Category Archives: Structural

Dynamic Wave Load on the Platform

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Naturally, wave loads acting on the platform are dynamic. For certain level of water depth, static equivalent load may be used for consideration. However, it is not applicable for such platforms that have deeper water depth and flexible behavior. Appropriate analysis should be carried out by fulfilling dynamic requirement. Consideration of Dynamic Amplification Factor (DAF) is a proper approach of describing the actual dynamic action of structure platform.

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Basically, dynamic analysis should be done in order to find out natural period of structure. By comparing its period with wave period and damping ratio, DAF value is able to be obtained. The dynamic amplification factor is calculated by the formula below.

DAF=[(1-(Tp/Tz)2)2+(2*µ*(Tp/Tz))2]0.5

Where, Tp = Platform’s Natural Period, Tz = Wave Period, µ = Damping Ratio = 0.2

The first three modes which are obtained during dynamic analysis give the fundamental natural period of the global platform in X-direction, Y-direction, and torsion respectively. However, only first and second modes are defined as period of structure (Tp1 and Tp2) so as to calculate Dynamic Amplification Factor (DAF1 and DAF2) along the respective direction. These DAFs are used in procuring DAFs for different wave directions.

In spite of the fact that some engineers consider only the first mode, both first and second modes should be basically used. The two DAFs are used to obtain DAFs in different wave directions as follows:

DAF(θ)=(DAF12*cos2(θ)+DAF22*sin2(θ))0.5

Where, θ = wave directions in degree (0, 30, 60, 90, 120, etc.)

These DAF are used as load factor in particular analysis which requires wave load case. In inplace analysis, wave and current are using same load case for certain direction. In addition, these load cases are utilized for generated wind load. Therefore DAFs could not be applied as load factor in it. Wave kinematic factor can be revised by multiplying it with DAF so that current and wind are not increasing. For conservative reason, some engineers are still using DAF as load factor in environmental load case.

BNK