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Sirehna's Contribution to WP5 and WP6 - Status and Progress Fabian Pécot

Sirehna's Contribution to WP5 and WP6 - Status and Progress Fabian Pécot. WP5. Status. ………………. Completed ………………. Completed ……….….…... Running. Estimation of the pressure drag coefficients Determination of the pressure distribution Determination of the motions of the buffer bell

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Sirehna's Contribution to WP5 and WP6 - Status and Progress Fabian Pécot

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  1. Sirehna's Contribution to WP5 and WP6 - Status and Progress Fabian Pécot

  2. WP5 Status ………………. Completed ………………. Completed ……….….…... Running • Estimation of the pressure drag coefficients • Determination of the pressure distribution • Determination of the motions of the buffer bell • and the forces at the mooring points of the buffer bell

  3. WP5 Estimation of the pressure drag coefficients

  4. WP5 - Estimation of the pressure drag coefficients Pressure drag force over the DIFIS system generated by the current : FD = 0.5..S.CD.U² where:  : water density (kg/m3), S : projected area (m²), CD : pressure drag coefficient, U : current velocity (m/s) To determine the pressure drag force it is necessary to know the pressure drag coefficient CD  Objective : provide approximate CD values Literature References : - Fluid dynamic drag - theoretical, experimental and statistical information– Hoerner -1965 - Principles of naval architecture – Vol. II – Resistance, Propulsion and Vibration

  5. WP5 - Estimation of the pressure drag coefficients Riser tube riser tube = infinite cylinder CD depends of the Reynolds number : Re = (U . D) /  where U : current velocity (m/s), D : diameter (m),  : cinematic viscosity (m²/s)

  6. WP5 - Estimation of the pressure drag coefficients • Different flow regimes (function of the Reynolds number) : • Re  1.103 laminar flow regime • 1.103  Re  2.105 sub-critical flow regime CDmax = 1.2 • 2.105  Re  5.105 critical flow regime CDmax = 1.2 • 5.105  Re  5.106 super-critical flow regime CDmax = 0.6 • 5.106  Re post-critical flow regime CDmax = 0.6 • Diameter of the riser tube between 1 and 2 m. • Current velocity between 0.1 and 3 knots (between 0.05 and 1.5 m/s). •  Reynolds number between 5.104 and 3.106. • If Re < 5.105 CDmax = 1.2 can be retained • If Re > 5.105 CDmax = 0.6 can be used

  7. WP5 - Estimation of the pressure drag coefficients Influence of the roughness Roughness ,  critical flow regime occurs at lower Re numbers (< 2.105) Furthermore, the fall in the CD values (during the critical regime) becomes less and less significant as the roughness  And, at post-critical regime, CD values tend to 1 (instead of 0.6) External wires will be added to provide structural axial strength. These wires will give additional roughness. This additional roughness can be taken into account by increasing to 1 the CD for the post critical regime.

  8. WP5 - Estimation of the pressure drag coefficients • Influence of the angle of attack • For an angle of 70° between the cylinder axis and the flow direction 2 cases can be retained : • Re  2.105 (sub-critical regime), CD can be multiplied by 0.8, • Re  2.105 (critical or post-critical regimes), CD are unchanged. Sub-critical regime Critical regime

  9. WP5 - Estimation of the pressure drag coefficients Buffer Bell buffer bell = smooth cylinder with a finite length The drag coefficient of a finite cylinder is : CDfinite = K . CD where K is a reduction coefficient. It depends of the length-diameter ratio of the cylinder and of the Reynolds number For the configuration studied (velocity), the regime is post-critical  CD = 0.6 (for an infinite cylinder with the same diameter than the buffer bell) For the ratios L/ and the flow regime considered, a conservative value of 0.8 for K can be retained.  a pressure drag coefficient of 0.8 x 0.6 = 0.48

  10. WP5 - Estimation of the pressure drag coefficients Dome dome = cone with a circular section To roughly estimate the drag coefficient of the dome (for early design), the shape of the cone can be approximated by a superposition of cylinders Therefore, the total pressure drag force can be expressed as follows : The projected area Si and the current velocity Ui depend of the height. The drag coefficient CDi corresponds to the drag coefficient of a cylinder. A conservative value of 0.6 can be retained (infinite cylinder, post-critical regime)

  11. WP5 Determination of the pressure distribution

  12. WP5 - Determination of the pressure distribution Theoretical pressure distribution due to the current over the riser tube Hypothesis : perfect circular cylinder and inviscid flow The pressure coefficient Cp Cp = (1-4sin²) • Effect of viscosity : •  separation of the flow •  lower average pressure on the rear half of the cylinder compared to the front half

  13. WP5 - Determination of the pressure distribution Pressure distribution due to the current over the dome Hypothesis : rigid circular cone and inviscid flow (perfect fluid) Calculation done by using the potential flow calculation tool : REVA Current direction

