Cassandra Guidelines

Corrosion Prediction with Cassandra S/UTG/013/03 Adam Petersen Richard Chapman Bill Hedges

Upstream Technology Group, Sunbury 01/03/03

GQS

In Greek Mythology Cassandra was the daughter of Priam and Hecuba. She was endowed with the gift of prophecy but fated never to be believed. She is regarded as the prophet of disaster…..especially when disregarded.

UTG Indexing Sheet BRANCH U T G

REPORT NO. S / U T

G

AUTHOR Adam Petersen Richard Chapman Bill Hedges

/

0

1

3

/

TELEPHONE + 44 1932 775912 + 44 1932 775944 + 1 868 623 2862 x5042

0

JOB NO. 1 1 9

3

LOCATION B H Sunbury B H Sunbury POS Trinidad

0

1

5

2

DATE 01/03/03

MAIN TITLE Corrosion Prediction with Cassandra

SUB TITLE

CLIENT Cassandra User Group

PRINCIPAL RECIPIENT

COMMISSIONED BY

PLEASE TICK SECURITY CLASSIFICATION TQA COMPLETED

X

X

UNCLASSIFIED

YES

CONFIDENTIAL

NO

KEYWORDS Cassandra, CO2, Corrosion, Rate, Prediction.

ACKNOWLEDGEMENT RECORD PAGE

FOR EXTERNAL CLIENT LISTING

DISTRIBUTION OVERLEAF

ABSTRACT PREPARED BY:

APPROVED BY:

AUTHORISED FOR ISSUE BY:

Adam Petersen

Bill Hedges

Don Harrop

. ISSUE DATE: 01/03/03 The information contained in this document is the property of bp Exploration. Due acknowledgement should be made if it is desired to refer to this information in publications or discussions with third parties.

DISTRIBUTION LIST

COPIES

NAME

Electronic

BDM/UTG Library

Electronic

Issued with Cassandra installer to user group

Contents 1.

Introduction ........................................................................................................................................... 5

2.

Simple Predictions using the Provided Spreadsheet.............................................................................. 5 a.

Basic Corrosion Rate Prediction........................................................................................................ 5

b.

Flow Sensitive Corrosion Rate Prediction......................................................................................... 6

3.

Programming Your Own Spreadsheets.................................................................................................. 6

4.

Corrosion Modelling.............................................................................................................................. 6 a.

The ’93 model (Flow Insensitive) ..................................................................................................... 7

b.

The’95 model (Flow-Sensitive)......................................................................................................... 7

c.

Arguments (Inputs)............................................................................................................................ 8 i.

Compulsory Arguments ................................................................................................................ 8 •

Unit Code .................................................................................................................................. 8

•

Temperature and Pressure ......................................................................................................... 8

•

CO2 Content .............................................................................................................................. 8

•

pH Models................................................................................................................................. 9

•

Liquid Velocity ......................................................................................................................... 9

•

Hydraulic Diameter................................................................................................................... 9

ii.

d.

Optional Inputs............................................................................................................................ 10 •

Scaling Temperature ............................................................................................................... 10

•

H2S Content............................................................................................................................. 11

•

Glycol Content ........................................................................................................................ 11

•

Water chemistry ...................................................................................................................... 11

•

Acetates................................................................................................................................... 11

Properties Available from CASS_RATE ........................................................................................ 12 i.

Corrosion Rates ........................................................................................................................... 12

ii.

Correction Factors ....................................................................................................................... 12 •

pH Correction Factors ............................................................................................................. 12

•

Fugacity Correction Factor...................................................................................................... 13

3

•

Glycol Correction Factor......................................................................................................... 13

•

Scaling Correction Factor ....................................................................................................... 13

•

Overall Correction Factor ....................................................................................................... 14

iii.

5.

Other Properties ...................................................................................................................... 14

•

Partial Pressures and Fugacity................................................................................................. 14

•

Scaling Temperature ............................................................................................................... 14

•

pH............................................................................................................................................ 14

Flow Modelling ................................................................................................................................... 14 a.

Fluid Property Prediction ................................................................................................................ 14

b.

Hydraulics and Flow Regime Prediction......................................................................................... 15

c.

Arguments ....................................................................................................................................... 15 i.

Compulsory Inputs ...................................................................................................................... 16 •

Temperature and Pressure ....................................................................................................... 16

•

Fluid Flow Rates and Properties ............................................................................................. 16

•

Pipe Information...................................................................................................................... 16

ii.

Optional Inputs............................................................................................................................ 16 •

d.

Gas Composition..................................................................................................................... 16

Properties Available from CASS_FLOW ....................................................................................... 16 •

Property Codes used in CASS_RATE .................................................................................... 16

6.

Limitations........................................................................................................................................... 17

7.

References ........................................................................................................................................... 17

8.

Appendix ............................................................................................................................................. 18 a.

History of Cassandra ....................................................................................................................... 18 i.

BP Cassandra ’98 ........................................................................................................................ 18

ii.

Cassandra 2001 ........................................................................................................................... 19

iii.

