Sealing a Corroded Seawater Vessel Without Hot Work or Drainage
A unique combination of materials and methods allowed an ASME-compliant repair to be executed on a leaking seawater vessel, saving on downtime by not having to drain the vessel.
In March 2015, an offshore oil & gas platform required a unique solution for the repair of a seawater bulk media filter vessel. Located in the Gulf of Mexico, USA, the bottom of the vessel was suffering from external damage due to heavily corrosive exterior and interior environments. The damage involved a combination of both thin and through-wall defects, posing a repair dilemma without emptying the entire vessel’s contents.
Background of the Process Vessel's Usage
In terms of functionality, seawater bulk media filter vessels simply remove the sediment and grit from the seawater pumped up to the platform. As the seawater enters at the top of the vessel, it passes through the bulk media, leaving the unwanted remains trapped in the beads of the media. Subsequently, this lean liquid is collected and drained for use in both on-board and off-board procedures.
Suspended particles in the seawater can cause continuous abrasion to the vessel lining, which—in combination with the highly corrosive saltwater—creates an extremely destructive environment. In this instance, it had caused the vessel to leak due to a failure with the underdrain system. (Learn more in Introduction to Managing Internal Corrosion in Process Vessels.)
Consequently, the asset owner decided to use a bolt to arrest the leak, until a permanent compliant solution was implemented.
An epoxy compound was used to plug the leak, and an epoxy composite to repair thin and through-wall defects.
Following investigation of alternatives, evidence suggested that competitor products were unable to provide a sufficient composite patch; therefore the customer selected a new generation-compliant composite repair system in order to complete the repair.
Specification and Repair of the Bulk Media Filter Process Vessel
After careful selection supported by an engineered design, the repair application was carried out in compliance with American Society of Mechanical Engineers (ASME) standards.
At the outset, the bolt plugging the hole was removed and the filtered water was drained, allowing complete access to the corroded area. The through-wall defect was plugged first using a malleable epoxy compound that was shaped to fit the prescribed affected area. Highly adhesive and water-resistant, the compound provided an excellent temporary solution to stop the process vessel’s leak.
The original vessel coating was then removed by hand sanding before the carbon steel substrate’s thin and through-wall failings were rebuilt using an epoxy composite. Designed to adhere to wet, oil-contaminated and underwater substrates, this surface-tolerant material was the most suitable repair solution for the aqueous environment.
Cleaned, blasted surface following initial repair.
Following the thin and through-wall repair, the substrate was then cleaned to the requirements of SSPC SP 10 (Near-White Metal) by abrasive blasting. (Discover more techniques in the article Substrate Surface Preparation for Corrosion Prevention.)
In reference to the design, three wraps (six layers) of the composite repair system were applied to the repair area to reinforce the bottom of the filter vessel. This composite wrap was chosen because of its application versatility and proven efficiency in repairing weakened and holed tank walls.
Using epoxy resin to create a mold of the vessel’s surface, the repair was consolidated by applying pressure against the template, creating a secure and tight seal around the defect.
The composite repair system applied per engineered design.
The customer chose the composite repair system because of the long design life and overall ease of the repair process versus traditional welding methods, which can incur a number of health and safety issues.
By using a composite patch, they were able to avoid emptying the entire vessel, including filter media, hence saving a considerable amount of downtime and labor.