Peripheral Protection Using a Horizontal Anode Trench Outside a Tank Dike

Peripheral Protection Using a Horizontal Anode Trench Outside a Tank Dike

Scenario: For a 50,000 m³ crude oil storage tank, the external cathodic protection of the tank bottom is obstructed by a concrete ring wall. Conventional anodes cannot be placed close to the tank bottom edge.

An MMO tubular anode (length 1.2 metres) is buried horizontally in a coke breeze trench 1.5 metres deep outside the ring wall. A 1×16 mm² cable is attached to the anode tail and exposed for 50 metres. This cable is laid along the trench bottom inside a φ50 mm PVC conduit, routed to a rectifier room 50 metres away from the tank farm.

The 50‑metre cable length is not arbitrarily chosen; it is derived from the boundaries of the explosion‑proof zone and the rectifier location. The rectifier positive terminal connects to this cable, and the negative terminal to the tank bottom grounding lug. The 16 mm² cross‑section ensures that at a maximum output current of 15 A, the temperature rise does not exceed 15°C above ambient. This cross‑section also meets the withstand capacity for a possible short‑circuit fault in the tank farm (prospective short‑circuit current approximately 1.2 kA, thermal withstand verified).

Installation precautions: No intermediate splices are allowed on the exposed 50‑metre cable. Where it crosses concrete pavements, a steel sleeve is added for protection. At the rectifier outlet, 5% extra length is reserved as slack to prevent core wire breakage due to soil settlement. Field operation data show that the protective potential at the tank bottom edge reaches –0.95 V (CSE), and the centre area is –0.88 V, with a potential difference of less than 100 mV, indicating uniform protection.

Notes:
CSE = copper/copper sulfate reference electrode.
NACE SP0169 = a standard of the National Association of Corrosion Engineers.
IP68 = ingress protection rating.
HDPE = high‑density polyethylene.
PVC = polyvinyl chloride.
All data ranges (e.g., 0.18 V/100m voltage drop, ±5% current density variation) are typical calculated values from engineering practice