MODELING OF IMPRESSED CURRENT CATHODIC PROTECTION ANODE ARRANGEMENTS FOR STORAGE TANK BOTTOMS

Transcription

1 MODELING OF IMPRESSED CURRENT CATHODIC PROTECTION ANODE ARRANGEMENTS FOR STORAGE TANK BOTTOMS Robert A. Adey, John M W Baynham and Cristina Peratta Computational Mechanics Inc 25 Bridge Street, Billerica, MA CM BEASY Ltd Ashurst Lodge, Southampton, Hampshire, SO40 7AA, UK 1

2 Overview Introduction Protecting tank bases Modeling Technology Tank base case study Simulation based design Impact of sand resistivity Revised design Conclusions

3 Introduction Above ground tanks are frequently used for the storage of Oil & Gas products and they can present a challenge to design an optimum corrosion control system

4 CP Design Options Deep Anodes Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

5 CP Design Options Shallow Anodes Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

6 CP Design Options Anode Grid Frequently a membrane is installed below the tank to prevent soil contamination in the case of leakage which has the consequence that the tank base is electrically insulated from the surrounding soil

7 CP Design Options Anode Grid Credit. John Fitzgerald. Corrpro, Freshwater Spills Symposium

8 Anode Grids One method of protecting the bottom of a tank is by use of an impressed current cathodic protection (ICCP) system There are a number of types of Cathodic Protection systems designed to protect the tank base in these circumstances They can range from a: Simple rectangular grid of anodes Single spiral anode Number of anode ribbons arranged in a circular grid connected to distribution bars

9 Design Considerations The details of the design of such a system are very important for optimal performance The consequences of: Can be: Too large a spacing between the anodes and/or between the current distribution bars An insufficient number of power feed connections Poorly chosen connection points for the cables Uneven distribution of protection potential on the tank base Or in the worst cases regions where corrosion of the tank base can take place An over designed system on the other hand can have significant economic consequences both in terms of installation cost and running costs

10 Design Optimization The optimal design of a CP system for a particular structure and environmental condition is not trivial, and may not necessarily be achieved by incremental changes of a prior design. The use of computer simulation in conjunction with suitable data obtained from the study of previous tank systems, allows the consideration of many design options and the effects of different soil or electrolyte conditions Such simulation allows selection in a systematic and predictable way of the most suitable design which provides good protection of the tank whilst minimizing cost

11 Objectives The main objectives of this work are to demonstrate that Simulation during the design stage of a tankbase ICCP system can be of considerable benefit to the designer Simulation results can assist in initial set-up of a system Simulation can be used to investigate the fault-tolerance of a design, its ability to perform adequately despite occurrence of faults, and the effects of any planned remedial actions

12 Computer Modeling Computer modeling is now widely used for a wide variety of applications including Ships and boats Offshore Oil & Gas structures Pipelines Concrete structures Storage tanks Wells Aircraft and Vehicles

13 Computer Modeling In general the input data for a model of a CP system consists of the following: Physical and geometrical properties of the electrolyte Anode geometry (sizes and locations) and surface coating Reference electrode set points and locations Condition of any coatings/paints on the tank base Polarization properties of the materials involved as active electrodes

14 Modeling Features: Different Materials, Coatings and Breakdown Factors Simply choose different materials, polarisation curves and coatings for each component Enter the electrical resistance of the material if it is not zero. Enter the current density or total current required for ICCP anodes

15 Modeling Features: Electrical Connections, Return Paths and Attenuation IR voltage drop in the supply and return path This is very important when there are nonzero resistances in the connections between structures, for example the resistances of feeder cable lengths, or if there are significant resistances in ribbon anodes (for example). Attenuation In the supply cable from the TRU to the cable connection point In the feeder cables from the cable connection point to the individual anode rings. In the anode rings

16 Case Study The study investigated the design of the CP system for a 42m diameter tank sitting on a 0.23m thick layer of sand with a membrane separating the sand from the surrounding soil

17 Model Description There are MMO coated titanium ribbon anodes arranged in concentric rings symmetrically distributed with fixed separation between them. The anodes are located 0.06m above the membrane. The ribbon anodes have cross-section 6.35mm by 0.635mm, and linear resistance 0.15 Ohm/m. Each anode is modeled as a cylinder with diameter x10-3 meters, giving a cross sectional area equal to the ribbon anode of dimensions 6.35mmx0.635mm.. Distance from the tank edge to the first ring is 0.23m

