Proof of Longevity of a Self-Cleaning ElectroChlorination Process System for Seawater Treatment
Authors: Haruna Ojonimi Samuel , Balk Ivar 
Published: April 2023

Subsea seawater treatment modules that use electrochlorination as pre-treatment were deployed with more than 20,000 hours of subsea operational run-time. Operational data and lessons learned indicate that the pre-treatment stage is key to the longevity of the overall subsea seawater treatment plant.

A self-cleaning, 4 years maintenance-free electrochlorination system has been refined and tested for six months. This paper will present the test set up, test results, findings, and lessons learned from current testing and previous operations, specifically around longevity.

An operational mode that allows for long-term subsea operation, without the need for system retrieval for cleaning with acid or alkaline is described in detail. Full-scale testing performed for 6 months established the various process parameters (current density, flow rate, polarity intervals and duration, electrode material, etc.) for minimizing and controlling the scale formation process to minimize subsea maintenance and proving the longevity. An automatic self-cleaning process and sequence have been established using the exact cell and power supply intended for real field application.

Electrochlorination is the process of producing hypochlorite by passing electric current through seawater. When producing sodium hypochlorite (disinfective agent) in situ by electrolysis, precipitation of insoluble salts like calcium carbonate and magnesium hydroxide will happen on the cathode. In the industry, this precipitate is typically called scale. The scale should be removed frequently from the process system, since this may otherwise reduce the confidence in cell longevity (if scale formation is not controlled). Therefore, electrochlorination is impacted by scale.

The electrochlorination test setup is identical to what is intended to be used in actual field application. Condition monitoring was established through video recording, camera pictures, continuous voltage, electrical current, and process flow parameter trends. The continuous data collection for the 6-month operation was analyzed on a weekly basis.

The test data was compared to operational data collected during two subsea deployments and showed that there is a significant improvement in electrochlorination system performance to achieve the minimum four-year, maintenance-free intervention period set for the subsea pre-treatment system.

This testing established operating parameters such as current densities, polar reverse sequence, etc. Also, intervals and duration for operating the cells to guarantee long-term maintenance-free subsea operation are part of the findings of this testing. The test data will feed into the operation, maintenance, and sparing philosophy for subsea seawater treatment systems.

Typical electrochlorinator (EC) systems are known to require frequent maintenance, requiring manpower and chemicals which are practically incompatible with subsea systems. The novelty of this paper is that a methodology for a maintenance-free, self-cleaning electrochlorination system has been successfully tested and proven for 6 months, enabling a minimum of 4 years maintenance-free subsea seawater treatment system operation. This would lead to significant capex and opex reductions for field applications.