ABSTRACT
In order to ensure the structural integrity of offshore structures, it is necessary to carry out periodic inspections. Fatigue cracking is one of the main deterioration processes and the inspection for cracks in welded joints forms a significant part of the inspection effort.
Recent developments in structural reliability theory, fatigue fracture mechanics and the availability of various probabilistic databases such as corrosion fatigue data, inspection reliability
data can be used
to provide a theoretical framework forinspection planning. In addition, advances in knowledge based system technology allows the incorporation of practical constraints with the theoretical results to provide an integrated practical solution.
This paper describes the develOpment of a knowledge based system incorporating the latest reliability based inspection planning methods. This development is the result of a large EC funded project under the THERMIE initiative and has had technical input from several organisatiorts in four European counthes.
INTRODUCTION
The offshore industxy currently requires that the structural integrity of fixed offshore platforms is ensured by inspecting periodically In the past decisions on inspection repair and maintenance (IRM) have bccn made by experienced engineers applying their judgemcni in the form of mles-of-thurñb together with the appropriate deterministic anal ses However it is now expected that, by employing recently developed techniques based
Dfl structural reliability methods considering the effects of
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1994 OMAE . Volume II, Safety and Reliability ASME 1994
uncertainties, inspection and maintenance scheduling can be made more rational.
In order to be able to apply reliability theory, it is vital to be able to model the failure criteria as well as the uncertainties involved in the process. In the case of fixed offshore structures in areas such as the North Sea, the major problem is that of the deterioration of the welded joints due to fatigue. Furthermore, in-service inspection data and its interpretation play a major role in the whole process. Thus for fixed offshore structures it is necessary to have
accurate fatigue fracture mechanics modelling
validated databases for all the modelling parameters, such as material properties
infonnation on NDT reliability such as accuracy and
sensitivity
However one of the problems associated with a theoretical approach is that it is not always possible to incorpOrate all the constraints that are present in a practical situation Knowledge based systems provide a means of encapsulating several different forms of information within a computer system and hence can overcome this problem to provide a practical inspection scheduling system.
This paper will review the offshore industry s requirements for an integrated approach to rational reliability based inspection scheduling and will describe a aintable software architecnre for implementing the chosen strategy The validation activites carried out on the methodology is also described.
This architecture is being implemented as part of an EC funded project Reliability based Inspection Scheduling for Fixed Offshore Structures (RISC) to produce a demonstrator computer system to aid offshore operators in implennienting this philosophy.
S. Dharmavasan and
M. H Faber
0. P. Dllkstra
S. M. C. Peers RCP-Denmark APS
NO Building and
NDE Centre Manager, Der mark Construction Research
Univ. College London Delft, The Netherlands
London, United Kingdom
D. Cervetto
E. ManfrediRegistro Italiano Navale University of Pisa
REQUIREMENTS FOR AN IRM SCHEDULING SYSTEM Any usable IRM methodology needs to take into account current (and future) legislation and guide-lines for the production of rational schedules of IRM actions For example, CertificatiOn Authorities allow a four to five year period over whibh to inspect important nodes; however all damaged Eiodes must be inspected regularly (MTD 1989).
In cenain countries there are no
definitive guide-lines instead, operators are required to provide sufficient justification for proposals.The main problems are associated with the substructure rather than the superstructure The latter is in general irrelevant to the problems of ensuring the structural integrity of the whole structure. For a typical offshore structure the number of substructure nodes can be as little as 300; however in the North Sea the total numberis approximately 1500-2000.
Annual inspection is usually carried out on a percentage of the agreed mportant nodes kflown or expected defects including repairs
areas of severe marine growth
As it is impossible to be able to inspect frequently all of the structure, design and analysis data is taken into account in order to identify primary or critical nodes. Inspection effort is concentrated on these nodes together with parts of the platform which are known to be at risk of damage, for instance due to collisions or other incidents.
