# The Recent Development on the Non-Destructive Inspection Reliability and Its Applications in Offshore Structural Integrity Analysis

• J. C. P. Kam

## Abstract

Due to the many design uncertainties involved in the long term fatigue loading and performance of offshore structural components, in-service non-destructive inspections (NDI) are usually incorporated as part of the maintenance programme to ensure the actual performance abides. by the design. However, the (un)reliability of NDI introduces another type of uncertainty into the assessment process. The aerospace and other industries, facing similar problems, carried out extensive studies in NDI reliability and statistical trials.

The differences in the structural configurations, and in the operational features mean that data exchange between industries are difficult. However, the exchange of modelling and analytical techniques is possible.

This paper briefly reviews the development of the fitness for purpose philosophy in structural maintenance and the associated research in NDI reliability. Emphasis, however, will be put on the scope of applications in offshore structures. Moreover, the paper will discuss some problems which are unique to the offshore industry and these require special considerations in obtaining and using the NDI reliability data. Some possible solutions to the above problems will also be discussed in this paper and illustrated with examples.

## Keywords

Fatigue Crack Growth Crack Size Structural Reliability Maintenance Strategy Offshore Structure
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

## Nomenclature

A, B

Weibull distribution parameters

$$\hat B$$

Bias factor for σ

Cj

Paris crack growth constants for segment j

$$\hat K$$t

Non-dimensionalised stress concentration factor

P(X)

Structural reliability (probability of failure) before updating

P(XIθ)

Updated structural reliability (probability of failure)

P(θ)

NDI reliability (probability of sizing /detecting defect present)

a, a’

An arbitrary crack size

ai,af

Initial and final crack growth sizes

acr

Critical crack size (Maximum size prior to failure)

aNDi

Critical non destructive inspection size

â

A measured crack size

da/dN

Crack growth rate

fy(X)

Probability density function of Y when Y = X

g

State function

mj

Paris crack growth exponents for segment j

p(a)

Probability density function of crack size a

p(XIθ)

Probability density function of X given an event θ

ΔK

Stress intensity range

Ф(X)

Cumulative standard Normal distribution at X

β

Reliability index

δa

An arbitrary, infinitesimally small crack size

µx

Mean of X

σ

Long term stress time history root mean square (RMS) value

σx

Standard deviation of X

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## References

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