The dynamic response of symmetric fixed-type offshore structures in deep water under
the action of random wind generated sea waves is investigated. The structure is
idealized as a discrete mass system consisting of set of nodal mass points interconnected
by linear elastic beam elements. The hydrodynarnic forces are computed according to
Morison's formula and the Pierson-Moskowitz wave spectrum is used to simulate
random waves. The analysis is carried out in the time domain to overcome the
nonlinearity in the drag force. This procedure avoids the complexity of the available
equivalent linearization techniques. For low wind speed the response spectntm is a
narrow band process central central frequency near the natural frequency of the structure
for the first mode which is the opposite for the case of shallow water where, the
existence of resonance is far remote. But, increasing wind speed shifts the energy in the
response spectrum toward lower frequency. Results include time histories for the
velocity, accelcration of the excitation force, deck displacement and response spectral
densities for different wind speed. |
