AC Corrosion Tests on Materials for Electrically Heated Flowlines
- Kristian Thinn Solheim (SINTEF Energy Research) | Martin Hoeyer-Hansen (SINTEF Energy Research) | Magnus Hurlen Larsen (Nexans Norway AS) | Øyvind Iversen (Nexans Norway AS)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- DEH, AC Corrosion, Subsea Structures, Experiment, Direct Electrical Heating, Steel, Pipeline Integrity
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- 26 since 2007
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This paper presents results from AC corrosion tests on cathodically protected steel flowlines. An important factor, which has an increased focus in the industry, is to understand AC corrosion and how it can be mitigated. Tests have been carried out at 100 and 200 Hz, where no current transfer design limits exist, as well as at 50 Hz. The results show that the corrosion rate is independent of transfer current density (0-400 A/m2) and power frequency (0-200 Hz) on steel. On the anodes, the corrosion rate increases as a function of transfer current density (0-80 A/m2) at 50 Hz. However, the correlation is less prominent at higher frequencies (100-200 Hz).
When a hydrocarbon production flowline is shut down, it gradually cools to a temperature level where hydrates or wax may form. If hydrates and wax are to form in a flowline, they may restrict the flow and in a worst-case situation block the entire pipe. Removal of hydrates is difficult and potentially hazardous. One of several field proven methods to prevent hydrates and wax formation is Direct Electrical Heating (DEH). Since the first installation in year 2000, approximately 30 flowlines have been installed with the DEH technology.
In a DEH-system, the flowline steel conducts electrical alternating current (AC) to generate heat in the flowline. For safety and reliability reasons, the heating system is electrically connected (”earthed”) to surrounding seawater through several sacrificial anodes. Typically, 40 % of the current is conducted by seawater, while the flowline steel conducts the remaining 60 %, (Nysveen et al., 2007). The electric current is transferred from the flowline steel to surrounding seawater via anodes connected to the flowline. This is indicated in Fig. 1.
When AC current transmits from metal to seawater, corrosion may occur. During the positive half-wave of the AC current, part of the current flow may be faradaic, i.e. contributing to the oxidation of solid metal to ions. In most situations, the current transfer across the metal-seawater interface is dominantly capacitive, and the faradaic component amount only to a minor part of the total current. The rate of AC corrosion depends on several parameters, including metal quality, cathodic protection level, power frequency, transfer current density (TCD), temperature, hydrostatic column pressure etc. The governing parameter in literature is usually considered to be the TCD. For DEH, this is the current density crossing the metal-electrolyte interface between flowline steel and seawater, as well as between anodes and seawater. Existing DEH systems are designed to use maximum limits for the TCDs under which AC corrosion has been considered acceptable. The limits are decided on a project basis by the operator. The most common acceptance levels for DEH have been 20 or 40 A/m2 for aluminum anodes and 100 or 240 A/m2 for carbon steel, (Lervik, 2004; Nysveen, 2007).
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