Soil response aspects, related to the seismic design of offshore platforms, are summarized and discussed. Among these aspects, special attention is given to ground shaking and to soil failure potential. Analytical methods that may be used in evaluating soil response and in establishing seismic design parameters are described.
Fixed offshore platforms are being designed and constructed in many parts of the world, including areas of potential seismic activity. Soil response considerations in these areas can be an important part of the seismic design of the platform and its foundations. Very little, if any, observational data are available regarding behavior of such platforms during strong earthquakes. However, many of the considerations are similar to those for structures located on land.
Important factors that influence soil response, for offshore or onshore structures, include: the regional seismicity and the distance of the structure to capable faults; and the type, physical characteristics, and geometric distribution of the foundation soils. A factor that is specific to the offshore environment is the presence of a body of water above the soil deposit. The effects of ocean waves deserve special consideration, due to the importance they may have in determining soil properties, and also because of the general similarities between wave and earthquake motions.
This paper presents a review of geotechnical aspects related to the seismic design of offshore platforms. Only template type structures supported on piles are considered, but most aspects of the basic parameters-discussed in this paper would also be applicable to structures placed directly on the sea floor. Representative examples are presented at the end of the paper for illustration purposes.
The occurrence of an earthquake results in energy propagating away from the earthquake source in the form of seismic waves traveling in the earth's crust. Part of this energy is transmitted to the soil materials at a site; from the soil, the energy is transmitted to structures through their foundations. Generally, only horizontal motions need to be considered, because:
horizontal seismic forces are more critical for the design than vertical forces, and
horizontal ground accelerations during earthquakes are usually larger than vertical accelerations (1. 2).
For the typical situation sketched in Fig. la, in which all soil and rock layers are horizontal, it is reasonable, to assume that horizontal motions are caused mainly by vertically propagating shear waves (3. 4). The rock acceleration time history can be considered as the input to the system. For free field conditions (i.e. far from the platform structure), the soil motions will be determined only-by the input and by the characteristics of the soil. The presence and thickness of water above sea floor will not influence ground response; water cannot transmit shear, and therefore the sea floor will behave essentially as a free surface. Therefore, the analysis reduces to that of a shear beam of soil subjected to a base excitation.
Other waves may be contributing to horizontal ground motions besides vertically propagating shear waves.
