TW16 floating offshore structures hydro handout tcm14 80891

31/10/2016 OIL & GAS Floating Offshore Structures Hydrodynamics and Structural Analysis 01 November 2016 01 November 2016 SAFER, SMARTER, GREENER Table of Contents  Wave theory  Wave conditions: Short-term vs long-term  Wave loads  Motions  Characteristic response  Design waves (for different unit types)  Air gap /model test  Design loads calculation  Structural analysis /evaluation (yield, buckling, fatigue) 01 November 2016 31/10/2016 Environmental Loads  Wind  Wave  Current  Tide 01 November 2016 Wave Theories 01 November 2016 31/10/2016 Basic Wave Terminology Crest, peak Wave elevation (m) Height -2 Trough -4 Zero-up-crossing period -6 -8 10 15 20 25 Time (s) 01 November 2016 Airy Wave Theory – Linear Regular Wave g T 2   a  sin( t  kx)  • wavelength • wave profile • horizontal particle velocity u  a ekz sin( t  kx) • vertical w  a ekz cos( t  kx) particle velocity Deep water 01 November 2016 31/10/2016 Stokes 2nd order Wave Theory   a sin( t  kx)  3  a  sin2( t  kx)   2   Surface elevation (m) Airy Stokes 2nd 50 100 150 200 250 -2 -4 -6 x-coordinate (m) 01 November 2016 Stokes 5th Wave Theory 01 November 2016 31/10/2016 Validity of Various Wave Theories 01 November 2016 Irregular Waves Linear theory is used to simulate irregular waves by superpositiong N    A j sin( j t  k j x   j ) j Then A2j  S ( j )  The instantaneous wave elevation is Gaussian distributed with zero mean and variance    0 S ( )d 10 01 November 2016 31/10/2016 Wave Conditions 11 01 November 2016 Short-Term Wave Height Distribution Cum.prob or Prob.density 1.4 1.2 0.86 0.8 Cum density 0.6 0.4 Hs 0.2 0 10 15 Wave height (m)   h     FH (h)   exp     H S   12 20 Wave elevation: Narrow-banded, Gaussian process Maximum: Rayleigh distribution 01 November 2016 31/10/2016 Pierson Moskowitz Wave Spectrum For Fully Developed Wind Sea S ()  H S2TZ 8   2 4   2      exp      TZ    TZ  TP TZ 13 01 November 2016 Jonswap Wave Spectrum 14 01 November 2016 31/10/2016 Two Peak Spectrum For Wind Sea and Swell 15 01 November 2016 Examples Now it is your turn  16 01 November 2016 31/10/2016 Short-Term Wave Conditions Example:  Time duration of hours  Zero-up-crossing period 10 s  Significant wave height 10 m  Estimate for most probable maximum wave height by setting the probability of exceedance N   FH (hmax ) N  3 3600  1080 10 hmax  H S 0.5 ln N  10 0.5 ln 10800  18.7m 17 01 November 2016 Long-Term Wave Conditions Wave Height & Period Scatter Diagram 18 Tz (s) Hs(m) 0.25 0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25 6.75 7.25 7.75 8.25 8.75 9.25 9.75 10.25 10.75 11.25 11.75 2.5 3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 21 0 0 0 0 0 0 0 0 0 0 0 17 708 725 83 0 0 0 0 0 0 0 0 0 0 18 755 1600 1151 310 16 0 0 0 0 0 0 0 0 0 328 873 1106 1010 640 187 33 0 0 0 0 0 0 0 98 365 607 744 642 514 407 235 79 0 0 0 0 0 0 0 19 70 198 283 304 293 263 271 256 194 117 31 0 0 0 0 0 39 72 97 78 101 75 86 75 91 91 61 30 19 0 0 0 0 18 15 16 28 16 17 14 16 14 27 22 13 10 0 0 0 0 0 1 1 5 1 0 0 0 0 0 0 0 0 1 0 0 01 November 2016 31/10/2016 Long-term vs Short-term Wave Statistics Long-term: Scatter Diagram Short-term: Single Seastate Short-term: Seastate Contour 100-year Hurricane: Hs = 15.8m Tp = 15.4s Wave period variation has to be included! 19 01 November 2016 Weibull Distribution of Significant Wave Height   h     FHs (h)   exp            1.92,   1.28,   0.53 20 01 November 2016 10 31/10/2016 Structural design checks – design wave analysis 5) Transverse acceleration of deck mass – For units without diagonal braces, the longitudinal acceleration of deck mass will introduce shear force and corresponding bending moments for the columns connecting the upper and lower hulls (deck and pontoon) – For units with diagonal braces the shear force will be experienced as axial force in the braces and shear force with corresponding bending moments in the columns 49 01 November 2016 Structural design checks – design wave analysis 6) Vertical acceleration of deck mass – In most cases, it is not critical for any global structural element in submerged conditions 50 01 November 2016 25 31/10/2016 Structural design checks – design wave analysis 7) Vertical wave bending moment on the pontoon – This response will reach its maximum value at head seas, θ = 0° 51 01 November 2016 Sectional Force RAO 52 01 November 2016 26 31/10/2016 Design Waves Max response = 90% fractile 53 01 November 2016 Buzz Group Discussion Unique features and challenges for other floaters: – TLP – Spar – FPSO 54 01 November 2016 27 31/10/2016 Global Characteristic Response for TLP Squeeze-Pry Loads 55 01 November 2016 Design Wave Selection  Section cuts: vertical and horizontal  Section force calculation – Wave periods: 2s – 30s – Wave headings 0deg-360deg, 15deg interval Motion RAO Sectional force RAO Squeeze-Pry load: Combine Fx and Fy with a vertical section cut 56 01 November 2016 28 31/10/2016 Motion Characteristics of Spar Pontoons, foundations and added mass/heave plates are crucial for moving the heave eigen-period outside the dominating wave periods 57 01 November 2016 Characteristic Response Calculation for Spar Global Response Characteristics: • Global bending: along the depth • Translational acceleration of decks/elevations • Vertical acceleration of decks/elevations • Split force between cylinders (for cells spar) 58 01 November 2016 29 31/10/2016 Global Responses for FPSO 59 01 November 2016 Multi-body Interactions and Airgap 61 01 November 2016 30 31/10/2016 Multi-Body Interaction 62 01 November 2016 Multi-body Interactions 63 01 November 2016 31 31/10/2016 Air Gap Calculation  Air Gap Requirement for Semi – Positive for 100-year event Run-up  Relative motion between structure & wave  Disturbed wave shall be used  Wave asymmetry factor  Local structure can be reinforced against wave slamming, if necessary Upwelling  Horizontal slamming force  Should be checked at early design phase  Calibrate against model test 64 01 November 2016 Air Gap Calculation - Example Offbody points in HydroD • Cover sufficient points below deck • Results for points too close to columns not reliable 65 01 November 2016 32 31/10/2016 Air Gap 66 01 November 2016 Structural Analysis 68 01 November 2016 33 31/10/2016 Design Conditions for Semi Design Phases to Be Considered:  In-Place: Operating, Survival, Accidental  Transit  Failure Modes: Yield, Buckling, Fatigue 69 01 November 2016 Accidental and Redundancy Requirements  Heeled Condition - Max 170 heel - Combined with 1-year environmental loads (allowable factor 1.0) or no environmental loads with allowable factor 0.75  Loss of one brace - Worst scenario to be considered - Combined with 1-year environmental loads (allowable factor 1.0) or no environmental loads with allowable factor 0.75  Local over-stress acceptable provided redistribution for forces accounted for 70 01 November 2016 34 31/10/2016 Design Loads  Wave Loads  Wind Loads on Deck Structure  Gravity (steel, equipment, tank loading, etc.) 71 01 November 2016 Structural Design Analysis 72 01 November 2016 35 31/10/2016 Ultimate Strength Capacity Check  Yield: based on rule scantling check – Longitudinal stress from global FEA – Lateral pressure: rule formulae  Buckling – Longitudinal stress from global FEA – Transverse stress from local FEA (webframe models) – Lateral pressure: rule formulae 73 01 November 2016 Ultimate Strength Check Each wave is represented by ‘real wave’ + ‘imaginary wave’ Combine dynamic wave cases with static cases by stepping through the wave and maximizing the stress at each structural element Scanning for max yield, max compression etc 74 01 November 2016 36 31/10/2016 Fatigue Load Cases  Apply unit waves in all headings and all periods (e.g 2s-30s)  Apply scatter diagram  Apply heading profile 75 01 November 2016 Fatigue Typical Fatigue Locations:  Pontoon to column connection  Column to brace connection  Column to deck connection 76 01 November 2016 37 31/10/2016 Buzz Group Discussion Unique features and challenges for structural analysis TLP, how is it different from Semi? 77 01 November 2016 Typical Parameters To Be Considered 78 Different TLP drafts Tendons to be modelled as springs Tendon flooding, tendon removal & hull compartment(s) flooding Tidal effects, storm surges, set down, subsidence, mispositioning, tolerances 01 November 2016 38 31/10/2016 Thank you DNV GL Oil & Gas www.dnvgl.com SAFER, SMARTER, GREENER 79 01 November 2016 39 ... November 2016 Wave Loads on Offshore Structures • Morison equation • Froude-Krylov theory • Diffraction theory 22 01 November 2016 11 31/10/2016 Loads on Slender Structures Morison Equation:... 31/10/2016 Diffraction Theory Diffraction theory is applicable to the large structures compared to the wave length, especially for the structures which span a significant portion of a wave length Perturbation... forces dominate compared to viscous forces  Potential theory  Fluid flow described by potential  Offshore structure described by surface panels  Pressure on panels from potential  Integrated