Soil-Response Considerations in Seismic Design of Offshore Platforms
Authors: Idriss I.M. , Dobry R. , Power M.S. 
Published: December 2007

Soil-response considerations in areas of potential seismic activity can be important to the design of an offshore platform and its foundations. Many of these considerations are similar to those for onshore structures. Here, the geotechnical aspects of seismic design of offshore platforms are reviewed, with special attention to template-type structures supported on piles.

Introduction

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 part of the seismic design of the platform and its foundations. Very little observational data are available regarding the 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 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 because of 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 discussed in the paper also would be applicable to gravity-type structures placed directly on the sea floor. Examples are presented at the end of the paper for illustration.

Geotechnical Aspects

The occurrence of an earthquake results in energy propagating away from the earthquake source in the form of 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. In many cases, only horizontal motions need to be considered because (1) horizontal ground accelerations during earthquakes are usually larger than vertical accelerations, and (2) horizontal seismic forces are often more critical for the design than vertical forces. In some cases, however, vertical motions are equally as important and should be considered.

For the typical situation sketched in Fig. 1, 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. The rock acceleration time history can be considered as the input to the system. For free-field conditions (that is, far from the platform structure), the soil motions will be determined only by the input and the characteristics of the soil. The presence and depth of water above the 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.

Waves other than vertically propagating shear waves may be contributing to horizontal ground motions. These include shear waves propagating at an angle to the vertical and surface waves propagating in a horizontal direction.

JPT

P. 244