The selection or specification of environmental criteria is one of the crucial steps in engineering offshore structures. In the various offshore areas the engineer is confronted with combinations of forces resulting from wind, waves, currents, earthquakes, ice, and submarine slides. Often, the selection of criteria for these forces must be done with scanty information. In the light of the 'unknowns and the natural variabilities of the environment, the rational selection of design criteria is not a simple problem. What has been termed as the "design criteria muddle" is one of the most pressing and controversial topics in both mobile rig and permanent platform design for newly developing offshore areas.
This paper attempts to develop a philosophy, logic, and computational framework in which design criteria can be selected, specified, or at least discussed. Two primary concepts are involved:
Uncertainty, variability and the resultant hazards of potential platform failures caused by the environment are an unavoidable reality. The hazards can only be minimized or, more realistically, the safety of the structures optimized to a degree consistent with available information and the justified investment of money.
The selection of environmental criteria should include an integrated consideration of the function, strength, reliability, and cost of the structures which will operate in the environment.
This paper is written for the engineer, platform owner, or design code writer faced with deciding what environmental criteria will be used for offshore platform designs. It is written within the context of presently applied design and analysis techniques.
The paper has a simple message: a logical and quantitative method is available to assist in the selection of design criteria. The method is not a panacea and does not represent scientific precision. It is limited by the data, engineering, experience, and judgement which are input.
To assist in expressing the concepts involved, a specific example will be used. A template-type, drilling and production platform will be the study model (see Figure 1). The platform will be located in 300 feet of water. It will be assumed that the critical forces on the platform are those lateral forces developed by waves. Consideration of different platform types and other environmental parameters is within the scope of the method discussed.
Generally, one has to be dissatisfied with the present state of affairs to warrant consideration of a change from that state. At present, the most common method of selecting design wave heights is the Lifetime Plus Method. Using this method, one decides on the maximum service life of the platform, and then designs for a wave height that is the most probable maximum that will be experienced during the life. If experience, lack of experience, or intuition indicates that the design wave is not big enough, the "plus" comes in and larger design waves are specified.