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BEASY attended the NACE Corrosion Conference in Vancouver this March where we were active at the exhibition as well as presenting and participating in the technical sessions and exchange groups.

It was great to meet many friends at the conference and discuss how modelling can be used to improve and optimize CP systems and mitigate interference.

At this year's conference there were a number of papers presented by BEASY staff and customers and BEASY CEO Tim Froome also gave a presentation on modelling of well casings at the Technical Exchange Group meeting. Some of the papers presented are described below.


Case Study of an ICCP Design and Installation for Well Casings Using Boundary Elements Software, Evaluating Different Deep Anode Architectures and Interference to Nearby Casings

Norberto Aldo Pesce & Norberto Antonio Pesce, Omnitronic S.A. Héctor C. Albaya Sistemas de Protección Catódica S.A. Guy Bishop & Andrés Peratta CM BEASY Ltd.

Impressed current cathodic protection (ICCP) systems can be used very effectively to achieve appropriate mitigation of well casing corrosion, but it may be difficult and expensive to define the most convenient anode distribution in order to adequately protect the desired casings and not interfere with nearby

Figure 1 Predicted current density on the well casing structures.

Once the system is installed, the cost of redesign may be high if experimental measurements indicate that the required protection is not achieved or that destructive interference is taking place. It is therefore both desirable and cost-effective to establish performance of the ICCP system before it is installed. Such assessment can be achieved through the use of mathematical modelling performed using numerical techniques. Of the available methodologies it is the boundary element method that is applied in this work.

To perform such simulation a model of the well casings is constructed (reflecting the directionally drilled profile) and the multiple anode ground-beds, including the return-path resistances along the cables and well casings. In order to effectively capture the electrical behavior of the currents, the resistivity of the ground through which current passes must be identified. Running alternative scenarios of the models to readily quantify the effects of alternative assumptions, provides valuable understanding to the CP system designer of probable behavior, and contributes to design robustness.

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How Interference Can Impact The Life Of CP Systems. An FPSO Case Study

Robert Adey, Cristina Peratta, John Baynham, CM BEASY Ltd

The design rules used to design CP systems in the main do not take into account the interference between the anodes provided to protect a structure or interactions between the structures themselves. CP systems will always interact with each other to some extent when they are in the same electrolyte even when there is no metallic electrical connection and this can radically affect the protection provided to the structure and the life of the CP system.

A case study is presented involving the design of the CP system of an FPSO (Floating production storage and offloading vessel). The aim of the study was to verify the performance of the CP system to ensure that the structure was protected for the design life and the anodes had sufficient capacity. Computer modelling was used to simulate the performance of the CP system which comprised of an ICCP system and sacrificial anodes. The study identified some interesting and unexpected interactions which required the design of the CP system to be modified.

Figure 2 Comparison of the protection potentials on the Moon Pool-Turret-Chain Connectors-Chains
for two different connection resistances at End Of Life


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Interference Between Sacrificial CP Systems In The Marine Environment

John Baynham, Tim Froome CM BEASY Ltd

It is generally accepted that destructive interference can occur between impressed and sacrificial cathodic protection systems, which can result in difficulty with control of the ICCP system, and which can also cause problems with performance of the sacrificial CP system

Less well known is the possibility of interference between multiple sacrificial CP systems. Such interference can occur under a number of different circumstances, and may for example arise when different design approaches are used for sub-structures which are then to be integrated together. Each CP system may be competently designed for stand-alone operation, but when combined together, protection potentials and life of the systems may be affected by interference. Consequences may include early consumption of anodes and more positive potentials towards end of life.

Figure 3 Projected anode life (in years) calculated in year 0 for the 330 anodes


Such effects can readily occur when different types of anode material are used, but may also occur when anodes in both CP systems are made from the same anode material.

This paper shows how mathematical modelling has been used to simulate performance of both stand-alone and integrated sacrificial systems. It goes on to identify interference effects between such systems, and investigates a number of different situations in which destructive interference can occur. Conclusions are reached of practices that can be adopted to avoid such destructive interference.

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Using Predicted Corrosion Damage To Determine Stress Concentration, Fracture And Crack Growth

Sharon Mellings, Andres Peratta, John Baynham, Tim Froome CM BEASY Ltd

Structure surfaces damaged by corrosion may develop stress concentrations which lead to initiation of cracks and possible crack growth.

Simulation of the galvanic effects leading to corrosion takes account of the properties of the electrolyte as well as the structural materials, to determine electric fields within the electrolyte, attenuation in the return path, and the surface current densities and potentials. If dissimilar materials are present or a CP system is not adequately designed, areas may exist where anodic current occurs on a structural surface, causing mass loss from the surface. The magnitude of the anodic current density, determined from simulation, can be used to determine surface shape change.

Such shape change generally results in indentations, which act as stress-raisers. Simulation to determine magnitude of the stress concentration can identify likely sites for crack initiation. The possibility of crack growth, and the time taken for the growth, can be determined using fracture and crack growth simulation.

This paper explores the combined use of galvanic simulation and fracture/crack growth simulation.

Figure 4 Predicted geometry changes in the specimen due to corrosion damage

 

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