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TECHNICAL PAPERS

# Risk Based Acceptance Criteria for Joints Subject to Fatigue Deterioration

[+] Author and Article Information
Daniel Straub

Swiss Federal Institute of Technology, IBK, ETH Hönggerberg, 8093 Zürich, Switzerlandstraub@ibk.baug.ethz.ch

Michael Havbro Faber

Swiss Federal Institute of Technology, IBK, ETH Hönggerberg, 8093 Zürich, Switzerlandfaber@ibk.baug.ethz.ch

The FDF is defined as the ratio of the calculated design fatigue life to the service life of the structure (3).

Dependency between fatigue performance at different hot spots is investigated in the next section.

In the example in Sec. 4 this example requirement will be replaced by the optimal allocation of the risk reducing measures.

It is here assumed that $I$ and $CD$ are constant with respect to the action $α$. This is typical for inservice structures.

J. Offshore Mech. Arct. Eng 127(2), 150-157 (Oct 04, 2004) (8 pages) doi:10.1115/1.1894412 History: Received November 05, 2003; Revised October 04, 2004

## Abstract

Different approaches to determine the acceptance criteria for fatigue induced failure of structural systems and components are discussed and compared. The considered approaches take basis in either optimization (societal cost-benefit analysis) or are derived from past and actual practice or codes (revealed preferences). The system acceptance criteria are expressed in terms of the maximal acceptable annual probability of collapse due to fatigue failure. Acceptance criteria for the individual fatigue failure modes are then derived using a simplified system reliability model. The consequence of fatigue failure of the individual joints is related to the overall system by evaluating the change in system reliability given fatigue failure. This is facilitated by the use of a simple indicator, the Residual Influence Factor. The acceptance criteria is thus formulated as a function of the system redundancy and complexity. In addition, the effect of dependencies in the structure on the acceptance criteria are investigated. Finally an example is presented where the optimal allocation of the risk to different welded joints in a jacket structure is performed by consideration of the necessary maintenance efforts.

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## Figures

Figure 1

Annual probability of fatigue failure in the last year of service given no prior fatigue failure as a function of the FDF (applying the probabilistic model from Straub (12))

Figure 2

Illustration of system deterioration model: One possible realization

Figure 3

System reliability model (with 2 fatigue critical hot spots)

Figure 4

System reliability model (with 2 fatigue critical hot spots) using conditional collapse events

Figure 5

Acceptance criteria for independent hot spots in a structure containing N fatigue critical hot spots (RSRintact=2.05,ψ=0.1)

Figure 6

Conditional probability of collapse given fatigue failure as a function of the RIF for different RSRintact, according to Appendix B

Figure 7

Distribution of the number of fatigue failures during service life

Figure 8

Two illustrative systems

Figure 9

Distribution of the number of fatigue failures in the last year of service

Figure 10

Expected failure cost as a function of the maintenance effort

Figure 11

Maximum annual probability of failure for different maintenance expenditures

Figure 12

Relation between the RSR and the annual probability of collapse

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