Bridge Fatigue
Fatigue services can be applied to road and rail bridges, including new design, existing-bridge reassessment and strengthening studies.
Relevant structures and details may include:
- Steel plate-girder, box-girder, truss, arch and cable-supported bridge components
- Steel-concrete composite girders, cross-girders and connection details
- Orthotropic steel decks and deck-to-rib or rib-to-crossbeam details
- Welded attachments, stiffeners, cover plates and termination details
- Bolted, riveted and welded splices and gusset connections
- Diaphragm, cross-frame and lateral-bracing connections
- Bearing, hanger, link and movement-sensitive details
- Reinforcement, prestressing steel and concrete fatigue where required by the bridge code
- Distortion-induced and secondary-stress fatigue mechanisms
- Existing cracks, repaired details and inspection-sensitive locations
Potential analysis tasks include traffic fatigue-load model application, influence-line or moving-load response, stress-range spectrum development, dynamic allowance, detail-category selection, cumulative damage, fatigue-usage evaluation, inspection interval support and assessment of repair or strengthening alternatives.
Bridge work may be developed under applicable IRC provisions, including project-relevant IRC loading and steel, composite or concrete bridge standards; AASHTO LRFD Bridge Design Specifications; EN 1991-2, EN 1993-1-9 and EN 1993-2; or the governing owner and railway criteria. The adopted edition, traffic model, design life and detail classification are established in the project basis.
Offshore Structure Fatigue
Fatigue can be assessed for fixed offshore and topsides systems, including:
- Jacket legs, braces, piles and conductor-support framing
- Tubular K, T, Y, X and multi-planar joints
- Topsides primary framing, deck beams, modules and skids
- Welded plate girders, stiffeners, brackets and local attachments
- Bridges, flare booms, boat landings, access and secondary steel
- Crane pedestals, equipment supports and vibration-sensitive framing
- Local details affected by wave, wind, current, machinery or operational cycles
- Transportation and pre-service fatigue accumulated before installation
- Existing structures subject to corrosion, modification or life extension
Methods may include deterministic wave fatigue, spectral fatigue, time-domain fatigue, fatigue screening, tubular-joint stress concentration factors, nominal or hot-spot stress, S-N classification, thickness and environmental corrections, design fatigue factors, cumulative damage and fatigue-life reassessment.
Applicable project frameworks may include DNV-RP-C203, ISO 19902, API fixed-offshore provisions, DNV offshore structural standards, NORSOK requirements and client specifications. Metocean scatter data, wave-transfer functions, hydrodynamic coefficients, damping, marine growth, corrosion allowance and operational occurrence data must be defined or approved as project inputs.
Riser Fatigue
Fatigue analysis may be undertaken for steel catenary, top-tensioned, rigid or other project-defined riser systems and critical interfaces. Flexible risers, umbilicals and composite systems require appropriate cross-section models, supplier data and specialist acceptance criteria.
Potential riser fatigue sources and locations include:
- Vessel motion, wave, current and VIV response
- Touchdown-zone and sag-bend response
- Hang-off, flex-joint, stress-joint and connector regions
- Girth welds, taper joints and local geometric transitions
- Pressure and temperature cycling
- Installation, pull-in, start-up, shutdown and operating cases
- Slugging, wave-frequency and low-frequency cyclic response
- Interference, support and accidental contact conditions where defined
Analysis may include global dynamic response, response extraction at fatigue-critical locations, transfer to local or hot-spot models, rainflow cycle counting, short-term damage, long-term environmental integration and cumulative fatigue life. Relevant standards may include DNV-ST-F201, API RP 2RD and project-specific riser criteria.
Pipeline Fatigue
Pipeline fatigue scopes may include:
- Free-span VIV and direct-wave fatigue
- Single-span, multi-span and interacting-span assessment
- Girth-weld stress and fatigue at span shoulders and response maxima
- Pressure, temperature, start-up and shutdown cycling
- Thermal expansion, walking and global-buckling fatigue interaction
- Installation, lay, pull-in, tie-in and spool fatigue cases
- Support, sleeper, crossing and local-contact fatigue
- Dent, corrosion or geometric-anomaly fatigue where a recognized assessment method applies
- Reassessment using survey, monitoring and operational history
Relevant design bases may include DNV-ST-F101, DNV-RP-F105, DNV-RP-F109, DNV-RP-F110, API RP 1111, ASME B31.8 and client criteria. Hydrodynamic and VIV response models, soil interaction, operating cycles and intervention criteria are confirmed before analysis.
Fatigue Assessment Methods
- Nominal-stress and code detail-category methods
- Structural hot-spot stress and stress-concentration-factor methods
- Local finite-element stress extraction where justified
- S-N curve selection and environmental correction
- Linear cumulative damage using an approved cycle-count basis
- Rainflow counting of time-history stress response
- Deterministic, spectral and time-domain fatigue integration
- Screening, refined assessment and sensitivity studies
- Inspection-informed fatigue reassessment using operational history
- Fracture-mechanics or flaw-tolerance assessment where crack, toughness and inspection data permit
Effective-notch or highly localized stress methods are used only where the governing procedure, mesh requirements and acceptance data support them.
Required Inputs
- Geometry, detail drawings, weld classes and fabrication information
- Material grade, toughness and corrosion environment
- Governing code, design life and fatigue safety format
- Traffic, wave, wind, current, vibration or operating-cycle data
- Dynamic, hydrodynamic, vessel-motion or VIV response inputs where relevant
- Stress concentration, joint classification and thickness information
- Inspection history, crack records, NDT findings and previous repairs
- Modification, corrosion, marine-growth and as-is condition data
Deliverables
- Fatigue design-basis memorandum
- Load-cycle, occurrence and stress-range definition
- Global and local analytical models
- Detail-category, SCF and S-N basis register
- Fatigue damage and life calculations
- Governing-location and sensitivity summary
- Inspection or intervention recommendations where appointed
- Fatigue assessment report and calculation package