over veiw on desighn of offshore pipelines

01/05/2023CE-6397 SAFE INSTALLATION OF OFFSHORE PIPELINES OVER DEPRESSIONS

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SAFE INSTALLATION OF OFFSHORE

PIPELINES OVER DEPRESSIONS

By:Anupoju Vinod Kumar

M150492CE


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The first “pipelines” were laid in China about 1000 B.C., while the first oil pipeline was built in Switzerland, 1860, it was above 10 km long and 2” in diameter

Credit for the development of pipeline transport: Vladimir Shukhov and the Branobel company in the late 19th century, and the Oil Transport Association

INTRODUCTION

https://en.wikipedia.org/wiki/Vladimir_Shukhov

https://en.wikipedia.org/wiki/Vladimir_Shukhov

https://en.wikipedia.org/wiki/Branobel


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Stresses and Forces acting on the offshore pipelines


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External pressure During Installation and Operational condition It reaches ultimate stresses during installation This external pressure is caused due to wave

loads, current etc. Internal pressure Developed during operation conditions Pipe is exposed to same pressure in all

directions Axial and circumferential stresses are developed

uniformly.


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Causes due to the size, length, and layout of the pipeline, roughness of pipe line and the terrain of the ocean floor.

Induces strain in pipeline that causes stresses at joint

Increase in length will be around 1.5 Inch with temperature difference 110 degrees which pipe line installed at ambient temperature

Temperature stresses


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Longitudinal Compressive stresses

If the axial force increases (F1 < F2 < F3), there will be more deformation of the pipe.Deflection of the mid-span point and compressive stresses will increase and downward buckling of pipe may occur.


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Drag force occurs on pipe line due to current, moving wave towards the pipe line.

Induces instability to the pipeline by moving the pipe line from original position

Lift the pipeline from its original position

Drag Force, inertia Force and lift force


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Effects of stresses in offshore pipelines


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Buckling due to external pressure


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Due to axial force pipe can buckle downwards in a free span, sideways on the seabed or upwards for buried pipelines.

Vertical buckling of a pipeline is called upheaval buckling

For upheaval buckling to occur, the pipeline must first have an initial imperfection.

Buckling due to axial force


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Continuation…


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Hoop stress is greater than the allowable stress, the pipe may burst due to internal pressure.

Burst due to internal pressure


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DESIGN OF OFFSHORE PIPE LINES


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The “regional survey” has to be done to get the sea floor bathymetry.

Route Selection


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Selection of pipe line diameter is the ability to carry fluids at the design flow rates, within the allowable pressure.

For Gas pipe line:

Where Q = cubic ft of gas per 24 h, ID = internal pipe diameter in inches, P1 = pisa at starting point, P2 = pisa at ending point, L = length of pipe line in miles.

Diameter Selection


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For Oil pipe line:

Continuation

For Use

Throughputs( bbl per day)

Pipe outside diameter (inch.) Pressure drop (psi per mile)

0 to 2000 3 1/2 -

2000 to 3000 4 1/2 32

3000 to 7500 6 5/8 16

7500 to 16,500 8 5/8 10.5

16,500 to 23,500 10 3/4 8.5

23,500 to 40,000 12 3/4 7


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API 5L grade X-65 has become the steel grade of choice for deep water offshore pipelines.

X-52 grade for Arctic, has more ductile in nature

Wall Thickness and Grade


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i. Internal pressure containment (burst).ii. Collapse due to external pressure.iii. Local buckling due to bending and external

pressure.

Continuation…


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The relationship between internal pressure(Pid) and the wall thickness(t) is given by:

MSP = maximum source pressure, which equals to well head pressure at the well pipeline interference.

= Internal gas weight from source of elevation to elevation of intrest

F = Construction design factor of 0.72 for the submerged component and 0.60 for the riser

T = Temperature de-rating factor obtained Table 2 E = longitudinal joint factor. Obtained from Table 3

Internal pressure containment (burst).


