First shrink-fit riser flanges set for North Sea debut

Subsea Riser Products (SRP), an Acteon company, has successfully attached the first flanges to pipe sections that will make up the world's first ultrahigh-pressure, large-bore drilling riser for Venture Production in the North Sea. The flanges were joined to the pipes using a novel shrink-fit process that the company developed specifically with the fabrication of risers like this in mind.

The Venture Production riser will be used for drilling high-pressure, high-temperature wells in the North Sea from a jackup rig equipped with a surface blowout preventer (BOP). The surface BOP promises significant cost and operational advantages, but to complete the wells at the seabed, a drilling riser is needed that offers full-bore (18¾-in.) access and can contain the full pressure of the reservoir, which could be in excess of 12,000 psi.

The riser has 13 main sections, each 30 ft long, plus fatigue-critical, tapered stress and tension joints at either end. As is common practice, the individual pipes will be connected to each other using bolted flanges. A challenge arose in how to attach the flanges to the main body of the pipes.

SRP states that the conventional option of welding was not practical because a commonly weldable grade of steel such as 65 ksi would require a pipe-wall thickness of approximately 75 mm to meet the pressure rating. Welding pipe of this thickness is difficult, particularly when the requirements for the Venture riser call for final weld properties and fatigue performance to comply with NACE sour-service requirements. Additional challenges arise in manufacturing pipe of this thickness and the enormous weight of a complete riser string of this nature.

While the wall thickness can be reduced by making the pipes from higher-yield-strength steel such as 110-ksi steel, challenges arise in controlling the hardness of the final weld.

Connecting the shrink-fit flange to the pipe.

To resolve this problem, Venture worked closely with a group of three Acteon companies to design a unique riser with flange connections that are shrink-fitted to the pipes: Claxton Engineering, which specializes in drilling riser rental and sales; 2H Offshore, which designed the riser and conceived the idea of shrink-fitting the flanges; and the newly established SRP, responsible for the detailed design and manufacture of the shrink-fit flange.

The fitting process is fairly straightforward, consisting of first heating a flange piece with its short stubs to greater than 400°C, which causes it to expand sufficiently to fit over the end of the  19¼-in. inside diameter, 80-ksi steel riser pipe section.  The flange and the pipe are then carefully brought together—in a process that takes just 30 seconds—using a purpose-built jig to ensure the precise alignment of the two parts. Once in place, the assembly cools to create a high-quality structural connection and a gas-tight seal.

SRP says that while the connection process itself is elegantly simple, a high level of attention must be paid to the process details to ensure successful mating.

Important considerations include the following:

• The profiles and surface finishes on the two components are critical
• The heating process needs to be very carefully controlled
• It is important to ensure the precise alignment of the two components during the mating process.

"The development of the shrink-fit flange has been the key to this entire project," said SRP Managing Director Simon Luffrum. "We have had a good deal of interest in the technique from a number of other operators. Now that we have proved the technique, it is likely to find application in a variety of riser-related projects in the future."

For more information, contact Simon Luffrum at +44 (0)1483 745030 or visit www.subseariserproducts.com.