Eike Lehmann

Marine Structural Analysis

Structural analysis has been integrated into the design process of marine structures to a far greater extent than conventional shipbuilding. The effectiveness of the design of, for example, a semisubmersible or a tension leg platform, significantly depends on its design weight. Design weight, in turn, depends on the scantlings of the structural members obtained from structural analysis based on a rational approach. An example may clarify this.

Dimensioning of Marine Steel Structures

Dimensioning involves, on the one hand, a multiplicity of regulations, guidelines and standards, and on the other, the application of rational analysis and evaluation procedures, which have been dealt with in detail in Chap. 3–6. The method of dimensioning on the basis of regulations, often used in design practice mainly to simplify the official approval process, is in many ways founded on rational analysis and evaluation procedures. Generally, it differs from rational procedures not in its principle, but in its form, which is that of a greatly simplified, and therefore at times confusing or even incomprehensible analysis procedure. For this reason, in this chapter we wish to discuss some regulations which are relevant to the dimensioning of marine structures, using selected examples from dimensioning practice, to demonstrate the relationship with the analysis and evaluation procedures already discussed.

Environmental Conditions Affecting Marine Structures

As outlined in some detail in Chap. 2, in comparison with similar structures onshore, the characteristics peculiar to marine structures are closely limited by the hostile environmental conditions at sea, particularly the relatively high, horizontal seaway loadings, which are frequently ten times higher than wind or current loads. As far as seismic loads are concerned, there is actually a special physical feature of marine structures by which hydrodynamic masses and damping determine dynamic behaviour. However, the earthquake problem is not specific to marine technology, so it will not be included in this study. This section is a rational evaluation of the marine environment represented by waves, wind and currents.

Evaluation of Marine Structures

As mentioned in the introduction to Chap. 5, we will concentrate mainly on evaluating the characteristics of marine structures determined by the sea, and on some effects of wind and currents. While in Chap. 5 we dealt with stochastic analysis of the marine environment itself represented by these influences, in this chapter we analyse its effects on marine structures. Because the seaway could be considered with classical stochastic (probabilistic-statistical) methods within the framework of short- or long-term analysis (see Sect. 5.2.2.1, 5.2.2.2), we will also use this concept here for analysing the seaway effects, which can be considered specifically as decisive for the dimensioning of marine structures.