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OCEN 201 Introduction to Ocean & Coastal Engineering Offshore Structures Jun Zhang [email protected] Offshore Structures •Drilling rigs: Exploration of oil and gas Stay in a place for a few months (Mobil or movable) - Jack-up drilling rig - MODU (Mobil Offshore Drilling Unit) • Production platforms: Production of oil and gas Stay in a place for at least a few years (usually 20 -30 years) - Ground-base structure ( <500~800 m) - Floating Structures (> 800 m) Fig. 3-2 Example of jack-up drilling rig Legs are retractable Fig. 3-3 A semisubmersible Drilling Rig •Mooring system •or Dynamic positioning DP Dynamic Positioning Fig. 3-5 Steel Jacket Platform in 20 – 200 m waters < 500 m Cannot be moved OFFSHORE PLATFORM Fig. 3-6 Concrete Gravity Structure OFFSHORE STRUCTURES OFFSHORE PRODUCTION & DRILLING • AUGER TLP • OCEAN CLIPPER OFFSHORE PLATFORM • SPAR • FIXED JACKETED DRILLING RIG & SPAR Articulated Tower Fig. 3-10 pp56 New version Fig. 12 pp61 Single Anchor Leg Mooring System Fig. 3-11 pp56 New Version Fig.13 pp62 Wave Forces on Offshore Structures • Morrison Equations • Diffraction/Radiation Theory* (Potential theory, neglect water viscosity) • CFD (Computational Fluid Dynamics)* (Navier-Stokes Equations, considering water viscosity) Keulegan-Carpenter Number (Non-dimensional) describing the relation between an oscillatory flow and a cylinder K U mT / D U m Peak (amplitude of) vlocity of the flow T Period D Diameter K 25 Particle movement is much greater than D 5 K 25 K 5 Particle movement is smaller than D D / L 0.2 Wavelength is much greater than D D / L 0.2 Wavelength is not much greater or smaller than D Morrison Equations & Modified ME 1 dun dFin Cd d A un un Cm dV 2 dt Cd ----- Drag Coefficient, dV ------ Volume Cm ---- Added Mass Coefficient d A ----- Projected area (normal to current) Additional term due to votex induce (lateral) force 1 2 dFl CLd A un cos(2 f t ), 2 fD S0 , S0 ------- Strouhal Number un CL ----------- Lifting Coefficient Wave Forces on A Vertical Cylinder Velocity & acceleration are a function of z & t Force (function of t) 1 dun dFin Cd d A un un Cm dV 2 dt 2 D d A Dd z , dV D d z , R 4 2 0 0 1 Fin dFin Cd D un und z h h 2 0 dun 2 + D Cm dz h dt 4 (3 4) Wave Forces on A Vertical Cylinder Wave Forces on a Horizontal Cylinder Velocity & acceleration are a function of t only 1 dun dFin Cd d A un un Cm dV 2 dt 2 D d A Dd z , dV D d z , R 4 2 Fin 0 h dFin 2 dun 1 L Cd D un un D Cm 2 4 dt L------ Length of the cylinder Wave Forces on a Horizontal Cylinder Example of Problem 3-1 pp73 (old v. pp64) Computing the horizontal load on a vertical cylinder Drag coefficient of a cylinder pp 72 & 75 (old v. pp63 & 64) Added-Mass coefficient of a cylinder pp 72 (old v. pp64) Wind & Current Forces Steady & oscillatory portions Steady current forces 1 Fin Cd A U U Inline force 2 1 2 Fl CL A U cos(2 f t ), Transverse force 2 Wind Forces 1 Fin aCd A U U , U U U 2 U Steady wind velocity U Fluctuated velocity (wind gustiness) Applied at the center of pressure Z P Zp U ( Z P ) U 10 10 0.113 0.125 U 10 - wind velocity at 10 m above the sea Forces on Pipeline Due to Wave & Currents Fd Fi F f w sin 0 (3.33) horizontal Fn Fl w cos 0 (3.34) vertical F f Fn (3.35) Coeff. of friction Minimum submerged weight (remain at sea bed) Fd Fi Fl w (3.36) cos sin Fd Drag force; Fi inertia force Fn Normal force; Fl lifting force F f Friction force; w Submerged weight Fi Fl Fd Ff Fn W Free Body diagram of A Pipe under the impact of Wave & Currents