Guidance Notes On Fracture Analysis For Marine And Offshore Structures 2022

Section1Introduction1term stress range distribution. An introduction of the types of fractures, possible countermeasures, crackdrivers, and fracture assessment methods, are briefly outlined.In the current context, the primary use of fracture mechanics is for evaluating the impact of a defect onstructural integrity during the service life. The ABS Guide for Nondestructive Inspection can be referred tofor the applicable requirements for nondestructive testing. Another application of fracture mechanicsapproach is the prediction of remaining life of a structure containing a defect, in conjunction with a riskbased inspection plan. The ABS Guide forRisk-Based Inspection for Floating Offshore Installationsdescribes the use of fracture mechanics analysis in the risk-based inspection program.It is noted that fracture mechanics is not intended to be used for design purposes. However, there are somestructures that require fracture analysis as part of the design process (e.g., Tendons on a Tension-LegPlatform (TLP)). Three major elements to the fracture mechanics assessment, including applied loads,materials fracture properties, and crack growth and fracture assessment, are described in these GuidanceNotes.3Abbreviations and References3.1AbbreviationsCFDComputational Fluid DynamicsCDFCrack Driving ForceCTCompact TensionECAEngineering Critical AssessmentFADFailure Assessment DiagramFALFailure Assessment LineFCGRFatigue Crack Growth RateFEFinite ElementFPIFloating Production InstallationsHAZHeat Affected ZoneIMOInternational Maritime OrganizationLCLoad CaseLEFMLinear Elastic Fracture MechanicsLGCLiquefied Gas CarriersMARPOLInternational Convention for the Prevention of PollutionMVRMarine Vessel RuleRAOResponse Amplitude OperatorSCFStress Concentration FactorSIFStress Intensity FactorSOLASInternational Convention for the Safety of Life at SeaTLPTension-Leg PlatformVIVVortex-Induced VibrationABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 20228

Section3.241Introduction1ReferencesReferences to specific sections of industry standards are based on the versions listed below.i)ABS Rules for Building and Classing Marine Vessels (Marine Vessel Rules)ii)ABS Rules for Materials and Welding (Part 2)iii)ABS Rules for Building and Classing Floating Production Installations (FPI Rules)iv)ABS Guide for Spectral-Based Fatigue Analysis for Vesselsv)ABS Guide for Fatigue Assessment of Offshore Structuresvi)ABS Guidance Notes on SafeHull Finite Element Analysis of Hull Structuresvii)A. Almar-Næss: Fatigue Handbook – Offshore Steel Structures, TAPIR publishers, 1985viii)BS EN 1999-1-3: Eurocode 9 – Design of Aluminum Structures – Structures Susceptible toFatigueix)BS 7608:2014: Guide to Fatigue Design and Assessment of Steel Products., 2014.x)BS 7910:2910: Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures,2019xi)API 579-1/ASME FFS-1:2016: Fitness-For-Service, 2016Overview of Fracture Analysis ProcedureThe general procedure for fracture analysis is given in Section 1, Figure 1. The common stress analysismethods are employed for marine and offshore structures subjected to wave-induced loads and vibrationinduced loads. The stress analysis results are applied for the crack propagation analysis and fractureassessment based on fracture mechanics theory.For marine and offshore structures, the most dominant source of fluctuating loads are waves. However, insome particular cases, other sources, such as slamming-induced vibration, engine and propeller inducedvibrations, vortex-induced vibration (VIV), wind, and operational loads, may be significant and need to beincluded in the analysis. Two stress analysis methods for wave-induced loads and vibration-induced loadscan be used to obtain the long-term stress range distribution for marine and offshore structures. For waveinduced loads, common analysis approaches including stochastic approach, deterministic approach, andsimplified approach are used to generate the long-term stress range distributions to determine the stressrange histogram. For vibration-induced loads, common analysis approaches including direct calculationapproach and measurement-based approach are introduced to generate the long-term stress rangedistributions to determine the stress range histogram. In most cases, the structure is simultaneouslysubjected to the combined wave- and vibration-induced loads, indicating that the total long-term stressrange histogram is the sum of the effect caused by these two types of loads.In Fracture Mechanics (FM) based Engineering Critical Assessment (ECA), when the orientation andposition of a flaw are known, with given structural configuration and material fracture properties, the stressrange histogram can be applied to calculate the crack propagation under dynamic loads using Paris’ law fora defected structure. At each cycle of crack extension, the crack driving force, such as the stress intensityfactor, can be calculated and then employed together with material fracture resistance properties to checkwhether the flaw has reached a critical length, which may cause the final failure of the structure due tounstable growth. The Failure Assessment Diagram (FAD) is employed and provides an estimatedremaining fatigue life. Thus, fracture analysis of a flaw or flaws can be applied to predict the remaininglife of a structure by evaluating the number of cycles expected for an existing flaw to develop to an extentthat will cause the final rupture of a structure, structural member, or component. This information also canbe used to evaluate the need to take mitigating actions such as repair or reduced operating load conditions.ABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 20229

