08_bottom stability analysis

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ISSN: 1829-9466 2007 Journal of the Indonesian Oil and Gas Community. Published by Komunitas Migas Indonesia

On Bottom Stability Analysis of Partially Buried Pipelineat Near Shore South Sumatera West Java Pipeline

M. Munari1

, R. Gantina2

, H. Ibrahim3

, K. Idris4

, T. Fahrozi5

1PT Perusahaan Gas Negara, Indonesia. Email: [email protected]

2PT Perusahaan Gas Negara, Indonesia. Email: [email protected] Perusahaan Gas Negara, Indonesia. Email: [email protected]

4Ocean Engineering, Institut Teknologi Bandung, Indonesia. Email: [email protected] Saipem Indonesia. Email: [email protected]

Abstract. Lateral stability of a pipeline is achieved byensuring a balance between external lateral forces and

seabed friction. Stability assessment of submerged

pipelines has been commonly performed in accordancewith DnV RP E305 1988 requirements. Based on thesequence of installation process, which is applied to

the pipeline, an increase of embedment depth willpotentially occur since the pipe will be flooded with

water. This process causes the pipelines submergedweight to increase significantly. Based on OTC 5851paper Forces on Sheltered Pipelines the embedmentleads to an increase of soil resistance and reductionsof hydrodynamic forces experienced by the pipe due tothe less exposed area. The reductions in hydrodynamic

forces are accounted through the modification ofhydrodynamic coefficients. To have this embedment

taken into stability assessment, DnV RP E305 hasstated several requirements to be fulfilled. Anotheraspect that would contribute to pipeline stability is thenature of environment condition of pipeline

surroundings. This has been done by assessing thevisual data of the pipeline. Thus, the most suitable

conditions of the pipeline have been considered. Thispaper exemplifies the consideration of those conditionswithin the assessment of submerged pipeline stability.

Keywords: pipeline lateral stability, submergedpipeline, seabed friction, sheltered pipeline,hydrodynamic coefficients, on-bottom stability, DnVRP E305 1988, OTC 5851.

1. Introduction

An independent on-bottom stability analysis on PTPerusahaan Gas Negara (PGN) offshore pipelinesystem at East Lampung shore has been carried out

based on the measurement of existing environmentalcondition. The analysis is also based on current datacollected by the main contractor during pipeline systeminstallation.

The purpose of this paper is to exhibit the variousanalyses performed within the pipeline zone. The

stability analysis was carried out in accordance to Ref

[1] requirements, PGN specifications, and existingenvironmental parameters. The pipeline had beeninstalled for about four month after its initialinstallation and has been scheduled to be flooded bywater for hydro-testing purpose. Thus, the analysis will

consider both the existing (hereby called installation)and operating conditions, which include:

1. Reviewing the visual documentation of ROVSurvey;

2. Soil bearing capacity analysis; and3. On-bottom stability analysis for various water

depths within the zone of concern.

2. Input Data and Criteria

The met ocean data used in the analysis were collected

from both PGN supplied data and installationcontractor. Environmental data and design criteria usedin the analysis are stated in the following sub-section.

2.1 Water Depth

Based on Ref [2], the water depth within the zone of

concern is varying from approximately 13 m to 16 m.

2.2 Geotechnical Data

Geotechnical data at the zone of concern are supplied

by the installation contractor. The sand was found to

have bulk density () of 1674.00 kg/m3

and apparent

soil density of (s) 649 kg/m3.

2.3 Wave Data

Wave data at the zone of concern is provided in Table

1. Wave data was collected prior to the pipeline design.

Munari, Gantina, Ibrahim, Idris, Fahrozi - 46
http://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=bmailto:[email protected]://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=bmailto:[email protected]://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=bhttp://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=bmailto:[email protected]://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=bmailto:[email protected]://sg.f575.mail.yahoo.com/ym/[email protected]&YY=80831&y5beta=yes&y5beta=yes&order=down&sort=date&pos=0&view=a&head=b


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Table 1. Wave data at the zone of concern

Return Period (year)Description Notation Unit

1 5 10 25 50 100

Significant Wave

Height Hs m 1.90 3.29 3.58 3.84 3.96 4.13Period Ts sec 5.60 7.34 7.65 7.92 8.04 8.21

