The objective of this paper is to review the available computerized structural analysis techniques to predict the response of offshore platform structures under all the important dynamic environmental loads. The analysis techniques and mathematical models are outlined, the assumptions and limitations are discussed, and possible extensions and improvements are indicated.
The first dynamic environment treated in this paper is that caused by ocean wave action. The advantages and shortcomings of the deterministic and random wave analysis techniques are discussed and illustrated by some comparative results on a deep-water bottom-fixed offshore platform.
The second loading environment discussed in some detail in this paper is that caused by earthquake tremors. The formulation of mathematical models, selection of significant normal modes, and techniques for combining of modal responses are discussed.
It has been recognized that as offshore oil platforms structures are placed in deeper water, the effects of environmental loads, such as ocean waves and earthquakes, will have to be predicted more accurately to ensure a safe design for these structures. The need for more accurate prediction of dynamic response of the offshore platform due to environmental loads is emphasized by the following considerations:
the taller slender structures are more susceptible to dynamic magnification of response;
the deeper water platforms represent a significantly higher investment; and
present public policy emphasizes protection of environment against oil spills.
The need for more accurate prediction of response, together with the dynamic nature of the environmental loads, calls for dynamic analysis methods instead of the static analysis methods which were used in the design of shallow platform structures in the Gulf of Mexico. However, a mere specification of dynamic analysis does not really define the method of analysis, since there exists a broad band of dynamic elastic analysis methods for structures under transient loading. This band of methods ranges from a response spectrum analysis of a one-dynamic degree-of-freedom system to random vibration analysis of complex multi degree of freedom systems. This paper will examine the applicability and utility of the available dynamic analysis methods to predict the dynamic response of offshore platform structures.
The transient loading problems on offshore structures may be classified into two groups according to the type of loading:
External time-dependent loads applied to various points on the structure which are anchored to an immovable base. An example of this class of problems is wave loading imposed on an offshore platform and wind loading imposed on the exposed portions of ocean structure.
Transient motion of the base of a structure which is free of external applied loads. Examples of this class of problems are earthquake loading on offshore structures in still water, shock loading of internal equipment of ships attacked by underwater explosions, and shock loading on equipment inside any ocean structure subjected to wave motion and slamming.