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Cathodic protection is required by law in many countries for applications including gas pipelines, well head casings, tanks, vessels and marine structures, such as jetties. Furthermore, remote and off-grid sites face the challenge of providing reliable and low-cost power. This article describes the relative merits of impressed current cathodic protection (ICCP) when powered by mains, diesel generators and solar or wind powered systems with batteries.
ICCP is one of the techniques used to control corrosion of steel structures, and is used widely in the oil and gas, marine and ports industries, and offshore wind farms, where it protects assets such as underground or buried pipelines from natural deterioration. As a result, it protects safety and process continuity, as well as the environment, as it reduces the risk of leaks from oil and gas pipelines and infrastructure.
Overall, it provides confidence and efficiency for operators who can apply ICCP to provide a constant trace current to slow down the rate of corrosion. This is particularly important for operators of remote and off-grid sites, where it can be challenging to schedule a visit by a qualified technician for inspection and maintenance of vital assets.
How impressed current cathodic protection works
Four components are needed for corrosion to take place through a natural galvanic reaction: a cathode, anode, electrolyte and an electrical pathway. ICCP works on the principle of overcoming the galvanic current with an opposing current.
In a typical ICCP system, a transformer/rectifier draws power from the mains and converts it from AC to DC. It then provides a constant trickle of direct current via anodes in the ground, with current flowing towards the structure to be protected. As a result, this system can prevent the natural oxidation of steel structures.
Depending on the level of current applied, ICCP will slow the rate of corrosion. Some systems can even extend asset life indefinitely as they reduce the rate of corrosion to almost zero. A single system can protect a length of approximately 50km of pipeline in a desert (where soil resistivity and moisture levels are low) but this can drop to 100 metres for structures immersed in sea water.
Installations that require protection include pipelines (it is, in fact, a legal requirement in many countries for gas lines in particular). It is also applicable for tanks, vessels, well casings, jetties and any other submerged metallic structure.
For many installations, a system can be down for several months without posing a major risk, but the more constant and reliable the supply of current, the better. Some of the different methods of powering ICCP include:
Mains power: Around 90 percent of ICCP systems are powered by the grid. It provides high reliability, known and constant current and voltage, and low risk of outages. ICCP systems require relatively low power compared with other industrial loads such as motors for pumping systems. However, it is only possible to specify a mains-fed ICCP system at sites where grid infrastructure exists, or where a grid connection can be delivered for a modest investment that is equivalent, or lower, than the cost of other power solutions.
Diesel genset: This is the traditional option for remote sites that do not have access to the grid and uses a diesel generator to provide power, either intermittently or constantly. This solution is relatively inexpensive if a grid connection is not available. However, such sites have high operational costs due to the need for a technician to visit regularly to refuel, inspect and maintain the genset. In addition, when specialised maintenance is needed, the operator will need to call on the services of a qualified technician to supply and fit spare parts, etc., and this can be a logistical challenge in some locations.
Renewable energy: This option uses solar photovoltaic panels (PV) or wind turbines to generate power to support the ICCP. Because renewable energy does not consume fuel, it has the advantage of having low operating costs that offset the relatively high installation cost. It is best suited to sites that are rich in renewable energy.
PV panels are well established in this application, having been used for 20-30 years in ICCP installations. They have the additional benefit that they generate DC power so there’s no need for a rectifier to convert AC to DC.
However, when the sun doesn’t shine or the wind doesn’t blow, power drops off and corrosion will restart, which has the potential to increase risk in the long term. As a result, many operators integrate a battery system to store renewable energy and release it when needed. For a solar-powered system, the battery will charge during the day and release energy overnight and on overcast days – and it’s a similar principle for wind-powered systems, which charge the batteries on windy days. Typically the cost of adding a battery is significantly less than the value of the infrastructure that the ICCP system is protecting so it is well worth the investment.
However, it’s important to select the battery carefully as not all batteries are tough enough to provide reliable service in a remote off-grid site, where temperatures can vary widely and impact performance and lifetime.
