Stainless steel tubing on O&G platforms is used in process instrumentation, hydraulic lines, chemical inhibition, and utility applications. These tubing are used in a wide range of service conditions regarding for instance temperature, flow rate and pressure. Many of these applications are on topside of offshore platforms and corrosion of 316L stainless steel tubing has been occurred in several of them, mainly in tropical sea waters, due to marine atmospheric corrosion.
The two prevalent forms of marine atmospheric corrosion in 316L tubing on topside applications are pitting corrosion and crevice corrosion, which happens beneath clamps, support trays and connections. Both corrosion processes depend on the chloride concentrations and moisture level. Tubing surface contamination also leads to corrosion problems. Such contamination may be caused by iron particles from welding and grinding operations, surface deposits from handling, drilling and blasting, and from sulfur-rich diesel engine exhaust systems. Periodic testing of seawater fire fighting systems, especially in combination with insufficient fresh water rinsing, may also leave undesirable chloride salts deposits on the tubing surface.
Today's minimally alloyed 316L tubing, with close to 10.0% nickel, 2.0% molybdenum and 16.0% chromium, may experience corrosion more readily than the more generously alloyed 316L tubing products produced decades ago. Variations such as 317L, with higher Mo are also not enough to withstand the corrosive environment. Since higher alloys such as Titanium and Nickel alloys are less attractive because of prices and availability, and normally coatings are not effective for hydraulic and instrumentation tubing because they represent a high potential risk of crevice, duplex grades appear as a costeffective good solution.
Previous experience in tropical waters, such as Gulf of Guinea and Gulf of Mexico, have proven that the 316L tubing have service life shorter than 5 years and in some cases less than 1 year, with risks of sudden failures that can cause life hazard accidents and serious damages on O&G platforms equipments.
The use of austenitic stainless steel 316L for hydraulic and instrumentation tubing on topside of offshore platforms is very common, because of its wide availability and well known corrosion resistance properties. However, the 316L corrosion resistance has proved insufficient in the case of marine atmosphere, particularly in tropical sea waters.
Recent studies conducted by Shell (Okeremi & Simon-Thomas, 2008), by Swagelok, Shell and BP (Schiroky, Dam, Okeremi and Speed, 2009) and later on by BP (Kopliku & Mendez, 2010) indicate that these materials failed due to localized corrosion processes, being crevice and pitting the main forms of corrosion damage experienced followed by much less frequent cases of stress corrosion cracking. This is influenced by the presence of chlorides and temperature, which are common characteristics in Oil & Gas offshore exploration and production and are very pronounced in tropical sea waters. One example in these studies is a FPSO in the Gulf of Guinea which showed severe signs of corrosion in small diameter tubing in several locations in the topside facilities, less than a year after the FPSO was moored in place offshore (Okeremi & Simon-Thomas, 2008). A comparative study of various alloys for marine environment tubing applications on topside reinforces, through laboratory tests and field trials, insufficient corrosion resistance of standard Mo alloyed austenitic stainless steels like 316L, 317L and 317LMN and also nickel alloys like Alloy 825. (Kopliku & Mendez, 2010).