The Application Of Computer Modeling To Improve The Integrity Of Ballast Tanks

Transcription

1 Paper No The Application Of Computer Modeling To Improve The Integrity Of Ballast Tanks Robert Adey, Guy Bishop, John Baynham, CM BEASY Ltd Ashurst Lodge Southampton, SO40 7AA, UK ABSTRACT Generally additional cathodic protection (CP) systems based on sacrificial anodes (and in some naval vessels Impressed Cathodic Protection Systems (ICCP)) are installed in ballast tanks to provide protection to the areas that may become unprotected by degraded coatings. Because of the complex geometry of the tanks and the presence of pipework, equipment and in some cases ladders and walkways the correct placement of both sacrificial & ICCP anodes is essential to get a good potential distribution that ensures no areas are either under or over protected. Computer modeling has become widely used in the maritime corrosion industry to predict the performance of CP designs and to ensure adequate protection is provided to the structure over its life. In this paper a case study is presented where computer modeling is used to verify and optimize the design of the corrosion control system of a ballast tank and to predict how it will perform over the service life of the tank. Case studies will be presented for both a sacrificial CP system and ICCP design. Key words: Galvanic corrosion, Cathodic Protection, Ballast tanks, ICCP INTRODUCTION Ballast tanks filled with seawater often experience problems with age-related coating degradation and incipient failure. Corrosion control in a seawater ballast tank environment is complicated by the function, physical design, operation and maintenance of the tank. Ballast tank corrosion is the third leading corrosion cost driver for Navy ships for example and it is estimated that the US Navy spends approximately $230 M / annually in corrosion maintenance for ballast tanks. Corrosion control in commercial shipping is equally important and the

2 surveyor of one of the leading classification societies has been quoted as saying that Effective corrosion control in segregated water ballast spaces is probably the single most important feature, next to the integrity of the initial design, in determining the ship s effective life span and structural reliability. 1 Because of the complex geometry of the tanks and the presence of pipework, equipment and in some cases ladders and walkways the correct placement of both sacrificial & ICCP anodes is essential to get a good potential distribution that ensures areas are not either under or over protected. Too many anodes can cause premature coating degradation, paint failure & coatings which delaminate by a process called cathodic disbonding so the number, type and placement of anodes has to be carefully matched to the barrier properties of the coating, which will change with time. Insufficient anodes will lead to the tank being under protected and a potential threat to the integrity of the vessel. The design of the CP system is complicated by the changes in the condition in the tank over its life where at times it may be full, half full or empty which can lead to accelerated corrosion due to the thin electrolyte film on the surface when the tank is not full. The design of ICCP systems for ballast tanks is particularly problematic because the location of the anodes and reference electrodes (RE) is complicated by the potential shadowing caused by the internal structure. For existing structures and vessels potential surveys using a half cell (either zinc or silver/silver chloride reference electrodes) can be used to identify good anode placement and some studies have been undertaken using Physical Scale Models to identify optimum CP designs. 2 However both these methodologies are relatively costly and time consuming and for one of the methods can only be applied to an existing tank. The recent widespread use of computer modeling in predicting the performance of cathodic protection systems provides a relatively inexpensive solution to determining the optimum design as the performance of the CP system can be predicted over the life of the tank. The robustness of the design can be ensured by modeling various scenarios representing the possible conditions, environments and damage the tank may experience over its life. COMPUTER MODELING Computer modeling has developed over a number of years and is now widely used to verify the performance of CP systems in the maritime environment and onshore. 3,4,5,6 The basis of the techniques used in this study is the boundary element method and the BEASY CP software package. (1) A computer model simulates the physics of galvanic corrosion and the features of a cathodic protection system. The model simulates the electrode kinetics on the metallic surfaces, the coating barrier, the electrolyte (seawater) resistivity and the 3D geometry of the electrolyte and the metallic structure immersed in the electrolyte. The time required to build a model very much depends upon the complexity of the ballast tank and if electronic CAD data is available describing the structure. The starting point for building a model is the geometry of the tank. As shown in Figure 1. All the metallic surfaces are defined, including the type of material they are constructed from and the coatings applied to the surfaces. The polarization properties of the different metals used in the structure and the seawater resistivity are defined. The geometry is 1

