Finite Element Modeling for Cathodic Protection of Pipelines under Simulating Thermocline Environment in Deep Water Using a Dynamic Boundary Condition

Abstract

A finite element model (FEM) was established to calculate the cathodic protection (CP) potential distribution on pipelines, under the environment of the thermocline structure in deep water. Supplemental boundary conditions were applied on the interfaces of two different mediums in the thermocline model. To solve the problem of time-dependent polarization curves due to calcareous deposits formed on the steel surface during CP in seawater, a dynamic boundary condition based on Ohm's law was proposed, instead of polarization curves or the Butler-Volmer equation. A good agreement was obtained between the calculated and experimental potential distributions, which proved that the model was practical and feasible. The anode placement studies revealed that the anode could supply a more efficient protection when it was placed at the thermocline zone rather than the surface zone or the deep water zone.Cathodic protection (CP) is one of the most effective methods for decreasing the corrosion of offshore structures and pipelines in seawater. There are two methods to supply direct current for CP systems, impressed current CP systems and sacrificial anode CP systems. Impressed current is usually supplied by a rectifier, which converts alternating current from the power lines to direct current. In a sacrificial anode CP system, single or multiple anodes distribute the cathodic current to the protected structure. 1 Numerous national, international and commercial standards have been published for the CP design of offshore structures at present. [2][3][4] Oil and gas production is now rapidly moving into deep water, which introduces significant changes in the chemical and physical properties (pressure, temperature, dissolved oxygen, etc.) that can influence CP processes. Therefore, a number of laboratory and field studies have been performed to investigate the influence of deep water properties on CP processes. 5-12 Compared with pressure, temperature plays a much more significantly influential role in CP, and most chemical and physical properties of seawater like dissolved oxygen depend on it. 6,13-15 Moreover, temperature observably affects the formation of calcareous deposits, which are important to an efficient CP system. [16][17][18][19] The profile of temperature in deep water can be described in the terms of thermocline structure, which consists of three layers: the surface zone, the thermocline zone and the deep zone, respectively. 11,12 Here the surface zone represents the surface water of the ocean with the highest temperature. Separating the deep zone from the surface zone is the thermocline zone, where the temperature changes abruptly with the depth. The temperature shift in these zones can lead to different polarization behaviors of materials. When offshore structures like pipelines or oil and gas well casings are passing through the thermocline environment, different parts of offshore structure in different layers will be galvanically coupled which can influence the CP on the entire structure. Therefore, in order to ...

Optimizing the cathodic protection system of pipeline under simulating thermocline environment in deep water by finite element method

“…Firstly, the CP potential and current distribution with single anode was simulated and studied, which has been used to validate the numerical model in the previous work . The anode was, respectively, placed at the SZ, where the coordinate x was 0 cm, the TZ ( x = 76.1 cm) and the DZ ( x = 203 cm).…”

“…In order to simulate the CP system of pipelines through the thermocline environment in deep water, a physical model was established on the similarity theory and dimensional analysis which has has been previously described (Figure ) …”

“…The numerical model was based on the physical model discussed above, which has been previously described . The distribution of CP potential was solved by Laplace's equation where E is the CP potential in the domain Ω .…”

“…While the reports presented were mostly developed in the homogeneous medium environment, and not suitable for the heterogeneous circumstance of thermocline structure in deep water. Therefore, in order to study the CP process of the pipeline passing through the special environment in deep water, a physical model was built on the basis of the thermocline structure in deep water and scaled down previously . In this paper, specific attention has been focused upon the influence of anode number and positions on the CP procedure and its effectiveness.…”

“…Therefore, different number of sacrificial anodes were coupled to the pipeline, and positioned at various simulation regions. The cathodic protection current and potential distributions were studied by a numerical approach based on the finite element method (FEM), which has been validated by the laboratory experiments . The optimum positions of sacrificial anodes were determined to optimize the sacrificial anode CP system under the heterogeneous environment in deep water.…”