IADC Conference, RIO 2005 Fatigue Monitoring of Deepwater Drilling Risers. Mateusz Podskarbi, Ricky Thethi, Hugh Howells; 2HOffshore Inc

Transcription

1 IADC Conference, RIO 2005 Fatigue Monitoring of Deepwater Drilling Risers Mateusz Podskarbi, Ricky Thethi, Hugh Howells; 2HOffshore Inc

2 How Far the Eye Reaches? Slide No. 2

3 Drilling Riser Fatigue Fatigue loading from: current flow induced vortex induced vibrations (VIV); wave action; vessel motions; operational sources (i.e. pressure changes). VIV responsible for the majority of the fatigue loading. Slide No. 3

4 Drilling Riser Fatigue Life Distribution VIV LONG TERM FATIGUE LIFE ALONG RISER LENGTH Life (years) 1.00E E E E E E E E E E E E E E Conductor Wellhead/BOP/LMRP Conductor Connector Buoyant Joints Slick Joints Length along Riser From Mudline (ft) 13.7 ppg, 1.75 Mlbs TT, 0.3% damping 13.7 ppg, 2.0 Mlbs TT, 1.0% damping 8.56 ppg, 1.5 Mlbs TT, 1.0% damping Min Allowable Life (40 years Conductor, 60 years Riser) Slide No. 4

5 Background of VIV Fatigue Uncertainties in VIV predictions. Impact of the predictions on the operations. Non-scalability of the tank test results. Risers are run in deepwater and high currents all the time. Opportunity to obtain full scale measurements. Riser dynamic response monitoring can be used to assure structural integrity and improve operational envelopes. Slide No. 5

6 Monitoring Objectives Ensure the fatigue structural integrity of the riser. Reduce risk expensive subsea architecture below vessel. Improve operational guidelines: Guide operational decisions in response to high current events. Calibrate analytical VIV predictions and improve riser VIV design practice and operating windows. Ensure appropriate maintenance and inspection strategies. Slide No. 6

7 Monitoring Approach Stand-alone vs Real-time Slide No. 7

8 Monitoring Approach Pros and Cons Stand-alone Pros Simple project interface No umbilical or subsea connections Low CAPEX Readily available Cons Synchronization No real time data May require ROV retrieval higher OPEX Battery and memory limitations Low power instrument options vs. Real-time Pros Real time data access and decision Best data quality No need for retrieval Can accommodate high power instruments Cons High cost Complicated interface Longer lead time Need for umbilical and subsea connections Slide No. 8

9 Monitoring Instrumentation Motions vs......strain Fiber-Optic Bragg Gratings Foil Strain Gauges 3D Acceleration 2D Angular Rates 2D Inclination Induction (LVDT, DVRT) Sensors Slide No. 9

10 Monitoring Instrumentation Pros and Cons Motions Pros Simple structure interface Low cost Measure global response Proven track record in deepwater Cons Data processing to calculate stresses Strain Pros Direct reading of strain Little processing to get fatigue Cons High cost Complicated interface with pipe No response measurements in uninstrumented regions Little track record in deepwater Low subsea reliability of certain instruments Slide No. 10

11 Case Study 1 Objectives: Monitor fatigue at Lower Stress Joint of the CWOR. Real time to assist riser management. Solution: Fiber-optic strain monitoring system connected to riser controls. Slide No. 11

12 Case Study 2 Objectives: Assess conductor fatigue accumulation. Solution: Stand-alone motion logger to monitor stack vibrations. Slide No. 12

13 Case Study 2 cont Vessel Vibrations Lower Stack Excitation LMRP Vibrations Slide No. 13

14 Case Study 3 Objectives: Global response of the riser to assess the fatigue damage accumulation and calibrate VIV analysis. Solution: Set of stand-alone motion sensors. Slide No. 14

15 Case Study 4 Objectives: Measure VIV real-time to optimize vessel drift to mitigate VIV during riser running. Use measured data to set inspection intervals and avoid unnecessary costly inspections. Obtain global VIV response of riser to determine fatigue response over entire riser length. Solution: Combination of real time strain and motion sensors connected to a dedicated umbilical. Slide No. 15

16 Case Study 4 cont Slide No. 16

17 Case Study 5 Objectives: Assess the amount and nature of axial movement between adjacent auxiliary lines. Solution: Stand alone LVDT float sensor. Slide No. 17

18 Summary VIV fatigue damage is a major issue potentially very dangerous. No such thing as standard monitoring system. Matching riser response with monitoring system capabilities. KEY DELIVERABLES: Ensuring structural integrity and reduce risk of operations. Assisting maintenance and inspection scheduling. Improving operational guidelines. Improving design techniques. Slide No. 18