Cathodic Protection Pipeline Design for Desert and Sabkha Soils

A buried pipeline crossing the Arabian Peninsula can pass through dry dune sand, gravel plains and coastal sabkha within a few tens of kilometres. Each of these soils puts a very different demand on the corrosion protection system. This article explains how coating and cathodic protection (CP) work together on desert pipelines, how the choice between sacrificial and impressed current systems is made, and what to check when buying CP materials.

Why desert sand and sabkha behave so differently

Dry dune sand typically has very high electrical resistivity, often well above 10,000 Ω·cm. It is not very corrosive in itself, but it makes CP current hard to deliver: anodes buried in dry sand have high resistance to earth, and galvanic anodes produce little useful current. Resistivity can also change sharply with depth and season, especially where rare rainfall or irrigation wets the surface.

Sabkha is the opposite. These salt flats, common along the Gulf coast, combine a shallow saline water table, a crust rich in chlorides and sulphates, and repeated wetting and drying. Resistivity can drop below 100 Ω·cm, and anaerobic layers can support sulphate-reducing bacteria (SRB). Bare steel corrodes quickly here, and coating defects at sabkha crossings are a common location for external corrosion. A single pipeline that crosses both environments needs a CP design that handles both very high and very low resistivity sections.

Coating is the primary barrier

CP is designed to protect steel at coating defects, not to replace the coating. The current demand of the whole system, and therefore the size of the rectifiers and groundbeds, depends directly on coating quality. For onshore line pipe, the usual factory systems are three-layer polyethylene (3LPE) or polypropylene (3LPP) to ISO 21809-1 and fusion-bonded epoxy (FBE) to ISO 21809-2, with field joints protected to ISO 21809-3 using heat-shrink sleeves or liquid epoxy. You can source the API 5L line pipe and field-joint coatings and repair materials as one package so the systems are compatible.

In the GCC, coating selection should account for high pipe and soil temperatures, UV exposure during long storage in open yards, and sand abrasion during handling and backfilling. Rock shield or padding with screened sand reduces damage in rocky ground.

Sacrificial anodes or impressed current?

Both approaches make the pipeline the cathode of an electrochemical cell. The difference is the source of the driving voltage.

In practice, long desert pipelines are almost always protected by impressed current, while galvanic anodes are used locally, for example at sabkha sections, casings, short spurs or during the construction phase before the permanent system is energised. Zinc suits low-resistivity sabkha; high-potential magnesium suits moderate resistivity. Zinc anodes lose efficiency at elevated temperatures, so they are not a good choice next to hot lines.

Deep-well vs shallow groundbeds

The groundbed is the set of anodes that discharges current into the soil in an ICCP system. Common anode materials are mixed metal oxide (MMO), high-silicon chromium cast iron and graphite, usually installed in calcined petroleum coke backfill.

A deep-well groundbed places the anodes in a vertical borehole, often tens of metres deep, to reach lower-resistivity strata or the water table beneath dry surface sand. It has a small footprint, spreads current more evenly and reduces interference with nearby structures. It needs a vent pipe to release gases generated at the anodes and careful construction; AMPP (formerly NACE) SP0572 covers design and installation. A shallow groundbed, with anodes laid horizontally or in short vertical holes, is cheaper and easier to inspect, but only works well where surface soil resistivity is low, such as in parts of the sabkha, and it creates a wider voltage gradient around the anodes.

Protection criteria and how to measure them

ISO 15589-1 is the main international standard for onshore pipeline CP, and many projects also reference NACE SP0169. The principal criteria are:

• −850 mV versus a copper/copper sulphate electrode (CSE), measured IR-free, for carbon steel in most soils.
• −950 mV CSE where anaerobic conditions and SRB activity are likely, which is relevant to sabkha.
• Less negative values, −750 mV and −650 mV CSE, permitted by ISO 15589-1 in high-resistivity aerobic soils, which can apply in dry sand.
• A minimum of 100 mV cathodic polarisation as an alternative, not applicable at high temperature, in SRB-active soils or where interference is present.
• A limiting potential to avoid overprotection, commonly −1,200 mV CSE IR-free for high-strength steels, to reduce the risk of hydrogen embrittlement and coating disbondment.

The −850 mV criterion refers to the polarised potential, not the "on" reading. Measurements should remove the IR drop using synchronised current interruption (instant-off readings) or coupons installed at the test post.

Interference, isolation and hot lines

AC and DC interference

Desert pipelines often share corridors with high-voltage overhead lines, which can induce AC voltages and cause AC corrosion at small coating defects; ISO 18086 gives assessment criteria and limits. DC interference comes from foreign CP systems and other DC sources and is addressed in ISO 15589-1 and EN 50162. Mitigation includes gradient control mats or zinc ribbon, solid-state DC decoupling devices for earthing connections, and bonds agreed with the owners of nearby structures.

Insulating joints and flanges

The protected pipeline must be electrically isolated from plant earthing, wellheads and unprotected structures, otherwise CP current drains away. Monolithic insulating joints are preferred on buried sections; above ground, flange insulation gasket kits with sleeves and washers are common. Install surge protection across insulating joints, using Ex-rated devices where the joint sits in a hazardous area, and test the isolation before and after commissioning.

High temperature and coating disbondment

Lines downstream of wellheads or compressors can run hot. Higher temperature increases CP current demand and accelerates cathodic disbondment of the coating. ISO 15589-1 requires a more negative criterion above 40 °C, and coatings must be qualified by cathodic disbondment testing at the design temperature. Disbonded high-dielectric coatings such as polyethylene tapes can shield the steel from CP current, so corrosion can proceed under the coating while potentials at the test post look acceptable.

Monitoring and commissioning checklist

1. Soil resistivity survey along the route, including depth profiles at proposed groundbed sites.
2. Test posts at regular intervals and at road and pipeline crossings, casings, insulating joints and AC corridors.
3. Permanent reference electrodes and coupons where IR-free readings are hard to obtain.
4. Remote monitoring units on rectifiers and key test posts, reporting output and potentials by GSM or satellite.
5. Commissioning survey with instant-off potentials, followed by close-interval potential survey (CIPS) and DCVG coating survey.
6. Periodic checks of rectifiers, isolation, bonds and corrosion monitoring probes, with records kept for trend analysis.

Sending an RFQ for cathodic protection materials

For a useful quotation, include the pipeline diameter, length and coating type, the soil resistivity data, design life, operating temperature, the selected criteria and any design report or bill of materials. List rectifier ratings and power source, anode type and quantity, backfill, cables, test posts, reference electrodes, insulating joints and remote monitoring units. If you need help reviewing the scope for cathodic protection systems, contact us with your RFQ and the project specification.