Design of subsea rigid pipelines The free span · PDF file · 2013-12-06Design of subsea rigid pipelines – The free span probl em 4th COPEDI Forum / 2013 Comitê Offshore para Pesquisa,

Design of subsea rigid pipelines – The free span probl em

4th COPEDI Forum / 2013

Comitê Offshore para Pesquisa, Desenvolvimento e Inovaç ão

11/11/2013

Design of subsea rigid pipelines Design of subsea rigid pipelines The free span problemThe free span problem


CONTENT :

• Overview of pipelines design process;

• Free span definitions;

• Free span project;

• Free span mitigation;

• Structural integrity monitoring;

• History of failure;

• Design methodology - DNV RP F105;

• Sensitivity studies;

• Conclusions.


Route Selection:

• Geofisical survey

• Geotechnical survey

• Engineering analyses

SubseaSubsea PipelinePipeline Project Project


For example, the route can be optimized to minimize the free span occurrence byavoiding seabed unevenesses. This may result in a longer route but with a better

budget because less supports will be need.

On-Bottom roughness simulation

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15 supports-70,0

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5 supports

Saipem

The geofisical and geotechnical survey will result in information about seabed unevenness and soil characteristics. This will be used along with other information (such as the presence of blockages) to optimize the route and minimize the overall pipeline costs.

.


SubseaSubsea PipelinePipeline Project Project

This chart shows the main

steps of a rigid submarine

pipeline design. The goal here

is not to go into detail on this

chart, but to see that the

process is long and the free

span analysis is only one of the

subjects of the pipeline design.


• Fluids and Flow characteristics

• Material Selection

• Calculation of the steel thickness (due to internal pressureand hydrostatic collapse)

PipelinePipeline Design Design –– StructuralStructural AnalysesAnalyses


• Installation analysis

• On-Bottom Stability

• Free Span analysis

• Global buckling

PipelinePipeline Design Design –– StructuralStructural AnalysesAnalyses


FreeFree SpanSpan AnalysisAnalysis

L

• Definition: A pipeline is called to be at free span when a pipe segment is not supported by the seabed.

• Reasons: Uneven seabed, pipeline crossings, tie-in to subseastructures, sleeper, soil scouring, sand waves, etc.

• Outcome: Failure due to maximum stress or fatigue due to VIV or direct wave.

span shoulder


The free spans are calculated for 3 soil properties estimations (LB, BE, UB ). The soil stiffness is an important parameter, that determine de pipe accommodations at the soil and hence the free span length.

After the pipe is laid, the as-laid survey is performed in order to confirm the foreseen free spans. Then, the necessary corrections are done. After that corrections the as-built survey is performed in order to register the pipe final condition.




This table shows a typical result of a free span analysis. It ispossible to see the position of the free span along the

pipeline, its length and if it is acceptable or not.


When a free span is not acceptable, mitigating solutions shall be adopted. Nowadays, there are several techniques available on the market. The most used is the placement of supports in intermediate positions of the span. It reduces the length of thespan increasing its fundamental frequency and consequently taking the span out of the VIV excitation zone.

FreeFree SpanSpan MitigationsMitigations

L



Another solution that begins to be more used recently is the placement of vortex suppressors at the free span. The most common are the strakes and fairings. This solution reduces, or virtually eliminates the VIV fatigue at the span. Recently, Petrobras adopted the use of strakes in spans caused by sleepers, since the use of supports are not allowed in this situation.



Another solution is the rock dumping, where rocks with controlled sizes cover the pipe restraining the flow around the pipe and its movement. This solution is more adopted in the north sea due to the availability of raw materials and ships.


Sand waves regions are usually a problem because the sand mounts are moving and hence the traditional support mitigation would not be effective.So the objective of that apparatus is to disturb the flow around the pipe with that fin in order to precipitate the deposition of sediments under the pipe.

Another approach to the sandwaves problem could be to

estimate the characteristic lengthand duration of the spans with

studies of the sand wavesbehaviour.


This solution smooths the soil profile in order to reduce the length of

the spans and smooth the shoulder span curvature. This operation

can be performed before or after the pipe lay.


This profile shows how the soil smooth can reduces the free span length.

Saipem-245

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KP(m)

WD

(m)

Pipeline on trenched profile 3m Pipeline on trenched profile 2m Pipeline on trenched profile 1m Pipeline on natural seabed

Trenched Profile 3m Trenched Profile 2m Trenched Profile 1m Seabed profile


Petrobras makes inspections every 5 years at the subsea pipelines. At that inspections one of the verified items are thefree spans conditions. The free spans that are not complying with the design requirements must be assessed and if necessary, intervention works must be done.