18 Circuit Model Ribbon Anode 1 R1 R0 R0 R1 N ribbon anodes Ribbon Anode N RN R0 RN Tank Base A key element in predicting the performance of the CP system is the electrical connections between the power supply (TRU) the distribution cables and the anode ribbon as shown Ro is calculated using the resistivity of copper and cable dimensions. The ribbon anode 1 is the outermost one. Ribbon anode 2 is the following ring anode and subsequently RN is the smallest and last ring. Ri, for a ring anode i is calculated using the resistivity of the ribbon anodes and the distance to the connector.

19 Feeder Cables Feeder cables were attached to both ends of each half ring of the anode ribbons

20 Polarization Data For The Tank Base Potential [mv] Current density [ma/m**2]

21 Initial Simulation In the initial simulation for the tank standing on sand with a resistivity of 50,000 Ohm-cm the ring spacing was set at 2m which gave 11 rings under the tank bottom A series of simulations were performed in which the TRU voltage was increased from 10V to 50V and the results evaluated to determine if the potential on the tank base was within the target range (-850mV and -950mV on the tank base)

22 Potential On The Tank Base This result shown that 2m of ring separation is not adequate when the soil resistivity is 50,000 Ohm-cm The blue indicates the potentials on the base closest to the anode and the red the more positive potentials in the gaps between the anodes

23 Potential On The Tank Base This view of the same result shows the potential values plotted with variation by colour and height. The TRU was increased from 10 to 50V and although the most negative value changed significantly, the most positive values did not change by more than 20%.

24 Revised Design As it was impossible to achieve the desired potentials with the initial design with anode spacing of 2m a new design was proposed with a spacing of 0.5m Therefore there were now 41 anode rings under the tank and the design was simulated as before to determine the TRU voltage required to achieve the required potentials on the tank base

25 Revised Design Results are shown for the case of TRU Voltage of 20v which can be seen now achieves the required potential (< -850) on the tank base

26 Design Robustness The design cases considered in the initial study and revised design assumed the sand resistivity was 50,000 Ohm-cm In order to test the sensitivity/range of application of the revised design a new case was considered where the sand resistivity was reduced to 5,000 Ohm-cm.

27 Sensitivity Study: Resistivity In this case the required protection was achieved with a TRU setting of 5V Predicted potential on the tank base with TRU 5V for the sand resistivity of 5,000 Ohm-cm

28 Design Robustness The model also provides insights into how the internal workings of the system which can used to optimize the design Such data includes the IR drop in the: Supply cables Feeder cables Junctions Ribbon anodes This data can also be used to simulate the impact of a failure of part of the system on the overall system performance

29 Ribbon Anode Voltages The voltages in the ribbon anodes can be predicted by the model to see the impact of the IR drop The simulation clearly shows how the voltages reduce with distance from the feeder cable connection points

30 Understanding The Behavior Of The Design Area of least protection where losses through the anodes are greatest Area where losses in supply cables are least CONNECTION POINTS CONNECTION POINTS Area where losses in supply cables is greater but compensated by the losses through the anodes being less

31 Reference Electrodes In this case the reference electrodes were not included in the model However, it is possible to define them at user selected positions and the modeling process used to identify potentials at those chosen reference electrode locations. These calculated potentials at reference electrode locations, can then be used by the CP designer in order to establish the system set points in order to achieve the desired target potential on the tank base.

32 Coating Breakdown Factors In this study the tank base was modeled as bare steel However by applying a breakdown factor to the tank base coating, the model can be used to assess the performance/robustness of the CP system at the start and end of life The impact of temperature can also be considered by modifying the polarization data

33 Summary A computational model has been introduced which combines a numerical model of the physics of a galvanic corrosion system with an electrical circuit model The model is capable of simulating the interaction between the electrode kinetics on the metallic surfaces in contact with the electrolyte, the IR drop through the electrolyte and the current flow through the TRU and feeder cables. The model has been applied to predict the protection provided to a tank base by a cathodic protection system. The use of the model to optimize the design and test its robustness under a range of conditions has been demonstrated