Pipeline Cathodic Protection Design

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8474L-082-CN-3533-101-0.doc TABLE OF CONTENTS 1.0 INTRODUCTION ...3 2.0 SUMMARY OF RESULTS ...4 3.0 DESIGN PARAMETERS...6 4.0 METHOD OF ANALYSIS ...8 5.0 CONCLUSION ...14 6.0 REFERENCES ...15

APPENDIX A COMPUTER PRINTOUTS

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8474L-082-CN-3533-101-0.doc

1.0 INTRODUCTION

This calculation note presents the offshore pipeline external corrosion protection design for the two of 30-inch crude oil carbon steel insulated pipeline from the Pipeline End Manifold (PLEM) system located on the seabed below the Single Point Mooring (SPM) buoy to the landfall (up to KP 3.960) as part of the Dung Quat Refinery Project. The scope of work includes:

i. Determining the type of anti-corrosion coating required for pipeline against external corrosion throughout its design life.

ii. Design of aluminium sacrificial bracelet anodes required for the pipeline against external corrosion for the design life of 20 years.

1.1 ABBREVIATIONS

CP - Cathodic Protection

CPR - Crack Propensity Ratio

ID - Inside Diameter KP - Kilometre Point OD - Outer Diameter 3LPP - 3 Layer Polypropylene Thk. - Thickness yrs - years

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2.0 SUMMARY OF RESULTS

2.1 EXTERNAL ANTI-CORROSION COATING

The selected external anti-corrosion coating system for the 30-inch pipeline from the Pipeline End Manifold (PLEM) system to the landfall is presented in Table 2.1.1.

Table 2.1.1 External Anti-Corrosion Coating System

Section Type of Anti-Corrosion Coating

KP 0.0-3.200 2.5 mm thick 3LPP

2.2 CATHODIC PROTECTION

The pipeline cathodic protection requirements for the 30-inch pipeline from the Pipeline End Manifold (PLEM) system to the landfall are presented in Table 2.2.1.

Table 2.2.1 Cathodic Protection Requirements for Pipeline

Description Unit KP 0.0 - KP 1.390 KP 1.390 - KP 2.000 KP 2.000 - KP 2.610 KP 2.610 - KP 3.200 Anode Type - I (Submerged) II (Submerged) II (Buried) III (Buried) Anode Inside Diameter mm 832.6 (Note 4) Anode Outside Diameter mm 942.60 982.60 1042.60 Anode Thickness mm 60.0 80.0 110.0 Anode Length mm 361.41 369.62 433.37 Net Weight / Pair (Note 1) kg 140.0 200.0 340.0 Spacing (Pipe Joint) - 12 8 9 Total Anode Quantity (for 2 lines) set 20 10 14 12 Total Net Weight kg 2800.0 2000.0 2800.0 4080.0 Crack Propensity Ratio (Note 2) - 1.13 0.46 0.20 Anode

Material - Aluminium-Indium Alloy Bracelet (Half-Shell Type) Notes:

1. One (1) pair of anode is made up of two (2) pieces of half-shell bracelet anode.

2. Crack Propensity Ratio (CPR) is a measure of the ratio of anodes length, diameter and thickness. A CPR value of less than 5 will ensure a lesser probability for the occurrence of cracks in the anodes.

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3. Vendor shall propose steel core dimensions and shall include in their calculations.

4. Positive tolerance on OD of pipe (3mm) and 0.1mm thick coating inside the anode has been considered in anode ID calculation.

2.3 INSULATION JOINTS

Monolithic insulation joints have been considered at the following location.

• One each at the shore line on 30-inch pipelines between onshore and offshore pipeline section.

• One each on the 30-inch onshore pipeline at the transition of above ground to below ground at pig launcher/receiver station.

This report addresses only design of Cathodic Protection System of offshore pipelines. Cathodic Protection design for onshore pipelines will be carried out by others.