Cassandra ................................................................................................................................ 19

b.

Property Codes for CASS_RATE ................................................................................................... 20

c.

Property Codes for CASS_FLOW .................................................................................................. 21

4

S/UTG/013/03

1. Introduction This report accompanies the latest version of BP’s corrosion prediction model Cassandra, which is based on the CO2 corrosion models published by de Waard et al [1-3]. Included with the corrosion prediction function CASS_RATE are a number of pH prediction models (one of which is the BP-developed BPpH) and a multiphase flow prediction function CASS_FLOW. CASS_FLOW is built from fluid property predictions and two hydraulic flow models for horizontal and vertical flow, the BP GRE Mechanistic model (see section 5.b for description) and the BP coded Ansari [4] model. The document Corrosion Prediction Modelling [5] described and included instruction for use of the first version of Cassandra. This document is based on the original release but has been brought up to date to reflect the differences in the latest release of the program. Corrosion Prediction Modelling also contains advice on how a predictive model can be used for design purposes and describes topics including the probabilistic approach to corrosion prediction modelling, inhibitor availabilities and corrosion risk, subjects that are not covered here. Therefore, this report should be viewed as a supplement to the original document. Further details relating to the history of the Cassandra program can be found in appendix a. A second, extensive source of information for this report is the help file that comes as part of the installer for Cassandra. This latest release of the Cassandra software contains the same corrosion model as earlier releases, but in response to requests from BP Corrosion Engineers for increased flexibility, the interface between the user and the program has been significantly altered. The excel format allows the engineer to configure calculations in a way that is most appropriate for the individual’s needs. Intermediate properties can also be interrogated enabling results to be sense-checked. Thus the intent of this latest version of Cassandra is that the user will build their own spreadsheets to meet their specific needs. However, the installer does include a simple spreadsheet that enables the most basic of corrosion predictions for those who only want a quick indication of the likely corrosion rate.

2. Simple Predictions using the Provided Spreadsheet A spreadsheet is supplied with the Cassandra installer that contains two of the simplest forms of the corrosion prediction tool to enable quick predictions of corrosion rates.

a. Basic Corrosion Rate Prediction The most simple corrosion prediction model within Cassandra is the so-called '93 model. It is based on experimental results from stirred autoclave tests and consequently does not take the effects of flow on corrosion into account.

5

S/UTG/013/03 A simple version of the '93 model has been set-up; it requires input of temperature, pressure and acid gas content along with the water chemistry. If these values are entered (cells G11 to G28), the predicted corrosion rate will be displayed in cell G30. If Status:OK is not displayed below the corrosion rate an error has occurred, details of which can be found in the Cassandra help file under Error Messages in Cassandra.

b. Flow Sensitive Corrosion Rate Prediction Another corrosion model within Cassandra, known as the '95 model, is based on data from flow loop tests and is sensitive to flow effects. The most basic form of this model is included in the example spreadsheet. The ’95 model requires all of the entries that are needed for the ’93 model (cells G11 to G28) in addition to the flow parameters of liquid velocity and hydraulic diameter (cells K30 and K31). The predicted corrosion rate will be displayed in cell K33. In a similar manner to the ’93 model Status:OK should be displayed below the corrosion rate, if this is not returned the error messages section within the help file should be consulted. The flow sensitive model is not valid for all flowing conditions, and if the '95 corrosion rate is less than the '93, it should be ignored.

3. Programming Your Own Spreadsheets The spreadsheet that comes with the Cassandra installer allows prediction of corrosion rates for simple examples only; there are many other features within the program, for example, the effects of glycol, or the option of over-riding the internally predicted scaling temperature. To access the full potential of Cassandra in terms of technical features, as well as flexibility, users must build their own spreadsheets. A reasonable knowledge of MS Excel is required for this, but adequate guidance can be located in the Getting Started section of the help file, and in MS Excel help under Array formulas and About cell and range references. The excel program will run more slowly when the number of calculations increases, but this will rarely cause concern because the number of calculations at which this effect becomes significant is in the order of hundreds. The Cassandra program consists of four functions; CASS_RATE for corrosion prediction and pH calculation; CASS_FLOW for fluid property prediction and multiphase flow modelling; the other two functions CASS_VERS and CASS_DLLV return information regarding the version of the program components, which can be useful information when dealing with other Cassandra users.

