Chapter 10 Pipeline 2012-02-21

8/22/2019 Chapter 10 Pipeline 2012-02-21

1/43

1-2-

3-

4-5-:

6-:

7-8-

9-

10-


2/43

Pipelines Located on Seabed


3/43

1-


4/43

Diameter range :

10cm-2m

Transporting oil , water, gas

Wall design:

Simple steel to sophisticated design

Their content:

High temperature , High pressure

Robot systems called pigs

Clean pipeline , remove obstruction , repairs

2-


5/43

Installation :

On the floor , in trench , buried

Trench keep pipeline in place laterally

Burial

provide thermal insulation

Protection against seafloor hazard

Trenching and Burial done by

Sub-sea jetting

Ploughing systems

-


6/43

Soil weakens during

Laying

buckling

walking

Storm loading

Ploughing and trenching


7/43


8/43

Pipeline route selection

Provide security

Allow the asset to be installed Interaction with other users of the seabed

Assessment of the geological setting

An oceanographic survey

A Geohazard assessment(earthquake-prone regions) A geotechnical survey

A borrow search

-


9/43

Hazards for pipelines

Fishing & Anchoring vessels

:


10/43


Free span


11/43


Submarine landslide and debris flows


12/43


Trenching action of an iceberg keel


13/43


Seabed contain fluid expulsion


14/43

Problem for pipelines

Geological faults Earthquakes and water waves

Liquefaction of a sandy seabed

Allowing a light pipeline to rise from buried depth

Allowing a heavy one to sink in

Hard , rocky seabed

Abrade a pipe

Do not allow to fix pipe in position

Very soft seabed

Allow pipeline to sink in

Existing pipelines

Shoreline

Shallow water movement can be significant

-


15/43

-

o

o

o

o

o


16/43

Towing method:(a) surface tow

(b) controlled depth

3-


17/43

Pipe-laying operation

S-lay method


18/43


J-lay method


19/43


Reel system


20/43

:


21/43

:


22/43

Trenching and burial technologies

Offshore pipelines can be laid

On the seabed

In pre-formed trenches

In trenches formed during laying

In trenches formed after laying


23/43

Jet trenching systems


24/43

Offshore ploughs


25/43

Burial assessment

Burial assessments are carried out prior to commencing a project in

order to determine the ease or difficulty with which a pipeline may be

trenched or buried in a seabed, and to determine the technologies

required and the timescales, and costs.

Burial assessment can include:

Trenchability

Ploughability

Rippability


26/43

Consideration include:

Bearing capacity

Cuttability

Mouldability

Erodability

Sedimentation

Trench side stability

Density , permeability and dilation


27/43

o Lateral bucklingThe pipeline moves laterally across the seabed, sothat an initially straight

pipeline will become curved. Movementscan be up to 20 or more pipeline

diameters, and can affect pipeline lengths of several hundred metres. The

pipeline moves into a shapewhose length is longer than in the unbuckled

position; this relievesthe induced stresses.

4-


28/43

o Upheaval buckling

A buried pipeline curves up, andmay break out above the seafloor,

creating a free span that can extend several metres above the

seabed. The curve is longer than the original shape, so the axial

stresses are relieved. The breakout defeats the objective of burial,

which was to protect the pipelineagainst hazards above the seafloor.


29/43

o Walking

Because the internal fluids carry the temperature and pressure in the

pipe, changes in the temperatureand pressure progress axially along

the pipe. Repeated changescause a cyclic effect, in which a pipeline

on the seafloor expands, moves a small distance axially, then

contracts and moves back;but not all the way due to soil friction. Over

many cycles, the pipelinemoves along its length.


30/43

Consequences

Fatigue

Plastic failure in bending

Rupture of thermal insulation

Cracking or tearing

Water ingress

Loss of flow capacity


31/43

Mechanics of buckling

Lateral buckling :

When the pipeline is started up, the

changing temperature and pressure

cause axial compressive stress to

develop,and the pipe tends to move

upwards where the soil resistance is

least,usually at a place where there is

an upwards overbend in the pipeline

due to the imperfections.


32/43

Mechanics of buckling

Upheaval buckling :


33/43

Geotechnical analysis

Yield locus concept

Yield envelope joins

Load-states that large

displacement occur


34/43

Simplified embedment mechanism

Conventional bearing capacity mechanism,'wished in place' at a

shallow embedment D

Pipeline considered to be aStrip footing

Using the general bearing capacity

equation, the vertical bearing capacityV per unit length of a strip footing of

this width on the surface of a clay of

uniform shearstrength Su is :


35/43


An issue with this calculation is that the penetration of pipelinewill cause heave.


36/43

A conventional mechanism at an embedment deeper than

It is convenient to consider the

pipe as a strip footing at this

depth with a width of D/2. The

general bearing capacity equation

gives :


37/43

Another potential mechanism at deeper embedment (e) and very

deep embedment (f)


38/43

Mechanism for creation of the berms in lateral buckling


39/43


Concept for vertical break-out


40/43

Photographs taken of an experimental in which pipe was

installed in sand in a glass-ended box


41/43

Preventative and remedial

To prevent buckling and walking

Decreasing the driving forces

Increasing the resisting forces

4-


42/43

Preventative and remedial


43/43

Control lateral buckling

Documents

Chapter 10 Pipeline 2012-02-21