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3.0 DESIGN PARAMETERS

3.1 DESIGN DATA

Unless otherwise stated, the design data are as per Crude Oil Pipeline Design Basis [Ref. 1] and the requirements of DNV RP B401 [Ref. 3]

Table 3.1.1 Design Parameters for Cathodic Protection Design

Description Unit KP0.000– _KP1.390 KP 1.390 – _{KP 2.000} KP 2.000 – _{KP 2.610} KP 2.610 – _{KP 3.200}

Pipeline Data

Pipeline OD mm 762.0

Pipeline Wall Thickness mm 15.9

Segment Length m 1390 610 610 590

Corrosion Coating Type - 3LPP

Corrosion Coating Thk. mm 2.5

Insulation Thickness (HDPU) mm 23.0

Insulation Density kg/m3 180.0

Outer Case Thickness (HDPE) mm _8.2

Outer Case Density kg/m3 ₉₄₀

Concrete Coating Thk. mm 60.0 80.0 80.0 110.0

Design Life yrs 20 (Note 8)

Anode Data

Anode Thickness (Note 2) mm 55.0 75.0 75.0 105.0

Bracelet Gap Width mm 100.0

Density of Anode kg/m3 2730

Closed Circuit Potential V -1.05 (w.r.t Ag/Ag/Cl)

Driving Potential Difference volts 0.25 0.15

Anode Design Temperature (Note 3) °C 32

Anode Current Capacity (Note 4) Ahr/kg 2176.0

Anode Material Type - Aluminium-Indium Alloy

Initial mA/m2 157.0 27.0 Mean mA/m2 77.0 27.0 Current Density at 25o_C (Note 5) Final mA/m2 97.0 27.0 Initial % 1.00 Mean % 5.00 Coating Breakdown

Factor (Note 6) _Final _% _7.00

Anode Utilization Factor - 0.80

Surrounding Resistivity

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8474L-082-CN-3533-101-0.doc Notes:

1. The anode shall be designed to employ the factory limitations for producing the anode. 2. Selected anode thickness is 5mm lower than the corresponding concrete coating thickness. 3. The design temperature is taken as maximum seabed temperature.

4. Anode electrochemical efficiency shall be determined by the following formula as recommended by DNV RP B401:

Efficiency = 2500 – 27 [Anode Temperature (°C) – 20] Ahr/kg Assuming anode efficiency is 2500 Ahr/kg at 20°C.

5. The current density has been increased by 1 mA/m2 for each 1oC that outside steel temperature exceeds 25oC.

6. The coating breakdown factors due to ageing of coating during design time are calculated in accordance with DNV RP B401. Since coating break down factor due to damage is very small, those are ignored. 7. As per DNV RP B401, soil resistivity has been considered five times of seawater density (for sandy soil). 8. Though the pipeline mechanical design life is 60 years, pipeline cathodic protection is designed to 20 years

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4.0 METHOD OF ANALYSIS

4.1 ANTI-CORROSION COATING SELECTION CRITERIA

The selection of the external coating system for the pipeline has been made taking into considerations cost, fit for purpose, project specification requirements [Ref. 1] and at the same time satisfying the following criteria:

Maximum pipeline operating temperature Coating impact resistance

Cathodic disbonding

Concrete coating suitability (for concrete coated pipe)

Field joint coating material and its compatibility with the proposed coating Field experience

4.2 CATHODIC PROTECTION ANALYSIS

The external corrosion coating provides passive protection to the pipeline against corrosive actions of seawater. However, this may still be subjected to breakdown due to harsh environment, incorrect pipe handling and others, exposing the surface of the pipe to corrosion. An active protection mechanism, in the form of anodes, shall be provided to supplement the FBE coatings as a corrosion protection measure. The provision of anodes is discussed in the following section.

The cathodic protection (CP) analysis is performed to determine the dimensions and quantity of sacrificial bracelet anode required to protect the pipeline against external corrosion throughout its design life.

The following sections describe the methodology and governing equations for the CP design. Aluminum based material have been selected for the anode.

