Đang tải... (xem toàn văn)
Thiết kế đường ống trên bờ (tiếng anh)
G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES TABLE OF CONTENTS 1.0 INTRODUCTION 2.0 SUMMARY 3.0 SOIL RESISTIVITY SURVEY 4.0 DESIGN PARAMETERS 5.0 CATHODIC PROTECTION DESIGN CALCULATIONS Page 2 of G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES 0 c: 1.0 INTRODUCTION This report defines the design calculations and requirements of Cathodic Protection System for one No. 17" existing gas pipeline and three new LPG product pipelines. An impressed current cathodic protection system will be installed to supplement the' corrosion coating in providing corrosion control to the pipeline. The impressed current Cathodic Protection system for existing gas pipeline will be upgraded to suit the new requirement for existing gas pipeline as well as new LPG product pipelines. 2.0 SUMMARY Cathodic protection is specified for the onshore section of the subject pipeline from Dinh Co to Thi Vai for new LPG pipeline and from Long Hi to Phu My for existing gas pipeline- length and other parameters are as given in Secsion 4.0 of this document. Both galvanic anode and impressed current cathodic protection systems were considered during this design. An impressed current cathodic protection system has been selected on a technical basis. The cathodic protection system has been designed in accordance with internationally accepted standards and compliance the codes and standards listed in section 1.2 o specification. A conservation design approach has been used including a 30 mA/m2 current density and 95% coating efficiency. For future addition a 20% spare output capacity has been provided ( NACE-1967). It has been determined that a single impressed current system required upgrading at the Phuoc Hoa LBV and at the Dinh Co Station would provide full cathodic protection of new LPG pipelines and existing gas pipelines. The soil resistivities at this location justified the installation of an effective surface anode groundbed. The groundbed will be located approximately 100 m from the pipeline and position perpendicularly to the pipeline in accordance with the project specification (BS-7361 ). A rectifier will be used to energize the groundbed. They.415. VAC; three phases power supply to the rectifier will be provided from the � 415;volt switchboard. Elec€rical isolation of the pipeline will be provided by the installation of insulating flange sets Insulating flange set shall be provided with Explosion-proof surge diverters to prevent damage due to lightning or power surges. Test stations will be provided to monitor and adjust the cathodic protection system . Test station also have to be provided at following location • Both side of the major river or road crosing. • At all insulating joint. • At HT overhead line crossing. • At all vulnerable location where interferance is possible These test stations will be located at maximum intervals of 1.5 km. All cable connections to the pipeline will be made using the brassing thermit weld process. 3.0 SOIL RESISTIVITY SURVEY Soil resistivity measurements have been previously carried out by others at 65 location along the pipeline ROW. According to the information included in the contract documents. this testing was performed using the Wenner 4-pin method at depth of 0.75, 1.5, and metres using a .M-416 instrument made in the Soviet Union. Test equipment must have maximum AC & DC ground current rejection feature. Soil resistivities are critical to the proper design of a cathodic protection system. They are used as a guide to determine the P9-CPS.05-01.0 Page of I G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES corrosiveness of the soil and also used to select ground locations and configurations. Soil resistivities are important regardless of whether and impressed current or a sacrificial anode cathodic protection system is utilized. It should be understood that this cathodic protection design has been based on the soil resistivities determined by others. Any inaccuracies in the reported resistivity values versus the actual resistivity values could have a significant impact on the effectiveness of the cathodic protection system. This can only be determined during commissioning of the system when the Contractor shall check the.soil resitivity inspection accordance with specification. 