Wave directionality and extreme wave loading

Wave directionality and extreme wave loading

Van Weert, P.J.G.

Harland, L.A. (mentor)
Holthuijsen, L.H. (mentor)
Tromans, P.S. (mentor)
Battjes, J.A. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

1997-12-01

For an accurate description of the wave loading on an offshore structure, detailed knowledge of the wave-induced kinematics is a precondition. To fully describe the ocean environment is, however, a complex problem. First of all because of the fact that wave action is a random process. The second complication is wave directionality. With wave directionality, or wave spreading, we refer to the observation that the wave energy propagates in numerous different directions, which is the reason for the shortcrestedness of the ocean surface. However, standard engineering methods (as recommended by design guidelines such as API-LRFD, DNV) are based on uni-directional deterministic wave models, thereby disregarding both above mentioned aspects. The reason being that full spread sea simulations are too complex for the design of space frame structures. Therefore, in standard recipes the wave spreading is accounted for by using a kinematics reduction factor. The major limitation of this reduction factor is, that it is only valid for wave loading on a single vertical stick. For structures that are relatively large on a wave length scale, application of this factor can introduce considerable errors. This is caused by the fact that the kinematics reduction factor does not incorporate effects of shortcrestedness. The most probable surface elevation in the vicinity of a large directionally spread wave crest, has been compared to the most probable surface elevation in the vicinity of a crest of identical height, resulting from a uni-directional wave model. This reveals that, in the mean wave direction, a multidirectional wave is on average higher than a uni-directional wave. In a transverse direction, which is perpendicular to the mean wave direction, a multi-directional wave model results in a surface elevation which is on average lower. This has important consequences for the wave loading on a large offshore structure, since the kinematics reduction factor does not take into account the systematic difference in wave height between both models. Application of the kinematics reduction factor which is only valid for a single stick, leads to underestimation of wave forces when a structure is relatively large in the mean wave direction, whereas it leads to overestimation of wave forces for structures which are relatively large in a transverse direction. A method is proposed for calculation of a modified value of the kinematics reduction factor for any arbitrary structure, that does take into account effects of shortcrestedness. The method is based on the following assumptions. Firstly it is assumed that large wave forces are caused by large wave crests. The location of a large wave crest that maximises the loading on the total structure can be calculated. Dependent on the position of the columns of a structure relative to the wave crest, a local reduction factor can be calculated for each column. The global reduction factor for the entire structure is found by averaging the local reduction factors. Each column contributes to the global reduction factor according to its contribution to the total load on the structure. Application of the method gives good results. If the reduction factor found with the method is compared to the factors that is found from numerical computations, the difference is in the order of magnitude of I - 2 %, versus 5 - 6 % errors for conventional reduction factors. It should be noted that, for drag dominated conditions, a 5% difference in the reduction factor will induce a 10% error in the design wave load on an offshore structure.

wave load
directional spectrum
wave spreading

http://resolver.tudelft.nl/uuid:c193a166-cb39-43e2-9d97-f87e7f618e90

Student theses

master thesis

(c) 1997 Van Weert, P.J.G.