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Steel-in-Steel Doublewall Piping for
Subsea Applications

Unisert's newest member in the IT3 family of piping products is a steel-in-steel subsea pipe system that features an internal bonded joint. Unisert has developed and tested a new technology for quickly connecting doublewall steel pipe. Especially suited for offshore deep-water applications, the patented S-SK Collar connector allows the field installer to join the internal steel carrier pipe without welding. In doing so, the external steel containment pipe can be easily welded over the bonded joint once the two pipes are connected. One circumferential weld on the external pipe replaces the more cumbersome and time-consuming process of one internal weld and up to four external welds. Half-sleeves or slide-over-sleeves are no longer needed to complete the connection of the external containment pipe.

The S-SK Collar is a bonded joint that features a combination of different contact surface finishes, interference, metal grades and epoxy compounds. The S-SK Collar is constructed of steel with a locking-thread serration and a slight taper. The "locking-thread" serration provides minimum resistance during insertion and excellent resistance to separation. This serration design also provides excellent geometry for epoxy fill, sealing and lap shear resistance. An o-ring provides an additional measure of protection.

Testing has proven its ability to function properly under difficult installation conditions and operating pressures of 7,500psi. Hydrotests were performed to simulate anticipated pressure cycles over a 30-year design life. The S-SK joint was initially tested to 10,000psi. In each test cycle the joint was pressurized to 7,500psi and temperatures were maintained at approximately 180°F. A final test was conducted at 10,000psi and no leaks or adverse effects were observed.

The key to this new piping system's success is the integrity of the S-SK joint, which is assured.

While Unisert's S-SK Collar was designed primarily for subsea applications where the J-Lay procedure is utilized, the joint can be used in any application where doublewall containment using steel for both the carrier and containment pipes is preferred. Excellent negative buoyancy, high pressure handling capabilities, and the ability to operate in low temperature environments combine to make this piping approach the right answer for moving highly corrosive oil and gas products subsea.

United States Coast Guard –Letter with regard to Steel Piping System


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