  14. WP5 - Determination of the pressure distribution Pressure distribution due to the current over the dome Calculations done by using the Navier-Stockes solver Fluent version 6  the viscosity of the fluid is taken into account Hypothesis : rigid circular cone water (at 3°C, 4000m depth): : 1043.7 kg/m3,  : 1.73 10-3 kg/m.s uniform current speed : 0.0297 m.s-1 k-e turbulence model + standard wall function 2 different cases have been modelled: dome lying on the seabed and dome being 80 m over the seabed

  15. WP5 - Determination of the pressure distribution Dome lying on seabed Cp = Pressure / (0.5 *  * velocity^2) Cp distribution and streamlines Comparison of Cp obtained with potential and viscous methods viscous : large area of lower pressure compared to potential

  16. WP5 - Determination of the pressure distribution Dome at 80m from seabed

  17. WP5 Motions of the buffer bell and forces

  18. WP5 - Motions of the buffer bell and forces Seakeeping calculations on the buffer bell have been performed with our seakeeping code AQUA+ AQUA+ solves the 3D linear radiation-diffraction problem for any body, with or without mooring links, modelled through equivalent stiffness. It has a statistical post-processor ALEA+ to calculate the statistical behaviour in given wave spectra. Objectives : provide first information on the behaviour of the buffer bell and forces in mooring lines, for some variants, to be used within the early design phase.

  19. WP5 - Motions of the buffer bell and forces • Inputs data : • 3 types of buffer bells : A1250 and A6250 (cylindrical), B1250 (cylindrical with spherical top end) • Vertical position below the surface : 30 m • Depth : 4000 m • Axial Stiffness of the Dyneema lines : K33 (6 lines), 4*K33 ,8* K33, • Net buoyancy : buffer bell filled with oil or filled with water • Vertical position of CoG : 60% or 50% of the height • Incidence of the waves : 0 to 180 degrees (step: 15°) • Outputs data : • Transfer Functions of motions of the buffer bell • Extreme motions and forces in mooring lines

  20. WP5 - Motions of the buffer bell and forces 3 types of buffer bell have been studied type A - 1250 m3 type B - 1250 m3 type A - 6250 m3

  21. WP5 - Motions of the buffer bell and forces • Additional stiffness (mooring) and added damping • The mooring of the buffer bell is achieved by the 6 dyneema lines of the riser tube  a linearised additional mooring stiffness matrix (6x6) has been determined for each configuration • The viscous effects have been taken into account by adding linear damping coefficients (the damping coefficients used are identical to those used by MARIN)

  22. WP5 - Motions of the buffer bell and forces First results - Natural periods • Large surge and sway natural periods  outside the range of wave periods • The naturals periods of the other motions could be inside the range of wave periods Influence of the axial stiffness If the stiffness   the natural heave period 

  23. WP5 - Motions of the buffer bell and forces First results - motion RAOs - Surge (at CoG) - axial stiffness : K33 (6 lines) A1250 A6250 A1250 : maximum 0.2 m/m for T = 9.5 s (coupling with pitch) A6250 : maximum 1 m/m for T = 20 s (coupling with pitch)

  24. WP5 - Motions of the buffer bell and forces First results - motion RAOs - Heave (at CoG) - axial stiffness : K33 (6 lines) A1250 A6250 A1250 : maximum 3.5 m/m for natural period A6250 : maximum 4 m/m for natural period

  25. WP5 - Motions of the buffer bell and forces First results - motion RAOs - Pitch (at CoG) - axial stiffness : K33 (6 lines) A1250 A6250 A1250 : maximum 2°/m for natural period A6250 : maximum 3.7°/mfor natural period

  26. WP5 - Motions of the buffer bell and forces First results - influence of the stiffness - A1250 - incidence 0° • No influence for surge and pitch motions • Heave • K33 (6 lines) : 3.5 m/m, T=16.4s • 4*K33 : 0.8 m/m, T = 8.2s • 8*K33 : 0.2 m/m

  27. WP5 - Motions of the buffer bell and forces First results - influence of the stiffness - A6250 - incidence 0° • No influence for surge and pitch motions • Heave • K33 (6 lines) : 4 m/m, T=27.3s • 4*K33 : 3 m/m, T = 13.6s • 8*K33: 1.2m/m, T=9.6s

  28. WP5 - Motions of the buffer bell and forces Extreme Motions A1/3 with ISSC Spectrum Incidence 0° • Maximum surge single amplitude • A1250 : 0.86m (Hs =12m and T0=10.5s) • 7% of Buffer bell diameter • A6250 : 2.46m (Hs=11 m and T0=11.5s) • 17% of Buffer bell diameter • Maximum pitch single amplitude • A1250 : 9° (Hs =12m and T0=10.5s) • A6250 : 6.5° (Hs=9 m and T0=12.5s)