4.0 4.1 4.1.1 Y • DESIGN PARAMETERS The following sub-sections include parameters which have been used for design of the proposed cathodic protection system. GENERAL PIPELINE DETAILS These calculation for upgrading existing CP system for new LPG pipelines and existing gas pipeline will be done through two (2) cathodic protection system design. Portion 1: (CP Station at Dinh Co location ) Gas pipeline (Long Hai to Ba Ria) Length of pipeline 16.5 km a Pipeline diameter 406.4 mm 1 /17 Pipeline number 1 Coating Coal tar enamel LPG pipelines (Dinh Co to Ba Ria) Length of pipeline 7.5 km Pipeline diameter 168 mm C L � Pipeline number 3 Coating Polyethylene 4.1.2 Portion 2: (CP Station at Phuoc Hoa location ) Gas pipeline (Ba Ria to Phu My) Length of pipeline 21.5 km Pipeline diameter 426 mm Pipeline number Coating Coal tar enamel LPG pipelines (Ba Ria to Thi Vai) Length of pipeline 17 km Pipeline diameter 168 mm Pipeline number 3 P9-CPS.05-01.0 Page 4 of I ' Rev. D /,( G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES Coating Polyethylene 4.2 CATHODIC PROTECTION DESIGN LIFE In accordance with the contract documents, the impressed current cathodic protection system will have a design life of 30 years. The cathodic protection Contractor shall demonstrate the design life of his proposed CP system. 4.3 ANODE TYPE 1 T' 'um tubular anodes with a mixed metal oxide coating will be installed. These anodes have proven to have superior operating characteristics over silicon iron and graphite anodes. Titanium / mixed metal oxide anode are also of lighter weight and capable of significantly higher current outputs and longer life. 4.4 COATING EFFICIENCY A common method to assess the pipe coating condition is to use a factor referred to "percent bare". For a normal factory applied coaltar enamel coating system which would be inspected and repaired prior to back filling. of the pipe, a value of a approximately I to 2 percent bare would be experienced immediately subsequent to construction. However, during the service life of the pipeline with expansion and contraction of the pipeline due to thermal effects, soil movement, water ingress through the coating etc, the condition of the coating will deteriorate. A conservative coating efficiency of 95% has been used for this cathodic protection design. This means that over the 30 year life, the cathodic protection system will have the capacity protect an average of 5% of the total surface area of the pipeline. 4.5 CP CURRENT DENSITY The National Association of Corrosion Engineers (NACE) Recommended practice RP-0169-92 and DNV RP B401 specifies a current density of 10-30 mA/m2 for bare steel structures in soil. The soils along the proposed ROW for this pipeline are very aggressive with high moisture content, high salinity and low soil resistivity. Therefore, this design uses a conservative 30 mAIm2 current density. 4.6 SOIL RESISTIVITY 4.6.1 Dinh Co Location As mentioned in previous Section 3.0, soil resistivity testing along the pipeline ROW has been previously carried out by others. A review of this data indicates that points R-18 and R-19 are located relatively near Dinh Co: Depth (m) Soil Resistivity (ohm cm) R-18 0.75 8000 1.50 4200 P9-CPS.05-01.0 Page 5 of 14 . G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES 3.00 1050 R-19 0.75 3600 1.50 1300 3.00 500 The deep-well anode system will be installed for cathodic protection, The average resistivity value for the above two points at a 1.5m depth is 2750 ohm-cm and will be used t For groundbed design at Dinh Co. 4.6.2 Phuoc Hoa Location Also mentioned in previous Section 3.0, soil resistivity testing along the pipeline ROW has been previously carried out by others. A review of this data indicates that points BH23 aid BH24 are located relatively near Ba Ria ( at Phuoc Hoa ) where the impressed current CP system will be located. The soil resistivity values reported at these locations are : Depth (m) Soil Resistivity (ohm ern) BH23 0.75 1300 1.50 1500 3.00 1000 BH24 0.75 1500 1.50 1500 3.00 800 The average resistivity value for the above two points at a 1.5m depth is 1500 ohm-cnl _:d will be used for groundbed design at Phuoc Hoa. 4.7 CATHODIC PROTECTION CRITERIA NACE Recommended Practice RP-0169-92 addresses cathodic protection criteria Dr underground and submerged metallic piping systems. Applicable excerpts of this st.and=d relative to CP criteria for this project are as follows- A negative (cathodic) potential of at least 85OmV with the cathodic protecti'Dn applied. This potential is measured with respect to a saturated copper / copper ulfat.e reference electrode contacting the electrolyte. Voltage drops other than those across the structure to electrolyte boundary must be considered for valid interpretation of this voltage measurement. A negative polarized potential of at least 850 mV relative to a saturated copper / copper sulfate reference electrode. P9-CPS.05-01.0 G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES A minimum of 100 mA of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte. The formation or decay of polarization can be'measured to satisfy this criterion. 