Anaiysis of the structure is carr d out both at the design stage and when damage is detected or changes made to the structure. Currently this is essentially deterministic. However, probabilistic and reliability analysis is allowed in some counthes as justification for a proposed schedule. It is expected that future guidelines in many more countries will allow non-deterministic analysis.
There are of course economic constraints on the acttad operation of inspections. For nstance: the costs of moving or employing more thah One inspection vessel are usually prohibitive. Hence this adds to the scheduling constraints
the part of the platform which may be inspected is the area closest to the diving vessel
a restrictiOn on the number of divers as only a small number can operate from one vessel
whenever possible inspection is carried out within a weather window
however other operations on the structure (such as drilling) take priority
Finally, there is trade-off between high-performance, efficient inspection vessels with the associated short weather window for inspections and the increased sensitivity to bad weather spells and breal downs with the associated higher level of management to tackle possible problems (Christer 1989, Rives 1986).
A rational IRM scheduling methodology must consider all the above requirements and allow operators to deal with them effectively.
THE RISC ARCHITECTURE
The methodology developed in the RISC project has been implemented as a software system.
The problem of being able to process and utilise correctly all the required data and information could be a major factor in limiting improved scheduling of inspection for fixed offshore platforms. There is information overload: i.e. too much data and information to be taken into account, and this alone makes it difficult for the operator to make full and rational use of it.
The development of a knowledge based system (KBS) provides a solution to the above problems. KBS technology was developed precisely
in an attempt to provide a computer
framework or structure for the use of little documented and extensive information.Some of the factors to be taken into account to develop a KBS for RISC are as follows:
flexibility of the representation scheme for the different types of information
effective handling of incomplete information selective utilisation of knowledge fOr efficiency
the ability for users to control the problem solving sequence ease of maintenance and extension as new information is gathered and as inspection procedures change
Overall architecture
The components of the RISC System are shown schematically in Figure 1. FIGURE 1: RISCARCHITECTURE
I
! '
REMJNING STRENGTH INSPEC11ON SCHEDULEThe functional sub-systems are given in Figure 2.
vow a
-FIGURE 2 RISC FUNCTIONS
The Structure Set-Up sub-sytem is run to input or generate and check the required information and data required to be able to carry out IRM scheduling:
geometrical data operator's predures
SCF calculations loading analysis
The Structural Analysis sub-system carries out the
f011owing:
selects the joints to be analysed and the appropriate analysis route to f011ow, given the current state of the structural model and operators prefcrences
prepare input data and execute the analysis module for each joint as required
re-direct the ourput from the analysis module to the structural model
The Analysis Results Evaluation sub-system is used to
interpret the results and choose the most appropriate action (inspection and repair) for the component in question. This will be dependent on operators preferences (including regulations and guide-lines) and the past history of the node in question, as well as infothiatin on the reliability of the inspection technique used. The Scheduling & Planning sub-system is invoked to produce a useable schedule, by
providing initial schedules, which take account of operators requirements and procedures
provide warnings in the case of actions on critical and/or likely to fail jOints which cannot be scheduled
allow the user to make modifications to the schedule and re-analyse (eg cost) as requested
Note that the schedule provided will be a list of actions which should be carried out over the next inspection periods. The detailed time-tabling
is carried out by the inspection
sub-contractors.The Structure Updating sub-systcm provides the user with a means of entering inspection and other data to update the stuetural model:
inspection data from engineering assessment reports
repair and damage data from platform damage status register changed properties of individual nodes of the structure. including minor changes in geomeu'y
operator's subjective ifleasures of a node's criticality
Msjor changes to the geometry of the structure, because of majOr damage or repairs, are input after re-analysis of the structure has taken place. Certain parameters can only be obtained from external analysis. For example, the measure of
criticality of any node, dependent on the geometry of the
structure, is obtained through redundancy analyses. However, an operator's subjective, measure representing requirements to always inspect or to always include in analysis, can be used in the short-term to overcome lack of complete information on the structure.The Systcm Management sub-system provides the operator with facilities to make modifications to the infOrmation on the structure, analysis modules and scheduling constraints.
