Metocean criteria for fatigue assessment Rafael V. Schiller 5th COPEDI Seminar, Oct 8th 2014.
Metocean requirements along the lifecycle of a project Metocean criteria for fatigue Analysis techniques and Datasets SE Brazil Industry challenges 2
Metocean requirements along the lifecycle of a project Engineering E&P Phase Rig Selection and Riser Design The end use of metocean criteria determines the type of analysis and data sources used. Overview of key metocean processes and hazards for the location. Operational Planning Extreme wind, wave and current (1, 10, 100-year return). Monthly, seasonal and all-year operational wind, wave and current statistics. Current profile characterisation. Offshore Drilling and Operations Overview of key metocean processes and hazards for the location. Monthly, seasonal and all-year operational wind, wave and current statistics. Production - Floating Structures - Fixed Structures Monthly persistence statistics for wind, wave and current. Principal current direction. 3
Metocean requirements along the lifecycle of a project Engineering E&P Phase Rig Selection and Riser Design Overview of key metocean processes and hazards for the location. Monthly, seasonal and all-year operational wind, wave and current statistics. Monthly persistence statistics for wind, wave and current. Operational Planning Monthly, seasonal and all-year operational wind, wave and current joint occurrence statistics (wind speed and direction, wave height and direction, wave height and period, current speed and direction). Monthly, seasonal and all-year maximum current profile. Monthly, seasonal and all-year sea water temperature statistics. Offshore Drilling and Operations Description of wave spectra. Production - Floating Structures - Fixed Structures 4
Metocean requirements along the lifecycle of a project Engineering E&P Phase Rig Selection and Riser Design Extremes (typically for 1, 10 and 100-year return). Directional wind speeds. Directional Hs and associated parameters (Tp, Tz, Hc, Hmax, THmax). Directional current speeds and current profile. Maximum and minimum total water level (tide + surge + wave crest). Tidal water levels (HAT, LAT, MSL etc). Operational Planning Wave scatter diagrams (joint occurrence of Hs and Tp) by direction. Individual wave number of occurrences (H and T) for deterministic fatigue analysis. Offshore Drilling and Operations Characterisation of directional wave spectra. Contour plots giving combinations of significant wave height and peak periods for a 100-year return period. Wind/wave/current angular separation data. Production - Floating Structures - Fixed Structures Air and sea temperature. Ice, iceberg and snow, if any. 5
Metocean criteria for fatigue Fatigue analysis: Looking at a progressive effect of continual forcing (waves, winds, and currents) of all sizes impacting the structure. Waves Criteria: Joint frequency distributions of significant wave Height (Hs) and either peak period (Tp) or mean zero-crossing period (Tz). Assessment of individual wave height (H) and period (T) of all the waves during the specified period. Joint frequency distribution: All-year, monthly, omni-directional, directional (Hs vs direction). Regionalization: including/excluding hurricanes, storms, etc. Wind Criteria: joint frequency distributions, wind speed vs direction. Current Criteria: Joint frequency distributions, current speed vs direction, typically at the surface, mid-depth and near-bed. Current profile characterization. Associated parameters: Maximum wind/current speed associated with each wave fatigue bin. 6
Data sources Measured Satellite Data Modelled 7
Example of metocean criteria for fatigue: SE Brazil Campos Basin. 23.00 S, 40.50 W. Water depth 1500-2000m. Waves and winds: WorldWaves database Based on ECMWF wave models and calibrated against remote-sensing and in-situ data. 10y of wave and wind parameter time series. Hs, Tp, mean direction, Te (Total, wind-sea and swell). 10m wind speed and direction. Location Currents: Global HYCOM Global Ocean Circulation model developed by the HYCOM consortium. 10y of current speed and direction in multiple depths. 8
Waves and winds: Directional histograms (coming from) winds All waves Wind sea Swell 9