3 preferably created from a CAD system and then imported into the modeling software where all the data is defined and the computational meshes are created on the surfaces. Figure 2 Figure 1 View of the geometry of the ballast tank showing the location of some of the anodes and the tank structure. Note some of the tank walls have been removed so the inside details can be displayed The most important aspect of any modeling study is the definition of the cases to be investigated. This can vary from a simple study to: Verify that the chosen design will protect the structure over its life Compare the effectiveness of different designs Test the robustness of the design by simulating possible damage and failure scenarios the tank may experience over its life Figure 2 View of the tank geometry showing the boundary element mesh on the surface of the structure In general once the model is built it makes sense to use it to test all the design options and the sensitivity of the design to various parameters as it is cost effective to run additional cases. To predict how the design will perform over the life of the structure a time dependent simulation is performed where the assumptions regarding the rate of degradation of the coatings with time are input into the model. This data can be based on the design code used or based on damage experienced on similar structures. The simulation starts at year zero and predicts the protection provided to the structure and the currents provided by the individual anodes. The simulation continues step by step calculating at each step the consumption of the anodes

4 (including the reduction in size) and including the assumed degradation of the coatings until the design life is reached. If during the simulation the anode is consumed it no longer contributes to the protection of the tank. The simulation provides data on the protection provided to the structure over time and the consumption of the individual anodes. With this information areas where protection may not be maintained and imbalances in the anode consumption can be identified and used to improve and optimize the design. CASE STUDIES In this study computer modeling was used to verify the protection provided by the initial design over the design life of the tank, optimize the design of the CP system and investigate the option of an ICCP based design. Case 1 Design Verification The ballast tank CP system was designed in this case according to DnV RP401 using the total area of the metal surfaces, assumptions about the degradation of the coatings over time and the calculated anode resistance. The problem with this design approach for complex structures is that: The anode resistance is based on an analytic equation which does not take into account the actual geometry of the tanks, all the materials present and only very approximately allows for interference between anodes Tanks frequently have other structures and piping passing through the tank which complicate the design of the protection system The ability of anodes to deliver current to protect the tank will depend upon their location in the tank and the geometry of the tank which is not considered in the design calculations Consequently designs can suffer from: Unprotected areas of the tank due to shadowing Inconsistent consumption rates of the anodes leading to some anodes failing to achieve the design life and thus failing to provide protection over the design life Overprotected areas which can cause accelerated coating degradation The advantage of using a computer model to assess the performance of the CP design is that the model is based on the corrosion physics and can accurately represent the real geometry of a tank and the characteristics of the anodes. Therefore the model can simulate how the anode current is distributed through the water in the tank and the potentials achieved on the metallic surfaces: Predicting the impact of shadowing. (when nearby structures increase the length of the path the current has to take in the seawater between the anode and the structure to be protected) Predicting the actual current delivered by the individual anodes Predicting the impact of additional structures present in the tank such as walkways Predicting the impact of piping, gratings and other structures with different corrosion properties such as corrosion resistant alloys

5 Like all models there is uncertainty in the input data for the model and this should be carefully considered when designing the study. However through the use of sensitivity studies the predicted performance envelope of the design can be determined. It can also be used as a Virtual Ballast Tank to test the robustness of the design by simulating possible damage and failure scenarios and the tank environmental conditions it may experience over its life. The 9 anodes calculated from the design were located in the tank and the coating degradation data defined. In the model an area of damage was assumed near the main bulkhead where the coating had a breakdown factor of 50%. The coating was assumed to be initially good with 2% coating breakdown and this was assumed to increase by 1.15% per year. To predict the lifetime performance of the design a simulation study was performed by time stepping through the years where at each step the coating breakdown factors were increased to simulate the coating degradation, the consumption of the individual anodes was calculated and used to reduce their size which also reduced their ability to deliver current. In cases where the anode had been fully consumed it was automatically eliminated from the simulation. The simulation also assumes that the tank is completely full with water therefore the timescales would have to be adjusted to take into account the actual operational history. For example by adjusting the timescale if the tank is empty 50% of the time or by running additional simulations with the tank only half full if that is how the vessel frequently operates. Figure 3 Predicted potentials on the tank walls and members at the design midlife The results of the simulation at the mid design life is shown in Figure 3. The figure shows the potentials achieved on the walls and structure of the tank with some of the walls removed to improve the visualization. The areas near the anodes colored blue shows potentials more negative than mv vs. Ag/AgCl and the red areas towards the top right corner of the tank are in the range -800 to -900 mv vs. Ag/AgCl thus indicating that the CP system is providing protection with the assumed level of coating degradation. Table 1 Predicted anode consumption and remaining life at the mid design life Case 1 location ID Mass Utilisation Factor Mass Now Mass Loss Rate Active? Remaining Life Predicted Life (Years) at Current Consumption Rate 25 Percentage Consumed Pecentage Consumed (Kg) (Kg) (Kg/Year) (Years) (%) Year 13 (MidLife) Cl yes X X X X X X X X X X 10% Fr.77_ yes X X X X X X X X X 12% X Fr.77_ yes X X X X X X X X X X 11% X Fr.78_ yes X X X X X X X X X X 10% X Fr.78_ yes X X X X X X X X X 12% X Shell_ yes X X X X X X X 15% X Shell_ yes X X X X X X 16% X Table 1 shows how each of the anodes in the tank have been consumed by the mid design life. As can be seen there is variation in the predicted anode consumption which ranges from

