Wave Forces On Pile Sections Due To Irregular And Regular Waves
Authors: Susbielles G.G. , Van Den Bunt J.R. , Deleuil G.G. , Michel D.G. 
Published: October 2008

Wave action on pile-sections and entire piles has been studied. Twenty-nine waves and ten spectra were generated and the corresponding forces were measured. The obstacle was a single vertical pile with three force-meters located at different levels. For regular waves, most of the results can be justified by use of CD and CM values from the Keulegan and carpenter relationship.

For irregular waves, a good fit between measured data and computed values was obtained for spectral analysis (Borgman method). Hydrodynamic coefficient values are consistent with the corresponding results for regular waves. No positive conclusion can be drawn from a deterministic analysis of random isolated waves.

Orbital velocity measurements are of great help, and their absolute necessity for interpretating full-scale studies is quite obvious.

INTRODUCTION

Since 1947, offshore platforms have been installed in the Gulf of Mexico. Wave-action on these structures has always been a matter of concern for design engineers. Wave-force computations were very questionable. For big diameter piles, forces are now known with satisfactory precision. If the diameter decreases enough to necessitate calculation of drag forces (due to eddy-streets), few available methods provide good approximations.

The Morison-O'Brien formula is currently used to remedy this situation but the output depends on orbital velocity and acceleration of water particules, and two coefficients CM and CD (CM, added mass, CD drag) are a function of many parameters. (Mathematical equation available in full paper)

Numerous model tests and in-situ measurement programs were made to test the validity of Morison's formula. Precise measurements were almost impossible to obtain and several major difficulties had to be overcome. Phenomena were note reproducible and some important parameters (such as orbital velocities) were unobtainable. Doubt remains due to the scattering of CM and CD values for the same obstacle submitted to the same wave.

The series of model tests performed with the equipment of the Delft Hydraulics Laboratory cannot answer all listed questions because natural phenomena are necessarily uncertain. It does prepare for full scale measurements.

For this reason the following quantities were checked :Horizontal component of the force on a pile-section. Several force-meters were attached to a single vertical pile.Horizontal component of total force acting on a pile. Measurements performed with regular waves were repeated with random waves to ensure adequate comparisons.

After a brief description of the experimental design, with a list of regular waves and spectra characteristics, some of the results will be presented and discussed. Although their interpretation was completed for regular waves, the interpretation for random waves is still incomplete and a comparison between the two series must be further elaborated.

EXPERIMENTAL DESIGN

General

The investigation concentrated on a single cylindrical pile in long-crested waves. The diameter of the pile was 1.0 m, the water depth 20.m (full scale). The pile extended from the bottom to a point above the highest wave-crest generated during the tests. Force meters of 0.6 m were located at 4.0, 10.0 and l6.m above-the bottom.