Using impressed current cathodic protection in remote and off-grid sites
Corrosion Management magazine
Cathodic protection in remote and off-grid sites
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.
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
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).
Some of the different methods of powering ICCP include:
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.
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.
Nickel battery system to store 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.
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. 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 Nickel 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.
Important considerations when specifying batteries
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.