OCEN 201Introduction to Ocean &

Coastal Engineering

Offshore StructuresJun Zhang

Jun-zhang@tamu.edu

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. 3-2Example of jack-up drilling rig

Legs are retractable

Fig. 3-3A semi-submersibleDrilling 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

Fig. 3-10 pp56New version Fig. 12 pp61

Articulated Tower

Fig. 3-11 pp56New Version Fig.13 pp62

Single Anchor Leg MooringSystem

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 /

Peak (amplitude of) vlocity of the flow Period Diameter

m

m

K U T DUTDK

25 Particle movement is much greater than

5 255 Particle movement is smaller than

/ 0.2 Wavelength is much greater than / 0.2 Wavelength is not much greater or smaller

DK

K DD L DD L

than D

Morrison Equations & Modified ME

12

----- Drag Coefficient, ------ Volume ---- Added Mass Coefficient----- Projected area (normal to current)

Additional term due to votex induce (lateral) force12

in d A m V

d V

m

A

l L

dd C d C ddt

C dCd

d C

nn n

uF u u

F 2

0 0

cos(2 ),

, ------- Strouhal Number

----------- Lifting Coefficient

A

L

d f t

fDS S

C

n

n

u

u

Wave Forces on A Vertical CylinderVelocity & acceleration are a function of z & t Force (function of t)

2

0 0

02

12

, , 4 2

12

+4

(3 4)

in d A m V

A z V z

in in d zh h

m zh

dd C d C ddtDd Dd d D d R

d C D d

dD C ddt

nn n

n n

n

uF u u

F F u u

u

Wave Forces on A Vertical Cylinder

Wave Forces on a Horizontal CylinderVelocity & acceleration are a function of t only

2

0

2

12

, , 4 2

12 4

------ Length of the cylinder

in d A m V

A z V z

in inh

d m

dd C d C ddtDd Dd d D d R

d

dL C D D Cdt

L

nn n

nn n

uF u u

F F

uu u

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

2

1 Inline force2

1 cos(2 ), Transverse force2

in d

l L

C AU U

C AU f t

F

F

Wind Forces

0.113 0.125

10

10

1 , 2

Steady wind velocity

Fluctuated velocity (wind gustiness)Applied at the center of pressure

( )10

- wind velocity at 10 m above the sea

in a d

P

pP

C AU U U U U

U

UZ

ZU Z U

U

F

Forces on Pipeline Due to Wave & Currents

sin 0 (3.33) horizontal

cos 0 (3.34) vertical (3.35) Coeff. of friction

Minimum submerged weight (remain at sea bed)

(3.36)cos sin

d i f

n l

f n

d i l

F F F w

F F wF F

F F Fw

Drag force; inertia forceNormal force; lifting forceFriction force; Submerged weight

d i

n l

f

F FF FF w

lF

nFfF

dFiF

W

Free Body diagram of A Pipe under the impact of Wave & Currents