Design for Robustness or Damage Tolerance - an important aspect of Structural Integrity Management Torgeir Moan, AMOS/CeSOS, NTNU, Trondheim, Norway

Transcription

1 1 Nuclear reactor Buildings Bridge Design for Robustness or Damage Tolerance - an important aspect of Structural Integrity Management Torgeir Moan, AMOS/CeSOS, NTNU, Trondheim, Norway Spacecraft Aquaculture Plant Aircraft Oil & gas platforms Wind turbines

2 2 Outline Introduction Ronan Point appartment building, 1968 Ranger I, GoM, 1979 Critical event - Definition of robustness and Robustness in draft EN 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 Comments on the draft of EN1990 Wider aspects of robustness Handling uncertainties in SIM Concluding remarks

3 3 Definitions of Robustness EN 1990, Sect. 5.4 Related terms: - Damage -/fault tolerance - Resilience 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 (2007), ISO 2394 (2015), EC1(2002) and ISO (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

4 4 EN 1990 ( ) Suggestion: - the robustness criteria should be operationalized - target level for the safety implied by the robustness criteria should be defined

5 5 Historical notes Design against accidental actions Ronan point apartment building, 1968 British building codes, Local strength to resist 34 kpa explosion pressure - Strength to resist car impact at street level D Ronan Point apartment building, 1968 ECCS / model codes, generally suggesting design for robustness but without specifying how it should be achieved Focus on accidental actions; buildings made of large concrete panels (not in-situ concrete structures) and therefore the ties between the panels. Failure of column Security against terrorism?? World Trade Centre Robustness req. applied to high consequence class buildings

6 Historical notes on design criteria for offshore structures Progressive (Accidental) Collapse Limit State 6 Ranger I, Gulf of Mexico, 1979 Ranger I C D Alexander L. Kielland 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

7 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 (2007) Bases for design of structures General requirements Int. Standardization Organization, London. - ISO 2394:2015, General principles on reliability for structures - ISO (and the related offshore standards: ISO19902; ISO 19904; ISO ) - Moan, T. (2009) Development of accidental collapse limit state criteria for offshore structures. Structural Safety, 31( 2), Fist presented in the Workshop on Risk Acceptance and Risk Communication at Stanford University in CPNI (2011). Review of international research on structural robustness and disproportionate collapse, the Centre for the Protection of National Infrastructure (CPNI). HMSO, Kӧhler, J., Narasimhan, H. and Faber, H. (2010) Proc. Joint Workshop of COST Actions TU0601 and E55, Ljubljana, Slovenia Sept. 2009, ETH, Zurich, Switzerland. - EC1 Basis of structural design. European Standard EN EC1-1-7 (2006) Actions on structures. Part 1-7: Accidental actions. EN Caspeele, R. et al. WG6_T1 Final Deliverables from WG6.T1 Robustness Andre, J. et al. Robustness in Eurocodes Project Team WG6.T1. Background document

8 8 Service experiences: - We learn more from incidents and accidents than successes Fault tree Critical event - Fatalities Event tree - Environmental damage - Property damage 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

9 9 SIM Strategy Cause Less than adequate safety margin to cover normal inherent uncertainties. Gross error or omission during life cycle phase: - design (d) - fabrication (f) - operation (o) Or, deficiency in design standard Unknown phenomena Causes of structural failures and risk reduction measures Structural Integrity Management Measures - Improve Design Criteria (Increase characteristic load, safety factors in ULS, FLS; - Improve inspection of the structure (FLS) - Improve skills, competence, selfchecking (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) - Research & Development None Quantitative measure of risk Structural reliability analysis Quantitative risk analysis

10 10 SIM Strategy Crack control measures. Type of structure Jacket Semisubm. TLP Ship Type of joint Tubular joints Plated brace Plated column -pontoon Tether FDF 1) Residual fatigue life 2-10 Some- Sign Some Some Small Ultimate reserve strength Inspection method Normally NDE,U 3 By ALS 2) Limited By ALS Plated col.-p. 1-3 Some Limited Plated longt. 1-3 Sign. - LBB, 4) NDE LBB,NDE IM 5) LBB NDE 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 ) ni Dc Dallowable; Dallowable 1/ FDF N ic

11 11 Approaches to achieve acceptable life cycle structural integrity 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.

12 12 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 )

13 13 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 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

14 14 Extension of the initial ALS to cover abnormal environmental actions 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 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

15 15 Comment on the current draft of EN 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. It is suggested to more explicitly include such an approach in terms of a Limit State in EN Sect. 5.2 (See ALS criterion in ISO 19900/Norsok N-01) 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).

16 16 Comments on the target safety level in EN 1990 EN1990 «refers» to safety (reliability) target levels in terms of P f 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.

17 17 Comments 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 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 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

18 18 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.

19 19 Handling uncertainty in Structural Integrity Management: Risk and Reliability Assessment Normal uncertainties due to fundamental variability and lack of data Probablity of failure (P f ) in as special case: - Random R and S with lognormal distribution ln R / S P P R S ( ) ( ) f 2 2 V V R S - denotes mean value ; s - denotes standard deviation V = s/ coefficient of variation (- ) = standard cumulative normal distribution Including the effect of human errors Risk = p i C i 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 ) PFSYS P FSYS D Ajk PE P D A jk P A jk P FSYS Dlm P D lm jk where A jk(i) are mutually exclusive - accidental actions (i) at location (j) and intensity (k) and D lm are damage at location (k) with a magnitude (l). PE represents the payloads and environmental actions to consider for the damaged structure. lm

20 20 Concluding remarks 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 Direct ULS design against accidental actions should also be permitted 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