Section1Introduction1FIGURE 1Fracture Analysis ProceduresABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 202210

SECTION 2Applied Loads and Stress Range Distribution1GeneralThe stress range distribution caused by dynamic loads is the key crack driving force for crack propagationanalysis. Dynamic loads on marine and offshore structures can be subdivided into three major categoriesaccording to the loading frequency or cycle: Wave-induced Loads: Wave‐induced pressure due to vessel motions in the seaway generates cyclicstresses with relatively low frequency in the order of 0.1 Hz, or periods around 10s. For an operationallifetime of 25 years, the total number of wave load cycles a vessel experiences can be between 107 and108. Vibration-induced Loads: Higher‐frequency loads caused by engines, propellers, and other rotatingmachinery result in forced vibrations with a high number of load cycles, typically 1010 or more in theiroperating lifetime. In addition, wave impacts (slamming), as well as small regular waves, may excitehull girder vibrations (whipping and springing) on ships and ship-shaped floating productioninstallations (FPIs) on the order of 1 Hz. Loads Due to Loading/Offloading: Changes in cargo loading/offloading conditions and associateddrafts generally cause loads to fluctuate in rather low frequencies. For instance, the cargo loading mayvary between hours for ferries and weeks for long distance ships, thus resulting in loading/offloadingcycles depending on the service and type of vessel during operation lifetime.When individual or combined dynamic loads on the structure susceptible to crack initiation andpropagation are identified, stress analyses using the finite element (FE) method can be performed tocalculate the stress and time history of the structure. Subsection 2/2 discusses recommended practiceswhen carrying out stress analysis by FE analysis. To capture the high stress concentration, a refined mesharrangement is needed for the local FE model of critical locations such as the weld toe. Weld hot spotstress should be evaluated in accordance with the deterministic procedure described in 2/2.2.Stress range can be expressed using a time history or in a statistical format. When a stress range timehistory is obtained from a stress analysis, the rainflow counting approach introduced in Subsection 2/3 maybe used to obtain the stress range histogram. If a statistical method such as the simplified approach isconducted, the long-term stress range distribution may be assumed to follow the Weibull distribution,which is introduced in Subsection 2/4.2Stress AnalysisStructural analysis should be performed under wave-induced loads, vibration-induced loads, or loading/offloading induced loads. In structural analysis, the stress can be calculated using either simplified Rulebased equations or numerical simulations. For structures with the geometric complexity of marine vesselsand offshore structure, the FE analysis is usually conducted to determine the stress distribution at criticalABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 202211

