Special loads

In addition to the aero- and hydrodynamic loads discussed in sections 4.5 and 4.6 there are still other loads that may act on members of the structure around the interface of air and water. These fall into two categories, i.e. special hydrodynamic loads due to:

- intermittent inundation of members - wave breaking

- wave slam - wave slap

and loads due to ice, which may be distinguished into loads due to: - sheet ice

- ice floes - icebergs.

All these loads require detailed specialist treatment. Discussion of these loads is beyond the scope of this handbook; for more information the reader is referred to the literature. It should be noted though that, generally speaking, most of these topics are far from conclusively resolved. Let it suffice here to say a few words about each of them.

Horizontal members located around the still water line at an elevation between the maximum wave crest and the minimum wave trough of the design storm may intermittently be in air and submerged. Consequently they will only experience hydrodynamic loading during a part of the wave cycle. When they are in air these members are only subjected to their own weight; when they are submerged they are loaded by the net effect of their own weight, upward buoyancy loads and both horizontal and vertical wave loading due to the varying wave kinematics during the passage of a wave. This is a discontinuous and non-linear loading situation. Especially the repeated stress variations that are experienced at the top and the bottom of the member (the 12 and 6 o’clock positions in the member’s cross section) as a result of self-weight, buoyancy and vertical water particle velocities may cause fatigue damage. This is the reason why horizontal framing levels should be located high enough above and low enough below the mean still water level to cause only occasional submergence or emergence, respectively, thus limiting both the number and the magnitude of such stress cycles.

The kinematics in breaking waves are poorly understood, and as the kinematics are the prime factors that control wave loads the loads due to breaking waves are also quite uncertain. Additionally, there are different types of breaking waves, while furthermore the air entrained in spilling or plunging breakers may greatly affect the loads experienced.

Loads due to wave slam and wave slap are impulsive (impact) type loads when an object is suddenly immersed in water. Wave slamming is generally associated with the vertical immersion at some velocity. The classical example is when the bow of a ship re-enters the water after having emerged from it due to excessive relative vertical motion. The bottom of the bow section may then be subjected to significant impact loads. Analogously, in case of a space frame structure wave slamming is associated with fixed horizontal members in a frame that is more or less perpendicular to the wave direction and a water surface that is rapidly

Slamming loads are thus (near to) vertical. Wave slap, on the other hand, is generally associated with loading that may occur during the breaking of waves on members or (flat) surfaces that also lie in a plane perpendicular to the wave direction. Wave slap is likely to be most severe when the objects are (near to) vertical, but it may also happen on members and surfaces with other orientations. The direction of loading due to wave slap is thus often approximately horizontal, but may vary with the orientation of the object. The hydrodynamics of wave slam and wave slap are very complex and again not well understood. Furthermore, as the loading is impulsive in nature with very short rise times of milliseconds and overall short durations, the phenomena involved are affected by the interaction between the loading and the dynamic response of the structure. Therefore loading and structural behaviour are interlinked, and the problem is actually more a dynamic response problem than a pure loading problem. Ice forces are similarly very difficult to predict. They depend on the mechanical properties of the ice (which may vary widely with conditions and the type of ice), on the configuration of the ice cover on the water (sheet ice, ice floes), on the current condition and on the failure mechanism of the ice as it is broken by the structure (compression, shear or bending failures). Furthermore, there is firm evidence that the build-up of ice loads due to the moving ice or their release when the ice breaks may be accompanied by significant dynamic response. Icebergs are massive floating bodies of irregular shape, with more than 90% of their volume under water. Any structure is highly unlikely to survive a collision with an iceberg. The usual strategy in offshore areas where icebergs are a hazard is therefore to try and avoid this from happening rather than to try and resist icebergs. For fixed structures this may be achieved by warning systems and altering the course of the iceberg by towing it out of the structure’s path, while floating structures may have quick release mechanisms to allow them to move rapidly off location.

Chapter 4- The offshore environment and environmental loading 4-68

4.8 References

4.1-1 Lamb, T.W. and Whitman, R.V., Soil Mechanics; Series in Soil Engineering, John Wiley & Sons, New York, 1969.

4.1-2 Verruijt, A., Grondmechanica; Delftse Uitgevers Maatschappij b.v., 3e druk, 1990; ISBN 90 6562 045 1.

4.1-3 Verruijt, A., Offshore Soil Mechanics; lecture notes, Delft University of Technology,

Faculty of Civil Engineering, 1992.

4.2-1 Hoven, I. van der, Power spectrum of horizontal wind speed in the frequency range