Extreme wave elevations beneath offshore platforms, second order trapping, and the near flat form of the quadratic transfer functions

J. R. Grice, Paul H Taylor, Rodney Eatock Taylor, Jun Zang, D A G Walker

OAI: oai:purehost.bath.ac.uk:publications/c118812e-7a1e-4b93-9951-fb2afcde058a • DOI: https://doi.org/10.1016/j.compfluid.2015.06.025

Extreme free surface elevations due to wave-structure interactions are investigated to second order using
Quadratic Transfer Functions (QTFs). The near-trapping phenomenon for small arrays of closely spaced
columns is studied for offshore applications, and the excitation of modes by linear and second order interactions
is compared. A simple method for approximating near-trapped mode shapes is shown to give
good results for both linear and second order excitation. Low frequency near-trapped mode shapes are
shown to be very similar whether excited linearly or to second order. Approximating surface elevation
sum QTF matrices as being flat perpendicular to the leading diagonal is investigated as a method for
greatly reducing lengthy QTF calculations. The effect of this approximation on second order surface elevation
calculations is assessed and shown to be reasonably small with realistic geometries for
semi-submersible and tension-leg platforms.

second order QTF approximation wave-structure interactions Near-trapping