Section2Applied Loads and Stress Range Distribution2locations. The calculated hot spot stress at the weld toe capturing the stress concentration is then employedfor the fracture assessment.2.1FE Modeling for Marine and Offshore StructuresEvaluating localized stresses for marine and offshore structures can be challenging. When building FEmodels for stress analysis, three types of elements are typically used to discretize the geometry of thestructure:i)Truss or rod elements with axial stiffness onlyii)Beam elements with axial, shear and bending stiffnessiii)Membrane and bending plate elements, either triangular or quadrilateralMesh size is one of the important FE modeling considerations for discretizing the geometry of thestructure. Typically, a one-longitudinal spacing mesh size is recommended for a global FE model. Moreinformation and guidance on global mesh discretization can be found in 2/9.3 of the ABS Guidance Noteson Safehull Finite Element Analysis of Hull Structures.For a local FE model, those structural details which are simplified or ignored in the global model arereinstated to obtain more detailed stress distributions. A finer mesh size of “t t” of shell elements shouldbe applied immediately adjacent to those hot spot locations, where t represents the member thickness. Theperformance of elements degrades as they become more skewed. If the mesh is graded, rather thanuniform, the grading should be applied in a way that minimizes the difference in size between adjacentelements.A refined local stress distribution can be obtained from a fine-meshed FE analysis. Reference canbe made to the ABS Guidance Notes on SafeHull Finite Element Analysis of Hull Structures. Section 2,Figure 1 shows an example for a global FE model of three tanks in a ship and Section 2, Figure 2 gives anexample showing a local FE model at a critical location.Generally, weld toes are critical points particularly in need of stress checks, and the hot spot stressapproach has been widely implemented to assess the stresses at those critical locations via a linearextrapolation manipulation on the calculated FE stress results. Introduction of the hot spot stress approachis given in 2/2.2.If the IACS Common Structural Rules (CSR) are applied for bulk carriers and oil tankers, the calculationof hot spot stress refers to Section 5A-9-5 of the Marine Vessel Rules.ABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 202212

Section2Applied Loads and Stress Range Distribution2FIGURE 1Global FE Mesh ModelFIGURE 2Local FE Mesh Model with Refined ElementsABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 202213

Section2.22Applied Loads and Stress Range Distribution2Hot Spot Stress Approach for Weld ToeA hot-spot stress is defined at one particular hot spot in a structural detail where fatigue cracking isexpected to initiate. The hot-spot stress includes stress risers due to structural discontinuities and thepresence of attachments but excludes the effects of welds. To determine hot-spot stresses, the mesh sizeneeds to be finer than 1/10 of longitudinal spacing (e.g., plate thickness size).The hot spot stress approach for ship structures, as shown in Section 2, Figure 3 and Figure 4, extracts andinterprets the stresses of elements “near weld toe” and obtains a stress at the weld toe. The principalstresses at the hot spot are then calculated based on the extrapolated stresses and used for fatigue crackpropagation analysis. Assuming that the applicable surface component stresses, Si, at Pi have beendetermined from FEM analysis, the corresponding stresses at “hot spot” can be determined by thefollowing procedure:i)ii)Select two points, L and R, such that points L and R are situated at distances t/2 and 3t/2 from theweld toe, i.e.:XL t/2,XR 3t/2Let X XL and compute the values of four coefficients, as follows:C1 C2 C3 C4 X X2X X1X X1X X1X X3X X3X X2X X2X X4X X4X X4X X3////X1 X2X2 X1X3 X1X4 X1X1 X3X2 X3X3 X2X4 X2X1 X4X2 X4X3 X4X4 X3The corresponding stress at Point L can be obtained by interpolation as:iii)SL C1S1 C2S2 C3S3 C4S4iv)SR C1S1 C2S2 C3S3 C4S4Let X XR and repeat the step of ii to determine four new coefficients. The stress at Point Rcan be interpolated likewise, i.e.:The corresponding stress at hot spot, S0, is given by:S0 3SL SR /2ABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 202214

Section2Applied Loads and Stress Range Distribution2FIGURE 3Extrapolation of Dynamic Stress Range at Weld ToeFIGURE 4Determination of Hot Spot Stress at Weld ToeWhen CSR criteria is chosen to apply on oil tankers or bulk carriers, an alternative hot spot stress approachand requirements are given for

ABS GUIDANCE NOTES ON FRACTURE ANALYSIS FOR MARINE AND OFFSHORE STRUCTURES 2022 vi. Fatigue Crack Propagation Analysis and Fracture Assessment.61. FIGURE 1. Flowchart for Option 1 Fracture Assessment.59. FIGURE 2. Failure Assessment Diagram \(FAD\).60. FIGURE 3