Scope

We were contracted to assist with the design of a retrofit Cathodic Protection (CP) system to extend the safe operating lives of four in-field pipelines (two of which were in piggy-back configuration) and a mid-water arch Gravity Base Structure (GBS) by 8 years, and a Gas Export Pipeline (GEP) by 13 years.

To reduce the cost of offshore retrofit work, the use of sleds comprised of stand-off sacrificial anodes proposed, instead of the bracelet and flush type anodes originally installed on the pipelines and structures.

RJDP’s scope included the following:

Determining the mass of anodic material (number of anodes) required to meet mean current demand for the remainder of the pipelines’ design lives, and the quantity and spacing of anode sleds required per pipeline to overcome voltage drop.
The number and mass of anodes required to protect the GBS.
Provide results with conclusions and recommendations, and compile the retrofit anodes’ specification in a report for the end-customer.

What are sacrificial anodes?

Sacrificial anodes are made from highly reactive metals like zinc, magnesium, or aluminium and are used to protect less reactive metals, such as steel, from corrosion in subsea environments. Attached to structures like pipelines and offshore platforms, these anodes corrode preferentially, effectively sacrificing themselves to extend the lifespan of the assets they’re attached to. They are especially crucial in the subsea oil and gas industry for maintaining structural integrity, minimising safety risks, and reducing maintenance costs. Unlike other corrosion prevention methods, sacrificial anodes work passively and are easy to install, making them a cost-effective and energy-efficient solution.

Example of a sacrificial anode skid (source)

Our approach

Data review and initiation

We reviewed all customer provided documentation to comprehend the subsea pipelines, structures, and existing CP systems.
Key input data and documents were stored and indexed in a custom database for review, approval, and easy access.
We prepared quality control forms for calculation review and sign-off.

Initial and cumulative coating breakdown factor

Our team understood that initial and cumulative coating breakdown factors have a significant effect on the required anode mass and size in a CP system and the voltage drop across a pipeline, and that these values needed to be reasonably determined.

The initial coating breakdown factors of the four in-field pipelines were sourced from the end-customer’s general specification. Annual cumulative factors were estimated from these initial figures, based on the observed rate of anode depletion and CP drop between the date of installation and the year in which the CP system was considered to be unsatisfactory.
Coating breakdown factors for the GEP were not available from the supplied data so were estimated using DNVGL methods.

Pipeline anodes

The retrofit CP calculations were completed in accordance with DNVGL-RP-F103 and DNVGL-RP-B401.
The use of retrofitted 129 kg slender stand-off anodes was assumed (the dimensions of which were sourced from a reputable supplier).
Pipelines were divided into sections based on their coating so the mean and final coating breakdown factors and current demands of each pipeline section could be calculated.
The number of anodes required to meet each pipeline’s mean and final current demands over its remaining design life was calculated; the maximum of these two values defined the total anodic mass required.
The number and spacing of anode sleds required to protect each pipeline was calculated based on the voltage drop over their lengths (accounting for risers, spool-pieces, pipeline, and subsea structure metalwork as appropriate).

GBS anodes

The CP requirement for each GBS was determined empirically based on each structure’s 20 year CP design life and the observation that their original 150 kg of anodic material had nearly depleted after 12 years of operation; an approximate mean rate of depletion of 12.5 kg per year.
A nominal anodic material mass was calculated by multiplying the mean rate of depletion by the remaining 8 years of design life. To account for the increased current demand attributed to continued coating breakdown and the current drain from adjacent uncoated mooring chains, a safety factor was applied to this nominal mass, providing a reasonable degree of conservatism.

Conclusions

The two individual in-field pipeline required at least one anode sled comprised of two 129 kg anodes. 124 kg of anodic material being required to meet the mean current demand during the final 8 years of the pipelines’ design lives, while two anodes were required to exceed the final current demand.
The piggyback lines required three sleds spaced 992 m apart with a total of eight distributed anodes. 769 kg of anodic material being required to meet the mean current demand during the final 8 years of the line’s design life, while eight anodes were required to exceed the final current demand
The GEP required two sleds spaced 867 m apart, each with two 129 kg anodes. 402 kg of anodic material being required to meet the mean current demand during the final 13 years of its design life, while three anodes were required to exceed the final current demand.
100 kg of anodic material was needed for CP of the GBS up to the end of its design life. It was recommended that two 129 kg anodes were used per GBS to account for the increased current drain caused by continued coating breakdown and the uncoated mooring chains.

Summary

Our team was contracted to design a cost-effective sacrificial anode retrofit solution to extend the lifespan of four in-field pipelines, a GEP, and a mid-water arch GBS. Using comprehensive data analysis and industry standards, we successfully determined the required mass and arrangement of anodic material, ensuring corrosion protection safely met design life demands.

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Design of a retrofit cathodic protection system for subsea pipelines and structures