AZZ Specialty Welding’s Unifuse™ Automated Weld Build-Up Process in FPSO Repairs


FPSO (Floating Production Storage and Offloading Vessels)

An FPSO is a floating production system that receives fluids (crude oil, water and a host of other things) from a subsea reservoir through risers, which then separate fluids into crude oil, natural gas, water and impurities within the topsides production facilities onboard. Crude oil stored in the storage tanks of the FPSO is offloaded onto shuttle tankers to go to market or for further refining onshore.

Most FPSOs/FSOs are based on tanker ship hulls and can be secured to the sea bed via a variety of mooring systems, the choice of which is determined by the specific environment. They are suitable for a wide range of water depth, environmental conditions and can be designed with the capability of staying on location for continuous operations for 20 years or longer. [1]

The Customer Challenge

As part of their operation, FPSO’s have to be serviced by transport ships on a regular basis.  These transport ships travel to the FPSO’s location and moor to the vessel to allow for safe transfer of the stored hydrocarbons in the FPSO.

To protect both vessels, fenders are used to cushion possible contact between the transfer shuttle and the FPSO.  In challenging weather these fenders can abrade the protective coatings from the vessel shell which leads to salt water corrosion induced wall thinning.  This issue creates a difficult choice for the customer.  Either take the vessel out of production or make repairs at dry-dock, or find a way to repair the shells at sea.  Many FPSO owners choose to make small scale localized repairs when the base material becomes too thin, but when larger affected areas are found, there is currently no viable repair while in-operation at sea available other than exchanging the plates or installing temporary patches.

The Best Solution

For this customer, AZZ WSI evaluated the condition of the vessel and determined that by making modifications to one of their proprietary automated welding systems, they would be able to work within the inner hull of the FPSO while at sea to deposit layers of weld metal to restore the base metal thickness to design requirements.  Because of the significant deposition rates achievable with this technology, justification for this approach vs. the dry-dock option was clear.  The customer estimates that they experienced multi-million dollar savings by utilizing this innovative solution.

Although this AZZ WSI weld overlay technology is commonly used in refineries, petrochemical plants, pulp mills, and other industries where they use high pressure, high temperature pressure components, it is not very common in the shipping industry.  Pressure vessels are typically constructed and repaired in accordance with the ASME Boiler and Vessel Code.  Ocean going vessels generally utilize standards from the certifying agency, which impose compliance requirements on repairs to accommodate the ocean environment.

For this project, AZZ WSI had to manage a development program to test and verify welding procedures in accordance with the insurer’s requirements.  These rigorous requirements ensure that the repair being implemented will result in a safe and reliable vessel.  In addition to normal tests performed on procedure qualification samples, the testing protocol also included impact testing on coupons welded in simulated above and below the water line with varying water temperatures.  In addition, AZZ performed a local and global distortion analysis to ensure that the distribution of weld zones was done properly to manage residual stress and distortion.

To date, the majority of this scope has been completed with excellent safety, quality, and productivity results.  This has been a significant financial win for the customer.

AZZ Specialty Welding – Proven Results. Automatically. 

With over 2000 total workscopes executed globally with Unifuse™ for pressure components, AZZ Specialty is the world leader in pressure equipment life extension. Whether the component needs to be repaired emergently or during a planned outage, AZZ has unparalleled innovative leadership backed by proven results. AZZ Specialty Welding has the most extensive portfolio of successfully completed, field weld projects and qualified procedures to handle specific challenges. Find out more about our engineered services offering at or follow us on LinkedIn.


[1] Web Content MODEC.  About and FPSO