The hose end non return valve (so-called NSV) is the last safety component of mooring and oil transfer during an offloading operation between a FPSO (Floating, Production, Storage and Offloading Vessel) and a Tanker.
Recurrent spring failure occurred during offloading where spring are compressed (under stress to keep damper piston open). All material failure performed characterized a brittle failure without evidence whether cracking was only driven by H2S presence or other agents also assisted such as microbiological (stagnation) and general corrosion by oxygen.
Aiming a long and safety spring integrity performance, a new spring material selection design was necessary to comply with offloading operation requirements where environment (medium) and spring material behavior were the most important guides to select the possible material solutions.
Two different spring materials configurations aiming long term application were specified. The first solution comes from a supplier where the spring is a low alloy steel (Cr-V) coated by a HNBR while the second solution is a spring fabricated in SS316 cold worked protected by zinc anodes.
Material tests and spring mechanical performance were defined to validate the alternatives in a laboratory phase. With positive results, such solutions have been applied at operation so that such spring materials design are working without any failure so far.
The function of the NSV (hose-end non-return valve, first used in the North Sea) is to create a safety connection between the FPSO and the shuttle tanker during mooring and oil transfer. This connection is made using the Loading System at the Bow of the tanker vessel through the BLS (Bow Loading System). After carrying out all necessary checklist and communicating with the tanker, the marine crew from FPSO start to pay the offloading mooring cable that connects on Dynamic Positioning Shuttle Tanker (DPST). When the mooring is complete, the offloading line connects to the DPST through the BLS which connects via NSV (Figure 1).
