9.4 and 9.5
)
exp(
1
(
)
(
max max n c CFn
L
A
l
A
A
= generalised logarithmic decrement as defined in 9.1.8
)
exp
max(-
n
dA
=
A
0 0.5 1 1.5 2 2.5 3 3.5 0 10 20 30 40 50 KC Cf St D f k1 n crit v 9.8.2 Spoiling devices
9.8.2.1 Spoiling devices are often used to suppress vortex shed-
ding locking-on. The principle in the spoiling is either a drag reduction by streamlined fins and splitter plates (which break the oscillating pattern) or by making the member irregular such that vortices over different length becomes uneven and irregular. Examples of this may be ropes wrapped around the member, per- forated cans, twisted fins, or helical strakes, Figure 9-12.
Figure 9-11
Helical strakes and wires
Figure 9-12
Force-deflection curve for 3/4 inch stranded guy-wire with geo- metrical configuration as shown
9.8.2.2 In order for the spoiling devices to work they shall be
placed closer than the correlation length for the vortex shedding.
9.8.2.3 The efficiency of the spoiling device should be deter-
mined by testing. The graphs for in-line and cross flow motion can be directly applied for the spoiling system by multiplying with the efficiency factor.
9.8.2.4 Using spoilers the marine growth may blur the shape
and may make them less effective. The changed shape shall be taken into account in the analysis.
Typical examples of the efficiency of helical strakes are given in Table 9-3.
9.8.3 Bumpers
For pipes closely spaced to a wall or to a greater pipe, bumpers may be used to limit the maximum response. Besides reducing the amplitude it will break up the harmonic vibrations. 9.8.4 Guy wires
9.8.4.1 Use of pretension guy wires has proven effective to
eliminate resonant vortex shedding. The guy wires should be attached close to the midpoint of the member and pretensioned perpendicularly to prevent cross flow oscillations.
9.8.4.2 The effect of guy wires can be summarized as follows:
— Increase member stiffness and hence natural frequency (small effect)
— Hysteresis damping of wires (large effect)
— Geometrical stiffness and damping of wires (large effect) (due to transverse vibrations of wire)
— Nonlinear stiffness is introduced which again restrains res- onance conditions to occur.
— The wires have to be strapped and pretensioned in such a way as to fully benefit from both hysteresis and geometri- cal damping as well as the non-linear stiffness. The preten- sion for each guy wire should be chosen within the area indicated on Figure 9-12. Total pretension and number of wires has to be chosen with due consideration to member strength.
— An example is shown in Figure 9-12 where a 3/4 inch wire is used to pretension a member with 30 m between the member and the support point. A tension (force) of 2.5 kN will in this case give maximum non-linear stiffness. — Instead of monitoring the tension, the wire sagging may be
used to visually estimate the tension. In the example shown, a sag of around 0.45 m corresponds to the wanted tension of 2.5 kN. Strakes Wire Pitch D d D d
Table 9-3 Efficiency of helical strakes and helical wires.
No. of windings Height of Strakes Pitch Lift Coefficient CL Drag Coefficient CD Helical Strakes 33 0.11 D0.11 D 4.5 D15 D 0.238 0.124 1.61.7 Helical Wires 3-4 3-4 3-4 3-4 0.118 D 0.118 D 0.238 D 0.238 D 5 D 10 D 5 D 10 D 0.2 0.2 0.2 0.2 1.17 1.38 - - No spoilers 0.9 0.7
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