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Oil & gas platforms
Aquaculture
Plant
Wind
turbines
Nuclear reactor
Spacecraft
Aircraft
BuildingsBridge
Design for Robustness or Damage Tolerance- an important aspect of Structural Integrity Management
Torgeir Moan, AMOS/CeSOS, NTNU, Trondheim, Norway
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Outline➢ Introduction
- Definition of robustness and
Robustness in draft EN1990
- Historical notes on robustness
➢ Service experiences and
Causes of Structural Failures &
Risk Reduction Measures
➢ Recent R&D and Regulatory requirements
➢ Crack Control Measures
➢ High Reliability (and Robust) Organizations
➢ Development of Accidental Collapse Limit State
Criteria
- Norsok N-001; ISO 19900
➢ Comments on the draft of EN1990
➢ Wider aspects of robustness
➢ Handling uncertainties in SIM
➢ Concluding remarks
Critical
event
Ronan Point
appartment
building, 1968
Ranger I ,
GoM, 1979
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3Definitions of Robustness
• EN 1990 , Sect. 5.4
• ISO19900 (2013) defines robustness by
“the ability of a structure to withstand accidental and abnormal events
without being damaged to an extent disproportionate to the original cause”,
• Other motherhood codes for structures: ISO 22111 (2007), ISO 2394 (2015),
EC1(2002) and ISO 19900 (2013):
- refer to resistance against accidental and abnormal events :
fire, explosions, impact ; or the consequences of human error
• Damage tolerance is also a crucial property for deteriorating structures to ensure a
reliable monitoring/ inspection and repair approach
• On this basis the following definition is suggested for consideration:
“the ability of a structure to limit the escalation of accident scenarios - caused by
accidental actions and abnormal strength due to fabrication or deterioration
phenomena - into accidental conditions with a magnitude disproportionate to the
original cause”
Related terms:- Damage -/fault tolerance
- Resilience
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EN 1990 (16.11.2017)
Suggestion:
- the robustness criteria should be operationalized
- target level for the safety implied by the robustness criteria
should be defined
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5Historical notes
Design against accidental actions• Ronan point apartment building, 1968
• British building codes, 1970
- Local strength to resist
34 kPa explosion pressure
- Strength to resist car impact at street level
• ECCS / model codes, 1978
- generally “suggesting “ design
for robustness
but without specifying
how it should be achieved
D
Failure of
column
Ronan Point
apartment building,
1968
Robustness req. applied to high consequence class buildings
Focus on accidental actions; buildings
made of large concrete panels
(not in-situ concrete structures)
and therefore the ties between the panels.
Security against terrorism?? • World Trade Centre
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Ranger I, Gulf of Mexico, 1979
Also many other accidents before
1980:
Alexander L. Kielland accident, 1980
• Norwegian Petroleum Dir. (NPD)
Regulations for Risk Analysis, 1981
• NPD Regulations for Struct. Design,
1984 : Introduct. of PLS (ALS)
• Later introduced in the ISO19900 code
Historical notes on design criteria for offshore structures
Progressive (Accidental) Collapse Limit State
Ranger I
Alexander L. Kielland
C
D6
43
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7 Recent R&D and regulatory assessments- FAA “Airworthiness requirements”. FAR 25b. US Federal Aviation Administration.
- Gallagher, J.P. (1985) USAF Damage Tolerant Design Handbook:
Guidelines for the Analysis and Design of Damage Tolerant Aircraft Structures”
Flight Dynamics Laboratory Air Force Wright, Wright-Patterson AFB, Ohio, USA.
- ISO 22111 (2007) “Bases for design of structures – General requirements”
Int. Standardization Organization, London.
- ISO 2394:2015, General principles on reliability for structures
- ISO 19900 (and the related offshore standards: ISO19902; ISO 19904; ISO 19906 )
- Moan, T. (2009) “ Development of accidental collapse limit state criteria for offshore
structures”. Structural Safety, 31( 2), 124-135. Fist presented in the Workshop on Risk
Acceptance and Risk Communication at Stanford University in 2007.
- CPNI (2011). Review of international research on structural robustness and
disproportionate collapse, the Centre for the Protection of National
Infrastructure (CPNI). HMSO, 2011
- Kӧhler, J., Narasimhan, H. and Faber, H. (2010) Proc. Joint Workshop of COST Actions
TU0601 and E55, Ljubljana, Slovenia 21-22 Sept. 2009, ETH, Zurich, Switzerland.
- EC1 Basis of structural design. European Standard EN1990.
- EC1-1-7 (2006) Actions on structures. Part 1-7: Accidental actions. EN1991-1-7.
- Caspeele, R. et al. WG6_T1 Final Deliverables from WG6.T1 Robustness.08.05.2017
- Andre, J. et al. Robustness in Eurocodes – Project Team WG6.T1.
