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  • T.Moan MARE WINT Sept.20131

    Structural reliability and risk analysisof offshore structuresBy Torgeir Moan, CeSOS and Department of Marine Technology, NTNU

    T.Moan MARE WINT Sept.20132

    List of content Introduction - Facilities- Regulatory framework - Accident/failure experiences

    - Safety ManagementStructural Reliability Analysis

    - Introduction- Estimation of failure probability of components- Uncertainties in Load effect (S) and Resistance (R)- System reliability- Time variant Reliability- Summary of Reliability methods- Practical use of Structural Reliability Analysis- Guidelines for Reliability Analysis

    Reliability-base-Calibration of codes for new applications - Relation between prob. Of failure and safert factors- Reliability based Calibration of safety factorsFatigue reliability- Background- Fatigue life models (based on SN- and Fracture Mechanics)- Reliability updating approaches

    T.Moan MARE WINT Sept.20133

    List of content - continuedFatigue reliability - continued- Inspection scheduling - Calibration of fatigue design criteria- Fatigue reliability of gear components

    Quantitative risk assessment- Risk Analysis Framework- Internal and external hazards- ALS design check- Ship Collision risk- Accidental Actions on wind turbines

    T.Moan MARE WINT Sept.20134Wind turbines vs other marine structures

    Facilities for wind vs oil and gas technology

    Number of units one of a kind versus mass production.

    Safety issues:No hydro carbons and people on board wind turbines

    The wind energy sector is a marginal business

    Return are more sensitive to IMMR (O&M) costs (access)

    Integratingknowledge

    4Introduction

  • T.Moan MARE WINT Sept.20135

    Regulatory framework - general

    5

    to avoid: Fatalities or injury Environmental damage Property damage

    Regulatory regime (depends on economy; accident potential):

    Regulatory principles- Goal-setting viz. prescriptive- Probabilistic viz. deterministic- First principles viz.

    purely experientialOverall stability Strength Escapeways/ lifeboats

    Offshore oil and gas Wind turbines- National regulatory

    bodies; - Industry: API, NORSOK, - Classification soc. - ISO/IMO

    - National Regulatorybodies

    - Classification societies ??- IEC

    Introduction

    T.Moan MARE WINT Sept.20136

    Introduction

    ExperiencesOil and gas platforms- significance of the oil and gas industry to the world econmy- need for technology development for deeper water, challengingnatural and industrial environment,

    - ageing facilitiesWind turbines

    CeSOS NTNU

    Gathering of experiences development of procedures/methods/data

    Failure - and accident dataSafety management procedure- safety criteria, (limit states) including accidental limit state- risk and reliability analysis of design, inspection/monitoring

    Methods (hydrodynamics, structural analysis)Data (strength data for tubular joints)

    5

    T.Moan MARE WINT Sept.20137

    A Case of structural failure - due to natural hazards ?

    Severe damage caused by hurricane Lilli in the Gulf ofMexico

    Technical-physical causes:Observation: Wave forces exceeded the

    structural resistance

    Human organizational factors:Design- Inadequate wave conditions or load calculationor strength formulation or safety factors

    Fabrication deficiencies

    due to- inadequate state of art in offshoreengineeringor,

    - errors and omission during design or fabrication!

    CeSOS NTNU

    6Introduction

    T.Moan MARE WINT Sept.20138

    a) Alexander L. Kielland fatigue failure, progressive failure and capsizing, North Sea, 1980

    c) Piper Alpha fire and explosion, NorthSea, 1988

    b) Ocean Ranger, flooding and capsizing, New Foundland, 1982.(Model during survival testing)

    d) P - 36 explosion, flooding and capsizing, Brazil, 2001

    Lessons learnt from total losses of platformsIntroduction

  • T.Moan MARE WINT Sept.20139

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    MobileFixed

    (World wide in the period 1980-95, Source: WOAD 1996)

    Accident experiences for mobile drilling and fixed production platforms(Number of accidents per 1000 platform years)

    Operational errors

    Design orFabricationerrors

    CeSOS NTNU

    7Introduction

    T.Moan MARE WINT Sept.201310

    In-service experiences with cracks in fixed offshore platforms (See Vrdal, Moan et al, 1997...)

    Data basis- 30 North Sea platforms, with a service time of 5 to 25 years- 3411 inspections on jackets- 690 observations of cracks

    The predicted frequency of crack occurrence was foundto be 3 times larger than the observed frequency; i.e.conservative prediction methods

    On the other hand:- Cracks which are not predicted, do occur. Hence, 13 % of observed fatigue cracks occurred in jointswith characteristic fatigue life exceeding 800 years; due to - abnormal fabrication defects(initial crack size 0.1 mm !)