Calculations have been performed using TPGM’s in-house computer program ‘Anode-B401 Version 3’ [Ref. 4]. This program calculates the number of anode for cathodic protection in accordance with the requirements of DNV RP B401 [Ref. 3].

4.2.2 CALCULATION OF REQUIRED NUMBER OF ANODES Calculation of the number of anodes shall satisfy four criteria: a) The number of anodes required to initially polarize the pipeline.

b) The number of anodes required to provide the maintenance current throughout the life of the design. c) The number of anodes required to provide adequate protection on the final day of the design life. d) Maximum allowable anode spacing.

4.2.2.1 Total Surface Area of Pipeline (Ac)

Ac = π x D x Ltot (Eq. 4.2.2.1.1)

Where,

D = Pipeline outside diameter Ltot = Pipeline segment length

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4.2.2.2 Anode Inside Diameter (Dia)

Dia = D + 2 tC (Eq. 4.2.2.2.1)

Where,

D = Pipeline outside diameter tC = Pipe corrosion coating thickness

4.2.2.3 Anode Outside Diameter (Doa)

Doa = Dia + 2 ta (Eq. 4.2.2.3.1)

Where,

Dia = Anode inside diameter

ta = Anode thickness

4.2.2.4 Anode Length (La)

Required anode length based on the weight and diameter of the anode specified in design, La =                                       ₊ ₋ − π /2 D b/2 sin os c t 2b t t D D M ia a a a ia a a 1 2 (Eq. 4.2.2.4.1) Where,

Ma = Assumed mass of each anode (net)

b = Side gap width of anode Da = Anode material density

4.2.2.5 Anode Surface Area at Initial Life (Aai)

With the defined dimensions, anode surface area at the start of design life, Aai can be found by:

Aai = π DoaLa – 2 La _               ×         − × 180 π /2 D b/2 1 sin 2 2 D oa oa (Eq. 4.2.2.5.1) Where,

Doa = Anode outside diameter

La = Anode length

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4.2.2.6 Anode Surface Area at End of Life (Aaf)

With the defined dimensions, anode surface area at the end of design life, Aaf can be found by:

(

)

[

]

(

)

                        π             − + − + − − + π = − 180 . 2 ) u 1 ( t 2 D 2 / b sin 2 . 2 u 1 t 2 D L 2 L u 1 t 2 D A a ia 1 a ia a a a ia af (Eq. 4.2.2.6.1) Where,

U = Anode utilization factor

La = Anode length

b = Side gap width of anode

4.2.2.7 Anodes Required To Initially Polarize The Pipeline

Initial anode current output =

ai

A

0.315xρ E (Eq. 4.2.2.7.1) Where,

E = Potential difference between anode and the linepipe protection ρ = Resistivity of surrounding

Aai = Anode surface area at initial of life

4.2.2.8 Anodes Required To Protect The Pipeline At The End Of Life Final anode current output =

af A 0.315X E ρ (Eq. 4.2.2.8.1) Where,

E = Potential differences between anode and the linepipe protection ρ = Resistivity of surrounding

Aaf = Anode surface area at end of life

4.2.2.9 Total Number of Anodes Required To Polarize The Pipeline

Number of anodes (N) = 1000 I f A i ai ci c ci × × × (Eq. 4.2.2.9.1)

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Where,

ici = Initial current density; Current required to polarize an unprotected pipeline located on sea

bottom

Ac = Total surface area of pipeline segment

fci = Initial coating breakdown factor

Iai = Initial anode current output

4.2.2.10 Anodes Required To Provide The Maintenance Current Number of anodes (N) = a f cm M u 1000 8760 t f A icm c × × × ε × × × × (Eq. 4.2.2.10.1) Where,

icm = Maintenance current density; The current density required maintaining

protection on the pipeline. Ac = Surface area

tf = Design life

fcm = Mean coating breakdown factor

ε = Anode electrochemical capacity u = Utilization factor

Ma = Assumed mass of each anode (net)

