Tanks criticality assessment by the dependability method

Tanks criticality assessment by the dependability method
H. Hammoum, K. Bouzelha, H. Ait Aider, N.E. Hannachi
Civil Engineering Department, Mouloud Mammeri University of Tizi Ouzou, 15000, Algeria.
Abstract
In industry, performing dependability evaluation along with other analyses allows anticipating and
making tradeoffs. This field has gained its current distinguished standing mainly during the last halfcentury in areas as defense, aerospace, space, nuclear,…It is now crucial and critical to all sectors of
the industry, even more in civil engineering. Dependability is able to taken into account explicitly the
failures, uncertainties, hazards ... as far of knowledge that holds about them. It also highlights the fact
that there is no possible dependability process as there is no awareness of the studied system.
With this approach "concrete tank" system study has been conducted, in terms of functions to be
fulfilled and of what it has been designed to understand how it works. The FMEA (Failure Mode
Analysis and Effects) which will be discussed in this paper consider systematically one after another,
each component of the tank and its failure mode analysis. A failure mode corresponds to the state in
which the considered tank will not fulfill its major function: Each unachieved function or badly
achieved is matched up a given a failure mode. These must be quantified according to different
parameters. This is followed by prioritization of failure modes by a causal graph (see figure 1 below).
Finally this method will evaluate the criticality of the failure of each function component before
proceeding to the evaluation of the overall criticality of the tank failure.
Key words : Tanks, criticality, dependability method, FMEA, Failure
1. Introduction
The techniques of dependability have emerged since some time in the civil engineering field
particularly in the field of dams (Peyras 2003, Serre 2005). The event tree method is preferred for
modeling. It provides dependability measures corresponding to subjective probabilities, obtained by
expert judgment. These techniques are powerful approaches for the diagnosis and analysis of risks and
provide benefits to the modeling of reservoirs and aging mechanisms.
2. External functional analysis
By external functional analysis, we will translate the needs that are satisfied by the reservoir (the
system) as functions: the main functions and constraint functions.
The system of civil engineering that we study is composed of the tank itself (civil engineering
structure out of the ground), its foundation footing, its hydraulic equipment, its drainage system and
the subgrade as shown in the figure follows.
SYSTEME
Trappe
0,10
Figure 1 : Definition and boundary of the system (Reservoir)
2.1. Determination of external environments interacting with the system
By examining the environment of the tank, we establish an inventory of hardware components that can
act on the tank and examine their interactions. We came to the table below.
Tableau 1 : definition of outdoor environments
Type
Geology of the site
Meteorology
Upstream human activity
Exceptional external event
Environment near the tank
Several other systems
Downstream human activity
External environments
Foundation soil
Sun
Temperature
Wind
Rain
Snow
Cold
Humidity
Frost and Thaw
Cities, villages, houses, agriculture, industry
Structures at upstream linked to water : sources, pumping stations,
treatment station, reservoirs and dams
mechanical shock
Earthquake
Vegetation
Man: Operator, subscribers, farmers
Standards: recommendations related to security and rules of art
Downstream structures linked to the water : pumping station, tanks
2.2. The main functions and constraint of the global system
By external functional analysis, we obtain the main functions and constraint accomplished by the
system as a whole. We materialize the interactions between the system and its environment using a
functional diagram block in which we differentiate between contact relations (represented by straight
lines) and flow (represented by arcs).
The main functions reflect the object of the action of a system. The analysis is reduced to a single
main function "The tank is used to contain water".
Soleil
Homme : Exploitatnt, abonné, agriculteur
METEOROLOGIE
Température
Vent
Pluie
Neige
Froid
Normes : Construction et sécurité
Activité de l'homme en aval :
station de pompage, reservoirs
Industrie, agriculture, habitations
Humidité
Gel et dégel
Seisme
Activité de l'homme en amont :
Forages, sources, barrages,
stations de pompage, reservoirs,
Géologie du site
Végétation
Figure 2 : Bloc diagramme Fonctionnel du système
About the constraint functions, they are obtained by examining the external environments interacting
with the tank. All these constraint functions that provide the stability of the tank, can be summarized
into one that is: "The tank withstands."
Tableau 2 : definition of the constraint functions
External environment
Geology of site
Meteorology
Human activity
(Downstream and Upstream)
Exceptional external event
Environment near the tank
Standards of art
Constraint functions
The tank resists to attack from ground
The tank resists to deformations and soil settlement
The tank resists to sun's UV
The tank resist to deviations in the temperature
The tank resists to wind attacks
The tank resists to rain attacks
The tank resists to snow attacks
The tank resists to cold attacks
The tank resists to Frost and Thaw attacks
The tank resists to external environment (Downstream and Upstream)
The tank resists to earthquake and hydrodynamic effect
The tank resists to mechanical shock
The tank resists to vegetation attacks
The tank must resist to hydrostatic effects in accordance with
codes and design standards in effect, during normal conditions
operating
The tank resists to chemical attacks
3. Internal functional analysis
After analyzing global system, we are now looking the role and participation of its components. Each
of them providing functions contribute to the overall functioning of the tank.
3.1. Structural analysis
Structural analysis is used to list all the constituent components of the tank, to identify their physical
location in the structure and identify interactions with other components. It consists in cutting the tank,
depending the limits given in the definition, to components, in order to build an accurate vision. We
present in this study the division of an elevated tank composed of a dozen components, which are
detailed below.