4. Corrosion Modelling CASS_RATE is the main function of Cassandra, designed to predict the pH and return corrosion rates according to the de Waard models, or BP’s interpretation of them, called the BP models in subsequent text. The models are based on two papers reported at NACE corrosion conferences and have come to be known by the years in which they were reported, i.e. ’93 and ’95 [2,3]. There are slight differences between the de Waard and BP models. For example they treat fugacity differently; in the original de Waard models the partial pressure of CO2 is used although the use of fugacity is 6

S/UTG/013/03 discussed and recommended to account for the non-ideal behaviour of the gas. De Waard et al developed a correction factor to accommodate non-ideal gas but were not consistent in its use and their equations alternate between the use of partial pressure and fugacity. The BP models uses the fugacity in the original equations and no fugacity correction factor is applied (see equations 4.1, 4.3 & 4.4 below). There are other differences between the de Waard and BP models that are described below, but because the models are fundamentally similar they should predict similar results. The recommended pH and corrosion prediction models are obviously the BP versions, but the others can be consulted for sense checking of results and may be useful when dealing with engineers from other companies.

a. The ’93 model (Flow Insensitive) The ’93 models use an equation based on equation 4.1 to first calculate an uncorrected corrosion rate. This corrosion rate is then adjusted by multiplication with correction factors due to the effects of the water pH, gas fugacity, scaling temperature and amount of glycol. These correction factors are discussed in the outputs section below.

log10 Vcor = 5.8 -

1710 + 0.67 log10 (fCO 2 ) T + 273

(4.1)

where T is the temperature in °C, fCO2 is the fugacity of carbon dioxide in bar, and Vcor is the corrosion rate in mm/y. The data on which the model is based is from gently stirred, autoclave or cell experiments, which although valuable does not accurately replicate pipeline conditions. Thus the ’93 model is often referred to as a ‘low flow rate’ model.

b. The’95 model (Flow-Sensitive) The ’95 model is based on experimental data from pipe flow and more accurately represents conditions in a flowline or well. It calculates a corrosion rate in mm/y, Vcor, by combining the contribution of the kinetics of the corrosion reaction, Vr, with the contribution from the flow dependent mass transfer of dissolved CO2, Vm, in the manner described by the following equations:

1 1 1 = + Vcor Vr Vm

(4.2)

where,

log10 Vr = 6.23 −

1119 + 0.0013 T + 0.41 log10 ( fCO 2 ) − 0.34pH act T + 273

(4.3)

and,

Vm = 2.45

U 0.8 (fCO 2 ) d 0.2

(4.4)

where:

7

S/UTG/013/03 T is the temperature in degrees Celsius. fCO2 is the fugacity of carbon dioxide in bara. pHact is the actual pH. U is the liquid velocity in ms-1. d is the hydraulic diameter in metres.

c. Arguments (Inputs) The inputs to the Cassandra functions are known in MS Excel as arguments and are described by both terms in accompanying documents. Some of the arguments for the CASS_RATE function are compulsory, such as the temperature and pressure, whereas others are not part of the main equation and not essential for predicting a corrosion rate, for example, the water chemistry is not required if the pH is already known and entered manually. All of the arguments are highlighted in bold text and described below: i.

Compulsory Arguments •

Unit Code

The first argument requested by the function is the Unit Code and has the options C and F, these specify whether the arguments and results will be expressed in °C/bara/m or °F/psia/ft respectively. The program must also be instructed which outputs are required. This is done by specifying Property Codes entered as an array of text. Typically these will be set to return corrosion rates, and an overall status indicator, but a range of other quantities may also be returned. All of the property codes are listed with their descriptions in the appendix b. •

Temperature and Pressure

The Local Temp and Total Pressure for the point of interest in the system need to be entered in units of °C/bara or °F/psia, depending on the selection of the unit code. The pressure itself is not used in the prediction, but is used in combination with the gas composition to determine the concentration of dissolved gases. If the pressure is greater than the bubble point of the fluid, the bubble point pressure should be entered instead of the total pressure. •

CO2 Content

As stated above, the molar/volume percentage of carbon dioxide from the separated gas (Percent CO2) is used to determine the concentration of the dissolved gas in the water phase. Therefore, even if there is no gas present at the particular point, the CO2 content of the gas phase that the liquid was last in contact with should be used.

8

S/UTG/013/03 •

pH Models

A number of different pH prediction models are available within Cassandra. A code identifying which model is required must be specified via the pH Model call. There are four options: 1. BP

- the BPpH model calculates the pH with the individual ion concentrations and balances the charge by adding either sodium or chloride ions. There is an option to treat any acetate as either the sodium salt or the acid (discussed below).

2. DW

- the de Waard and Milliams model calculates the pH of salt-free water containing dissolved CO2; it is calculated with the following equation and is valid for temperatures between 10 and 80°C [3].

pH(CO 2 ) = 3.82 + 0.000384 T - 0.5 log10 (fCO 2 )

(4.5)

- the Oddo and Tomson model is based on equation 4.6 and is valid for temperatures up to 200°C and pressures up to 1200 bara. The equation does not give accurate results if [HCO3-] is below 50 mg/l [6].

3. OT

−   [HCO 3 ]   + 8.68 + 0.00405 (T * 9/5 + 32)... pH = log10  fCO2 *14.5 * 61000   

+ 0.000000458 (T * 9/5 + 32) 2 - 0.0000307 (P *14.5)...  TDS  - 0.477    58500 

1/2

(4.6)

 TDS  + 0.193    58500 

where the T is in °C and P is in bara. 4. MANUAL

- for direct entry of the pH into the Ph Manual argument if it has been determined or measured elsewhere. If any of the other pH models are selected any entry in the Ph Manual argument is ignored.