4.2.2.11 Anodes Required To Protect The Pipeline At The End Of Life

Number of anodes (N) = 1000 I f A i af cf c cf × × × (Eq. 4.2.2.11.1) Where,

icf = Final current density. The current density required maintaining protection on the

pipeline under end of life operating conditions. Ac = Surface area

fcf = Final coating breakdown factor

Iaf ₌ Initial current output per anode

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4.2.3 ANODES SPACING

The anodes shall be evenly spaced along the pipeline. For pipelines with inlet product temperatures in excess of 45°C, closer spacing of the anodes until the product has cooled to less than 45°C is acceptable. The spacing shall not exceed the minimum allowable spacing. The anode spacing, Sm (in meters) can be found by:

(

mean final

)

tot m N , N Max L S = (Eq. 4.2.3.1) Where,

Ltot = Pipeline segment length

Nmean = Number of anodes to meet mean current demand

Nfinal = Number of anodes at final design life

The number of spacing in pipe joints is given by:

SLL S

S_jo_int_s = m (Eq. 4.2.3.2)

Where,

Sm = Anode spacing

SLL = Standard linepipe length per joint

The recommended anode spacing, SR, is the calculated number of spacing in pipe joint, Sjoints rounded down to an

integer.

To obtain the recommended anode quantity, NR, the following equation is used:

R tot R S SLL L N × = (Eq. 4.2.3.3) Where,

Ltot = Pipeline segment length

SLL = Standard linepipe length per joint SR = Recommended anode spacing

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4.2.4 CRACK PROPENSITY RATIO (CPR)

Crack Propensity Ratio (CPR) is used to adjust the ratio of anode length, diameter and thickness so that the anode will be manufactured with lesser probability of cracks. It is advised to design anode of which CPR value is 5 or less.

₃ a ia a t x 5 D x L x CPR = π (Eq. 4.2.3.4) Where, La = Anode length

Dia = Anode inner diameter

4.2.5 COATING BREAKDOWN FACTORS

The coating breakdown factor specified the amount of damage that is expected to occur to the pipeline’s corrosion coating at various stages of the pipeline’s life. The breakdown factor is determined by the type of coating, predicted operating temperature of the pipeline, whether the pipeline is concrete coated and an estimation of the amount of physical damage that will occur to the pipeline.

4.2.6 ANODE ELECTROCHEMICAL EFFICIENCY

The performance of a sacrificial anode material is dependent on its actual chemical composition. At increased temperatures, the self-corrosion of the anodes is greater and therefore their efficiency decreases. In practice, the theoretical capacity of sacrificial anodes is not fully available for cathodic protection. Refer to Section 6.6 of DNV RP B401 [Ref. 3] for further details.

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5.0 CONCLUSION

The selection of external anti-corrosion coating system is made in view of the following criteria:

• Subsea Pipeline

The 3LPP coating is selected for the subsea section of this concrete coated 30-inch pipeline. The requirements of sacrificial bracelet anode are determined to satisfy the following criteria: i) The number of anodes required to initially polarize the pipeline.

ii) The number of anodes required to provide the maintenance current throughout the design life of the pipeline. iii) The number of anodes required to re-polarize the pipeline if such layers are partly and periodically damaged

at final of design life.

i) The maximum allowable anode spacing. ii) Ease of installation.

As a conclusion, the pipeline external corrosion protection design consists of passive corrosion protection system via the coating of the pipeline and active corrosion protection in form of cathodic protection system, as presented in Table 2.2.1, can sufficiently protect the pipeline for the design life of 20 years.

