Overlay Alloy Technology for FPSO Seawater Compartment Components

Literature Overview

This study, published in 2005 in the journal "China Shipbuilding" by Xue Daliang from Dalian New Ship Heavy Industry Ship Design Research Institute, addresses a critical engineering challenge in floating production storage and offloading (FPSO) vessel construction. The seawater compartment of an FPSO is exposed to a highly corrosive marine environment, and the selection and application of overlay alloys on structural components must ensure long-term integrity under cyclic loading, chloride-induced corrosion, and potential sulfide stress cracking conditions. The paper examines the feasibility of applying corrosion-resistant overlay coatings on carbon steel and low-alloy steel components forming the seawater piping system and structural penetrations within the FPSO hull.

Core Technical Content and Process Parameters

The research focuses on the weld overlay of austenitic stainless steel and nickel-based alloy consumables onto the interior surfaces of seawater compartment components. The primary welding process employed is manual metal arc welding (SMAW) with stainless steel electrodes such as E308L and E316L, supplemented by gas tungsten arc welding (GTAW) for critical junction areas and thin-wall sections. The base material is typically AH36 or DH36 marine-grade steel conforming to ABS and DNV standards.

The overlay design must account for the thermal mismatch between the carbon steel substrate and the austenitic overlay layer. Dilution control is critical; excessive dilution reduces the chromium and molybdenum content in the weld metal, compromising the pitting resistance equivalent number (PREN) required for seawater service.

Defect Analysis and Countermeasures

Common defects encountered in FPSO seawater compartment overlay work include hot cracking in the overlay layer, lack of fusion at the interface, and undercut at the toe of the overlay weld. The primary root cause is the high sulfur and phosphorus content in the base material combined with rapid cooling rates. Countermeasures include strict preheating, reduced welding current, and the use of low-hydrogen consumables. Residual stress from overlay welding can also induce hydrogen-assisted cracking in the heat-affected zone of the base steel, particularly at temperatures below 200 °C during subsequent cooling.

Engineering Practice Integration

In FPSO fabrication, the seawater compartment overlay work is typically performed in a controlled shop environment prior to outfitting. The quality assurance plan should include interface bond strength testing per ASTM A263, intergranular corrosion testing per ASTM A264, and a minimum of one full-penetration RT examination on a coupon weld for every production batch. The overlay thickness must be verified by ultrasonic testing (UT) using a dual-probe technique to detect interface discontinuities. Field experience has demonstrated that a two-pass overlay with 316L provides adequate protection for seawater service, while three-pass overlay with Inconel 625 is recommended for areas subject to biofouling and potential hydrogen sulfide exposure.

Study Insights and Reflections

The significance of this work lies in its practical orientation toward offshore FPSO construction, where repair options after commissioning are extremely limited. The overlay design philosophy must be integrated early in the engineering phase, with careful consideration of weld accessibility, inspection feasibility, and the long-term corrosion allowance. Engineers should note that the marine environment imposes additional requirements for cathodic protection compatibility, and the overlay alloy must not create galvanic coupling issues with adjacent aluminum or zinc-coated components. The residual stress management through appropriate post-weld heat treatment is equally important to prevent stress corrosion cracking in chloride-containing seawater.