Wave Impacts due to Steep Fronted Waves

Bas Buchner and Arjan Voogt

Damage to Schiehallion Bow• Early storm,

max ship response 2-6 hours later

• Estimated sea state: Hs=14m Tp=11s

• Severe steepness

Experiments on floating FPSO

PSU PSL PPU

PPL

Upper bow segment

Lower bow segment

PZU

PXU

PXL

-PZL

segmentedbow

Schematic bow tests

Typical stages during impact

time step

between

profiles

is 0.31 sMWL

6 m

R4 R5R3R2R1 R6

Spatial wave profile

Impact

Time trace of wave steepness

ddt wave

time trace velocity

ddx steepness

spatial wave profile

Vertical Free Surface Velocity

wave celerity

Time trace of d/dtSignificant wave impact

time [s]

Ver

tica

l fr

ee

surf

ace

ve

loci

ty [

m/s

]

d/dtlinear 6

2nd order 10measured 12

measured2nd order

linear

Wave modeling to get d/dt

Exceedance probability

Vertical free surface velocity [m/s]

Pro

babi

lity

of E

xcee

danc

e

Local impulse versus

Conclusions

• Wave steepness drives the impact

• Linear wave theory underestimates wave steepness

• 2nd order wave theory results in improved prediction

• Good probabilistic models needed to predict extreme wave steepness