Length Ls m 47.2 72.2 76.7 80.5 82.1 84.4Steepness (H/L)s 0.04 0.05 0.05 0.05 0.05 0.05

Maximum Individual Wave

Height Hmax m 3.42 5.92 6.45 6.92 7.13 7.43Period Tmax sec 7.28 9.54 9.95 10.30 10.45 10.67

Length Lmax m 82.9 142.0 154.5 165.6 170.5 177.7Steepness (H/L)max 0.04 0.04 0.04 0.04 0.04 0.04

Table 2. Current velocities at the zone of concern

Consideration Average WD (m)Surface Velocity

(m/s)Velocity at 1m Above Seabed

(m/s)

6.22 0.910 0.701

7.20 0.910 0.686Installation Phase

6.53 0.910 0.696

9.25 0.910 0.662

9.50 0.910 0.660

9.53 0.910 0.659

13.00 0.910 0.631

Operation Phase

18.04 0.910 0.602

2.4 Current Data

Current data is supplied by installation contractor. Thedata was collected during the pipeline installation, Ref

[3], at the zone of concern. Current data can be seen in

Table 2. To show the increase of current speed betweendesign and installation, Table 3 that is extracted fromRef [4] consists of current data collected prior topipeline design.

2.5 Hydrodynamic Coeffic ient

In accordance with Ref [1], the drag coefficient, liftcoefficient, and added mass coefficient are taken as

0.7, 0.9, and 3.29, respectively.

2.6 Pipe Specifications

Pipeline used at the zone of concern has parametersthat are described in Table 4. These values are takenfrom Ref [3] p. 17.

3. Methodology

3.1 General

Lateral stability of a pipeline is achieved by ensuring abalance between the external lateral forces and the

seabed friction. Assessment of lateral stability hascommonly been performed in accordance with Ref [1]

applied through simplified computer calculation. Thisis a recommended practice, which gives two methodsfor static stability assessment: Generalized Stability

Analysis and Simplified Static Stability Analysis.The latter is used in present application.

The pipeline stability is mainly a function ofhydrodynamic forces, submerged weight of the

pipeline, and soil characteristics. Since the pipeline hasbeen resting at the sea floor for about four month, theanalysis will consider installation condition based ondocumentation and results of latest survey. Theoperating condition will also be analyzed since thepipeline is going to be flooded for hydro test. Thus,

based on documents received in supporting presentanalysis, both qualitative and quantitative assessments

will be exercised.

The pipeline weight that will be used in installationcondition is the weight of a new empty pipeline since itadequately represents the current condition. Inoperating condition, the weight of fluid within thepipeline is accounted, and the reduction of pipe wall

thickness due to 10% corrosion is considered as well.



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Table 3. Current velocities for the zone of concern prior to pipeline design

Return Period (year)Description Notation Unit

1 5 10 25 50 100

Current Speed *

0% water depth v0 m/s 0.68 0.70 0.71 0.73 0.74 0.7610% water depth v10 m/s 0.67 0.67 0.67 0.67 0.67 0.67

20% water depth v20 m/s 0.67 0.67 0.67 0.67 0.67 0.6730% water depth v30 m/s 0.66 0.66 0.66 0.66 0.66 0.6640% water depth v40 m/s 0.65 0.65 0.65 0.65 0.65 0.6550% water depth v50 m/s 0.64 0.64 0.64 0.64 0.64 0.6460% water depth v60 m/s 0.62 0.62 0.62 0.62 0.62 0.6270% water depth v70 m/s 0.60 0.60 0.60 0.60 0.60 0.6080% water depth v80 m/s 0.57 0.57 0.57 0.57 0.57 0.5790% water depth v90 m/s 0.53 0.53 0.53 0.53 0.53 0.53100% water depth v100 m/s 0.47 0.47 0.47 0.47 0.47 0.47

*) average depth is 14.86 meters

Table 4. Pipe parameters

Parameter Value

Pipe outside diameter 32 (812.8 mm)Wall thickness 0.75 (19.05 mm)Corrosion allowance 3 mmPipe grade SAWL DNV 450 I FUDSpecified Minimum Yield Strength (SMYS) 450 MPaSpecified Minimum Tensile Strength (SMTS) 535 MPaAnti-corrosion Coating 3LPE (2.5 mm thick)