JCnowledge Based.Systems
The architecture of a KBS is usually defined as comprising three parts: the knowledge base, a user interface and an inference engine.
The knowledge base contains the mformation that is common to a type of problem: the heuristics used, classes of components or concepts and procedures. A knowledge base can and often does contain several types of infOrmation. In practice, a KBS does not usually have a single knowledge base: knowledge is modulared wherever possible for practical reasons. First of all, modularising ensures that knowledge bases may be lOaded only when needed to reduce memory requirements.
In common with any complex computer program. a suitable man-machine interface or user-interface is required for any expert system. A üserinterface will need to provide explanation facilities, prompt for input, let the user to control the reasoning process whenever possible and display results in a graphical
format.
In the same way that a human expert is required to reason with and deduce new information, a KBS has an inference engine to reason with the knowledge in a knowledge base and with input.
Knowledge Bases
The knowledge bases in the RISC system contain information to be used by the analysis and scheduling procedures. In particular it is the explicit data and information keyed to the structure in question which will enable rational decisions to be made during the production of a schedule.
Structural datg The structural model used by the RISC system stores data of the following broad classifications:
design and manufacturing data inspection data
environmental data
The system cannot hold all inspection information in detail. On the other hand, providing various means of including this information is particularly important for data on cracka in tubular joints, as it has been fOund that there is significant variation in the information recorded by operators. Requirements include the storage of information on the inspection carried out in a format
closely associated with methods of reporting inspectiOn results, such as direct input from CAD drawings, written reports, and, in the future, direct input from the inspection equipment.
Inspection data to be stored for a node includes: inspection technique used
procedural details (eg.dates, and by whom) defects and cracks found (eg. type, position and size)
For most non-critical joints little detail is required (Rives 1986): only information on the existance of a defect or crack can be used.
Other inspections stich as after incidents and swan-round surveys would include data on general structural damage or state with the type and position.
Environmental and other loading data can be updated with data on the actual weather and structural conditions:
wind and wave loading for global structure structural loading on individual joints and members re-distributed loads due to changes to the superstructure This data may be updated either by direct structural analysis after making changes tO the geometrical data or from on-line
monitoring data. Records of reported incidents, such as heavy items dropped overboard, should also be kept to provide some
indication of areas of the
structure which would require inspection after these incidents(Dunn, 1983).Scheduling and Planning The scheduling of inspections and repairs uses the following information:
Guide-lines and regulations Scheduling heuristics Resource constraints Cost information
In addition, knowledge of inspection and repair strategies must be stored.
Analysis Procedures A knowledge base of the analysis modules which are available for use contains information on:
Analysis routes: expected overall analysis route as defined by the operator and expert engineers
Reliability Fatigue and Fracture Mechanics module SCF module
Loading analysis module Database access and handling
For all of the above, information is stored on the required input, execution of the module.and intepretation of output. RELIABILITY TECHNIQUES
Overview
Within the last decade, reliability based methods have become recognised tools in offshore engineering. This is most significantly reflected through the increasing role played by reliability methods in the formulation of codes, classification notes and also recently in the formulation of "safety cases".
For offshore structures in the North Sea, the dominant
deterioration process is fatigue crack growth. The fatigue life of welded connections in offshore structures is therefore animportant design criterion and a governing factor for the
planning of inspection repair and maintenance actions. Due to the considerable costs associatedwith IRM of offshore
Structures, it is desirable to optimise maintenance planning suchthat costs are minimized whilst ensuring that risks are kept within acceptable limits. This implies that the estimation of fatigue crack growth, as well as the effect of IRM actions, must be accurate.
