NI 664, Sec 1
6 Bureau Veritas June 2021
The Society has gained a significant experience on RBI from
previous projects (Goyet, et. al., 2004; Biasotto & Rouhan,
2004), providing benefits to develop guidelines.
Moreover, the issue of risk-based approaches for the integ-
rity management of hull structures in general is addressed in
some Joint Industry Project (JIP) (e.g. FPSO Structural Perfor-
mance JIP and LCM JIP) and many technical reports and
research papers deal also with that topic. Experience-based
data related to the risk exposure of hull structures are also
available, especially for ship structures (e.g. SSC − 416,
2001 and SSC − 421, 2002). They may be used also for the
floating offshore units in virtue of the similarities between
their respective hull structures.
1.3.2 The existing international standards (e.g. API-RP-
2FSIM) address the following issues regarding the RBI of the
hull structure:
• implementation of RBI
• structural data requirements
• common degradation and damage mechanisms, and
failure modes
• critical structural components and locations for the
inspection
• hull inspection specifications.
However, the following key issues are still not addressed for
the specific case of the hull structure of offshore units:
• risk assessment
• rate of degradation
• risk-based inspection intervals.
Guidance on risk assessment may be found in most of the
international standards dealing with risk assessment or other
related topic such as risk management and risk analysis.
The Society has published indicative corrosion rates with
respect to compartment type (NI593). Indicative corrosion
rates are also available for the hull structure of ships in
some published guidelines (e.g. TSCF) and that may be con-
sidered for offshore units in virtue of their similarities with
ship structures.
There is currently no guidance for risk-based inspection
intervals to be applied to the hull structures of floating off-
shore units when qualitative risk assessment methods are
used. Quantitative risk assessment methods, which com-
pute the change in risk level over time, may be used to
establish inspection intervals in terms of achievement of a
target risk level. However, quantitative risk assessment
needs sufficient data and computational effort to provide
accurate results, which may not always be possible. There-
fore, whatever the type of risk assessment used, competent
person should be involved to set appropriate risk-based
inspection interval and scope for each part of the hull struc-
ture based on expert judgment and experience.
1.4 Overview of the Society’s semi-quantita-
tive risk assessment method
1.4.1 The Society has developed a semi-quantitative risk
assessment method for developing risk-based inspection
strategy for hull structures.
The method allows global risk level to be computed for the
hull compartments so as to define inspection intervals and
general inspection requirements. It allows also local risk
levels to be computed for the structural components of the
hull compartments so as to identify risk drivers and critical
inspection locations which should be considered to provide
more details on the inspection strategy.
In particular, the method:
• compute the LoF using a rule-based scoring approach,
where the set of scoring rules is established based on
design data, expertise and experience on the risk expo-
sure of the structure
• assess the CoF using a categorization in terms of life-
safety consequence, environmental consequence and
financial loss
This method may be used as a complement to the qualita-
tive risk assessment process required by the class for the
hull structure of a offshore floating unit within the Society
rule. In this case, the results obtained from the application
of this method should be used as input to the risk assess-
ment process required by the class.
This method must not be an alternative to the qualitative
risk assessment process required by the Society. However,
for those units not within class rule regime, a procedure has
been established to develop an inspection strategy directly
from the risk data provided by the risk assessment method
( App 1).
1.5 Organization of the document
1.5.1 The existing Guidelines for performing RBI for hull
structures are set out in Sec 2.
The semi-quantitative risk assessment method developed by
the Society for developing risk-based inspection strategy of
hull structures is presented in Sec 3.
The procedure for developing an inspection strategy of hull
structures directly from the results of the semi-quantitative
risk assessment is set out in App 1.
Typical examples degradation and damage mechanisms, and
failure modes of the hull structure are provided in App 2.
The activities that should be implemented for the inspection
of the hull structure are set out in App 3.