08_Bottom Stability Analysis

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

On Bot tom Stability Analysis of Partially Bu ried Pipeline Pipeline at Near Near – Sho Shore re South Sout h Sumatera Sum atera – West West Java Pipeline Pipeli ne 1

2

3

4

5

M. Munari Munari , R. Gantina , H. Ibrahim , K. Idris , T. Fahrozi 1

PT Perusahaan Gas Negara, Indonesia. Email: [email protected] 2 PT Perusahaan Gas Negara, Indonesia. Email: [email protected] 3 PT Perusahaan Gas Negara, Indonesia. Email: [email protected] 4 Ocean Engineering, Institut Teknologi Bandung, Indonesia. Email:[email protected] Email: [email protected] 5 PT Saipem Indonesia. Email: [email protected]

Abstract. Lateral stability of a pipeline is achieved by ensuring a balance between external lateral forces and seabed friction. Stability assessment of submerged pipelines has been commonly performed in accordance with DnV RP E305 1988 requirements. Based on the sequence of installation process, which is applied to the pipeline, an increase of embedment depth will potentially occur since the pipe will be flooded with water. This process causes the pipeline’s submerged weight to increase significantly. Based on OTC 5851 paper “Forces on Sheltered Pipelines” the embedment leads to an increase of soil resistance and reductions of hydrodynamic forces experienced by the pipe due to the less exposed area. The reductions in hydrodynamic forces are accounted through the modification of hydrodynamic coefficients. To have this embedment taken into stability assessment, DnV RP E305 has stated several requirements to be fulfilled. Another aspect that would contribute to pipeline stability is the nature of environment condition of pipeline surroundings. This has been done by assessing the visual data of the pipeline. Thus, the most suitable conditions of the pipeline have been considered. This paper exemplifies the consideration of those conditions within the assessment of submerged pipeline stability. Keywords: pipeline lateral stability, submerged pipeline, seabed friction, sheltered pipeline, hydrodynamic coefficients, on-bottom stability, DnV RP E305 1988, OTC 5851.

The purpose of this paper is to exhibit the various analyses performed within the pipeline zone. The stability analysis was carried out in accordance to Ref [1] requirements, PGN specifications, and existing environmental parameters. The pipeline had been installed for about four month after its initial installation and has been scheduled to be flooded by water for hydro-testing purpose. Thus, the analysis will consider both the existing (hereby called ‘installation’) and operating conditions, which include: 1. 2. 3.

Reviewing the visual documentation of ROV Survey; Soil bearing capacity analysis; and On-bottom stability analysis for various water depths within the zone of concern.

2. Input Data Data and and Criteria The met ocean data used in the analysis were collected from both PGN supplied data and installation contractor. Environmental data and design criteria used in 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 Geotechni cal Data Data

1. Introduction An independent on-bottom stability analysis on PT Perusahaan Gas Negara (PGN) offshore pipeline system at East Lampung shore has been carried out based on the measurement of existing environmental condition. The analysis is also based on current data collected by the main contractor during pipeline system installation.

Geotechnical data at the zone of concern are supplied by the installation contractor. The sand was found to 3 have bulk density (γ (γ ) of 1674.00 kg/m and apparent 3 soil density of (γ (γ s) 649 kg/m .

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


Table 1. Wave data at the zone of concern Description Significant Wave Height Period Length Steepness Maximum Individual Wave Height Period Length Steepness

Notation

Unit

H s T s L s (H/L) s H max T max Lmax (H/L)max

Return Period (year) 10 25 50

1

5

100

m sec m

1.90 5.60 47.2 0.04

3.29 7.34 72.2 0.05

3.58 7.65 76.7 0.05

3.84 7.92 80.5 0.05

3.96 8.04 82.1 0.05

4.13 8.21 84.4 0.05

m sec m

3.42 7.28 82.9 0.04

5.92 9.54 142.0 0.04

6.45 9.95 154.5 0.04

6.92 10.30 165.6 0.04

7.13 10.45 170.5 0.04

7.43 10.67 177.7 0.04

Table 2. Current velocities at the zone of concern Consideration

Average WD (m)

Installation Phase

Operation Phase

6.22 7.20 6.53 9.25 9.50 9.53 13.00 18.04

2.4 Current Data Current data is supplied by installation contractor. The data 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 between design and installation, Table 3 that is extracted from Ref [4] consists of current data collected prior to pipeline design.

2.5 Hydrodynamic Coeffic ient In accordance with Ref [1], the drag coefficient, lift coefficient, and added mass coefficient are taken as 0.7, 0.9, and 3.29, respectively.

