MIT2_019S11_OWE - 2.019 Desi 2 019 D ign of O f Ocean...

2 019 D 2.019 Design of Ocean Systems f O t
Lecture 9
Ocean Wave Environment
March 7, 2011

Ocean Surface Wave Generation
Waves important to offshore structure design and operation: Wind waves or gravity waves with wave period
T= 5 ~ 20 seconds, wavelength O(10)m to O(500)m.
Source of forcing: wind
air
Source of restoring: gravity
Source of damping: wave breaking and viscous effects
water
•When wind starts (0.5 ~ 2.0 knots), capillary waves form
(e.g Vp =24 cm/s → λ =1.73 cm)
•As wind becomes stronger, waves become longer
Wind energy
input into water:
Frictional drag
Separation drag
Bernoulli effect

Wave evolution
as wind blows
• Nonlinear wave-wave interactions cause energy to be transferred into longer waves
• Certain distance and duration (for wind to blow) are necessary for effective energy
transfer
• Equilibrium sea: when energy input from the wind is balanced by dissipation
• When wind input energy is larger than dissipation, waves grow
• When wind input energy is smaller than dissipation, waves decay. Short waves
decay faster.
Shorter waves are
steeper, and easier
to break

Required fetch and storm duration:
•Wind must blow over long
periods of time and large
distances to reach fully-
developed state.
•At fully-developed state,
U
w
~ V
p
(i.e. ω
limit
~ g/U
w
)
•Swell: waves are not
generated by local wind
•Sea: waves generated by
local wind

Standard Wave Spectra
S ( ω )
Based on measured spectra and theoretical results, standard spectrum forms have been developed:
Bretschneider spectrum:
S ( ω ) =
1 . 25
ω
4
H
2
¡ ¢
4
m
ω
m
4 ω 5
s
exp { − 1 . 25
ω
}
ω
m
is peak or modal frequency
H
s
is signi fi cant wave