Page 36 - Marine Structural Design

Chapter I Introduction 13

Corrosion resulted defects may significantly reduce ultimate strength and fatigue strength of
the structures. Various mathematical models have been developed to predict the future
corrosion development in structures such as pipelines, risers and plating. Various methods
have been applied by the industry to measure the amount, locations and shape of the corrosion
defects, as all these are crucially important for strength and fatigue assessment.
In many cases, the use of nonlinear analysis of loads and structural response and
risklreliability methods is required to filly utilize the design margins. The re-qualification may
be conducted using the strength and fatigue formulations, and the risWreliability methods
discussed in this book.

1.4 Risk Assessment
1.4.1 Application of Risk Assessment

Risk assessment and management of safety, health and environment protection (HSE) became
an important part of the design and construction activities.
Use of risk assessment in the offshore industry dates back to the second half of the 1970s
when a few pioneer projects were conducted, with an objective to develop analysis
methodologies and collect incident data. At that time, the methodologies and the data
employed, were those used for some years by the nuclear power industry and chemical
industry.
The next step in the risk assessment development came in 1981 when the Norwegian
Petroleum Directorate issued their guidelines for safety evaluation. These guidelines required
that a quantitative risk assessment (QRA) be carried out for all new offshore installations in
the conceptual design phase. Another significant step was the official inquiry led by Lord
Cullen in the UK following the severe accident of the Piper Alpha platform in 1988. Lord
Cullen recommended that QRAs be implemented into the UK legislation in the same way as in
Norway nearly 10 years earlier.
In 1991, the Norwegian Petroleum Directorate replaced the guidelines for safety evaluation
issued in 1981 with regulations for risk analysis. In 1992, the safety case regulation in the UK
was finalized and the offshore industry in the UK took up risk assessments as part of the safety
cases for their existing and new installations. In 1997 formal safety assessment was adopted by
IMO as a tool to evaluate new safety regulations for the shipping industry.
1.4.2 Risk-Based Inspection (RBI)
Based on risk measures, the development of a system-level, risk-based inspection process
involves the prioritization of systems, subsystems and elements, and development of an
inspection strategy (i.e., the frequency, method, and scope/sample size). The process also
includes making the decision about the maintenance and repair. The risk-based inspection
method may also be applied for updating the inspection strategy for a given system, subsystem,
or componentfelement, using inspection results.
The important features of the risk-based inspection method include:
The use of a multidisciplinary, top-down approach that starts at the system level before
focusing the inspection on the element levels;