Depending on the selection for the pH model, other inputs may become compulsory, for example, the ion definitions and concentrations when the BP pH model is selected. •

Liquid Velocity

The Liquid Velocity is compulsory for flow sensitive corrosion rate predictions using either of the de Waard or BP ’95 models, but is not required for the basic ’93 models. It should be stressed that the actual liquid velocity in either ms-1 or fts-1 is required, not the superficial velocity. The data from which the model was built lies between 1.5 and 13 ms-1. When the model is used outside of this range the results should be used with caution. The value can either be entered directly if calculated with a different with a multiphase prediction package, or calculated with the Cassandra multiphase flow model, CASS_FLOW. •

Hydraulic Diameter

Another required argument for the flow sensitive models is the Hydraulic Diameter in units of either m or ft, dependent on the unit code entry. In a similar manner to the liquid velocity, a value can be entered directly or calculated with CASS_FLOW. 9

S/UTG/013/03 ii.

Optional Inputs

•

Scaling Temperature

The corrosion reaction produces products, such as Fe2+ ions, which can precipitate from solution onto the metal surface to form scale. This formation of scale is dependent on several factors and is important as it may provide some corrosion protection. Temperature is the most important factor in determining if a protective scale is formed or not is. As the temperature increases the formation of a protective scale becomes favourable and the de Waard models introduced the concept of a Scaling Temperature, which is the temperature at which a protective film will form. The Scaling Temperature can be entered manually or calculated. If a value for the Scaling Temp is not entered, the program automatically predicts a value using equation 4.7 [3],

Ts =

2400 + 273 0.44log(fCO2) + 6.7

(4.7)

where Ts is the scaling temperature in °C. There is an opportunity to over-ride the internally calculated scaling temperature by manually entering a value in the Scaling Temp argument. Warning – if the CASS_RATE function is directed to a blank cell, MS Excel assumes that a scaling temperature of zero has been entered and a very low and incorrect corrosion rate will be returned.

Figure 4.1 Variation of corrosion rate with temperature. The figure depicts the conflicting trends from experimental data used by de Waard and results from IFE. The BP approach in light of this data lies between the two experimental data sets.

The treatment of the variation in corrosion rate above the scaling temperature is an area where the de Waard and BP models differ. These different approaches are shown in Figure 4.1 along with the trend of data from IFE, Norway. The two sets of experimental data, one used in developing the de Waard model and the second 10

S/UTG/013/03

from IFE, indicate opposing trends, the de Waard approach indicates that the corrosion rate will fall whilst IFE data suggest that the corrosion rate will continue to rise, albeit at a lower rate. In light of this conflicting data and with the aim of being neither too conservative nor optimistic, the BP model assumes the corrosion rate remains constant once the scaling temperature is exceeded. Cassandra achieves this by internally performing the calculation at the scaling temperature for any case where it is exceeded by the actual temperature. •

H2S Content

In a similar manner to the CO2 content the molar/volume percentage of hydrogen sulphide in the gas phase (Percent H2S) is used along with the total pressure to determine the amount of hydrogen sulphide in solution. If there is no H2S present the argument can be left undefined. It should be noted that Cassandra only takes into account the effect that H2S has on the pH of the solution and does not account for any changes in the corrosion mechanism (i.e. sour corrosion). •

Glycol Content

When glycol is present in the process stream it inhibits the corrosion reaction. This inhibition is accounted for with a correction factor that requires entry of the percentage by weight of glycol in the aqueous phase (Percent Glycol). The inhibitive effect is much smaller than that of specifically designed chemical corrosion inhibitors, such that when both are present the effect of the corrosion inhibitor dominates the effect of glycol and in these cases it should be assumed that the glycol content is zero. •

Water chemistry

When either the BP or Oddo and Tomson pH models have been selected the water chemistry becomes compulsory for pH prediction. The water chemistry requires two entries, firstly an array of text corresponding to the Ions: For the Oddo and Tomson model specification of bicarbonate concentration and total dissolved solids in terms of the ions; Na, K, Ca, Mg, Sr, Ba, Fe, Cl, SO4, Ac (for acetate) are required. The BP model allows a wider range of ions to be included; Mn, Al, Si, B, F, Li, Br, Zn, Cd, Pb, Cu, PO4, NO3 and also permits all ions to be specified by name, e.g. Sodium, (ions cannot be specified by name for the Oddo and Tomson model). The entry for the ions must be consistent with the specification of Concentrations, entered as a numerical array in the units of mg/l. The BP pH prediction program also has a dependency on the Water Gravity, if the value is unknown the argument can be left blank and the program will assume that the gravity is equal to that of pure water. •

Acetates

The effect of acetates on the corrosion process can be two-fold; acetates cannot only affect the pH of the solution, but possibly more importantly, the acetate species in solution can enhance the CO2 corrosion reaction. Research into the corrosion reaction enhancement is ongoing and in the future an enhancement factor for acetates may be introduced into the Cassandra program. Early results have shown that acetate concentrations of only 100 mg/L can have the effect of tripling the corrosion rate.