2.6 Pipe Specifi catio ns Pipeline used at the zone of concern has parameters that are described in Table 4. These values are taken from Ref [3] p. 17.

3. Methodology 3.1 General

Surface Velocity (m/s) 0.910 0.910 0.910 0.910 0.910 0.910 0.910 0.910

Velocity at 1m Above Seabed (m/s) 0.701 0.686 0.696 0.662 0.660 0.659 0.631 0.602

seabed friction. Assessment of lateral stability has commonly been performed in accordance with Ref [1] applied through simplified computer calculation. This is a recommended practice, which gives two methods for 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 of hydrodynamic forces, submerged weight of the pipeline, and soil characteristics. Since the pipeline has been resting at the sea floor for about four month, the analysis will consider installation condition based on documentation and results of latest survey. The operating condition will also be analyzed since the pipeline is going to be flooded for hydro test. Thus, based on documents received in supporting present analysis, both qualitative and quantitative assessments will be exercised. The pipeline weight that will be used in installation condition is the weight of a new empty pipeline since it adequately represents the current condition. In operating condition, the weight of fluid within the pipeline is accounted, and the reduction of pipe wall thickness due to 10% corrosion is considered as well.

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



Table 3. Current velocities for the zone of concern prior to pipeline design Description Current Speed * 0% water depth 10% water depth 20% water depth 30% water depth 40% water depth 50% water depth 60% water depth 70% water depth 80% water depth 90% water depth 100% water depth *) average depth is 14.86 meters

Notation

Unit

v0 v10 v20 v30 v40 v50 v60 v70 v80 v90 v100

m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s m/s

Table 4. Pipe parameters Parameter Pipe outside diameter Wall thickness Corrosion allowance Pipe grade Specified Minimum Yield Strength (SMYS) Specified Minimum Tensile Strength (SMTS) Anti-corrosion Coating

Return Period (year) 10 25 50

1

5

0.68 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

0.70 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

0.71 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

0.73 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

0.74 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

100 0.76 0.67 0.67 0.66 0.65 0.64 0.62 0.60 0.57 0.53 0.47

Value 32” (812.8 mm) 0.75” (19.05 mm) 3 mm SAWL DNV 450 I FUD 450 MPa 535 MPa 3LPE (2.5 mm thick)

3.2 Visual Ass essment Visual assessment was performed to asses the ROV record, which exhibits the latest pipeline condition, on qualitative basis. The assessment was carried out by witnessing the record of Ref [5] and note any distinguish findings based on both visual and audio record. The ROV record is continuous over time; selected visual findings were captured and interpreted. Capturing will be exercised in two ways. First, ROV record was captured for an approximate interval of 50m within the zone of concern. Each capture, which may 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.

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

The meaning of visual assessment was exercised by capturing distinguish findings found from the observed zone. Distinguish findings were identified by clear visual of pipeline that was fully buried, side visual of the pipeline and comments from the ROV operator regarding his opinion about the condition of the pipeline. The assessment was intended to show the level of sedimentation occurrence in the pipeline, which may increase the pipeline stability. Figure 1 and Figure 2 depict results that can be acquired from the assessment. Figure 2. Distinguished finding; found at the water depth of 15.71m



3.3 Soil Bearing Capacity Analysis Ref [6] stated that the stability of pipeline is governed by the “pipeline-soil” interaction. The soil bearing capacity intervenes in calculation of the vertical stability of a bottom-laid pipeline. The bearing capacity of a foundation soil can be expressed by the following well-known general formula, Ref [6]:

⎛ W s ⎞ ⎜⎜ − F L ⎟⎟ μ ≥ F D + F I ⎝ F w ⎠

(5)

The hydrodynamic force acting on the pipeline due to combined wave and current are given by the following expression, Ref [1] Section 5.3.8:

F L = 12 ρ w ⋅ D ⋅ C L ⋅ (U s cosθ + U C )

2

q = 12 γ ' LN γ + cu N c + γ ' dN q

(1)

The pipeline can be assimilated with a mat of infinite length and a width L (for L ≤ D) such that:

L = 2 d ( D − d )

(2)

In the case of sands, where the cohesion cu is zero, we have:

q = 12 γ ' LN γ + γ ' dN q

(3)

For this analysis, the soil bearing capacity factors were extracted from Ref [7] Section 7.3, assuming that the soil internal friction angle to be 25 degree. Due to its self-weight, the pipeline will be embedded from its initial installed-position into the soil until it reaches an equilibrium condition. In operating condition, the pipeline embedment for pipe filled with water will be considered. The linear bearing capacity of the soil beneath the pipeline can be defined by the following equation:

qL = W s

(3.4)