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Temperature ( Fo ) Temperature De - rating factor

250 or Less 1.00

300 0.967

350 0.933

400 0.900

450 0.867

Temperature de-rating factor, T, for steel pipe(Table 2)


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Spec No. Pipe class E factor

ASTM A 53 Seamless 1.0

Electric resistance welded 1.0

Furnace butt welded - continuous weld

0.6


ASTM A 134 Electric fusion arc welded 0.8

ASTM A 135 Electric resistance welded 1.0

ASTM A 139 Electric fusion welded 0.8

ASTM A 211 Spiral welded steel pipe 0.8


Electric resistance 1.0

Longitudinal joint factor, E (Table-3)


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The pipe collapse pressure Pc must be greater than the net external pressure.

collapse pressure Elastic collapse pressure Plastic collapse pressure

= safety factor 0.7 for seamless pipe.

Check for collapse pressure


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More severe during installation when bending and external Pressure effects are critical.

API RP 11 11 (1999) and DNV OS-F101 (2000) have adequate formulations:

For up to 50, the following equation API RP 1111 (1999) gives: g (

= critical strain (maximum compressive strain at onset of buckling)

= = Critical strain under pure bending g ( = (1+20 = collapse reduction factor = = Ovality

Local Buckling Due to Bending and External Pressure


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The bending strains shall be limited as follows:

= maximum installation bending strain = maximum in-place bending strain= safety factor for installation bending plus external pressure = safety factor for in-place bending plus external pressureand values are 2.0;

Continuation…


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The API RP 11 11 (1999) equation for calculating the propagation pressure(Pp)is as follows:

= 24*Sy*( propagation pressure Following satisfied: = 0.8 No buckle arresters are required.

Buckle propagation


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Buckle Arrestors


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Axial force in offshore pipelines can potentially be in the form of compression or tension.

During operation usually compression force is governed.

Parameters: ; ; ; ;

Bottom less depressions


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Qx,Qy components of seabed reaction force at x = L1

Continuation…


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for for The solution for differential equation 16 and 17 is: for

for

Continuation..


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By applying boundary conditions we get A,B,C and D values as ; ; ;

Continuation..


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Situation arises when compression and tension forces are equal.

Solution to these sections can be written as: for for By applying boundary conditions we get:

No axial force


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Friction factor depends on the type of soil, the pipe roughness, seabed slope and depth of burial.

Loose sand: tan(φ) (generally φ =30o) Compact sand: tan(φ) (generally φ =35o) Soft clay : 0.7 Stiff clay :0.4 Rock and gravel : 0.7

Soil Friction Factor


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Hydrodynamic Force Calculation


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α = angle made by the waves with pipe line. β = current flow angle made with flow

velocity. The drag force, lift force and inertia force are

calculated by the Morrison’s equations:Drag force Lift force Inertia force

Continuation…


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)ξ = safety factor, µ = friction factor. Recommended safety factors are: ξ = 1.05 for installation ξ = 1.1 for operation

Stability Criteria


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pipelines are installed on the seafloor by one of the four typical installation methods:

1. J-lay method2. S-lay method 3. Reel-lay method4. Towed pipelines

Methods of installation


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J-lay method


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Advantages:1. ultra deep-water pipeline installation.2. Suited for all diameters.3. Handling is easy during installation.Disadvantage:4. At shallow depths installations.

Advantages and Disadvantages


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S-lay method


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Advantages: All welds are done on horizontal position. Suited for all diameters

Disadvantage: Pipeline will rotate axially during

installation. Buckle arrestors induces concentrated

higher strains. Requires high horizontal tension.



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Reel-lay method


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Advantages: All welds are done on-shore. Well suited for smaller diameter lines. Very fast installation.

Disadvantages: For large diameter board is not sufficient. Very high pipeline strains During installation pipeline may coil on the

seafloor.



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Towed pipe lines


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Pipe line buckling equation over a free span is presented and the pipeline behaviour under compressive load is discussed.

Pipe line design aspects are discussed.

Conclusions:


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over veiw on desighn of offshore pipelines

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