Hydrodynamic loading and design aspects of offshore jacket platforms
The design aspects of offshore jacket structures are presented and discussed with a special emphasis on the different factors which affect wave loading calculations for these structures. An up-to-date review of a large amount of data on the hydrodynamic coefficients obtained from Laboratory experiments and wave projects is presented and the main data are tabulated. To assess the different aspects of the wave loading a set of computer programs were developed and used to perform various comparative studies for the existing methods of wave loading estimation. The analysis of the wave loading was carried out using a jacket structure of 119 members having 73m x 73m base representing a typical offshore platform, assumed to be working in 150m of water. The general method of wave loading calculation is based on Morison's equation taking into account the phase differences between the velocities and accelerations of the wave particles. The relative positions of the different members in space and time when the wave passes through the jacket were also considereG. Besides the drag and inertia forces, the lift (transverse) forces are also taken into account. The kinematics of the flow can be determined using Airy (linear) wave theory, Stokes 2nd order theory or Stokes 5th order theory. Constant drag and inertia coefficients (CD, CM), as recommended by Lloyd's Register of Shipping (LR), Det Norkse Veritas (DnV) and Bureau Veritas (I3V),can be used. Alternatively, variable hydrodynamic coefficients (CD, CM, CL) from Sarpkaya's experimental data for smooth and rough cylinders can be used. The drag interference effect and the current effect can be included in the calculations. Various interpretations as to how to apply Morison's equation in the design were examined which have shown the importance of taking full account of both the relative positions in space and time of the different members of the structure as well as the phase relationships in the wave. A comparison was made between the results of calculations using the recommended coefficients (CD, CM) of LR, DnV and BV which has shown that even small variations in these coefficients leads to appreciable differences in the loading estimation of up to 45%. The approach using variable coefficients (Sarpkaya's data), which are related to the local Reynolds number (Re) and Keulegan-Carpenter number (K) at the different points of the structure, was compared with the method of adopting constant coefficients (as recommended by LR) showed differences up to 26% in the wave loading estimation between the two methods. The effects of surface roughness, as well as the transverse (lift) forces, on the wave loading were also investigated and found to be very significant (eg 43% to 56% in the surge force) and should be considered in design. Three wave theories (Airy, Stokes 2nd order, Stokes 5th order) were compared in terms of wave profile, horizontal and vertical velocities and accelerations. The results have shown that the differences in predicting the wave kinematics by Airy and Stokes theories are large. The wave forces on the individual members as well as the total forces and moments on the complete structure calculated by the fifth order theory, showed 30-60% differences when compared with the results based on Airy theory. The experimental data on the interference effect between the cylindrical members were reviewed. The effect on the jacket loading was examined using some experimental data and found to be 6-9% reduction in the loading for rough cylinders. However, more experimental investigations are required in this area to deal with this problem properly. The effect of current speed and direction on the wave loading was examined by the commonly used practice of adding the velocity of current vectorially to the wave particle velocity when calculating the drag and lift forces. The results showed that the total forces and moments could be increased by 16-37% for a/1 mls current in the direction of the wave. Several static analyses of the jacket were performed using constant and variable hydrodynamic coefficients and two wave theories (Airy and Stokes 5th order theory). The initial differences in the wave loading due to the different coefficients and wave theories appeared again as appreciable differences in the maximum stress on the different members. This supported the necessity of calculating the wave loading accurately from the beginning. A general review of the reliability analysis method as applied to jacket structures indicated that the modelling of the wave loading needs further improvements to take account of the large uncertainties in the loading especially due to the hydrodynamic coefficients and non-linear loads.