Total sanctions seam-welded tubes
for west of Shetland umbilical
Reduced wall thickness cuts offshore installation costs
Super duplex tubes are the main cost item in a steel tube umbilical, and optimizing the wall thickness to minimize the material required can provide significant cost savings. Vallourec Umbilicals’ newly developed super duplex seam-welded tube offers a guaranteed high material yield and tighter manufacturing tolerances, enabling more cost-effective tube designs without
The first successful installation was the 18-km ( 11.2-mi) umbilical
that TechnipFMC supplied for Total’s Glenlivet gas/condensate field
west of Shetland, which came onstream this August via a tie-in to the
subsea production infrastructure serving the Laggan-Tormore fields.
Glenlivet was co-developed with the Edradour field in water depths
of 300-435 m (984-1,427 ft), but the benefits of seam-welded tubes
are equally applicable to longer-distance umbilicals in much deeper
water. Work on the new super duplex tube manufacturing process
started in 2008, and the recent industrialization followed an extensive
qualification program, which continues with various offshore operators, umbilical manufacturers and third-party type approval bodies.
The new process, developed in accordance with ASTM A789/
A789M-10a, employs the established super duplex stainless steel
grade UNS S32750 or 2507, used widely by umbilical manufacturers
for subsea applications. Main benefits are strong pitting and corrosion resistance; high levels of strength and hardness; and tight
dimensional tolerances properties (in the case of Glenlivet, ovaliza-tion is +/-0.03 mm).
There are four principal features of seam-welded tube manufacturing: a longitudinal welding line, an orbital welding line, hydraulic test-ing/cleaning/conditioning, and packaging. The longitudinal welding
line is configured to transform a raw material, flat super duplex stainless steel strip into a 100% NDT (non-destructive testing) controlled
coiled tube. The strip is formed into a round tube, which is welded
by a laser source and then heat treated and calibrated before being
controlled by ultrasonic testing and eddy current testing methods: the
latter ensure detection of any possible defect greater than 5% of the
wall thickness. Defect indications are automatically ‘red’ marked on
the tube prior to intermediate reeling on the longitudinal welding line.
Several intermediate reels packed with completed tube are welded
together (TIG, with super duplex filler material process) on an orbital
welding line to produce a final spooled product ready for pressure
testing and shipping. Any ‘red’ marks detected previously are at this
point removed and replaced by circumferential welds on the line. All
orbital welds are X-ray controlled in real time; the tube is then reeled
onto a customer process reel, which will undergo hydraulic testing,
cleaning and conditioning prior to shipment.
By applying these procedures, long-length super duplex umbilical
tubing can be produced with a minimum yield strength of 750MPa and
a minimum ultimate tensile strength (UTS) of 900MPa, for each tube
size and wall thickness. Vallourec undertook demonstrations of tube
property reliability as part of an extensive, Bureau Veritas-witnessed
qualification program and in line with stringent testing requirements
specified by TechnipFMC Umbilicals and Total.
The manufacturing process provides further benefits in terms of
manufacturing tolerances and general geometry of the tube. Constructing the tubes from strip raw material means that the tube wall
thickness tolerances are directly linked to the manufacturing tolerances
of the strip wall thickness, so from the strip manufacturing process a
tolerance below 5% is guaranteed. For seamless tube, wall thickness
tolerances selected for the design and calculations are usually around
10%, but for seam-welded tube the tolerances can be set at up to 5%,
depending on results of the qualification program. Residual ovaliza-tion is below 3%, which compares with 5% typically for seamless tube.
Yield strength, wall thickness tolerance and ovality all play an
important role in the tube performance. The qualification program
showed that tube wall thickness could be optimized and significantly
reduced, in turn lessening the weight and quantity of raw material.
Another benefit of reduced wall thickness is an increased bending
performance of the tube. The tube’s bending capacity drives the
manufacturing process, as well as the packing and installation method
Vallourec Research Center France
A seam-welded tube. (Courtesy Vallourec Umbilicals)