The embedment of pipeline into its base soil will be approximated using the above equations. The results of this analysis are provided in Table 5. Table 5. Results of analysis of soil bearing capacity Condition Pipeline Embedment Installation 41.86 mm Operating 79.25 mm

3.4 Pipeline Stability Analysis The lateral stability under hydrodynamic forces is governed by the lateral force of friction of the pipeline (coated with concrete) against the soil. Stability in this quasi-static method is given by the following expression, Ref [1] Section 5.3.5:

F D = 12 ρ w ⋅ D ⋅ C D ⋅ U s cos θ + U C

(U s cosθ + U C ) F I =

π D 2 4

ρ w ⋅ C M ⋅ A s sin θ

(6)

(7)

(8)

The limiting value of submerged weight can be found from, Ref [1] Section 5.3.6:

⎡ ( F D + F I ) + μ ⋅ F L ⎤ ⎥ ⋅ F W μ ⎣ ⎦ max

W s = ⎢

(9)

Hydrodynamic force can be computed and phase angle (θ ) can be found through iteration to give maximum submerged weight requirement (W s). Since the analysis assumes that the pipelines will experience embedment, it implies that the exposed area of pipeline will be reduced and the contact area between pipeline and soil will increase. Hydrodynamic forces acting on the pipeline will then decrease and the lateral force of friction will increase. Thus, the resultant of lateral forces will be reduced. This analysis uses Ref [8] to consider pipeline embedment in the calculation of hydrodynamic forces. Under the condition of “Partially Buried Pipelines” Ref [8] recommends modification to all of the hydrodynamic coefficients as the function of dimensionless quantity d / D (ratio of pipe embedment depth to pipe diameter). The coefficients decrease as embedment depth increases. Using the embedment depth calculated in Soil Bearing Capacity Analysis, d/D are 4.0% and 7.7% for installation and operating condition, respectively, with corresponding modification of hydrodynamic coefficients shown in Table 6. Table 7 provides the results of lateral stability analysis. The vertical stability of the pipeline is given by the following expression, Ref [1] Section 3.2.2:

[Ws + B] B

≥ 1.1


(10)


Condition Installation Operating

Table 6. Modified hydrodynamic coefficients Hydrodynamic Coefficients d/D (%) C D’ C L’ C M ’ 4.0 0.668 0.859 3.191 7.7 0.639 0.822 3.043 Table 7. Results of lateral stability analysis

Condition

Critical Water Depth (m)

Installation Operating

15.00 15.00

FOS Un-modified 0.9868 1.0210

Modified 1.0235 1.1091

Table 8. Results of vertical stability analysis Condition FOS Installation 1.56 Operating 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 to be most important at water depth of more than 15 meters. As stated in MIGAS regulation requirements, pipeline laying at sea bottom less than 13 m shall be buried 2 meters.

4. Result s and Analys is 4.1 Visual Ass essment The result acquired from capturing ROV record within the zone of concern by an approximate interval of 50 m is that the sediments were found at the surroundings of pipeline.

The main results that are found from lateral stability analysis after considering the modification of hydrodynamic coefficients during Installation Condition are that the stability can be achieved for the critical water depth with an FOS of 1.0235. While during Operating Condition FOS for this depth, 15.0m, is 1.1091. However, to validate the reduction in hydrodynamic forces due to pipeline embedment/partial burial, Ref [1] Section 3.3.5 states four considerations that should be accounted in the stability calculation. The analysis also shows that the stability is increasing as the increasing of water depth. The FOS found from vertical stability analysis is 1.56 for installation condition and 1.58 for operating condition.

5. Conclusions Several outlines can be withdrawn from the analysis: 1.

Sedimentation process has been occurring in the pipeline. This is found from observing the ROV survey data at the area of concern. The sediment will likely to improve pipeline lateral stability if the burial happens. No quantitative measures can yet be made from present assessment. To acquire the adequate measures from sedimentation phenomenon, specific field survey needs to be conducted.

2.

Pipeline embedment has increased the pipeline lateral stability. This fact can be seen from the results of Installation Condition and Operating Condition.

3.

Based on the sequence of installation process, which is applied to the pipeline, an increase in embedment depth will potentially occur since the pipe will be flooded with water. This process causes the submerged weight to increase significantly and the lateral stability as well. Since the hydro test will be carried out anytime soon, the

4.2 Soil Bearing Capacit y By considering the installation condition of the pipeline and the empty pipeline with no corrosion, the pipeline embedment is 41.86 mm. In operating condition, the pipeline weight during hydro test will cause an embedment of 79.25 mm. These calculations were performed using the assumption of 25° internal soil friction and the values of bearing capacity factors taken from Ref [7].