11

S/UTG/013/03

In terms of the effect of acetate on the pH, the BP pH model provides an option of assuming that the acetates were added in the form of acetic acid or the sodium acetate salt. The preference of these two options can be specified through the Acetates as acid argument, which can be defined either as ‘TRUE’ or ‘FALSE’. In the cases where the acetate is treated as an acid, the pH will be lower and subsequently the predicted corrosion rate will be higher. The default and recommended selection is ‘FALSE’ unless there is data to suggest otherwise (e.g. the measured pH of a de-gassed brine sample).

d. Properties Available from CASS_RATE The flexibility in Cassandra enables a number of properties to be displayed to the user. These can be used to determine the factors that are affecting the corrosion process, examine intermediate properties for quality assurance purposes, etc. The user defines which properties are returned by entering the property codes, written in bold type in the text below, into the CASS_RATE function (a full list is included in appendix b). i.

Corrosion Rates

It is recommended that the corrosion rates used for prediction are the BP versions, which are returned using the codes BP93 and BP95 for the ’93 and ’95 models respectively. The codes DW93 and DW95 will return predicted corrosion rates according to the original de Waard models. There are two other codes, VBP and VDW, these return the largest of the ’93 and ’95 corrosion rates for the BP or de Waard assumptions. ii.

Correction Factors

Correction factors are used in the ’93 corrosion models to adjust a corrosion rate that is derived using equation 4.1 for the effects of pH, fugacity, glycol and scaling. The property codes for correction factors are denoted by the first two characters CF, the third and fourth characters correspond to the property, for example FU for fugacity, and the final character identifies whether the factor is used in the de Waard or BP models (D or B respectively). •

pH Correction Factors

Equation 4.5 can be used to predict the pH of water containing dissolved CO2., if the water contains dissolved ions the actual pH of the water may differ significantly from this value. To accommodate for this effect the original de Waard model uses the following correlation to calculate the correction factor CFPHD [1]: log10 CFPHD = 0.32 (pH sat − pH act ) for

pH sat > pH act

log10 CFPHD = - 0.13 (pH act - pH sat )1.6 for

pH sat < pH act

(4.8)

(4.9)

where pHact is the ‘actual’ pH of the brine, i.e. that pH calculated using the model selected with the pH model argument or the pH entered manually. The value can be recalled by requesting the property code PHACT. pHsat is the pH at which the brine becomes saturated with Fe2+, its value recalled with PHSAT (equation 4.10).

pH sat = 5.4 - 0.66 log10 (fCO 2 )

(4.10)

12

S/UTG/013/03

The pH correction factor used in the BP model (CFPHB) is calculated slightly differently because it is felt that pHsat is difficult to define especially when supersaturation occurs (the condition where dissolved Fe2+ concentrations can exceed the theoretical saturation values by orders of magnitude, making the probability of a ‘fixed’ saturation pH unlikely). The BP method avoids this issue by using the BP pH model (that does include the effects of ions) for PHACT and the pH of a salt free solution of CO2, PHDW (equation 4.5) instead of PHSAT. This approach has the advantage that it is well defined and is valid over a wide range of conditions. •

Fugacity Correction Factor

The original de Waard model includes a dependency on the partial pressure of CO2, and corrects for the nonideal nature of CO2 with the correction factor, CFFUD. log10 CFFUD = 0.67 (0.0031 -

1.4 )P T + 273

(4.11)

where T is the temperature in °C and P the pressure in bara. The de Waard paper states that models must take account of the non-ideal nature, which the BP approach does by using fugacity in the original equation, therefore, no correction is needed and the correction factor, CFFUB, will equal unity. •

Glycol Correction Factor

A correction factor is used to account for the inhibitive effect of glycol on the corrosion reaction. The correction factor is identical for both the BP and de Waard approaches, consequently CFGLB will equal CFGLD: log10 CFGLD = A (log10 W - 2)

(4.12)

where A is a constant which varies according to the glycol (1.6 is recommended) W is the water content (%) in the water / glycol mixture •

Scaling Correction Factor

Figure 4.1 and accompanying text describes the different approaches to the behaviour of corrosion rate increase with temperature above the scaling temperature. The de Waard method uses the following equation to calculate a correction factor, CFSCD, which accounts for the protection provided scale that forms above the scaling temperature.  1  1  log10 CFSCD = 2400  T + 273 Tscale + 273 

(4.13)

where T>Ts , otherwise CFSCD = 1 Tscale is the scaling temperature in °C (equation 4.7) The BP approach assumes that the corrosion rate remains constant above the scaling temperature. Therefore the correction factor for scaling, CFSCB, is not used and is set equal to one (=1.0).

13

S/UTG/013/03

•

Overall Correction Factor

The above correction factors are combined into a single factor, equal to the product of the individual factors. These overall correction factors are calculated by Cassandra and can be returned with the property codes CFDW and CFBP for the de Waard and BP methods respectively. iii.

