OCEN 201
Introduction to Ocean & Coastal Engineering

Offshore Structures

Jun Zhang

Jun-zhang@tamu.edu

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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 ( 800 m)

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Fig. 3-2

Example of jack-up drilling rig

Legs are retractable

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Fig. 3-3

A semi-submersible

Drilling Rig

Mooring systemor Dynamic positioning

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DP -

Dynamic Positioning

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Fig. 3-5

Steel Jacket Platform in 20 200 m waters

< 500 m

Cannot be moved

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OFFSHORE PLATFORM

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Fig. 3-6

Concrete Gravity Structure

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OFFSHORE STRUCTURES

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OFFSHORE PRODUCTION & DRILLING

AUGER TLPOCEAN CLIPPER

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18.bin

OFFSHORE PLATFORM

SPARFIXED JACKETED

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19.bin

DRILLING RIG & SPAR

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20.bin

Fig. 3-10 pp56

New version

Fig. 12 pp61

Articulated Tower

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Fig. 3-11 pp56

New Version

Fig.13 pp62

Single Anchor Leg Mooring

System

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Wave Forces on Offshore Structures

Morrison EquationsDiffraction/Radiation Theory*

(Potential theory, neglect water viscosity)

CFD (Computational Fluid Dynamics)*

(Navier-Stokes Equations, considering water viscosity)

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Morrison Equations & Modified ME

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Wave Forces on A Vertical Cylinder

Velocity & acceleration are a function of z & t

Force (function of t)

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Wave Forces on A Vertical Cylinder

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Wave Forces on a Horizontal Cylinder

Velocity & acceleration are a function of t only

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Wave Forces on a Horizontal Cylinder

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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)

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Wind & Current Forces

Steady & oscillatory portions

Steady current forces

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Wind Forces

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Forces on Pipeline Due to Wave & Currents

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Free Body diagram of A Pipe under the impact of

Wave & Currents

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Keulegan-Carpenter Number (Non-dimension

al)

describing the relation between an oscil

latory flow and

a cylinder /

Peak (amplitude of) vlocity of the flow

Period

Diameter

m

m

KUTD

U

T

D

K

=

----

----

----

>

25 Particle movement is much greater

than

525

5 Particle movement is smaller than

/0.2 Wavelength is much greater than

/0.2 Wavelength is not much greater or

smaller

D

K

KD

DLD

DL