4.3 Pipelin e Stabili ty Lateral stability analysis using measured current data shows that the critical depth of 15.00 m, which is the depth of unburied pipeline. At this depth, FOS in installation condition is 0.9868 and in operating condition is 1.0210, while the required FOS according to Ref [1] is 1.0.


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

pipeline will tend to meet Operating Condition. Thus, the pipeline is concluded to be stable. 4.

Ref [1] states four considerations that should be accounted for in the stability calculation.

6. References [1] [2]

[3]

[4]

[5] [6]

[7] [8]

DNV “RP E305, On Bottom Stability Design of Submarine Pipelines.” October 1988. PGN. “30”∅ Pipeline Routing Alignment KP12.5 – KP15.0 (Sheet 7 of 42), Revision 1, th Doc No. 002-42-L-DG-1010.” September 27 , 2006. SSWJ Document. “Re-evaluation of On-Bottom Stability Based on Actual Current Observations During Pipe-laying Phase, Revision A, Doc No. 002-42-L-RE-2049.” October 26th, 2006. PGN. “Ocean Environment Analysis for Gas Transmission and Distribution Project.” May 2004 PGN. “ROV Survey – video documentation.” Tyrant, P. L. Seabed Reconnaissance and Offshore Soil Mechanics for the Installation of Petroleum Structures. Paris: Editions Technip, 1979. DnV. “RP F105, Free Spanning Pipelines.” March 2002. Jacobsen, V. “Forces on Sheltered Pipelines,” Offshore Technology Conference 5851. Houston: OTC, 1988.

Rikrik Gantina is an offshore engineer. He is involved in many offshore project and assessment from 2002-2003 in PAU-LAPI ITB. Since 2003, he has been involved in surveying, designing, installation, pre-commissioning and commissioning of offshore pipeline project phase I (32 inches, 105 Km) and Phase II (32 inches, 165 Km) in PT. PGN. He holds BS degree in Ocean Engineering from ITB. Krisnaldi Idris is lecturer/researcher and ocean engineer. He has been involved in various ocean engineering related projects, and has developed knowledge on the various fluid-structure interaction issues, including hydrodynamics around cylindrical bodies. He was graduated from ITB, and obtained MSc degree and PhD degree in Civil Engineering (emphasize in Ocean Engineering) at the Oregon State University. Taufik Fahrozi graduated from Ocean Engineering, ITB, in 2006. He is involved in several fixed platforms and pipeline analysis during his undergraduate years. He currently works as junior engineer at PT Saipem Indonesia.

8. Nomenclatures

7. Biographies Muhammad Munari , who graduated from Institute Teknologi Sepuluh November is a senior offshore engineer at PT. Perusahaan Gas Negara (Persero) Tbk. He is involved in many of PGN’s projects, onshore and offshore, from surveying, designing, installation, pre-commissioning and commissioning. Currently coordinating offshore section of PGN’s South Sumatera – West Java gas transmission and distribution project phase I and phase II. Hasanuddin Ibrahim , graduate from Ocean Engineering of Institut Teknologi Bandung (ITB) is an offshore engineer of PT. Perusahaan Gas Negara (Persero) Tbk. He is involved in PGN’s project for the last 4 years; especially offshore sections from surveying, designing, installation, precommissioning and commissioning. Currently finishing phase I (105 km 32”) and phase II (160 km 32”) offshore pipeline that connecting Sumatra and Java.

γ'

=

μ ρw θ

= = =

A s

=

B C D C D’ C L C L’ C M C M ’ cu D d

= = = = = = = = = =

F D F I F L F W FOS L

= = = = = =

Submerged density of the soil (γ’ = γ – 1); Soil friction factor; Mass density of seawater; Phase angle of the hydrodynamic force in the wave cycle; Significant acceleration perpendicular to the pipeline (= 2π U s/T u); Buoyancy of pipe; Drag force coefficient; Modified drag force coefficient; Lift force coefficient; Modified lift force coefficient; Inertial force coefficient; Modified inertial force coefficient; Cohesion of soil; Total outside diameter of the pipe; Depth of to which foundation is buried; Drag force; Inertia force; Lift force; Calibration Factor; Factor of safety; Width of foundation;


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

N c , N γ , N q q U c U s

W s

=

Dimensionless coefficients depending on the angle of friction of the soil;

= =

Maximum bearing capacity; Current velocity perpendicular to the pipeline; = Significant near bottom velocity amplitude perpendicular to the pipeline; and = Submerged weight of pipeline.


08_Bottom Stability Analysis

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