Trappe
10 : Coupole de couverture
9 : Ceinture supérieure
8 : Paroi circulaire
5 : Cheminée interieure
7 : Ceinture intérmediaire
4 : Coupole inférieure
6 : Cone de réduction
3 : Ceinture inférieure
2 : Support
1: Fondation
11 : Systeme de drainage
Figure 3 : Structural decomposition of an elevated tank
Tableau 3 : List of components of an elevated tank
Component number
1
2
3
4
5
6
7
8
9
10
11
Name of the component
Foundation
Tower
Lower beam
Lower dome
interior chimney
Reduction cone
Intermediary beam
Circular wall
Upper beam
Cover dome
Drainage system
Nature of the material
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Reinforced concrete
Rough stone drain
Concrete pipes
4. The Functional diagrams Blocks of the tanks
The functions of design that realize the system components imply contact and flows fucntions. They
express the interactions between these components, but also between components and external
environments. They are materialized through the Functional Diagrams Block. The three main
Functional Diagrams Block concern the categories of the following relationships:
4.1. Functional diagrams block defining contact relations
This Functional Diagram Block indicates the set of contacts between the components themselves and
between components and external environments. It can then examine the functions of contact (surface
preparation and support), but also possible transfers of flow between components.
4.2. Functional diagrams block defining the relationship of flow of charges
This Functional Diagram Block shows the various external actions acting on each component of the
tank. We differentiate the gravity forces, hydrostatic load, seismic load, the hydrodynamic effect,
accidental impact and underpressures acting under the foundation.
4.3. Functional diagrams block defining the relationship of hydraulic flow
Hydraulic flows reflect water circulation in the tank. We separate the flow associated with the function
of draining the system, the drainage of the overflow, the distribution of customers, the drainage of
water infiltration and runoff water.
Coupole de couverture
Trappe
Soleil
Ceinture supérieure
Température
Paroi circulaire
Vent
Pluie
Neige
Froid
METEOROLOGIE
Cheminée interieure
Activité de l'homme
Ceinture intérmediaire
Coupole inférieure
Cone de réduction
Ceinture inférieure
Support
Humidité
Homme : Exploitatnt, abonné, agriculteur
Gel et dégel
Canalisations
Seisme
Fondation
Végétation
Systeme de drainage
Normes : Construction et sécurité
Géologie du site
Figure 4 : Functional diagrams block defining contact relations
5. Analysis of Failure Modes and Effects Effets (AFME)
The FMEA (Failure Mode Analysis and Effects) is an inductive method of analysis of potential
failures of a system. It considers, systematically one after another, each system component and its
failure mode analysis.
5.1. AFME Process
FMECA-Process is used to study the failure modes generated by the process of design and
construction of the system. In the application to tanks, we realized the FMEA Process for elevated
tanks which has identified some 140 intermediate-grained modes of failure of process design and
construction. These defects revealed are likely to occur then new failures during the operational phase.
Tableau 4 : Extract from the structure of the FMEA Process applied to elevated tanks
5.2. AFME Product
With the FMEA Product we list the different failure modes in service, linked to the process of design
and
production
but
also
to
the
hazards
that
may
appear
during
its
operation.
For this, we fill rigorously each column of the table FMEA product as obtained: Component, function,
failure mode, cause, effect, symptoms and detection means. The approach taken is similar to that used
by the dams (Peyras, 2003) and that used on the embankments of protection (Serre, 2005).
Tableau 5 : Extract from the structure of the FMEA Product applied to elevated tanks
6. Representation of aging scenarios in the form of causal graph
After the FMEA, we can model scenarios of aging taking place in a tank, by linking the chronological
sequences of failure, representing the physical mechanisms occurring within the system and leading to
loss or degradation of functions. In a failure sequence, we link the causes for failure modes and effects
which are manifested by symptoms. This approach is summarized in the figure below.
Figure 5: Conceptual diagram of a failure sequence in an aging scenario
Figure 6, represents the oriented graph of the aging scenario "degradation of the waterproofing cover"
which is the concatenation of the failure of the two functions "snare hydraulic flow" and "limit the
hydraulic flow" in the form of a directed graph as follows:
Figure 6: Mechanism of aging "degradation of the waterproofing cover"
7. Conclusion
We retain, at the end of Functional Analysis, we have an accurate description of our tank, its
components and links between components with each other and their environment, that's what we
called the functional block diagrams. All informations from the functional analysis we have provided
the basis for the application of FMEA which in turn has provided a comprehensive list of failure
modes of components of a tank and their causes, effects and symptoms associated. These results
allowed us to model scenarios of aging using a representation by means of causal graphs.
This detailed analysis of components, allows us to arrive at maintenance plans for given priority by
component according to these levels of criticality. However, in the case of particularly complex
systems as is our case, where a container has a large number of components, the FMEA can be very
difficult to conduct and particularly burdensome given the large volume of data to process. It comes
that the cost of such studies is important and this approach is reserved for great structures which we
want to know the safety and prioritize actions. It is difficult to consider applying this approach to a
wide park of medium-sized tanks.
8. References
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contre les inondations, revue française de géotechnique, N° 115, 2° trimestre 2006.
D. Serre, P. Maurel, L. Peyras, R. Tourment, Y. Diab. Modèles de rupture de digues couplés à un SIG,
revue internationale de géomatique, volume 16, N° 3-4, 2006.
D. Serre, L. Peyras, R. C. Curt, D. Boissier, Y. Diab. Evaluation des ouvrages hydrauliques de génie
civil, revue canadienne de géotechnique, volume 44, 2007.
D. Serre, L. Peyras, R. Tourment, Y. Diab. Levee performance assessment: development of a GIS tool
to support planning maintenance actions, Journal of Infrastructure System, ASCE, Vol. 14, Issue 3, pp.
201-213, 2008.
G. Zwingelstein. La maintenance basée sur la fiabilité. Guide pratique d'application de la RCM. Paris:
Hermès Editions, 1996, 666 p.