6 10% to 16%. As discussed previously the design calculation calculates the total mass of anodes required without taking into account the actual distribution of the anodes in the tank or the geometry of the tank. Therefore it is not surprising that the consumption rates of individual anodes vary. Figure 4 Predicted consumption rates of the individual anodes in the tank at the end of the design life. Red indicates the highest consumption rate and blue the lowest Figure 4 shows a visualization of the anode consumption rates at the end of the design life. The variation in the consumption rates of the individual anodes can be clearly seen thus providing information on how the anode layout can be improved as seen in the optimization study discussed in Case 2. Figure 5 Predicted Current Densities in the tank at the Midlife and End of life

7 It is also interesting to look at which parts of the structure the current is going to and how this changes with time as shown in Figure 5. The blue color indicates the highest current demand which in this case is shown on the damaged area on the frame. Figure 6 Predicted potential in the tank at the end of the design life Figure 6 shows the predicted potentials at the end of the design life which indicate that the potentials in the area of the tank near the top right corner shown in the figure will become more positive than -800 mv Ag/AgCl. Table 2 Predicted consumption and remaining life of the anodes at the end of the design life Case 1 location ID Mass Utilisation Factor Mass Now Mass Loss Rate Active? Remaining Life Predicted Life (Years) at Current Consumption Rate 25 Percentage Consumed Pecentage Consumed (Kg) (Kg) (Kg/Year) (Years) (%) Year 26 (EndLife) Cl yes X X X X 49% X X X X Fr.77_ yes X X 61% X X X X X X Fr.77_ yes X X X 55% X X X X X Fr.78_ yes X X X 52% X X X X X Fr.78_ yes X X 61% X X X X X X Shell_ yes X 77% X X X X X X X Shell_ yes % X X X X X X X X Table 2 shows the consumption rates and remaining life of the anodes. The data shows that anode 7 is nearly consumed and the average consumption of all the anodes was 63%. Case 2 Design Optimization There are a number of factors which could determine the optimum design. For example it could be the minimum anode mass to provide protection, the best distribution of anodes to provide uniform potential or it could be the minimization of the occurrence of premature coating degradation. Modern coatings are not immune to the effects of cathodic protection as too many anodes can cause premature coating degradation. The number, type and placement of anodes have to be carefully matched to the barrier properties of the coating, which will change with time. The simulation model provides a Virtual Tank where all of these aspects can be investigated. In this next study the objective was to determine the location of the anodes which not only provided a good distribution of the potentials in the tank but also equalized the consumption rates of the anodes so they achieved a similar life. There are a number of strategies which could be applied to optimize the anode layout but in this case a simple approach was used where the anodes with the lowest consumption rate were moved to locations near the anodes with the highest consumption rate. This is a reasonable quick procedure to perform as in the modeling software tools are provided to move anodes or delete and insert anodes on the structure. This process was repeated a few times until an acceptable design was achieved which though not optimum was reasonably close for practical purposes.

8 The revised anode layout is shown in Figure 7 where it can be seen that the variation in the consumption rate has been significantly reduced compared with the initial design. The impact of moving some of the anodes to better locations can be more clearly seen in Table 3 where the predicted anode consumption and remaining life at the end of the design life is shown. In the initial design the anode consumption varied by 57% whereas in the optimized design the variation was 25%. Similarly the remaining life of the anodes for the optimized design was between 5-6 years whereas for the initial design it was between 2-11 years. Figure 7 Optimized anode layout showing initial consumption rates Table 3 Predicted consumption and remaining life of the anodes at the end of the design life for the optimized design location ID Mass Utilisation Factor Mass Now Mass Loss Rate Active? Remaining Life Predicted Life (Years) at Current Consumption Rate 25 Percentage Consumed Pecentage Consumed (Kg) (Kg) (Kg/Year) (Years) (%) Cl yes X 70% X X X X X X X Fr.77_ yes X 74% X X X X X X X Fr.77_ yes X X X 58% X X X X X Fr.78_ yes X X 63% X X X X X X Fr.78_ yes X X 63% X X X X X X Shell_ yes X X 68% X X X X X X Shell_ yes X X 63% X X X X X X The optimization of the design has not only improved the consumption rates of the anodes but also the distribution of the potential in the tank. [Figure 8, Figure 9] Comparing the End of Life Potentials shown in Figure 9 with that shown in Figure 6 it can be seen that the overall potential in the tank is improved as it is more negative by approximately 50 mv in the less protected areas of the tank. The areas of very negative potentials are also significantly less which reduces the risk of coating degradation.