Unfortunately the fatigue life of a welded conneàcion such as a tubular joint is influenced by a large number of uncertainties, such as wave loading, stress concentration factors, material properties and the size
and number of
initial defects. Furthermore, inspections and repair actions of joints are subject to significant uncertainties mainly due to the difficult conditions under which they are performed.Deterministic analysis methods in combination with, e.g., the partial safety factor code formats have proven their usefulness in assuring safe structural designs. However one area where probabilistic analysis methods provide better information is when the task is to identify which parameters are dominant. For this it is necessary to take into account all the involved uncertainties and to correctly modl the mutual dependencies between the uncertainties. Modern reliability methods serv as a tool for the consistent handling of uncertainties and, when combined with state-of-the-art models of fatigue crack growth, can provide a powerful tool for reliability-based IRM planning of offahore structures.
In order to ensure and maintain the safe operation of offshore structures inspection and maintenance actions are performed with adequate intervals. The costs associated with inspection and maintenance of offshore structures are very considerable mostly due to the difficult conditions under which inspection and maintenance
must be
performed. Inspection and maintenance actions should therefore be optimised with respect to inspection intervals, inspection techniques and potential repair actions.Description of limit state
In order to estimate the reliabilities and the expected costs several probabilities must be estimated. To this end modern reliability methods such as first order reliability method (FORM) and second order reliability method (SORM) (Faber et al. 1992) are adequate. These methods are based on the concept of a limit state function. The limit state function is a function g(x)of the outcome x of the uncertain variables X describing the problem, which is defined such that "safe" performance is characterized by g(x)>O and "unsafe" performance characterized by g(x)<O. Any event can be described by a limit state function. The problem of estimating the probability of a certain event can therefore be solvedby estimating the probability::P(g(X) 0).
When fatigue crack growth events are considered the limit state functions are formulated as
g(x)=0CR1T -a(t.x)
where is the critical crack size and a(t,x) is the actual crack size at time 1. Alternatively and often more appropriately, the limit state function can be formulated as
N
-1.0
N(aCRffX)
where N is the anticipated lifetime and N(aCRfl.X) is the time before a crack of size aCRif is obtained.
Reliability Aflalvsls Results
Several proposals for IRM planning can be found in the literature, see for example a review in (Faber et a]). Some strategies: are purely reliability-based (P&lerseñ ci al, 1992), whereas Others combine costs with reliability (Sorenson ci al,
1992).
The necessary input forIRM planning is: joint and weld geometry
stress history
material characteristics initial defects
previous inspection results inspection reliability
costs of failure, inspection and repair
The output for each joint and over a period of time, will be: expected remaining lifó
probability of failure and reliability measures expected costs of for maintenance plans reliability sensitivity measures
An example of a reliability-based IRM plan is illustrated in Figure 3. InspectiOns are planned whenever the reliability index decreases to a certain code specified value, At the planned inspections it is assumed that no damage is detected and the reliability is updated using this assiinption. This approach assures that a minimum reliability is maintained throughout the lifetime of the structure.
Using a costs based approach only the next inspection time is planned but the costs associated with failure, repair and inspection are taken into account (Figure 4). After an inspection is performed the inspection results are taken into account when planning the next inspectiOn. This scheme is often referred to as the adaptive scheme and is described in detail in Faber ci al (1994).
13'
Ti
Time to Next Inspection
FIGURE 4: COSTS-BASED IRM PLANNING
Updating of Structural Reliability based onInspeOtlon Jesults
Once inspection has revealed a defect, this information is tised to update the state of the component. The reliability of the component can be re-calculated by two differentmethods:
1 complete reliability analysis taking the observed as the new initial crack size
2. rapid Bayesian updating
Both require data on the reliability of the technuique and two scenarios are considered:
1 Non-detection of a crack this uses the information on probability of detection (POD) The assumption made here is that a crack exists which is too small to detect.
