Wave loads on cylinders have predominantly been determined by model tests in 2-D (uni-directional or long-crested) waves, and such tests form the basis for most practical offshore design work, directly or through various offshore design codes. The real ocean waves are normally 3-D (multi-directional or shortcrested) waves, and several investigations have shown that the horizontal wave forces on cylinders are significantly smaller in 3-D than in similar 2-D waves.
The question discussed here is the following: Is it safe to use the reduced 3-D loads in practical offshore design? With the present state of knowledge an attempted answer is: 3-D loads can be used in fatigue load design, but 2-D loads should still be used in extreme load design, because extreme waves evidently in some cases may have a pronounced 2-D character.
Historically, wave loads on offshore structural elements, of which circular cylinders are the most important, have predominantly been determined by model tests in 2-D wave basins, or in flume tanks with simulated wave motion instead of surface waves. Very few measurements have been carried out in real 3-D waves. The 2-D test results have been, and still are, applied in practical design, either directly, or by their use in verification and calibration of theoretical methods and offshore design codes.
Since 1980 a growing number of 3-D wave basins have been built and made available for the offshore industry, see e.g. Salter (1981), Huse and Tsrum (1 981), Aage and Sand (19841, and Isaacson and Nwogu (1989). Published results from these basins confirm the hypothesis that wave loads on cylinders in 3-D waves are significantly smaller than in 2-D waves with identical spectra or even identical wave elevation time series, see e.g. Isaacson and Nwogu (1 989), Aage et al. (19891, Bryndum et al. (1990).
With such knowledge at hand, the obvious question for the offshore engineer is then: Will it be safe to use the reduced 3-D loads in practical design of offshore structures? The answer to this question is not as simple as it sounds, however. At least the following three problems must be addressed:
Firstly, it must be confirmed that the load reduction in 3-D waves is a real phenomenon, not caused by theoretical or experimental errors ("the hydrodynamic problem").
Secondly, it must be known whether the real ocean waves are in fact 3-D, or if they in some cases can be 2-D waves ("the oceanographic problem").
Thirdly, it must be confirmed that the load reduction in 3-D waves has not already been taken into account as an implicit safety factor in the offshore design codes ("the offshore code problem").
The three problems will be discussed In the following sections.
Speaking about a load reduction in 3-D waves relative to 2-D waves is actually to turn the problem upside down. Ocean waves are generally 3-D waves, so we should more correctly speak of an increased 2-D laboratory wave load relative to the real 3-D wave load.