3.2 Visual Assessment

Visual assessment was performed to asses the ROV

record, which exhibits the latest pipeline condition, onqualitative basis. The assessment was carried out bywitnessing the record of Ref [5] and note anydistinguish findings based on both visual and audiorecord. The ROV record is continuous over time;

selected visual findings were captured and interpreted.Capturing will be exercised in two ways. First, ROVrecord was captured for an approximate interval of50m within the zone of concern. Each capture, whichmay also contain comments from the ROV operator,was interpreted qualitatively, such as:1. The pipeline was resting on the seabed;2. The pipeline was partially buried by sediment; and

3. The pipeline was fully buried by sediment.

The meaning of visual assessment was exercised by

capturing distinguish findings found from the observedzone. Distinguish findings were identified by clearvisual of pipeline that was fully buried, side visual ofthe pipeline and comments from the ROV operator

regarding his opinion about the condition of thepipeline. The assessment was intended to show the

level of sedimentation occurrence in the pipeline,which may increase the pipeline stability. Figure 1 andFigure 2 depict results that can be acquired from theassessment.

Figure 1. Distinguished finding; found at the waterdepth of 15.54m

Figure 2. Distinguished finding; found at the waterdepth of 15.71m



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Table 6. Modified hydrodynamic coefficients

Hydrodynamic CoefficientsCondition d/D (%)

CD CL CM

Installation 4.0 0.668 0.859 3.191Operating 7.7 0.639 0.822 3.043

Table 7. Results of lateral stability analysisFOS

Condition Critical Water Depth (m)Un-modified Modified

Installation 15.00 0.9868 1.0235

Operating 15.00 1.0210 1.1091

Table 8. Results of vertical stability analysis

Condition FOS

Installation 1.56Operating 1.58

The water depth at the zone of concern is varying from

approximately 13 m to 16 m.

Within the area of concern, untrenched area is found tobe most important at water depth of more than 15meters.

As stated in MIGAS regulation requirements, pipelinelaying at sea bottom less than 13 m shall be buried 2

meters.

4. Results and Analysis

4.1 Visual Assessment

The result acquired from capturing ROV record withinthe zone of concern by an approximate interval of 50 mis that the sediments were found at the surroundings of

pipeline.

4.2 Soil Bearing Capacity

By considering the installation condition of the pipelineand the empty pipeline with no corrosion, the pipelineembedment is 41.86 mm. In operating condition, thepipeline weight during hydro test will cause an

embedment of 79.25 mm. These calculations wereperformed using the assumption of 25 internal soilfriction and the values of bearing capacity factors takenfrom Ref [7].

4.3 Pipeline Stabili ty

Lateral stability analysis using measured current data

shows that the critical depth of 15.00 m, which is thedepth of unburied pipeline. At this depth, FOS ininstallation condition is 0.9868 and in operatingcondition is 1.0210, while the required FOS accordingto Ref [1] is 1.0.

The main results that are found from lateral stability

analysis after considering the modification ofhydrodynamic coefficients during Installation Conditionare that the stability can be achieved for the criticalwater depth with an FOS of 1.0235. While duringOperating Condition FOS for this depth, 15.0m, is1.1091. However, to validate the reduction in

hydrodynamic forces due to pipeline embedment/partialburial, Ref [1] Section 3.3.5 states four considerationsthat should be accounted in the stability calculation. Theanalysis also shows that the stability is increasing as theincreasing of water depth. The FOS found from verticalstability analysis is 1.56 for installation condition and1.58 for operating condition.

5. Conclusions

Several outlines can be withdrawn from the analysis:

1. Sedimentation process has been occurring in thepipeline. This is found from observing the ROVsurvey data at the area of concern. The sedimentwill likely to improve pipeline lateral stability if the

burial happens. No quantitative measures can yet bemade from present assessment. To acquire the

adequate measures from sedimentation

phenomenon, specific field survey needs to beconducted.

2. Pipeline embedment has increased the pipelinelateral stability. This fact can be seen from the

results of Installation Condition and OperatingCondition.

3. Based on the sequence of installation process,which is applied to the pipeline, an increase inembedment depth will potentially occur since thepipe will be flooded with water. This processcauses the submerged weight to increase

significantly and the lateral stability as well. Sincethe hydro test will be carried out anytime soon, the



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pipeline will tend to meet Operating Condition.Thus, the pipeline is concluded to be stable.

