Selection of Environmental Criteria For Offshore Platform Design
Authors: Bea R.G. 
Published: December 2007

How high should the design wave be? What magnitude of earthquake should be allowed for? Such questions daily confront the platform designer, the owner, and the code -writer. Here is a philosophy, logic, and computational framework in which design criteria can be examined and selected.

Introduction

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 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. panacea and does not represent scientific precision. It is limited by the data, engineering, experience, and judgment that attend it. To assist in expressing the concepts involved, a specific example will be used. A template-type drilling and production platform is the study model (see Fig. 1). The platform sits in 300 ft of water. It is assumed that the critical forces on the platform are those lateral forces developed by waves. Different platform types and other environmental factors may platform types and other environmental factors may be considered in the method discussed.

Lifetime-Plus Method

Generally, one must be dissatisfied with existing conditions before he will consider changing them. At present, the most common method of selecting design 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 the most probable maximum wave heights that will be experienced during that 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. The Lifetime-Plus method has served the offshore industry long and well. However, it is known to suffer from at least three major deficiencies:It fails to quantitatively recognize the possibilities and consequences of experiencing waves possibilities and consequences of experiencing waves both larger and smaller than that used in design.It fails to quantitatively recognize the strengths and weaknesses in platforms intended to resist the wave forces.It fails to place any limits on the "plus" part of the method. In general, one wants benefits (failures prevented) to equal or exceed the costs.

The prevented) to equal or exceed the costs. The Lifetime-Plus method has no means for recognizing benefits, costs, or the point at which a balance between those is reached. Dissatisfaction alone seldom justifies change. One must have something to change to that will be an improvement. The method outlined in the next section is proposed as a significant improvement over the Lifetime-Plus method.

Reliability Analysis Method

The method to be discussed in the remainder of this paper will be called a Reliability Analysis. Such an paper will be called a Reliability Analysis. Such an analysis is a logic framework in which the uncertain and variable aspects of loads and platform strengths are quantified and examined.

JPT

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