5.0 CATHODIC PROTECTION DESIGN CALCULATIONS 5.1 DINH CO LOCATION 5.1.1 Pipeline Total Surface Area The onshore portion of Long Hai to Ba Ria gas pipeline and Dinh Co to Ba Ria LPG pipelines has a total surface area as follows: Sa = (Pi) (d) (1) Sa (3.1416)-[(0.406,x 16500) + (3 x 0.168 x 7500)] Sa = 32921 (m2) Where: Sa = Surface area (m2 ) Pi = 3.1416 1 Length of pipeline (m) Diameter of pipeline (m) 5.1.2 CATHODIC PROTECTION CURRENT REQUIREMENT The cathodic protection system will have a DC output capacity as follows: (Sa) (Id) (Cb) It It It = ZSF 1000 (32921) (30) (0.05) X1.1 1000 54.32 (A) Say 55 amperes Where: Sa Total surface area (m2 ) Id = CP current density (mA/m2 ) Cb = Coating breakdown factor (%) SF = Safety factor (1.1) 5.1.3 CATHODIC PROTECTION ANODE REQUIREMENT Chosen output rating of 2 Ampere for each anode, the number of anodes: N = It/2 = 54.32/2 = 27.16 No. of anodes based on current requirement Page 7 of 14 m.1 G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES N = 30 Another, maximum anode current output for each anodeto be determined as follows : LogL= 3.3 - LogId Where : L = design life in years Id = Maximum anode current density (A/m2 ) For L = 3 0 years The anode has a dimension of 31.75mm dia x I000mm legth with a.total area of 0.1m2 Therefore, Id =.66.5 A/m2 Maximum current output of anode = 0.1m2 x 66.5 A/m2 =6.65A Hence 3 0 anodes will give 6.65 x3 0 = 199.5 A > Required 54.3 2 A noe wegt requ id re , W = W W = (0) (It)(y) Uf 0.1 (54.32) (30) 0.6 271.6 kg Anodes No. required based on weight requirement : N W/Wa 271.6 / 20 13.6 No. of anodes based on weight requirement, N = 14 Where Q Anode consumption rate (kg/amp-year) It = Cathodic Protection current ( A ) Y = Design life of the system ( years ) Uf = Anode Utilization factor ( 0.6 for a conservative design ) Wa = Weight of individual anode ( kg ) Hence anode No. chosen is 30 There are 20 canister anodes in existing groundbed at Dinh Co, so that 10 canister anodes which are the same existing anode specification will be required to add in this location. 5.1.4 GROUNDBED RESISTANCE Installation of 30 canister anodes on 5 metre spacing in 2750 ohm-cm soil results in an estimated groundbed resistance of 0.5844 ohm as follows: Rn = RI + R2 ( NACE - 1967, page 95 ) Where Rl Anode to backfill resistance R2 Backfill to soil resistance Pb BLcr - 1+ 2La R1 N.yLcrx6.28 [In do S In(0.656N)]) { NACE - 1967 ) R1' = 50 F n (8)(100) I + (2)(100) ln(0.656)(30)] (30)(100) (6.28) 3175 500 Ri 0.0152 (ohm) Rev. D - G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES Where: Pb La = da = S = N Backfill resistivity (ohm-cm) Length of anode- excluding backfill (cm) Diameter of anode- excluding backfill (cm) Spacing of anodes (cm) Number of anodes Ps 8Lc -1 + 2Lc R2 R2 NxLcx6.28 2750 [In In(0.656N)J) (NACE - 196J ) do S (8) (150) (2) (150) -1 + in(0.656)(30) J (30)(150) (6.28) [in 76 500 R2 = 0.5692 (ohm) Where: Ps Soil resistivity (ohm-cm) Lc Length of anode- including backfill (cm) dc Diameter of anode- including backfill (cm) S. Spacing of anodes (cm) Number of anodes. Hence Rn = 0.0152 + 0.5692 0.5844 (ohm) 5.1.5 TRANSFORMER RECTIFIER DC OUTPUT VOLTAGE It is estimated that the transformer rectifier will require a 45 volt output to achieve :Lie desired 54.32 ampere DC current output as follows: It.(Rn+Rc) + Bemf E 54.32x(0.5844 + 0.2) + 2 E 44.6 volts Say 45 volts Where : E Rectifier DC output voltage (volts) It = Rectifier output current (amp) Rn = Total groundbed resistance (ohm) Rc Total cable resistance (0.2 ohm) Bemf = Dropped voltage between pipeline and ground (-2.0 volts) Note : Pipe to earth resistance (Rpe) has not been considered since the coating resistari :e is so high.( >1010 ohm-mz ) that all current will pass through 5% bare areas through r±e pipeline coating. Rev. D G.U.P PHASE II - LIQUID PIPELINES, TERMINAL & JETTIES 5.1.6 TRANSFORMER RECTIFIER AC INPUT Actual DC power output required : Pd = 44.6 x 55.32 2422.672 (W) DC power requirement including 20% overvoltage provision (20% spare Capacity ) = 1.2x44.6x54.32=2907.2 (W) Now, considering overall system P.F (power factor) = 0.8 and Transformer Rectifier Unit efficiency (E.F) = 0.85 (85% ) For 3 phases, 415V, 50Hz supply input to Transformer Rectifier Unit : Input current 2907.2 Ip Ip = Jx415x0.8x0.85 5.95 A Hence, AC power input ( including 20% spare capacity) is : 43 x 415 x 5.95 x 0.8 3420.23 Watt 5.1.7 CHANGE IN EARTH POTENTIAL DUE TO FORCED DRAINAGE When CP current is injected into ground through anode bed, the current flow results in a potential gradient in the earth. _ The change in earth potential near the pipelines (Ps)(It) V = 27r(r) (2750)(54.32) V Where : Ps It r 5.2 PHUOC HOA LOCATION 5.2.1 Pipeline Total Surface Area 2 7zx 10000 2.38 Volts Soil resistivity ( ohm-cm ) Cathodic Protection current ( Ampere ) Distance between pipeline and anode bed (cm ) The onshore portion of Ba Ria to Phu My gas pipeline and Ba Ria to Thi Vai LPG pipelines has a total surface area as follows: Sa _ (Pi) (d) (1) Sa (3.1416) [(0.426 x 21.500)+ (3 x 0.168 x 17000)] Sa = 55691 (m2 ) P9-CPS.05-01.0 Page 10 of 14