Other Properties

As well as the correction factors described above, Cassandra can return other calculated properties that can be valuable for the purpose of sense-checking results. •

Partial Pressures and Fugacity

The partial pressures of the acid gases can be returned with PPCO2 and PPH2S, but due to the non-ideal nature of the gases the BP approach uses the fugacity of CO2 instead of the partial pressure. There are two codes that can be used to return the fugacity, FCO2A, returns the fugacity of CO2 at the system pressure. In cases where the system temperature is above the scaling temperature, the fugacity at the scaling temperature is used in the calculations and can be recalled with FCO2U. •

Scaling Temperature

There is uncertainty in the manner that the corrosion rate varies above the scaling temperature, in light of this fact it may be important to determine how similar the scaling and system temperature are and the effects of any assumptions. The scaling temperature that is internally calculated by the program using equation 4.7 is returned with the code CSCAT. The scaling temperature used by the program in calculations can be returned with ASCAT, and will only differ from the calculated scaling temperature if it has entered manually. •

pH

The pH value used in the calculations can be returned with PHUSD.

5. Flow Modelling The Cassandra flow modelling function, CASS_FLOW, can perform predictions of fluid properties and also hydraulics and flow regime. Many different properties can be predicted by the complex function, but the focus of this document is corrosion prediction and so a brief overview of the CASS_FLOW function is described below with particular attention to the properties used to predict corrosion rates. If there is any uncertainty or if further information is required about the CASS_FLOW function, the Multiphase Flow Group in UTG Sunbury should be contacted for guidance.

a. Fluid Property Prediction The CASS_FLOW function contains correlations for the prediction of black oil physical properties for conditions of temperature and pressure that are typical of reservoir and well applications; 30 to 150 °C and 70 to 550 bara. Some correlations apply to wider ranges, but these conditions can be considered representative and when values lie outside these ranges any key physical properties must be sense-checked. 14

S/UTG/013/03

The model requires both gas and oil to be present in the system, but unfortunately there is a limit on the relative values and is only valid for gas to oil ratios (GOR) up to a maximum of about 2000 scf/stb. When the GOR raises above this value the need for careful inspection of the derived properties increases. Any additional gas produced by the system, such as gas lift or injected gas that has broken through the reservoir, must be included. If unusual fluids are being modelled then advice should be sought from the Multiphase Flow Group in UTG Sunbury, as not only may the black oil fluid models not be applicable but also the flow correlations. It should be noted that the black oil viscosity correlations will not predict emulsion behaviour and therefore care should be taken if emulsions are suspected to exist or at values close to the inversion point for the fluid (if known). Specifically, the correlations included are: •

Glaso correlation for solution gas-oil ratio and oil formation volume factor prediction [7].

•

Lee for the prediction of gas viscosity [8].

•

Beggs and Robinson for the prediction of liquid viscosity [9].

•

Baker and Swerdloff and Hough et al for the prediction of surface tension [10,11].

b. Hydraulics and Flow Regime Prediction The hydraulic model available within Cassandra 2002 for estimating liquid velocity and hydraulic diameter relies on the fluid property predictions outlined above and an integrated model for the prediction of multiphase flow characteristics for which there are two options: For inclinations greater than or equal to 20° the BP-coded Ansari model [4] for the vertical upflow hydraulics is used whilst for inclinations less than 20° the BP GRE mechanistic model for near-horizontal multiphase flowline hydraulics is employed. The GRE model is a mechanistic pressure drop model for stratified flow (developed in the 1990’s at Sunbury) that reverts to Beggs and Brill pressure drop calculations if it determines the flow is not stratified [12]. The horizontal flow regime prediction is extremely sensitive to inclination. For slight inclinations where the angle is less than about 1°, the inclination needs to be specified to the nearest 0.1°. Unfortunately, the models are not designed for downflow conditions although in the absence of any other data the GRE model can be used with caution to angles as steep as –20°. Great care should be taken with the flow regime predictions provided by the function. The empirical nature of the function will specify a particular flow regime, but naturally the regime boundaries are not precise. Therefore, it is recommended that sensitivities be run to establish whether the operating point of concern is near a regime boundary, and if so both regimes should be considered and that which produces the most conservative results assumed.

c. Arguments Many of the arguments that are required for the CASS_FLOW function are also required for the CASS_RATE function; both functions can be directed to the same cell to ensure consistency. Of the many properties that are calculated by the function only two are used as inputs for the flow sensitive ’95 corrosion rate, the liquid velocity and hydraulic diameter.

15

S/UTG/013/03 i.