Background document 05.12.2016
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Obervations of accidents
(damages) :
➢Technical-physical point of view
- Loss of equilibrium or total
structural failure commonly
develops in a sequence of events
Identifying the root causes of
accidents:➢Human and organizational
point of view
- All decisions and actions
made – or not made during
the life cycle are the
responsibility of
individuals and organizations
Critical
event
Fault
tree
Event tree
- Fatalities
- Environmental
damage
- Property
damage
Service experiences:
- We learn more from incidents and accidents than successes
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Causes of structural failures and
risk reduction measures Cause Structural Integrity Management
Measures
Quantitative
measure of “risk”
Less than
adequate safety
margin to cover
“normal” inherent
uncertainties.
- Improve Design Criteria
(Increase characteristic load,
safety factors in ULS, FLS;
- Improve inspection of the
structure (FLS)
Structural reliability
analysis
Gross error or
omission
during life cycle
phase:
- design (d)
- fabrication (f)
- operation (o)
Or, deficiency in
design standard
- Improve skills, competence, self-
checking (for life cycle phase: d, f, o)
- QA/QC of engineering process (during d)
- Direct ALS design (in d)– with
adequate damage conditions arising
in f, o (NOT d)
- Event control relating to accidental
fires, explosions and ship impacts
- Inspection/repair of the structure
(during f, o)
Quantitative risk
analysis
Unknown
phenomena
- Research & Development None
SIM
Strategy
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Type
of
structure
Type
of
joint
FDF1) Residual
fatigue
life
Ultimate
reserve
strength
Inspection
method
Jacket Tubular
joints
2-10 Some-
Sign.
Normally NDE,U3
Semi-
subm.
Plated brace
Plated column
-pontoon
1-3
1-3
Some
Some
By ALS2)
Limited
LBB,4) NDE
LBB,NDE
TLP Tether
Plated
col.-p.
10
1-3
Small
Some
By ALS
Limited
IM5)
LBB
NDE
Ship Plated
longt. 1-3 Sign. -
Close
Visual
1) FDF - Fatigue Design Factor – by which the service life
is to be multiplied with to achieve the design fatigue life
2) ALS - Accidental Collapse Limit State
3) NDE - Non Destructive Examination Method; U-underwater
4) LBB - Leak before break monitoring
5) IM - Instrumental monitoring (by “an intelligent rat”)
Crack control measures.
; 1/ 0.1 1.0ic allowable allowable
ic
nD D D FDF to
N
SIM
Strategy
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High reliability organisations (HRO)
The HROs are those organisations that have operated
nearly error-free over long periods of time.
Studies (e.g. Weick, et al 1999) have shown that the reduction
in error occurrence is accomplished by the following
(1) command by exception or negation,
(Decision-making responsibility is allowed to migrate to the persons with the
most expertise to make the decision (employee empowerment)).
(2) robustness by redundant personnel, procedures and hardware
(3) procedures and rules. ( Procedures should be accurate, complete,
simple, well organised, and well documented. Rules should be adhered to). .
(4) selection and training,
(5) appropriate rewards and punishment and
(6) ability of management (key decision-makers) to
“see the big picture”.
Approaches to achieve acceptable life cycle structural integrity
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Development of structural
robustness requirements
➢ recognise that ALS/PLS criteria
represent one element in
the structural integrity management
and are based on
- a system model and
relevant failure modes
- accident experiences
➢ make the criteria operational
(possible to check compliance with)
➢ The main challenge:
determine the initial damage that
the structure should survive relating to its
- location
- magnitude
- probability
(reduce the probability
and intensity of e.g.
accidental actions,
structural flaws), as
discussed in N0145:
by involving competent
personnel, executing
QA/QC in the life cycle
phases, prevent fires
and other accidental
events from escalating )
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Step 1
• check capacity to resist abnormal or
accidental loads with annual exceedance
probability of 10-4 (allowed to cause
local damage only)
Step 2
• check that the structure in damaged
condition (step1 or specified damage)
does nor experience total collapse for
actions with annual exceedance
probability of 10-2 or 10-1 (when initial
damage is not correlated with the
environmental actions)
Action and resistance factors are 1.0
Example Procedure (NPD, 1984/Norsok):
Accidental collapse limit state (for equilibrium and
structural strength) - ALS, also denoted PLS
- local strength or system check
A
E
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➢ The ALS criterion was intended to ensure an annual probability of total
collapse of less than 10-5
➢ It turned out that the ULS criterion for environmental actions using 10-2
actions with an annual prob. of 10-2 and partial safety factors, did not
provide a similar safety level.
➢ Hence, including an “abnormal” environmental actions with a probability
of 10-4 was introduced. (This scenario of environmental condition should not be a simple extrapolation
of the 10-2 event, but represent e.g. a wave condition with a possible
“abnormal” condition, e.g. with steep wave with a large crest).