2 Detection and sizing of a crack the probability of sizing (POS) information is used to model this event.
Spurious reSults arc not considered for practical reasons as little data exists to model, this problem.
FRACTURE MECHANICS MODELLING Genéra[
Fatigue crack growth mOdels for welded structures, based on linear elastic fracture mechanics have been described recently 1y several authors (Straalen and Dijkstra 1991 Dharmavasan ci al
1991, Thorpe, 1986). In general, the crack growth model gives the relation between the crack grbvth rate (da/dN) and the fatigue loading parameter (AK) (stress intensity factor range).
-
Total Costs -. = FatureCostsRepairCosts
- - Inecten Costs
This relation has in general a sigmOida] shape as given in figure 5, with a threshold value at the lower LiK values and an unstable crack growth part at high tiK values.
Ti
T2 T3Ufetime
FIGURE 3: RELiABILITY-BASED JAM PLANNING
Region II Paris- Erdogqn m
da/dN = C(K)
Kmax=Kc
FractureMechanics
Model
FIGURE 6: FRACTURE MECHANICS MODEL FOR TUBULAR JOINTS
Fatigue cracks in tubular joints generally stans at the weld toe and grow in depth and width direction into the tube wall. So, the CFA assumes a semi-elliptical crack at a weld tOe and the real geometry is translated to a simplified model as given in figure 6. With the crack growing into both depth and width direction crack extension rules for both directions are needed In the CFA this can be dOne in two different ways, namely:
1 Crack growth model for both crack depth and width direction In this model equation 1 is used in both directions with the appropriate SIF range for the depth direction and for the width direction).
2. Forcing function for crack width direction. In thi model equation 4 is used only in the depth direction and the crack width is a function of the crack depth.
cf(a)
. (4)Both methods have their own advantages and disadvantages. The first approach is more correct from a theoretical point of view. However, the problem here is how to deal, with multi initiation and crack coalescence. The latter approach needs a
forcing function Of the type shown in equation 4 and the
available forcing functions are all based on experimental observations One of the questions requinng further investigation is whether the' forcing functiOns are applicable for situations different from the situations on which the formulae are based.
The RISC knowledge based system makes a choice out of these. two methods and the available solutions for the SIF (Peeñ et al, 1994).
The crack growth model mostly used is the Paiis-Erdogan relation (equation 5), which represents the linear part of the crack growth curve of figure 5.
(5) where C = crack growth coefficient
m = crack growth exponent'
The material constants C and m are dependent on the material and the environmental conditions (seawater)..
The CFA has a number of crack growth relations available and the most appropriate method used in the analysis.
Kth
log
FIGURE 5: CRACK GROWTH RELATIONSHIP The stress intensity factor range is the difference between the maximum SIF and the minimum SIF during a load cycle The SIF is a measure for the magnitude of the stresses near the crack tip and depending on the geometry, the load level and the crack dimension.
(2) where: cy = remotely applied stress
Y = correction factor depending on geometry and. loading conditions
a = crack depth
hitegration of the crack growth relation (equation 1) from an intial defect a. to the final size a, gives the lifetime N of the detail.
a,
N
f(iK)
(3)For the Component Fatigue Analyses (CFA) Of the RISC system a combination
of the FACTS program of UCL
(Dharmavasan, 1991) and the FAFRAM program of NO
(Dijkstra and Straa]en, 1991) was used. A brief description of the basis oftheCFA is given below. A more detailed description can be fOund in a paper on the CFA (Dijkstra et al, .1994).AppIicatIon to Tubular Joints
The RISC system is developed for offshore tubtilar structures and therefore the genera] fracture mechanics model is modified for steel welded tubular joints (Straalen and Dijkstra. 1991).
