Reliability of Offshore Structures
Authors: Fjeld Svein 
Published: December 2007

This paper discusses current practice for protection against potential failure sources. Variability, in loadings and in the strength of structural elements typical for offshore installations is discussed in terms of statistical properties. A reliability analysis based on modern probabilistic methods is conducted. The reliability, of different types probabilistic methods is conducted. The reliability, of different types of structures is compared according to load type, structural elements, materials, and design codes.

Introduction

The need for risk evaluation and reliability criteria today is gaining recognition in the offshore industry. Ideally, such criteria should be quantitative and should provide for unambiguous interpretation of safety targets and standards.Probabilistic reliability is the only meaningful concept that can be used to obtain a logical and objective distribution of risks and safety requirements. Probabilistic reliability as a tool specifies criteria for how safe offshore engineering equipment should be and for evaluating design and operational requirements from probabilistic criteria for the installation as a whole. The mathematics and concepts used for probabilistic calculations are defined in the Appendix.

Failure Sources

Failure sources of an offshore structure may be associated with design, choice or use of material, workmanship and maintenance, and operation of the structure. Failures are associated with accidental noncompliance with written or unwritten criteria for safe design, construction, or operation. This noncompliance may be caused by errors that may be classified conveniently as gross errors (blunders), systematic errors, and random errors, as classified by Gauss 150 years ago.Examples of gross errors are decimal points misplaced in the analysis, using low-grade steel where high-grade steel is specified, etc. All gross errors can be eliminated with adequate care. But no design safety factor, however large, will give adequate protection against the effect of gross errors.The only way to eliminate gross errors is with extensive supervision and inspection of all activities involved in the process of design, construction, maintenance, and operation of the structure. A reliable inspection must be based on the contractor's own supervision and control of his activities. In addition, the quality of the control has to be inspected by some independent authority conducting design evaluation and survey of the construction and operation. The expenditure of time and money for these activities contributes more to structural safety than large safety factors.Systematic errors are associated with causes that contribute to deviations on one side. These errors may be associated with empirical relationships between visual wave records compared with significant wave heights, recognized design formulas based on incomplete theory or experiment, strength properties determined by distorting recording devices, etc.Systematic errors associated with ignorance of real physical phenomena may result in fatal structural physical phenomena may result in fatal structural failures. This type of error might be feared especially in connection with rapid technological developments beyond current experience. Examples are the suspension bridge failures caused by ignorance of dynamic effects in the late 1930's and the fatigue ruptures of the Comet airplanes in the 1950's. In this context, ignorance errors are classified as gross errors, and for this reason the inspection authority must have superior scientific competence to discover them.

JPT

P. 1486