The philosophy and background of seismic design provisions for buildings are discussed. The applicability of design assumptions and provisions used for buildings to extremely tall, slender, and flexible offshore structures is reviewed. Offshore structures are much bigger in plan than most buildings and do not have a common foundation. Hence they may respond in torsion as well as translation to seismic motions. Moreover they are surrounded by water that causes different types of actions and response to dynamic inputs. The significance and possible effects on offshore structures of major earthquake motion characteristics are presented. Areas that require detailed study are described.
Earthquakes are of special interest because they can affect most components of a structural system and the equipment supported thereon. Earthquakes occur at irregular time intervals and transmit transient dynamic ground motions which can induce large forces in man-made facilities. The response of structures and their contents to such dynamic motion is a very complex phenomenon.
There has been a tendency in most parts of the United States to view earthquakes as a California problem. However, over 70 million people in 32 states are subject to major and moderate earthquake risks, that is, earthquake motions with effective peak ground accelerations exceeding 0.l0g. If it is assumed that the seismic design regionalization maps recently developed by the Applied Technology Council (ATC) can be extended offshore, about 41 percent of the East Coast, 80 percent of the West Coast excluding Alaska, all of the Gulf of Alaska including the Aleutians, the Bering Sea coast up to about 60 degrees north latitude about one half of Hawaii, and much of Puerto Rico are subject, to effective peak ground accelerations exceeding 0.10g.1
Until recently few offshore structures have been built in active earthquake areas. Most offshore structures have been constructed in the Gulf of Mexico, a re1atively Quiescent seismic area. A few structures have been built off the southern California coast and in Cook Inlet, Alaska. For the Cook Inlet structures, the forces exerted by ice packs were more severe than those from earthquakes. The design of most of the southern California structures followed the Uniform Building Code (UBC) requirements for buildings, with each company using earthquake engineering consultants, as appropriate.
The increasing energy crunch and impending future shortages have sparked an upsurge in interest in developing the potential oil reserves existing in the outer continent shelf (OCS). Activity is largely centered in the Gulf of Mexico, southern California coast, Gulf of Alaska, and the mid-Atlantic coast. Of these areas, those off southern California and the Gulf of Alaska are in seismically active areas. The earthquake problem is not unique to the United States, and offshore developments near New Zealand, Indonesia, and the Philippines are also subject to major earthquake motions.