StructuralStructural IntegrityIntegrity MonitoringMonitoring

LPN - On Bottom Roughness Analysis - Gravel Support s GD25, GD26, GD27, GD28 - AS LAID Condition

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KP (m)

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BOP (As Laid)

Post Lay IW Profile

Undisturbed Seabed Profile

GD27

GD25

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There are few failures registered. Two cases are well known:

• Cook inlet pipelines – Alaska region. It was caused by high tide currents that eroded the sand soil creating free spans that was subjected to that high tide currents. The result was a total of 14 failures from 1965 to 1976.

• Ping Hu 10” oil pipeline – Submarine pipeline from China east shore. Failure of 2 free spans (31m and 44m long) located approximately 2.5km from coast. According to the design, the pipe should be buried at thatregion, but it had been exposed due to extreme environmental conditions.

Source: OMAE2005-67453

HistoryHistory ofof FailureFailure

Design Design methodologymethodology -- DNV RP F105DNV RP F105

Crossing to the design

methodology. The important thing

to say is that the worldwide

procedure used to calculate

pipeline free spans is the DNV

RP F105. This flow chart was

take from that code and shows

that this methodology comprises

either fatigue calculation due to

VIV and wave and a ULS check.

But, it is well known that usually

the design is limited by the

fatigue calculation.


Free Span Data & Characteristics

Start

Span InterventionDetailed Analysis

“Screening” “Fatigue”RP-F105

“ULS Check”RP-F105 OS

F101

Stop

not OK

OK OK

OK

not OK

not OK

RP-F105


WaveWave Fatigue: Fatigue: Occurs at low depth zones, where the wave orbital

reaches the bottom.

Fatigue calculation procedure:

• Calculate period (T) and velocity (U) of

the water particle at the pipe level

• With U and T calculate hidrodynamics

forces at the pipe.

• Calculate the structural response to thehidrodynamics forces in terms of stress

range (∆σs) and cicles (fv).

• With ∆σs and fv calculate the fatigue damage at the pipe.

As the shape of the wave orbital changes while it approximate to the bottom, waves generates only in-line oscillations.

Fluid particlevelocity

In-lineOscillations


WaveWave Fatigue: Fatigue: Criticality grows fast at lower water depths

Fatigue life at base metal per span length

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

0 10 20 30 40

Depth (m)

Fat

igue

life

- ye

ars

(log

scal

e) L = 20m

L = 40mL = 60mFatigue limit

This graph is a calculation example showing how the fatigue life decreases rapidly at very low water depths.

Rio pipeline 2009 - IBP1168_09


Alternative approach to free spans at erodible seabed

00.10.20.30.40.5

0.60.70.80.9

1

0 500 1000 1500 2000 2500Time (Hours)

Fat

igue

dam

age

(-)

0

10

20

30

40

50

60

70

80

90

Leng

th /

Hei

ght x

10

(m)

Fatigue DamageFreespan lengthHs

0.1

1

10

100

1000

0 50 100 150 200 250 300 350 400

Freespan duration (days)

Fatig

ue d

amag

e

a

0.1

1

10

100

1000

0 50 100 150 200

Persistence above 60 m (days)

Fat

igue

dam

age

b

This approach was presented by Saipem for a pipeline placed in

a severe environment with wave action. This showed good

results, but demands wave and current time series, a model to predict soil behaviour and a statistical work. A validation should

be done and the following topics must be outlined:

• The accumulation of unacceptable fatigue damage may occur during a single storm event when free span may develop excessive leng th ;

• Temporary free spans above a critical length might accumulate larger fatigue

damage than shorter permanent ones.


VIV VIV atat freefree spansspans

The in-line VIV has smaller amplitudes, but twice the cross-flow

excitation frequency. Hence, it is usually what limits the free span

fatigue life. There are two reasosn for that: (1) The higher frequency

increase the fatigue damage; (2) The in-line VIV starts to be excitedwith smaller marine currents.0.3− 0.2− 0.1− 0 0.1 0.2 0.3

1−

0.5−

0

0.5

1Trajetória

y 0 t,

( )

x 0 t,

( )In-line

Cro

ss-f

low

• VIV can produce in-line and cross-flow oscillations.• Usually the span length is restrained by the in-line oscilations.

