The Earthquake Engineering Online Archive

Treatment of non-linear drag forces acting on offshore platforms

Dao, Bui Van; Penzien, Joseph

UCB/EERC-80/13, Earthquake Engineering Research Center, University of California, Berkeley, 1980-05, 128 pages (540/D28/1980)

The hydrodynamic drag forces acting on an offshore platform, as described by the Morison equation, are nonlinear and are coupled to the response of the structure. Using this formulation as a basis, comparisons are made with the drag forces derived from two different approximations: (1) a method which uncouples the drag forces from structural response, but retains their basic nonlinear character; and (2) the standard method of equivalent linearization. Drag forces are determined by the three different formulations mentioned above for a simple cylindrical single degree-of-freedom system, having a fundamental period and damping ratio comparable to real offshore platforms. Response to both harmonic and random wave motions are presented in the form of steady-state amplitudes and extreme value distributions, respectively. The adequacy of each method is discussed.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-80-13.pdf (11 MB)