ssc-367 - Ship Structure Committee

D.8.3 Devices and Spoilers
Devices that modify flow and reduce excitation can be fitted to
tubular structures. These devices (see Figure D-5) work well for
Isolated members but are less effective for an array of piles or
cylinders. Unfortunately, there is no relevant information
describing how the governing stability criteria are modified. The
most widely used devices are described below.
Guy Wires
Appropriately placed guy wires may be used to increase member
stiffness and preclude wind-induced oscillations. Guy wires should
be of sufficient number and direction to adequately brace the tubular
member; otherwise, oscillations may not be eljmlnatecl completely and
additional oscillations of the guys themselves may occur.
Strakes or Spoilers
$trakes and spoilers consist of a number (usually three) of fins
wound as a helix around the tubular. These have proven effective in
preventing wind-induced cross-flow oscillations of structures, and
there is no reason to doubt their ability to suppress in-line motion.
provided that the optimum stroke design is used. This comprises a
three-star helix, having a pitch equal to five times the member
diameter. Typically each helix protrudes one-tenth of the member
diameter from the cylinder surface. To prevent in-line motion,
strakes need only be applied over approximately in the middle onethird
of the length of the tubular with the greatest amplitude.
Elimination of the much more violent cross-flow motion requires a
longer strake, perhaps covering the complete length of tube. The
main disadvantage of strakes, apart from construction difficulties
and problems associated with erosion or marine growth, is that they
increase the time-averaged drag force produced by the flow. The drag
coefficient of the straked part of the tube is independent of the
Reynold’s number and has a value of CD = 1.3 based on the tubular
diameter.
D-25