DEEPWATER DRILLING RISER INTEGRITY - FATIGUE, WEAR, INSPECTION AND MONITORING by Dr Hugh Howells and Dave Walters 2H Offshore Inc Presented at OMAE, New Orleans, February 2000 Introduction Characteristics of deepwater drilling risers VIV - effects and implications Wear - reasons for concern Inspection of deepwater drilling risers VIV and wear mitigation Monitoring requirements Purpose of Drilling Riser Inspection Detect and quantify deterioration in integrity Sources of deterioration: fatigue damage accumulation impact / handling damage wear from drill string rotation corrosion Deep Water v Shallow Water Larger tensions Larger internal and external pressures Longer, heavier riser joints Exposure to severe currents Subject to vortex induced vibrations Larger curvature Deepwater 1 Year Return Period Currents Effects of VIV Elevation Above Seabed (m) 1500 1000 500 0 WoS Brazil GoM Loop West Africa High rates of fatigue damage Increased drag loading 0 1 2 Current Speed (m/s) 1
Fatigue Damage (1/Years) 2.00E-03 1.80E-03 1.60E-03 1.40E-03 1.20E-03 1.00E-03 8.00E-04 6.00E-04 4.00E-04 2.00E-04 VIV Fatigue (1) 21 INCH Drilling Riser - 6000ft - 12 ppg Mud Fatigue Damage (1/Years) 3.50E-02 3.15E-02 2.80E-02 2.45E-02 2.10E-02 1.75E-02 1.40E-02 1.05E-02 7.00E-03 3.50E-03 VIV Fatigue (2) 21 INCH Drilling Riser - 6000ft - 16.5 ppg Mud Cd Amplification 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 VIV Drag Amplification 21 INCH Drilling Riser - 12 ppg Mud - Operational Top Tension Implications of VIV High rates of fatigue damage Increased top tension Increased vessel and base loading Suppression devices Increased drag loading Increased curvature More wear More downtime Wear in Deep Water Larger mean angles Larger tensions Higher external pressures 2667/4445psi at 6,000/10,000ft Higher internal pressures 4393/7321psi 14ppg mud, 6,000/10,000ft Integrity of wall more important LOWER RISER Firm Soil LMRP BOP Wear Hotspots Lower Flex-Joint Conductor Casing Soft Soil UPPER RISER 2
Wear Control Flex joint angle limits 2 degrees mean, 4 degree max (API) Criteria based on historical performance Deepwater limits? 0.5 to 1 degree mean used by some drilling contractors Deepwater Integrity Issues Increased fatigue damage from severe currents Increased wear from riser curvature Increased susceptibility to handling damage from the use of longer, heavier riser joints Increased wall thickness integrity required to accommodate higher tensions and pressures Status of Drilling Riser Inspection Shallow water total kip-days 1 year usage approach Deep water increased wear and fatigue limited long term experience strategies need to be defined Deepwater Inspection Difficulties Joint length - 75-90ft Joint weight - 60-70kips Difficult to handle - damage to buoyancy More remote - longer turn around More joints More expensive Need to rationalise Riser in Rack Riser Joint Rack (1) 3
Deepwater Inspection Difficulties Joint length - 75-90ft Joint weight - 60-70kips Difficult to handle - damage to buoyancy More remote - longer turn around More joints More expensive Need to rationalise How to Rationalise? Fatigue based approach: Severity of operating conditions Inspection detail Inspection frequency Inspection coverage Wear Joint position Fatigue Crack Growth How to Rationalise? Circumferential Half-Crack Length, c (mm) Lower Riser - Surface Crack Growth 90.0 80.0 70.0 60.0 50.0 Initial a/c = 1.0 40.0 Initial a/c = 0.5 Initial a/c = 0.2 30.0 Critical Crack Size 20.0 10.0 0.0 0 10 20 30 40 50 60 70 80 90 100 Time (years) Fatigue based approach: Severity of operating conditions Inspection detail Inspection frequency Inspection coverage Wear Joint position Influence of Operational Practices Tension and tension variation reduced VIV, vessel capacity may limit, increased wellhead loading Joint rotation spread fatigue damage scope may be limited Flex-joint angle limits reduced limits give less wear more downtime Fatigue Damage (1/Years) 2.00E-03 1.80E-03 1.60E-03 1.40E-03 1.20E-03 1.00E-03 8.00E-04 6.00E-04 4.00E-04 2.00E-04 VIV Fatigue at Riser Base 21 INCH Drilling Riser - 6000ft - 12 ppg Mud 4
Riser Joint Rack (2) Deepwater Inspection Strategy Use current time based approach as basis Account for increased wear and fatigue Account for operational practices Variables to Accommodate Drilling in different water depths Different usage of different joints lower rated joints used more often Actual operating conditions may be more or less severe than predicted Requires rigorous usage logging Usage Logging Requirements Operations log riser history Riser stack-ups joint position and dates basis for selective inspection Operating conditions tension, mud-weight, drill string tension, current and wave verify operation and identify extreme events Summary Current inspection practices must be extended for long term deepwater drilling Rationalisation dependent on: depth and environmental conditions time each joint in service operational practices Logging necessary to verify correct operation and enable selective coverage Experience will enable refinement 5