Field observations of

pipeline as-laid embedment in soft clay

Zhao Liang, Geotechnical specialistINTECSEA Singapore

Outline

� Pipeline as-laid embedment introduction

� The estimation of the embedment in the current designing (SAFEBUCK III)

� Field observations

� Numerical simulation

� Soil resistance degradation back-analysis

� Conclusions

Pipeline as-laid embedment

Original Seabed

Lay barge

Stinger

As-laid embedment

Pipeline laying shape

TDZ

Scheme of pipeline installation

Pipeline as-laid embedmentWhy is it important?

Pipeline designings

Stability

Lateral buckling

Axial friction

Heat transfer

…

Pipeline as-laid embedment

Underestimated

Overestimated

More than US$50 million savingRefinement of the design parameters for pipe-soil interaction

Resistance of intact soil

Pipeline as-laid embedment

The pipeline as-laid embedment is usually significant larger than that predicted by its submerged self-weight alone.

→ The prediction of pipeline as-laid embedment is much complicated by its laying process.

Stress concentration

TDZ

Self-weight

Dynamic effects

As-laid embedment

Soil softening

The estimation of the embedment in the designing

SAFEBUCK III

Static pipe-soil resistance expression (fine-grained soil)

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= � ∙ ��� + ����(��)

�2

�′�

�,��� ��

Touchdown factor, n(1~3), (1.3~2.0)--DNV

wstatic

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wdynamic

fdyn, 1.0~2.5 1.0~8.0 (reported)

1. Estimate the stress concentration in the TDZ and the pipeline static as-laid embedment (wstatic) accurately using numerical simulation.

2. Back-analyze the dynamic factor (fdyn) from the field observations, provide more supports to select empirical factor in the designing.

Objectives of the study:

Field observations

Laying conditions:

• The pipeline was directly S-laid on the seabed in the shallow waters of Bengal Bay, Asia.

• The water depth in the studied region is from 50m to 100m.

• During the pipeline installation, the laying rate is relatively fast and Regular sea states with small variations in wave heading were reported.

Field investigations:

• The investigation was performed shortly after installation using sonar scanning and ROVs visual recordings.

• The embedment was collected at an interval of 10 meters along the pipeline route, and the pipeline is nearly 45km long in the investigated region.

Soil properties:

Sections 1 2 3 4soil

descriptionsvery soft to

firm clayvery soft to

firm claysoft clay

soft clay

moisture contant

(%)52 65 66 57

Submerged unit weight γ΄

(kN/m 3)6.37 5.42 5.43 6.09

Gradient of soil strength ρ

(kPa/m)0.947 0.725 0.425 0.525

Field observations

Pipeline properties and laying situations:

Properties/Parameters Section 1 Section 2 Section 3 Sec tion 4

Steel pipe diameter Ds (m)

0.813

Concrete coating thickness dc (mm)

70 95

Submerged pipe weight W p (kN/m) 2.823 4.029

Bearing pressure Wp/D (kPa)

3.197 4.437

Bending stiffness EI (MN·m2)

1.761×103 2.094×103

Investigated route length (km)

16 8.5 7 13

Average water depthzw (m)

98.8 81.5 65.88 53.3

Top tensionT0 (kN)

1800

Lay ratev (km/day)

3.5

Quantification of the field observations

Quantification of the field observations

sectionsField observations

wmin/D wavg/D wmax/D

1 0.475 0.579 0.683

2 0.605 0.715 0.830

3 0.600 0.695 0.789

4 0.633 0.717 0.802

Numerical simulation

Stress concentration in the TDZdepends on the pipeline laying shape, decided by lay tension, water depth, seabed stiffness and pipeline bending stiffness.

Step2: Lift up freelyTDP

z

x

Step3: Tension

Beam element node

Macroelement footing

Stinger

Step1: Gravity

Seabed

h

Step4: move

Step5: pipe-lay

Inflexion pointStinger

Tension

Abaqus simulation steps:

Numerical simulation

Pipe-soil interactions

Macroelement�������

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��(��)

�2

�′�

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First loading:

Unloading:

Reloading:

Numerical simulation

� Simulation-1

� Simulation-2

Numerical simulation results

TDZ

Summaries of pipeline as-laid embedment results

Summaries of pipeline as-laid embedment results

sections

Field observations Numerical results Dynamic factors

wmin/D wavg/D wmax/D wself-weight/D wstatic/D nmax fdyn-min fdyn-avg fdyn-max

10.475 0.579 0.683 0.289 0.377 1.371 1.260 1.536 1.812

20.605 0.715 0.830 0.419 0.552 1.420 1.096 1.295 1.504

30.600 0.695 0.789 0.475 0.609 1.360 1.000 1.141 1.296

40.633 0.717 0.802 0.430 0.567 1.434 1.116 1.265 1.414

Summary of pipeline embedment and dynamic factors

Factors Current recommended Results from this study

Touchdown factor (n)1~3 (SAFEBUCK III)

1.3~2.0 (DNV)1.36~1.43

Dynamic factor (fdyn)1.0~2.5 (SAFEBUCK III)

1.0~8.0 (reported)1.0~1.81

The back-analysis of soil resistance degradation

Intact soil resistanceRemolded soil resistance

• Soil resistance degradation factor, Sremolded

• Sremoldedcan be derived by finding the remolded soil resistance at which the pipeline embedment equals to the field observations.

• The stress concentration (n) in the TDZ decreases as soil become softening.

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Soil resistance degradation

sections

Static results Results with remolded soil

wstatic/D nmax wremolded/D nmax Sremolded

1 0.377 1.371 0.579 1.261 0.531

2 0.552 1.420 0.714 1.342 0.694

3 0.609 1.360 0.695 1.321 0.831

4 0.567 1.434 0.715 1.310 0.699

Soil resistance degradation

• Larger pipeline embedment would lead to higher soil lateral resistance, and the dynamic motions of the pipeline in the TDZ would be limited more due to higher resistance. So the soil resistance degradation becomes smaller.

• Conversely, it can be concluded that the lateral dynamic motions is the major causes of soil resistance degradation in the dynamic laying process.

When the pipeline static as-laid embedment (wstatic/D) becomes larger, the soil resistance degradation caused by the pipeline dynamic motions becomes smaller, and vice versa.

Conclusions

� wstatic, the pipeline static as-laid embedment on the intact soil can be well established using numerical simulation.

� The touchdown factor, n, is estimated to be in the range of 1.36~1.43 from this study.

� The dynamic factor, fdyn, is back-analyzed in the range of 1.0~1.81 from this study.

� The degree of soil resistance degradation is found to be smaller when the pipeline static embedment is larger..

Thank you!