Compulsory Inputs

The fields that are required for the flow prediction function CASS_FLOW include the Unit Code and a set of Property Codes corresponding to the relevant outputs that are required. A list of the property codes with descriptions is included in appendix c. •

Temperature and Pressure

Similar to the CASS_RATE function the Local Temperature, and Total Pressure at the point of interest are required. •

Fluid Flow Rates and Properties

The Gas, Oil and Water Flow Rates in mmscf/d or mbd are required, as well as the Gas, Oil and Water Gravities relative to air and water. •

Pipe Information

Some information about the pipe itself must also be entered; the Internal Diameter of the tubing or flowline must be entered in m or ft and the Roughness needs to be entered in the same units. A typical value for the roughness of an internal pipe surface is 4.57 x 10-5 metres. The Inclination of the pipe of flowline must be entered in degrees from the horizontal to not only be used in the calculations, but also select the appropriate multiphase model. ii.

Optional Inputs

•

Gas Composition

Part of the gas composition should be entered, if available. This includes not only the molar/volume percentage of carbon dioxide and hydrogen sulphide, which are also used for the CASS_RATE function (PercentCO2 and PercentH2S), but also nitrogen (PercentN2).

d. Properties Available from CASS_FLOW There are over thirty different property codes that can be returned from the CASS_FLOW function, of which only two are used for the corrosion rate prediction. The other properties are, in many cases, intermediates that can be used to sense-check results. The total list of property codes with brief descriptions and units is shown in the appendix c. The Multiphase Flow Group in UTG Sunbury should be contacted if further clarification or guidance is required. •

Property Codes used in CASS_RATE

The property codes used for predicting the corrosion rate are the liquid velocity, which has the code VLIQ and units of ms-1 or fts-1, and the hydraulic diameter, HDIAM, with units of either m or ft.

16

S/UTG/013/03

6. Limitations The Cassandra model is based on equations that have been developed over a number of years by de Waard et al. [1-3]. These equations can be used to produce precise corrosion rates, but the results should not be interpreted in this way. The corrosion mechanism is inherently complex and cannot be accurately predicted (or indeed measured) under all circumstances. This should be kept in mind when using corrosion prediction models for design. Within Cassandra there are a number of different models and correlations, the validity of which have not been assured over all conditions. The following table describes the limits of certain elements of the model. Element

Validity Range

de Waard pH calculation

10 to 80 °C

Oddo and Tomson pH calculation

Up to 200°C and 1200 bar

Flow sensitive corrosion rate BP95 and DW95

Liquid velocities between 1.5 and 13 ms-1

Fugacity of CO2

0.3 to 0.65 bar

Fugacity look up tables

Total pressures below 200 bar

Table 6.1 Validity ranges for some elements of the Cassandra Prediction Program

7. References [1] C. de Waard, U. Lotz and D. E. Milliams, Corrosion 47, 12, (1991) p976 [2] C. de Waard, U. Lotz, Paper 69, Corrosion ’93 (NACE International) [3] C. de Waard, U. Lotz and A. Dugstad, Paper 128, Corrosion ’95 (NACE International) [4] A. M. Ansari et al. SPE Production and Facilities, SPE 20630 (1994) p143-165 [5] A. J. McMahon and D. M. E. Paisley, Sunbury Report No. ESR.96.ER.066, (1997) [6] J.E. Oddo and M.B. Tomson, J. Petroleum Tech., 34 (1982) p1583-1590 [7] O. Glaso, J. Petroleum Tech., (1980) p785-795 [8] A. L. Lee, M. H. Gonzalez and B. E. Eakin, J. Petroleum Tech., (1966) p997-1002 [9] H. D. Beggs and J. R. Robinson, J. Petroleum Tech., (1975) p1140-1144 [10] Baker and Swerdloff, Oil and Gas Journ., (1956) p125 [11] Hough et al., Trans AIME, (1951) p57 [12] H.D. Beggs and J. P. Brill, J. Petroleum Tech., (1973) p607-617

17

S/UTG/013/03

8. Appendix

a. History of Cassandra The Cassandra Corrosion Prediction Tool is based on the equations developed by the Shell Oil Company. The first equations were published by C. de Waard & D.E. Milliams in the mid 1970’s with significant additions in 1991, 1993 and 1995. These equations became known as the de Waard & Milliams equations and were widely used in the Oil & Gas industry including BP. With time many companies modified the equations to account for their own preferences and /or experiences, which led to an inconsistent use of them. Moreover, the exact method of calculation was obscured by the use of different practices and correction factors. This proved increasingly problematic for BP when dealing with external engineering companies who often did the calculations for BP Projects. Consequently, in 1997 BP Sunbury began a project to formalize the way the calculations were done and provide transparency to them. An Excel based spreadsheet was built by Drew McMahon to generate the corrosion rates using standardized inputs. In May 1998 this was released to both BP and it’s contractors under the name of BP Cassandra ’98 (the winning name suggested by David Ray in a competition). It was released with a guidance document detailing the logic used together with advice on how to use the results during the design phase of a project (see reference 5). i.

