Risk Analysis Methodology for Developing Design and Assessment Criteria for Fixed Offshore Structures tg
Authors: De R.S. 
Published: October 2008

ABSTRACT
This paper presents a risk analysis methodology for developing design and assessment criteria for jlxed offshore structures based on consequence of failure. The responsebasedcriteria take into account global properties of the structure, e.g., rate of increase of wave load with wave height and the reserve strength beyond the design load. The longtermenvironmental features are modeled through wave height values in the high return period region that is of importance tostructural collapse. The paper describes how the risk-based j%mework was used in developing the recently proposed API criteria for assessmentof existing plaforms. The application examples should enable development of risk consistent criteria for novel applications, e.g., in different environmental conditions, with specific reserve strength characteristics or wave loading conditions.

INTRODUCTION
The structural reliability framework described in this paper was used in developing the recently drafted API criteria for assessment of existing structures. The related guidelines were issued on April 1, 1995, by the API as part of the draft Supplement 1 for WSD and LRFD versions of API RP2A. 1'2 They are contained in Section 17, Assessment of Existing Platforms. The assessment process, acceptance criteria, andthe underlying bases have been discussed in a number of recent papers?-s This paper shows how the annual failure probabilities and return periods reported in Reference 3 were calculated. The information provided in this paper allows risk consistent extrapolation of the assessment criteria to environmental conditions and special reserve strength conditions not addressed in the API work. Consequence of failure in terms of life safety andenvironmental impact forms the basis for the proposed assessment criteria. Another API effort is currently underway to develop consequence-based design criteria for new and reused offshore structures. The same reliability framework is being used to evaluate risk levels associated with the proposed consequence-based criteria.

RELIABILITY FRAMEWORK
The reliability framework focuses on the system failure rather than component failure. The main features are as follows:Models only the global features of the stmcture and the environment using a small number of parameters.Suitable for criterion development and calibration risk model uses lognormal probability models for which closed-form solutions are available.Provides criteria for "design-level" and "ultimate strength" analysis.Risk numbers calculated are notional and tied to therecipe used.Applicable to static response of steel template offshorestructures, e.g., drag dominated fixed structures.
Design-level analysis is similar to that used in new platform design, where component strengths are checked against designequations incorporating the usual safety factors, 1/3 increase in allowable stresses, and using nominal rather than mean yield stress. In ultimate strength analysis, a best estimate of the ultimate collapse capacity is sought excluding factors of safety and reducing sources of conservatism as much as possible.3 As a result, the environmental criteria for the ultimate strength check are more severe than those for the design-level check although the two levels of criteria represent comparable levels of failure probability.

Environmental Model
The environmental variability is modeled by the probability distribution of the annual extreme wave height.