    - inadequate inspectionCeSOS NTNU

    8Introduction

    T.Moan MARE WINT Sept.201311

    Failure Rates and Down Times of Wind Turbines

    Availability- 96 - 98 % on land- 80 % for early wind

    farms offshore

    -Need for robust design,(reliable and fewcomponents) & smart maintenance, but also improvedaccessibility

    - Larger turbine size? ( > 5 - 20 MW)

    Courtesy: Fraunhofer

    - Predict, monitor and measure degradation

    (Courtesy: Fraunhofer)

    Introduction

    T.Moan MARE WINT Sept.201312

    Risk Control MeasuresCause of failure Safety measure

    Inadequate design check toaccount for normal variability

    Increase safety factors or uselower Rc and higher Sc

    Human error and omission Design Fabrication Operation

    Unknown phenomena

    In general:-improve the quality of the initial job.-implement proper QA/QC

    possible ALS design checkALS design checkR&D

    Introduction

  • T.Moan MARE WINT Sept.201313

    Safety management (ISO 2394, ISO19900, etc)ULSFLS: D = ni/Ni DallowableALS

    Measures to maintain acceptable risk- Life Cycle Approachdesign, fabrication and operational criteria

    - QA/QC of engineering design process- QA/QC of the as-fabricated structure- QA/QC during operation

    (structural inspection )

    - Event control of accidental events

    - Evacuation and Escape

    CeSOS NTNU

    9Introduction

    T.Moan MARE WINT Sept.201314

    Safety with respect to - Fatalities- Environmental damage- Property damage

    Floatability / stability

    Structural strength of the hull

    Strength of (possible) mooring system

    Escapeways and lifeboatstationes etc for evacuation

    Regulatory requirements: - National Regulatory bodies;

    (MMS, HSE, NPD- Industry : API, NORSOK,- Class societies/IACS- IMO/ISO/(CEN)

    OR modeltests

    Introduction

    T.Moan MARE WINT Sept.201315

    Safety criteria for design and reassessment(with focus on structural failure modes) ISO

    Limit states Physical appearance of failure mode

    Remarks

    Ultimate (ULS)- Ultimate strength of structure, mooring or possible foundation

    Component design check

    Fatigue (FLS)- Failure of welded jointsdue to repetitive loads

    Component design check depending on residual system strength andaccess for inspection

    Accidental collapse (ALS)- Ultimate capacity1) of damaged structure with credible damage

    Plate thick-ness

    Collapsedcylinder

    Jack-up collapsed

    Fatiguecrack

    CeSOS NTNU

    10Introduction

    T.Moan MARE WINT Sept.201316

    Accidental Collapse Limit State for Structures (NPD, 1984)

    Estimate the damage due to accidental loads (A) at an annual exceedance probability of 10-4

    - and likely fabrication errors

    Check survival of the structure with damage under functional (F) and environmental loads (E) -at an annual exceedance probability of 10-2.

    Load & resistance factors equal to 1.0E

    P,F

    P,F

    A

    CeSOS NTNU

    11Introduction

  • T.Moan MARE WINT Sept.201317

    Ocean environment

    Analysis for demonstrating compliance with design criteria

    Functional loads- dead loads- -pay loads

    Accidentalloads

    Piper AlphaResponseanalysis- dynamic v.s.quasi-static/quasi-dynamic

    Sea loads

    Design criteria

    Load effects

    Collapseresistance

    SN-curve/fracturemechanics

    Ultimateglobalresistance

    Extrememoment (M)andaxial force (N)

    Localstressrangehistory

    Extremeglobalforce

    Designcheck

    ULS:

    FLS:

    ALS:Damagedstructure

    Analysis ofdamage

    IndustrialandOperationalConditions

    CeSOS NTNU

    Defined probability level

    12Introduction

    T.Moan MARE WINT Sept.201318

    Risk and reliability assessment

    Definition Reliability:

    Probability of a component/system to perform a required function

    Risk: Expected loss (probability times consequences)

    Recognised in the oil and gas industry- calibration of LFRD design approaches (1970s, 1980s)- RBI (Risk/Reliability Based Inspection)

    (methods in 1980s-; industry adoption in 1990s-)

    rational mechanics methods for design of structures, foundationsloads and resistances are subjected to uncertainties- normal variability and uncertainty; gross errorsdesign is deci