Summary picture of driving forces Eric Oliveira Ribeiro (PETROBRAS 4 th COPEDI Seminar) 10
Joint Probability Distributions Percentage, Number of occurrences. Selection of bins, steepness curves. Based on parameter time series: Hs, Tp, etc. All waves (windsea and swell) Fatigue bin with largest occurrence: 1<Hs(m)<2 and 9<Tp(s)<10 11
Fatigue analysis continued Alternative method: assessment of individual wave heights (H) and periods (T) that a structure is likely to encounter during a given return period. WAFO method Estimate an appropriate wave spectrum Hs and Tp. Simulate a Gaussian time series from the wave spectrum. Transform the Gaussian time series to non-gaussian using an appropriate transformation function. Need to account for wave non-linearities that are related to water depth decrease and increase of sea severity. Departure from Gaussian Assumption. Estimate individual H and T from the non-gaussian time series. T = (2πH/(9.81*S)) 0.5, S=steepness Battjes method Based on Hs and Tz (mean zero crossing period). The distribution of individual wave heights is estimated using the modified Rayleigh distribution. 12
WAFO example Omini-directional H and T joint probability distribution for a 10y period. All waves (wind-sea and swell). 10 years T (s) H 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 (m) 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 0 2 841926 4761921 8717639 9208327 6333496 3482138 1429778 488224 133830 31634 6475 2121 173 3 3 0 0 0 0 2 4 0 21450 752703 2101126 1432217 1421294 826700 322879 68917 9722 3340 220 17 0 0 0 0 0 0 4 6 0 0 21352 103443 86028 102250 63252 18111 3782 376 125 7 0 0 0 0 0 0 0 6 8 0 0 198 1786 3076 5555 5736 1579 259 7 0 0 0 0 0 0 0 0 0 8 10 0 0 0 64 273 642 208 67 14 0 0 0 0 0 0 0 0 0 0 10 12 0 0 0 0 0 28 22 0 0 0 0 0 0 0 0 0 0 0 0 12 14 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Fatigue bin with largest occurrence: 0<H(m)<2 and 6<T(s)<8. Additional JPDs per direction cluster: 0 to 337.5, every 22.5. 13
Currents: Directional histograms (towards) Surface: Brazil Current (CB) Soutelino (2008) 800m: Intermediate Western Boundary Current (CCI) 14
Current Profile Characterization A series of characteristic current speed profiles are derived for riser fatigue, operability and design. Reduced number of characteristic profiles. Based on Empirical Orthogonal Functions and on the direct method (Jeans et al. 2002, OMAE2002-28153). 15
Current Profile Characterization Example with bin resolution of 0.4 m/s. 19 characteristic profiles, 1 dominant profile with ~36% of occurrence. If bin resolution = 0.2m/s, 146 characteristic profiles. How many profiles are necessary? 16
Perspectives and Industry Challenges Recommended practice (e.g., API RP2). General guidelines are provided and detailed selection of analysis is usually based on the Industry experience. Regional requirements in relationship with regional climate. SE Brazil Multi-directional, bi-modal wave states. Multi-directional, depth-dependent offshore currents. Co-linear (or not) scenarios. Further challenges to determine the minimum number of representative recurrent sea states. 17
Accounting for directionality Mean wind-sea direction vs Mean swell direction Largely not from the same direction. Mean wave direction (wind-sea and swell) vs Mean surface current direction 18
Accounting for directionality Variations in current direction with depth. Coherent variations. Need to consider a Directional Current Profile Characterization. 19
Accounting for metocean datasets 20
Integrated approach for improved current datasets Combination of measurements and modeling to obtain a validated/calibrated current hindcast suitable for criteria. 21
Concluding remarks There are several methods in place and existing Industry experience regarding metocean assessment for fatigue. However, there is the need to continually improve: Statistical techniques. Metocean datasets. Regionalization aspects. Alignment across the Industry. Standardization. We need to minimized the gap between metocean and engineering groups. Fugro is keen to address those challenges with the COPEDI group and the Industry in Brazil. 22
Thank you. Any questions? Rafael V. Schiller Business Development Manager rschiller@fugro.com