Fluid particlevelocity

In-lineOscillations

Cross-flowOscillations

VIV Fatigue VIV Fatigue –– DNVDNV--RPRP--F105F105

nR fD

VV

⋅=


The methodology outlined at DNV suggest a

semi-empiric analysis based on experimental

results, dimensionless parameters and a

structural modal behavior.

VIV Fatigue calculation procedure:

• Calculate natural frequencies and respectivenormalized stress amplitude;

• Calculate reduced velocity (V R) associatedwith the incident flow and natural frequencies;

• Take the dimensionless VIV amplitude (A/D) related to calculated V R;

• Impose A/D over the modal normalized stress amplitude to obtain the stress range ( ∆σ∆σ∆σ∆σs);

• The respective vibration frequency (f v) is function of the natural frequency;

• With ∆σ∆σ∆σ∆σs and f v calculate the fatigue damage atthe pipe.RT TDUT 024 2012


Mod 3 – Modelo bi-rotulado do NMRI

UncertaintiesUncertainties atat VIV VIV calculationscalculations

Source: RT TS 020 2010

As the VIV fatigue calculation is based on a semi-empirical procedure, this has some

uncertainties. These graphs show how the response can vary from one experiment to

another and how the several VIV empirical calculations procedures reach different results.


Kristoffer Høyem Aronsen - Phd

Full line – DNV

Dashed line – Aronsen experiment for CL = 0 (rigidcylinder and forced oscillation)

These graph shows that the in-line VIV

phenomenon has a complicated behaviour

and the empirical VIV curve adopted at DNV

is an envelope curve.


VIV VIV atat freefree spansspans –– SensitivitySensitivity to to thethe spanspan lengthlength

1.0E-02

1.0E-01

1.0E+00

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1.0E+03

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

0 20 40 60 80 100 120 140

Span Lenght (m)

Spa

n Li

fe (

year

s)

Span Life - In Line VIV

Span Life - Cross Flow VIV

This graph shows the methodology sensitivity to the span length showing that the in-line fatigue life decreases rapidly when the spans grows over a

certain length.


SensitivitySensitivity studystudy to to manymany parametersparameters

25m long isolated free span.


SensitivitySensitivity studiesstudies

Maximum variationEffect of free span type 43764%Effect of effective axial force (Seff/Pcr) 29200%Effect of free span gap 5226%Effect of S-N curve 715%Effect of soil damping 478%Effect of soil type 345%

• DNV-RP-F105 presents an analytical expressionfor effective axial force that work well for isollatedspans in almost even seabed. But, for more complex cases it is important to make non-linearFE analyses.

25m long isolated free span

Approximated analytical structural response (DNV-RP-F105)

RT TDUT 024 2012

Structural response by FEM

• Important to correct estimate the effective axial force (Seff).

With that result we can observe the parameters that have more influence over the calculation.


SensitivitySensitivity studystudy to to thethe marine marine currentcurrent

Point FMax current = 1.4m/s

Point HMax current = 0.7m/s

This sensitivity study shows the importance to correct estimate the current velocity, as the variation at the results can reach 21k% when you double the maximum velocity at the current histogram.

Cases from RT TDUT 024 2012

in-line cross-flow in-line cross-flowInteracting spans (45 and 27m) 10,6 7200,0 0,2 0,2 6135%Isolated span (95 m) 19,0 30,0 0,09 0,05 21011%Interacting spans (40 and 60m) 10,0 2000,0 0,4 0,4 2400%

Variation for in-line

Point H (max 0.7m/s)Fatigue life (years)

Point F (max 1.4m/s)


ConclusionsConclusions• The free span analysis is an important subject at submarine pipelines;

• At very uneven seabeds the route definition can be refined to reduce theoccurrence of free spans;

• The free span analyses are highly influenced by the VIV phenomenom, which calculation has many uncertainties;

• At very shallow waters the wave have strong influence on the free span fatigue life;

• There are many mitigation solutions available to the free span problem. Themost used by Petrobras are the supports. Recently strakes have been usedat the specific case of sleepers free spans;

• The free span calculation procedure involves many parameters and theresponse is very sensible to them. The effective axial force is an importantone that is already being considered at the new projects. More effort shouldbe done in order to make more precise estimations of the currents and thesoil characteristics;

• Changes at the calculation procedure and metocean data should motivate a reassessment of the pipelines free spans.


Thank youThank you

Documents

Design of subsea rigid pipelines The free span · PDF file · 2013-12-06Design of subsea rigid pipelines – The free span probl em 4th COPEDI Forum / 2013 Comitê Offshore para Pesquisa,