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6.0 REFERENCES

1. Crude Oil Pipeline Design Basis Doc. No.: 8474L-082-JSD-3500-001 2. Concrete Coating Thickness

Doc. No.: 8474L-082-CN-3531-002

3. DNV RP B401

Cathodic Protection Design (1993)

4. Technical User Manual and Validation Report

ANODE-B401 Computer Program for Cathodic Protection of Submarine Pipeline Doc. No.: PL/TM/VR/07

5. Internal and External Cathodic Protection. Doc. No.: 8474L-000-JSD-1600-008

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APPENDIX A COMPUTER PRINTOUTS ANODE SIZING CALCULATIONS

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9 _{Pipeline Outside Diameter} _D _762.00_mm

10 _{Pipeline Wall Thickness} _d _15.9_mm

11 Pipeline Segment Length Ltot 1390.00m 12 Corrosion Coating Thickness + Insulation Thickness tc 33.7mm 13 Resistivity of Linepipe Material ρMe 2.00E-07Ohm-m

14 Anode Thickness ta 55.00mm

15 _{Anode Material} _{Aluminium-Indium Alloy}

16 _{Anode Material Density} _D_a _2730.00kg/m3

17 Assumed Anode Mass (Net) Ma 140.00kg 18 _{Resistivity of Surrounding} _ρ _0.21_Ohm-m

19 _{Side Gap Width of Anode} _b _100.00_mm

20 Potential Difference Between Anode And Linepipe E = Ec o

- Ea o

0.25volts

21 Standard Linepipe Length per Joint SLL 12.20m

22 Design Life tf 20Years

23

24 _{Current Density :} _Initial _i_ci _157.00mA/m2

25 _Mean _{At 32}o_C _i cm 77.00mA/m 2 26 _Final _{At 32}o_C _i cf 97.00mA/m 2 27

28 Coating Breakdown Factor : Initial fci 0.01

29 Mean fcm 0.05

30 Final fcf 0.07

31 32

33 _{Anode Electrochemical Efficiency} ε 2176.00A-hr/kg

34 _{Anode Utilization Factor} _u _0.80

35

36 _OUTPUT:

37 _{Surface Area of Pipeline Segment} _A_c _{3327.51 m}2 38 Anode Inside Diameter Dia 829.40 mm 39 Anode Outside Diameter Doa 939.40 mm 40 Anode Length (Based on Net Mass) La 361.41 mm 41 _{Anode Surface Area at Initial of Life} _A_ai _{0.9942 m}2 42 _{Anode Surface Area at End of Life} _A_af _{0.8942 m}2 43 Initial Current Output per Anode Iai 3.77 Amps 44 Final Current Output per Anode Iaf 3.57 Amps 45 Initial Current Demand Ici 5.22 Amps

46 Mean Current Demand Icm 12.81 Amps

47 Final Current Demand Icf 22.59 Amps 48 _{Maximum Allowable Anode Spacing} _s _{12.00 joints} 49

50

51 _Initial _Maintenance _Final _(m) _Joints _{Spacing (Joints)} _{Quantity (Nos.)} 52

53 _1.39 _9.21 _6.32 _150.93 _12.37 _12.00 ₁₀

54

55 _{CRACK PROPENSITY RATIO, CPR =} _1.13 _{< 5} _OK 56

57 _REMARKS:

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- Ea o

0.25volts

23

29 Mean fcm 0.05

30 Final fcf 0.07

31 32

35

36 _OUTPUT:

47 Final Current Demand Icf 9.92 Amps

48 _{Maximum Allowable Anode Spacing} _s _{12.00 joints} 49

50

53 _0.59 _2.83 _2.73 _215.62 _17.67 _17.00 ₃

54

57 _REMARKS: _{RECOMMENDED ANODE SPACING HAS EXCEEDED THE ALLOWABLE ANODE SPACING}

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- Ea o

0.15volts

23

29 Mean fcm 0.05

30 Final fcf 0.07

31 32

35

36 _OUTPUT:

50

53 _0.80 _0.99 _6.03 _101.16 _8.29 _8.00 ₇

54

57 _REMARKS:

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- Ea o

0.15volts

23

29 Mean fcm 0.05

30 Final fcf 0.07

31 32

35

36 _OUTPUT:

50

53 _0.69 _0.56 _5.35 _110.36 _9.05 _9.00 ₆

54

57 _REMARKS: