This paper describes a study that compared a limit-state design approach with the traditional working stress design approach for fixed steel offshore platforms in terms of member capacity, joint capacity, foundation capacity and fatigue analysis.

Favorable comparisons were made between the limit-state based Danish Offshore Regulations and the current (12th ed.) API RP 2A from a general viewpoint of reliability using safety index (ß) statistical philosophy 1-7 and, in more detail, for a sample platform based on North Sea site conditions.


During the past decade, design codes for fixed steel offshore platforms, particularly European codes, have increasingly addressed the use of a limit-state design as a suggested alternative 1:0 the traditional working stress design. As American Institute of Steel Construction (AISC) and American Petroleum Institute (API) are likewise developing limit-state formats, a comparison of the two types of design and review of the implications of the limit-state design to fixed steel offshore structures are appropriate 1,2,6.

A brief discussion of the limit-state design philosophy is presented and followed with a discussion of application for a particular limit-state code.

The Regulations for Permanent Offshore Steel Construction 8, which is commonly called the Danish Offshore Regulations (DaR), and the current (12th ed.) API RP 2A 9 were compared from a general standpoint of reliability and, also, in more detail for a sample structure based on North Sea site conditions. The detailed comparison investigated numerous members and joints in the deck, jacket and foundation; however, the authors do not pretend to suggest which code is more applicable for a given situation. It is recognized that API RP 2A was not intended for application in the North Sea, but is frequently used in such design.

The conclusion drawn from this comparison should provide designers with indications of what to expect when utilizing limit-state design criteria. The results also provide an indication where current working stress criteria may not provide uniform reliability.

Limit-State Design Philosophy

Limit-state design is probability-based to provide a relatively uniform component reliability by explicitly considering uncertainties in load and resistance. Alternate names for limit-state design are semi-probabilistic design, load factor design, and load and resistance factor design.

The Danish Offshore Regulations (DOR), March 1980, are based on limit-state design philosophy and specify the following structural capacity checking format for each component and each appropriate load combination:


Partial coefficients for resistance and loads from DaR are presented in Tables I and 2, respectively. Two types of failure mode analysis are required in DaR: Type-I failure which is of a ductile nature with additional load carrying capacity (e.g., strain hardening), and Type-II failure which is of a ductile nature without additional load carrying capacity. For comparison with the current API working stress design (whi.ch does not consider Type-II failure), only Type-I failure analysis is discussed in this paper.

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