MATERIALS DATABASES Overview
Since
the RISC output
relies heavily on probabilistic component fatigue analysis results, exhaustive data on materials behaviour are needed. A materials database was therefore developed containing corrosion-fatigue crack growth data in seawater for structural steels commonly used in the fabrication of jacket off-shore platfoñns. A Corrosion Fatigue Materials Data Base (CFMDB) onginafly created at the University of Pisa was further expanded to cover the need of the RISC Project (Bertini, 1994). For this application, the CFMDB was divided into two parts:the primary level Materials Data Base (MDB), containing both raw experimental data and all the data needed for material environment and test conditions characterisation; the high level MDB containing both the overall characterisation information and the multisegment Pari law parameters together with the experimentally observed scatterband.
Once the material environment and loading conditions are determined, the RISC component fatigue analysis module makes use of the high level database only.
Raw Experimental Data Materials Database
The primary level database was designed around a relational Data Base Management System to record and retrieve published arid unpublished experimental results.. The. data were obtained making use of base material and welded specimens subjected to various corrosion-fatigue loading conditions and envirorunents,
i.e.: various levels of cathodic protection, fully immersed or areated. etc. Materials chemical composition, heat treatment, welding parameters were recorded as well as tesi conditions and source data. Up to now several thousand data for 18 different types of structural steels are stored in the RISC CFMDB. The structure of the primary level database is shown in Figure 7
FIGURE 7 STRUCTURE OF PRIMARY CFMDB
Multi Segment Idealisatlon of the Corrosion Fatigue Crack Growth Behaviour
For each material, environment and loading condition considered, the Paris law coefficients C arid in of a
multi-segment bi-logarithmic curve (cxivation 5)are stored into the high level MDB. The maximum number of segments considered was five.
TheParislaw parameters and statistical scanerband (constant C and exponent m) were evaluated by curve fitting techniques applied to homogeneous raw data sets of the primary levCl MDB.
The grouping of data was chosen in such a way as to make the transition between the various segments as smooth as possible. Each group of data was Used for computing the Paris law parameters by iterative least squareregression.
INSPECTiON REUABILITY
The main purpose of inspection is to assess the current state of the structure and in particular of the component. For this, it is necessary to be able to interpret inspection data correctly (Topp, 1985). It must be remembered that underwater inspection is very difficult and hence it is essential to consider only realistic modelling of the inspection data. A substantial amount of work has been carried out in the NDE Centre, at University College London (UCL) on this modelling (Rudlin and Dover, 1990 and Dover and Rudlin, 1994). Extensive blind trials have been conducted undCr realistic conditions: in a water tank, on a library of tubular joints, comparing most inspection techniques with offshore divers.
Inspection Data
Currently much inspection data is collected and it has already become clear is that there are no standard forms across operator organisations. Some types of data collected every year are given in Table I.
TABLE 1: DESCRIPTION OF PERFORMED INSPECTION TYPES
Inspection Data Type Coverage
100%
swim-round survey for gene damage and marine growth
inventory flooded members provide
immediate information on the existence of through-thicbiess cracks
yes/no 100%
seabed debris survey inventory 4 locations scour survey to check integrity of
foundations
quantitative
-cathodic protection numeric one leg. anode conditiOn given as subjective
information either as a grade or as a percentage of eroded or rernáiñthq volume
graded (-3-4 grades)
one leg
marine growth - - numeric
bolted connections on the condition qualitative 100%
welded joints corrosion, pitting and grinding marks
qualitative selection welded joints undercut qualitative selection welded joints data on cracks
-detail of information vanes according rothe.inspectjon method
In the case of data on weld-toe cracks fOund on welded joints, currentlyinformation recorded on a crack will include:
indication length (and possibly depth - most current inspection techniques do not provide depth measuremcnts) clock:position relative to some pre-defined point on the jOint ground length, width and depth
The data is stored
in various formats and in
different locations, mostly as text and in tabular fOrm. In addition, some information may be kept in a pictorial form.The RISC system however, makes use of data from confirmed anomalies which has been to a certain extent interpreted by the maintenance department.