9 Figure 8 Predicted potential in the tank at the end of the design life for the optimized design Figure 9 Predicted potential in the tank at the end of the design life for the optimized design The results indicate that the number of anodes or the size of the anodes could be reduced to minimize the system and installation cost. Case 3 ICCP Ballast tanks are normally protected by a combination of coatings and sacrificial anodes but it is difficult in some cases to install the large quantities of the zinc anodes required. Another disadvantage is their potentially short service life. Therefore the use of a ballast tank impressed current cathodic protection system could be beneficial but their design is complicated by the internal structure of the tank as it can be quite complex. In naval vessels ICCP systems have been proposed as part of an integrated ICCP system for the vessel as the design is complicated by other issues such as the electromagnetic signature but in merchant shipping they have not been generally applied. However new ship designs with large complex ballast tanks may provide an opportunity for ICCP systems. In this case computer modeling is used to investigate the performance of an ICCP system design for the ballast tank. A single circular ICCP anode has been located centrally in the tank and it is assumed that it is connected to a reference electrode to control the anode current to achieve protection inside the tank for the various operational conditions in the tank over the life of the vessel. The predicted performance of the ICCP system at the end of life conditions is shown in Figure 10 where the anode current has been adjusted to provide potentials more negative than -800 mv vs. Ag Ag/Cl everywhere in the tank. As can be seen the ICCP system is capable of providing this protection with a current output of 4 Amps.

10 Figure 10 Predicted potentials in the tank at the end of life conditions with a single ICCP anode supplying 4 Amps In case 2 the optimized sacrificial anode system delivered approximately 5 Amps at the end of life conditions. Figure 11 shows the performance of the ICCP system with the anode also delivering 5 Amps. The potential distribution with the ICCP anode was not as uniform as that obtained in Case 2 with the sacrificial anodes however the most negative potentials are localized by the ICCP anode and can be mitigated by shielding. Figure 11 Predicted potentials in the tank at the end of life conditions with a single ICCP anode supplying 5 Amps CONCLUSIONS Computer modeling has been applied to understand the CP requirements of a ballast tank over the life of the tank, to verify the protection provided and to assess the robustness of the design to changes which may occur over its life. The performance of the CP system has been predicted for various scenarios including degradation of the coating over the design life and the robustness of the CP system to protect localized damage in the tank. The life of the anodes and the protection provided over the design life has also been predicted. Predicted anode consumption rates were used to optimize the location of the anodes in the tank CP system to ensure the anodes had similar consumption rates and to maximize the protection provided to the tank. The protection of the tank using an ICCP system was investigated as an alternative to a sacrificial system. The use of modeling has the potential of overcoming some of the design issues with ICCP systems as the model can be used to verify the protection provided by the

11 system, identify the optimum anode locations, RE locations and set points for the most complex tank geometry. In the Case studies presented the tank has been assumed to be full but the modeling study can be easily used to investigate all the potential operational conditions in the tank as well as the impact of piping and other structures passing through the tank. The main effort in any modeling study is the initial construction of the model geometry and associated material and electrolyte data therefore other cases can be easily considred. REFERENCES Lucas, K.E. et. al., Design of Seawater Ballast Tank Impressed Current Cathodic Protection (ICCP) Systems, Tri-Service Corrosion Conference, Paper No. 218, R. A. Adey, J. Baynham, Design and Optimization of Cathodic Protection Systems Using Computer Simulation, CORROSION/2000, paper no. 723 (Houston, Texas: NACE Int., 2000). 4. A Jain, C. Peratta, J. Baynham, and R. Adey, Optimization Of Retrofit Cathodic Protection (CP) Systems Using Computational Modeling By Evaluating Performance Of Remnant And Retrofit CP Systems, Taking Into Account Long-Term Polarization Effects. CORROSION/2011, (Houston, Texas. USA, NACE Int. 2011). 5. Jean Vittonato, John Baynham, General consideration about current distribution and potential attenuation based on storage tank bottom modeling study, CORROSION/2012. Salt Lake City 6. Thomas J. Curtin, Robert A. Adey, Steven Policastro, Integrating Cathodic Protection System Measurement Data and Corrosion Simulation Software to Predict Coating Damage Inside a Ballast Tank. DOD Corrosion Conference. NACE 2013