BP Cassandra ’98

The model, together with the guidelines, superseded previous BP documents covering this topic. They described BP's approach to Corrosion Prediction and its use during the design of pipelines and facilities. Important features were: 1. A clear, transparent method for calculating corrosion rates. 2. The introduction of probabilistic modelling as an alternative to the traditional deterministic approach (reflecting the reality that real systems experience a distribution of corrosion rates and not a single, discrete rate). 3. A move away from the use of corrosion inhibitor efficiencies to the use of availabilities. 4. The introduction of Corrosion Risk Categories as a way of classifying systems. The guidelines were divided into two sections; the first section introduced the new prediction spreadsheet called Cassandra ’98 as BP's implementation of the CO2 prediction models published by de Waard et al. It built on these models to include BP's experience of such systems. The spreadsheet was provided on a floppy disc with the guidelines. The second section discussed how the prediction model should be used for design purposes and introduced several improvements over previous guidelines. The new material and concepts originated from many sources including the results of Joint Industry Projects, in-house R&D and practical experience from both BP and other operators. To illustrate the points made, examples were provided from many BP assets worldwide.

18

S/UTG/013/03 ii.

Cassandra 2001

The Cassandra 2001 worksheet was developed based on feedback from the users of the Cassandra ’98 version and because Microsoft had changed the macro structures in the Excel program. It included updated macros and components, which were compatible with Windows 2000 software (BP COE3). In order to provide the required functionality for the new version it was necessary to write detailed macros, which inevitably reduced the ‘transparency’ of the calculations. Cassandra 2001 was designed to be compatible with the ‘Corrosion Prediction Modelling’ guidelines. Due to the loss of transparency of some of the calculations the need for reference to the guidelines was very important for users who wanted to understand how the calculations were made. New Features introduced were: 1. Option to store brine chemistries 2. Option to predict corrosion rates over a range of one parameter (e.g. Temperature). 3. Incorporation of a multiphase model iii.

Cassandra

The latest Cassandra has been developed to increase the functionality further and to fix a serious error in the multiphase program. The flexibility has been introduced by the use of array functions. It is targeted at experienced corrosion engineers. The date suffix has been omitted to avoid any confusion that an ‘out of date’ version may be being used.

19

S/UTG/013/03

b. Property Codes for CASS_RATE Property Code

Description

Units

PPCO2 PPH2S FCO2A ASCAT FCO2U CSCAT PHACT PHUSD PHSAT CFPHB CFGLB CFFUB CFSCB CFBP CFPHD CFGLD CFFUD CFSCD CFDW PHDW BP93 BP95 VBP DW93 DW95 VDW STATS

Partial pressure of CO2 Partial pressure of H2S Fugacity of CO2 (actual) Accepted scaling temperature Fugacity of CO2 (used) Calculated scaling temperature pH (actual) pH (used) pH of saturated CO2 in water pH correction factor (BP model) Glycol correction factor (BP model) Fugacity correction factor (BP model) Scaling correction factor (BP model) BP total correction factor pH correction factor (de Waard model) Glycol correction factor (de Waard model) Fugacity correction factor (de Waard model) Scaling correction factor (de Waard model) de Waard total correction factor de Waard pH BP 93 model corrosion rate BP 95 model corrosion rate (requires non-zero liquid velocity & hydraulic diameter) BP overall corrosion rate (requires non-zero liquid velocity & hydraulic diameter) de Waard 93 corrosion rate de Waard 95 corrosion rate (requires non-zero liquid velocity & hydraulic diameter) de Waard overall corrosion rate ( “ “ “ “ “ ) Return status and error flags

bara or psia bara or psia bara or psia °C or °F bara or psia °C or °F None None None None None None None None None None None None None None mm/y mm/y mm/y mm/y mm/y mm/y None

20

S/UTG/013/03

c. Property Codes for CASS_FLOW Property Code

Description

Units

GOR

Input GOR

scf/stb

API

API gravity of oil

°API

RS

Solution gas-oil ratio

scf/stb

BO

Oil formation volume factor

stb/rb

RSW

Solution gas-water ratio

scf/stbw

BW

Water formation volume factor

stbw/rb

VFW

Volume fraction of water

None

SGDG

Specific gravity of dissolved gas (air=1)

None

SGFG

Specific gravity of free gas (air=1)

None

ZFAC

Compressibility factor of gas

None

BG

Ideal gas equation constant

ft3/scf

QG

In situ volume gas rate

QL

In situ volume liquid rate

mmscf/d stb/d

VSG

Superficial gas rate

ft/s

VSL

Superficial liquid rate

ft/s

ODENS

Oil density

lb/ft3

WDENS

Water density

lb/ft3

GDENS

Gas density

lb/ft3

LDENS

Liquid density

lb/ft3

GVIS

Gas viscosity

cP

LVIS

Liquid viscosity

cP

STEN

Surface tension

dyn/cm

HL

Liquid holdup

None

DONR

Depth/radius

None

DPDL

Pressure gradient

psi/ft

FRGR

Frictional pressure gradient

psi/ft

ELGR

Elevational pressure gradient

psi/ft

Accelerational pressure gradient

psi/ft

No-slip liquid holdup

None

Hydraulic diameter

m or ft

ACCGR HLNS HDIAM VLIQ

Liquid velocity

FPAT

Flow pattern

None

Return status and error flags

None

STATS

m/s or ft/s

21