EXTREME HYDRODYNAMIC LOAD CALCULATIONS FOR FIXED STEEL STRUCTURES
P.R. de Jong* and J. Vugts, Delft University of Technology, *Presently working for Allseas
O.T. Gudmestad, Statoil, Stavanger, Norway
ABSTRACT
This paper discusses the expected differences between the planned ISO code for design of offshore structures and the present Standard Norwegian Practice (SNP), concerning the extreme hydrodynamic design load calculation for fixed steel space frame structures. Since the ISO code is expected to be similar to the API RP2A LRFD code [16], the provisions of API RP2A LRFD are used to represent the ISO standard. It should be noted that the new ISO code may include New Wave theory [11], [12] in addition to the wave theories recommended by the API.
Design loads and associated failure probabilities resulting from the application of the code provisions are compared for a typical North Sea structure, the Europipe riser platform 16/ll-E.
APPLICATDN OF THE DETERMINISTIC- SPECTRAL FATIGUE ANALYSIS METHOD USING PLATFORMS OPERATING IN NON-LINEAR WAVE LOADING CONDITIONS
Neil W M Bishop and Qi a Feng, Department of Mechanical Engineering, University College London, London WC1E7JE, United Kingdom
Mike G Kirkwood and Tim E Turner, Gas Research Centre, British Gas plc, Loughborough LEll 3QU, United Kingdom
ABSTRACT
It has previously been proposed [1,2] that the so-called deterministic-spectral approach may have advantages over the deterministic approach for the fatigue analysis of shallow water fixed jacket platforms. In particular it is asserted that the deterministic-spectral approach can handle the random nature of the sea more adequately. The deterministic approach is currently the most widely used method for such platforms.
In these earlier studies it was shown that the deterministic-spectral method can be
applied successfully to certain types of shallow water jacket structures in the Rough Field area of the North Sea. Since this earlier study much more work has been done verifying the approach. In particular an alternative jacket platform in the Morecambe Bay area h as been analysed which exhibits, because of its shallower water depth, more severe non-linear tendencies. Results for this jacket are presented in the paper.
Apparent discrepancies between the results obtained from the deterministic approach and those obtained from the deterministic-spectral approach have been noticed. As a result of these discrepancies, a more detailed and sophisticated transient analysis (when compared to previous analyses) has been performed which utilises different methods of decomposing the random sea surface elevation into its velocity, acceleration and associated force components. Results using this alternative method are presented.
As a further aid to verifying and calibrating the technique a procedure for in-situ monitoring of the Morecambe Bay jacket is currently being developed and an outline of this monitoring work is presented.