4. Ref [1] states four considerations that should beaccounted for in the stability calculation.

6. References

[1] DNV RP E305, On Bottom Stability Design ofSubmarine Pipelines. October 1988.

[2] PGN. 30 Pipeline Routing AlignmentKP12.5 KP15.0 (Sheet 7 of 42), Revision 1,

Doc No. 002-42-L-DG-1010. September 27th,

2006.[3] SSWJ Document. Re-evaluation of On-Bottom

Stability Based on Actual Current ObservationsDuring Pipe-laying Phase, Revision A, Doc No.002-42-L-RE-2049. October 26th, 2006.

[4] PGN. Ocean Environment Analysis for Gas

Transmission and Distribution Project. May2004

[5] PGN. ROV Survey video documentation.[6] Tyrant, P. L. Seabed Reconnaissance and

Offshore Soil Mechanics for the Installation ofPetroleum Structures. Paris: Editions Technip,1979.

[7] DnV. RP F105, Free Spanning Pipelines.March 2002.

[8] Jacobsen, V. Forces on Sheltered Pipelines,Offshore Technology Conference 5851. Houston:OTC, 1988.

7. Biographies

MuhammadMunari, who graduatedfrom Institute Teknologi SepuluhNovember is a senior offshore engineer

at PT. Perusahaan Gas Negara(Persero) Tbk. He is involved in many

of PGNs projects, onshore andoffshore, from surveying, designing,installation, pre-commissioning and

commissioning. Currently coordinating offshore section

of PGNs South Sumatera West Java gas transmissionand distribution project phase I and phase II.

Hasanuddin Ibrahim, graduate from

Ocean Engineering of Institut TeknologiBandung (ITB) is an offshore engineerof PT. Perusahaan Gas Negara(Persero) Tbk. He is involved in PGNsproject for the last 4 years; especiallyoffshore sections from surveying,designing, installation, pre-

commissioning and commissioning. Currently finishingphase I (105 km 32) and phase II (160 km 32)

offshore pipeline that connecting Sumatra and Java.

Rikrik Gantina is an offshoreengineer. He is involved in manyoffshore project and assessment from2002-2003 in PAU-LAPI ITB. Since2003, he has been involved insurveying, designing, installation,

pre-commissioning andcommissioning of offshore pipeline project phase I (32inches, 105 Km) and Phase II (32 inches, 165 Km) inPT. PGN. He holds BS degree in Ocean Engineeringfrom ITB.

Krisnaldi Idris is lecturer/researcherand ocean engineer. He has beeninvolved in various oceanengineering related projects, and has

developed knowledge on the variousfluid-structure interaction issues,including hydrodynamics around

cylindrical bodies. He was graduatedfrom ITB, and obtained MSc degree and PhD degree inCivil Engineering (emphasize in Ocean Engineering) at

the Oregon State University.

Taufik Fahrozi graduated from

Ocean Engineering, ITB, in 2006.He is involved in several fixedplatforms and pipeline analysisduring his undergraduate years. He

currently works as junior engineer atPT Saipem Indonesia.

8. Nomenclatures

' = Submerged density of the soil ( = 1);

= Soil friction factor;w = Mass densityof seawater; = Phase angle of the hydrodynamic

force in the wave cycle;

As = Significant acceleration perpendicularto the pipeline (= 2Us/Tu);

B = Buoyancy of pipe;

CD = Drag force coefficient;CD = Modified drag force coefficient;CL = Lift force coefficient;CL = Modified lift force coefficient;CM = Inertial force coefficient;

CM = Modified inertial force coefficient;cu = Cohesion of soil;D = Total outside diameter of the pipe;d = Depth of to which foundation is

buried;FD = Drag force;FI = Inertia force;FL = Lift force;

FW = Calibration Factor;FOS = Factor of safety;L = Width of foundation;



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Nc,N,Nq

= Dimensionless coefficients dependingon the angle of friction of the soil;

q = Maximum bearing capacity;Uc = Current velocity perpendicular to the

pipeline;

Us = Significant near bottom velocityamplitude perpendicular to the

pipeline; andWs = Submerged weight of pipeline.


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