Abstract

The results of a small scale experimental study to investigate the wave run-up and wave forces on a truncated cylinder, representing a single TLP leg, are presented. The investigation examines the wave run-up and in-line wave force conditions resulting from both regular and random waves. Surface adhering wave gages and a shear type load-cell design were developed for use in the study. Time series data around the cylinder and the sensitivity of the observations to wave steepness and scatter parameter are presented. For the random wave cases, attention is focused upon the relationship between the front and back of the truncated cylinder. Information on the transfer function, coherence, phase and total force are presented and discussed.

Introduction

The prediction of wave run-up and the associated wave forces on coastal structures and deepwater platforms has been of interest to the offshore community for many years. During this time, the engineering applications have included the design of piers, lighthouses, conical islands, fixed offshore platforms, and more recently the design of tension leg platforms (TLP's). As the focus for engineering design has changed the analytical and numerical tools available have significantly improved (Kim and Yue, 1989). However, these tools have been calibrated with only a limited amount of laboratory and field data.

The objectives of this study were to:

  1. to provide new laboratory data on truncated cylinders subject to regular and random waves, and

  2. to examine this data utilizing variety of statistical analysis tools in an attempt to provide new insight into the wave run-up problem.

In what follows, a perspective of where the results from this study fit into the current open-literature is provided through a brief review of previous experimental studies. Next, a description of the experimental setup and procedures is presented. The results for the regular and random wave cases are presented and discussed in sequence in separate subsections.

Previous Experimental Studies

Previous experimental studies have generally focused upon 1.) very large diameter structures in shallow water including man-made islands, and 2.) single and pile groups extending from the seafloor through the free-surface. Table 1 presents a comparative view of many of the earlier studies. Of particular interest are the range of parameters which have been examined.

The first three studies, listed in Table 1, are concerned with the wave elevation distribution around large circular islands and lighthouses. The tests conducted were for a rather limited range of wave interaction parameters with a scatter parameter greater than 0.6 in all cases. However results from all three investigations showed good agreement with linear diffraction analysis. Isaacson (1978) proposed a cnoidal wave theory approach to estimating the run-up on large circular cylinders in shallow water and showed that, though the cnoidal theory underestimated the experimentally measured wave run-up, it provided better estimates than linear diffraction analysis.

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