In this paper the influence of the geometry of gravity type platforms on wave loads is discussed. Results are given of wave load computations for a systematic series of platforms in a water depth of 150 metre. These results apply to regular as well as to irregular waves both for survival and operational conditions, and show clearly the possibility of geometry optimization with respect to hydrodynamic loads.
One of the most important problems with regard to the design of offshore gravity platforms is the stability of the structure. To ascertain a stable structure, the following conditions must be met under extreme loading conditions:
the apparent weight of the structure must be greater than the maximum vertical force (upwards), to prevent that the structure will be lifted the bearing capacity of the soil must be sufficient to resist the minimum vertical force (or maximum downwards directed force) as well as the pressure due to the maximum overturning moment.
to prevent tilting of the platform, the overturning moment may not exceed a certain limit, regardless of the soil properties the soil must provide sufficient friction to withstand the maximum horizontal loads on the platform.
Besides the extreme forces, also dynamic loads in operational conditions must be considered: these less severe, but more frequently occurring loads may be of importance with regard to liquefaction of sand or weakening of clay. The total loads on the structure consist of static loads (weight of the platform) and of the environmental loads due to waves, wind and current, which have a dynamic character. Since in the environmental loads the influence of the waves is overwhelming, it will be clear that optimization of a gravity platform means mainly minimization of the wave loads in survival and operational conditions. Of course, for a complete optimization a lot of other aspects should be taken into consideration, as there are strength of the structure, economics, stability during towing and installation, and limitations placed by building site and towing route. These aspects, however, are beyond the scope of this paper, and so we will focus on the overall hydrodynamic loads. For the sake of convenience, we will consider only gravity structures consisting of large caisson-type base, with a number of columns supporting the work platform (see Figure 1). The majority of the platforms now under construction or being designed is of this type. In the following sections we will discuss the nature of the hydrodynamic loads and methods to calculate them. Further, results will be presents of systematic computations, which show the sensitivity of the wave loads and resulting soil pressures for changes in the geometry of a particular structure.
The wave forces on the columns and on the base of a gravity type platform as depicted in Figure 1 are usually determined separately. For the calculation of wave loads on the large caisson inertial as well as gravitational effects much be taken into account, while the influence of viscosity is negligible.