  29. WP5 - Motions of the buffer bell and forces • Maximum heave single amplitude A1250 • K33 (6 lines): 10.5m (Hs=11 m, T0=11.5s) • 84% of Buffer bell diameter • 4*K33 (4*6 lines): 2m (Hs=12m, T0=11.5s) • 16% of Buffer bell diameter • 8*K33 (8*6 lines): 0.5m (Hs=12 m,T0=10.5s) • 4% of Buffer bell diameter • Maximum heave single amplitude A6250 • K33 (6 lines): 4m (Hs=9 m, T0=12.5s) • 28% of Buffer bell diameter • 4*K33 (4*6 lines): 8m (Hs=12m, T0=10.5s) • 55% of Buffer bell diameter • 8*K33 (8*6 lines): 2.5m (Hs=12 m,T0=10.5s) • 17% of Buffer bell diameter

  30. WP5 - Motions of the buffer bell and forces • Maximum tension single amplitude A1250 • K33 (6 lines): 500t (Hs=11 m, T0=11.5s) • 4*K33 (4*6 lines): 380t (Hs=12m, T0=10.5s) • 8*K33 (8*6 lines): 240t (Hs=12 m,T0=10.5s) • Maximum tension single amplitude A6250 • K33 (6 lines): 200t (Hs=9 m, T0=12.5s) • 4*K33 (4*6 lines):1600t (Hs=12m, T0=10.5s) • 8*K33 (8*6 lines): 1000t (Hs=12 m,T0=11.5s)

  31. WP5 - Motions of the buffer bell and forces • First conclusions for the configurations studied : • Maximum surge single amplitudes are 7%of buffer bell diameter for A1250 and 17% of buffer bell diameter for A6250 acceptable • Maximum pitch single amplitudes are 9° for A1250 and 6.5° for A6250. • The maximum allowable pitch motion is 9° • For A1250, the large single heave amplitude is 84% of buffer bell diameter for stiffness = K33 (6 lines)  not acceptable • For A6250, the large single heave amplitude is 55% of buffer bell diameter for stiffness = 4*K33 (4*6 lines)  not acceptable

  32. WP5 - Motions of the buffer bell and forces • Next calculations : • Buffer Bell A1250 and A6250 : • influence of the buoyancy • influence of the depth (finite depth) • influence of the vertical position of the CoG • Buffer Bell B1250 • Report to follow in December

  33. WP6 - Conceptual Hydraulic Calculations Task 6.1: Status ……………………………………. Completed ……………………………………. Completed …………………………..………... Running …………………………..………… No Preliminary study State of the art 2D CFD simulations 3D CFD simulation

  34. WP6 2D CFD Simulations - Reminder • Prestige case scenario • Depth: 4000m • Mass flow rate: 125 tons/day • Wreck: 1/2 ULCC • "Prestige grade" heavy fuel oil: • viscosity: 400 000 cSt • density: 1012 kg.m-3 • surface tension: 40 mN.m-1 • Sea water at 3°C • viscosity: 1.65*10e-6 m2.s-1 • density: 1043.7 kg.m-3

  35. WP6 2D CFD Simulations - Reminder (2) Has been decided not to model the leaks (not known) but the flow at the inlet of the device.

  36. WP6 New strategy: reminder • Validation through a "worst case" approach: • Oil is blocked at the riser inlet • What oil height is necessary in the dome to have sufficient buoyancy force ? • -> Problem is 2D • -> Smaller flow domain • -> Compare to allowable forces on the structure

  37. 2 m WP6 - Initial guess: 2 meters of oil - Corresponding force imposed to the structure : ~900 kg It seems to be a reasonable start - Grid: 28 000 quads Average cell size: 0.02 m - Inlet: target mass flow rate of 125 t/day -> Will the amount of blocked oil increase or decrease? - Inlet diameter: arbitrary - 0.5 m Oil from inlet aimed at merging with "blocked oil"

  38. WP6 Very tedious computations - Non standard - Massive use of Fluent Technical Support - After several weeks of investigations from Fluent France, it appeared that the large difference in viscosity between oil and sea water led to tremendous numerical issues for VOF model. => Solved by the use of a specific discretization scheme for VOF: CICSAM ("hidden" scheme, not accessible to "standard" Fluent users) Very small time step imposed by numerical instability Dt = 0.00025 s 100 seconds of physical simulation time ~ 4 days of CPU

  39. WP6 Riser diameter 2 m Results: - Amount of blocked oil decreases rapidly => additional computations for higher initial oil height not necessary in this configuration - Size of flow structures: 0.1 m to 1 m.

  40. WP6 Riser diameter 1 m Results: - Amount of blocked oil decreases slowly - Riser diameter too close to the size of flow structures => Avoid using 1 m diameter riser

  41. WP6 Limitation in the model: - Unknown oil / riser surface interaction - Hypothesis: no velocity at wall, which is probably not true. => Could be exaggerating friction Expected behavior: riser's wall will be covered with a thin layer of oil.

  42. WP6 • Next steps • Other calculations with the previous 2D model to study the influence of some parameters : mass flow rate, type of oil, height of blocked oil,… • Simulations to study the flow along the length of the riser tube (reflection is still in progress) • Preliminary report on the first results in December

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