Steel Stress Joints
Steel catenary risers (SCRs) are used to transfer fluids like oil, gas and injection water between platforms and the pipelines. They have been used for many years for fixed platforms (TLPs), and today the trend is that they are seen more and more as an attractive, cost saving riser solution for deep-water, flexible floating offshore facilities, such as semi-submersible platforms and FPSOs. Due to load and environmental conditions, the predominant solution for the SCR to vessel interface has been flexible joints and stress joints in titanium with only a relatively small number of installations with a steel stress joint. This article presents the possibilities that are available with (low alloyed) steel stress joints, and the recent developed manufacturing capabilities.
The manufacturing of large and long forgings has improved as a result of developments in steel making, forging and heat-treatment practice. This led to increased cross-section properties and larger forgings made out of most commonly used low alloy steel grades. These stress joints forgings have a specified minimum yield strength level of SMYS 80ksi with good fracture toughness. The properties are achieved and can be maintained as well, with increased yield strength to 90ksi, and still meet tensile strength and hardness limits for sour service. It was proven that the strength of the steel is maintained even after corrosion resistant alloy (CRA) clad welding and subsequent post weld heat treatment (PWHT). In finite element analysis (FEA) calculations for load and fatigue life length, a lower yield strength is used after PWHT. Steel grades and manufacturing processes are available for even higher strengths (SMYS up to 120ksi) with relatively good fracture toughness. This improves the ratio of yield strength, which in turn impacts the design to give the required bending properties and improves stress joint design in non-sour service applications, resulting in both a technical and commercial interesting alternative for flex-joints and Titanium stress joints. The development of CRA internal diameter (ID) clad welding of long heavy wall forgings capability increases the competitiveness of the use of steel as it limits the corrosion in high load and fatigue sensitive areas of the stress joint, especially with high quality welds.
In many cases simulated post weld heat treatment (SPWHT) was also performed, which has proven that the base material properties are maintained, also after welding and stress leaving. A significant possibility to improve designs with steel, if actual values are achieved instead of traditional material models in FEA work for steels structures. With the PWHT today, the yield strength drop is less than 2%, which results in 80ksi yield forgings. They remain 80ksi, also after PWHT. There is even room to increase strength with the use of new steel grades and manufacturing methods. Steel produced has a large range of properties. It is up to the designers to use the properties they need in their design. Today’s machining equipment are so well developed that machining these large sizes of forgings with narrow tolerances are possible. Of course, there is experience needed to understand how the forgings behave in relation to the machine tools, in order to keep control of cutting data and shape stability when a large amount of steel from a high-strength forging is removed.
CRA Clad Welding
With the recent possibilities to ID clad with low heat input (HI; <0.3kJ/mm), it is possible to PWHT with a comfortable margin to the tempering temperature of the steel. With this PWHT, the sour service requirements of the National Association of Corrosion Engineers (NACE) were met. The PWHT temperature was significantly lower as the SPWHT at which the original material was tested. With reference to the clad layer after PWHT, the yield strength of the CRA after PWHT was proven to be 90ksi, with an elongation of 44%. The tensile strength of the fusion bonding between the CRA layer and the A182 F22 base material, was found to be on the same level as the base material; 110ksi. This was all in line with yield properties of previous tested CRA samples. These samples were welded with a conventional GTAW (hot wire) process, and an HI of 1.1-1.7kJ/mm. Yield strengths of CRA after PWHT; 90 ksi, or a little more depending on soak time in the PWHT procedure. All the CRA samples had a chemical composition well below 5% Fe, and a fusion line at base material of FL +3mm. This internal bore cladding of stress joints is available for lengths up to 18.5m.
For cladding 18m long steel risers, SME Cladding, developed the Cladiator. The new welding station uses a twin torch set-up, with each torch using a patented twin electrode head. This ensures an extremely high quality weld with a high deposit rate. The process of the twin electrode furthermore ensures low heat input and dilution rate, enabling lower post-weld heat treatment (PWHT) temperatures and thus protecting base material properties. The riser also has to be pre-heated during welding and, after welding, the complete riser requires a post weld heat-treatment. A dedicated PWHT furnace was built for this task.
The aforementioned indicates a great potential for further development of designing steel SCR stress joints, for use with existing and new installations, as long as the new manufacturing capabilities are evaluated and introduced early on in a new development’s design phase.
Words by: Peter Jansson, Managing Director at Scana Subsea AB, Willem Konings, Technical Director and Alfred van Aartsen, Welding Engineer at Schelde Exotech BV – this article is based on the whitepaper published by the writers on behalf of SME-Cladding BV.