In the revised ISO 19900 standard the ALS limit states will be denoted:
- ALS1 : considering accidental damage due to accidental actions or
structural flaws (abnormal) resistance
- ALS2 : considering abnormal environmental condition
Extension of the initial ALS to cover
abnormal environmental actions
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Comment on the current draft of EN1990- EN1990 apparently focuses on direct design against accidental actions as
a conventional ULS strength check
- An alternative approach – using an alternate path approach –
is not spelled out – even if it is mentioned that design check
should be made during and after the accident.
The alternate path approach can more easily account for structural flaws
(abnormal strength) – e.g. relating to the effect of fabrication defects
or fatigue or other deterioration effect on the resistance) – which
is required to rely on an inspection/monitoring and repair strategy.
The redistribution of forces in an alternate path approach relies on
a ductile structure (esp. joints) and appropriate analysis tools.
Actual ALS (robustness) criteria should depend on the consequence class
(It is noted that robustness relating to fatigue can of course also be provided by using more
restrictive ULS/FLS criteria, but would depend on cost/benefit).
It is suggested to more explicitly include such an approach in terms of a
Limit State in EN1990 - Sect. “5.2” (See ALS criterion in ISO 19900/Norsok N-01)
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Comments on the target safety level in EN 1990
➢ EN1990 «refers» to safety (reliability) target levels
in terms of Pf and .
- Such measures refer to structural reliability analysis
considering normal uncertainties – and is suitable to deal with
the conventional ULS.
- the true (actuarial) risk – probability times consequences
need in principle to be estimated by
Quantitative Risk Analysis (QRA) . Hence the given target levels
are nor relevant since EN1990 deal with the broader aspect of risk –
e.g. by accounting for accidental actions
➢ No explicit target safety/«reliability» level
is defined for the robustness.
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17Comments on “redundancy” in view of robustness
or damage tolerance- Structural redundancy (i.e. load carrying capacity after removal of one or more
components) is sometimes considered to “imply” robustness. However,
- redundancy is not a quantitative measure of load
carrying capacity
- redundancy is a pure structural feature; robustness
depends on the structure as well as the actions
- some hazards (ship impacts, explosions…)
might cause partial damage or damage to or
failure of more than one component.
- the hazards and the corresponding damage might
occur in different locations
Even though this approach is based on risk analysis it is of course not needed for
“well known “ cases; i.e. it can be based on generic damage conditions
Car impact:
Failure of column
These facts suggest use of an approach based on risk
assessment, considering
- Various hazards, their probability in time and space
- Their implied damage and
- The residual strength of the structural system
after damage
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Wider Aspect of Structural Robustness
- In addition to the ALS criterion which aims at damage tolerance relating to
conditions caused due to human errors during operation and fabrication, it is
important to encourage designers to
provide robustness in cases where the structural performance is
sensitive to uncertain parameters. This is because the normal characteristic
values and partial safety factors in ULS requirements do not properly account
for such situations.
- Examples of such cases are:
- resonant dynamic response which is sensitive to damping;
- the ultimate strength of cylindrical shells under axial compression,
which is sensitive to imperfections and
- fatigue life estimates that is very sensitive to the local geometry and defects.
In addition to provide robustness against fatigue failure by the conventional
two-step ALS approach, use of a large FDF will also
provide robustness since implied lower stress level will lead to more time
to identify and possibly repair cracks.
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19Handling uncertainty in Structural Integrity
Management: Risk and Reliability Assessment➢ Normal uncertainties due to fundamental variability
and lack of data
➢ Including the effect of human errors
2 2
ln /( ) ( )
R S
R SV VP P R S
f
Probablity of failure (𝑃𝑓) in as special case:
- Random R and S with lognormal distribution
- denotes mean value ; s - denotes standard deviation
V = s/ – coefficient of variation(-) = standard cumulative normal distribution
The probability of system loss, relating to different accidental actions and
“accidental damages” identified as abnormal resistance, may be written in a
simplified manner,
( i ) ( i ) ( i )
FSYS jk jk jk lm lm
jk lm
P P FSYS | D A PE P D| A P A P FSYS | D P D
where Ajk(i) are – mutually exclusive - accidental actions (i) at location (j) and
intensity (k) and Dlm are damage at location (k) with a magnitude (l). PE
represents the payloads and environmental actions to consider for the damaged
structure.
Risk = pi ∙Ci
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Structural Integrity Management should, among other measures,
include
➢ Design using an Accidental Collapse Limit State
to ensure robustness
- relevant damage conditions (due to accidental actions and
abnormal strength) should reflect other efforts
to ensure structural integrity (QA/QC of design and analysis
w.r.t to novel phenomena and gross errors ; Inspection and
monitoring of the structure during fabrication and operation)
- the criteria should be formulated as an explicit limit state
corresponding to a relevant safety target level and
harmonized with available tools for action and structural analysis
➢ A simplified approach should be used for low consequence class
structures
➢ Encourage providing robustness in design in view of uncertain
parameters affecting actions and action effects
- damping in case of resonant dynamic behaviour
- local geometry in connection with fatigue analysis
Concluding remarks
Direct ULS design
against accidental
actions should
also be permitted