All of these methods can be compared with galvanic protection, which uses the natural galvanic potential of different metals to protect a structure with a sacrificial anode. It’s great for small structures like the hull of a ship or another accessible structure where the anode can be changed when it is depleted. However, this option is not so good for extensive buried infrastructure like pipelines or where an operator needs a constant and controllable current output.
The Total Life Cycle Cost of an ICCP system
When choosing the right solution for any particular site, it’s important to evaluate the total lifetime cost of the different options to identify the least costly.
This should take into account the initial purchase price of the installation, operational and maintenance costs, as well as the salvage cost that can be obtained at the end of an installation’s lifetime when components are sold for scrap.
Initial cost includes site surveys, engineering design and specification, delivery to site, installation and commissioning. Requirements are highly specific to the conditions for each installation. A typical ICCP system for a pipeline might draw 1 Amp at 5-10 Volts, whereas systems for well casings will draw 15-20 Amps at a similar voltage.
Specifications vary widely as the current drawn differs depending on the soil resistivity and moisture levels in the soil (or salinity of water for subsea installations), climate and the extent of the infrastructure to be protected.
During an installation’s lifetime, operational costs include the cost of fuel or power from the grid, as well as the cost of visits by certified technician to inspect, test and deliver maintenance services. These can be costly for remote sites, which require long travelling time and coordination, to ensure that technicians have the right tools and spare parts with them to avoid the need for repeat visits.
When mains power is available, installations based on transformer rectifier units are typically the least costly – but this is not always possible, particularly for operators of oil and gas pipelines that run through uninhabited regions. It is just not practical or cost-effective to run a power line across a desert or a remote mountainous region.
That leaves a comparison between systems based on diesel gensets and solar PV or wind power – and the high cost of fuel and logistics means that hybrid renewable power supplies are significantly more attractive as running costs mount up over a number of years.
An example of the use of a photovoltaic panel/battery system is given on the right.
Case study: Spie Oil & Gas Services
One operator that has adopted an ICCP system powered by solar PV and battery systems is the Hassi R’Mel gas field pipeline in Algeria. Located around 550 km south of Algiers in the Sahara Desert, the pipeline is 1,650-km long and stretches from the remote Hassi R’Mel gas field in Algeria to Qued Saf-Saf on the Tunisian border. The pipeline then feeds into Transmed’s supply link that flows from Tunisia to Italy to provide Europe with gas. The field currently represents a quarter of Algeria’s total gas output.
In 2018, Spie Oil & Gas Services installed an ICCP system that is powered by solar PV panels in conjunction with nickel technology battery systems, to ensure a constant unbroken 100 Watt supply to keep the cathodic protection systems operating.
The batteries were installed at 34 stations along the pipeline where they store energy from solar panels. During daylight hours, solar PV panels generate electricity to meet the demands of ICCP, run SCADA (Supervisory Control and Data Acquisition) systems and charge the batteries. When the sun sets and on overcast days, the batteries step in to maintain a continuous power supply. They are sized to provide up to five days of power to ensure the pipeline is protected even in rare extended periods of overcast weather.
There are some important considerations for engineers who specify batteries for remote sites, where a maintenance call-out can be costly and resource intensive.
Batteries at such sites need to be tough enough to withstand the extreme heat and cold of the desert day and night and the mechanical stresses of transport to the site. Operators typically want to choose batteries that have a proven track record and have demonstrated high reliability in similar operating environments.
When choosing battery technology and sizing batteries, it’s important to consider temperature, as it has a significant impact on battery performance and life expectancy. Nickel technology batteries are better able to withstand extreme high or low temperatures than leadacid technology. Although lead-acid batteries have a low purchase price, they have a limited lifetime, which is further shortened in hot climates. A lead-acid battery system designed to provide five days of autonomy will last 10-11 years at 25C, or 5-6 years at 35C. In comparison, nickel battery technology will last up to 20 years, so is less costly over the lifetime of an installation. This has a significant impact on Life Cycle Cost of an installation therefore when selecting a battery system, it’s important to use this as the deciding factor.
Typically for an ICCP installation, a battery will need to provide a minimum of 2 to 3 days of stand-alone power.
Issue 151 September/October 2019 - Corrosion ManagementRead magazine online