Sensitivity and Accuracy of Inspection TechnIques
To define the reliability of an inspection technique under certain conditions requires measurements of
sensitivity, or the probability of a crack being detected (POD) accuracy or the probability of a crack being sized accurately (POS)
To be able to measure the POD and POS requres producing extensive data on detectiOn of many cracks of different sizes. Only recently has enough representative samples been produced and sufficient ttials been carried out to be able to generate adequate data for POD and POS modelling.
IMPLICATIONS FOR CERTIFICATION AUTHORITIES Two aspects are to be taken into account when dealing with inspection planning. From a purely economic point of view, optimisation means a balance between the investment and the potential failure costs, considering the lifetime risk. From a Certification Authority point of view, the inspection planning is optimised when the annual risk of failure is lowered to an acceptable level. The two aspects are to be integrated in order to obtain a safe and economic inspection planning.
Inspecting offshore structures is considered vital by Certification Authorities in order to assess their original safety and to verify that it is maintained during their operating life. The joints to be inspected and the intervals between inspections are generally defined on the basis of the joint importance with respect to the overall safety and the calculated fatigue life of the joint. In most cases, the fatigue life is calculated following a deterministic approach.
In this respect, the most innovative aspects of the RISC project are considered as a significant achievement towards a rational optimised approach of IRM scheduling. In particular, reference is made to the possibility of utilising:
accurate models of fracture mechanics; a database for the main parameters;
a reliability approaOh which includes Bayesian updating for the results of the in-service inspections and accounts for the many uncertainties
and the
reliabilityof the
different inspection techniques.As regards the economic optimisatiori of inspection planning, Certification Authorities are generally aware of the economic problems involved in the inspection plarthing of fixed offshore structures. As a consequence, they are ready to evaluate the impact of optimal scheduling on rule based and current practice
based inspection planning, in order to assess the true possibility of integrating traditional inspection plans with cost-optimal ones. as long as the structural safety is not be jeopardised.
With this objective, Registro ltaliano Navale (RINA), a Certification Authority, reviewed the most significant phases of the project in order to obtain information from all tasks and, thus, contributing to thc decision making process from the
certification point of view. on the basis of the experience
achieved in the field of offshore platform certification, RINA validated the analysis software modules and the inspection scheduling methodology.The analysis
software modules were validated from a
methodological point of view according to the acquired expertise in steel offshore fixed platform and application of structural reliability methods. The validation provided by RINA was based on the positive.comparison with the results of:the validation of some of the analysismodules was carried out by using similar computer codes available at RINA.
well-known test cases with experimental data was used for the validation of CFA.
The inspcction scheduling derived from life-cycle, reliability-based cost optimisation procedure as validated in order to ascertain its substantial agreement with practice requirements needed for operational and certification purposes. Several sets of input parameters (such as inspection. maintenance and failure costs, number of inspection intervals, inspcction methodOlogies etc.) were used to assess their influence on optimal planning. CONCLUSIONS
This paper has described the development of an inspection, repair and maintenance planning sytem which incorporates aqvanced reliability analysis with fracture mechanics based limit state functions. In order tO provide realistic results, the system makes use of databases containing appropriate information for probabilistic modelling. Finally the whole analysis system and databases have been integrated into a knowledge based system to provide rational schedules which take practical constraints into account.
ACKNOWLEDGEMENTS
The work described here forms part of the EC funded
THERMIE project Reliability based Inspection Scheduling of Fixed Offshore Platforms. The partners and collaborators in this project are Technical SOftware Consultants (UK). University College London (UK), TNO-Building Construction Research (NL), Registro Italiano Navale (I), University of Pisa (I) and RCP-Denmark APS.In addition, the sponsors to the RISC project are: AGIP Spa. Amerada Hess Ltd, Elf Aquitaine, Kvaerner Earl & Wright Ltd, Instituto Mexicano del Petrolco, US Coast Guard and Stork Protech by.
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