The first shrink-fit riser flanges

Paul Boughton

Subsea Riser Products (SRP), an Acteon company, has attached the first flanges to pipe sections that will make up a  ultra-high-pressure drilling riser for use by Venture Production in the North Sea. The flanges were joined to the pipes at SRP’s premises in Sheffield, UK, using a novel shrink-fit process that the company developed specifically with the fabrication of risers like this one in mind.

SRP managing director Simon Luffrum said: “The development of the shrink-fit flange has been the key to this entire project. 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.”

The flange is first heated using induction coils to over 400°C, which causes it to expand sufficiently to fit over the end of the 19¼-in ID riser pipe section. The flange and the pipe are then carefully brought together – this part of the process takes no more than 30 seconds – using a purpose-built jig to ensure the precise alignment of the two parts. Simply allowing the
assembly to cool down creates a high-quality structural connection and a gas-tight seal.

Subsea Riser Products designs and delivers offshore riser products for critical drilling and production applications. Its range of specialist products provides significant advantages, including reduced overall cost, improved levels of performance, reliability and project-enabling
delivery scheduling. Through its core strengths of innovative mechanical design and highperformance materials, together with advanced manufacturing methods, the company delivers optimum value-for-money products that yield enhanced system integrity.

The novel application of an established technology lies behind the manufacture of the world’s first ultra-high-pressure, large-bore drilling riser The Venture Production riser is intended to 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 will provide significant cost and operational advantages. However, in order to complete the wells at the seabed through the surface BOP, 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.

Venture has liaised closely with a group of three Acteon companies, Claxton Engineering, 2H Offshore and the newly established Subsea Riser Products, in designing a unique riser, the notable feature of which is its flange connections, which will be shrink fitted to the individual

The riser has 13 main sections, each 30 feet 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. The question in the case of this ultra-high-pressure riser was how to attach the flanges to the main body of the pipes.

The answer was not easy, as the conventional option, welding, is not really practicable. Using a normally weldable grade of steel, say 65 ksi, would require a pipe wall thickness of around 75mm to meet the pressure rating. Welding pipe of this thickness is extremely difficult, especially
when the final weld properties and fatigue performance are governed by NACE sour service requirements, as in this case. Actually, welding is not the only problem. In the first place, manufacturing pipe of this thickness is far from straightforward, and the weight of a complete riser string of this nature would be enormous.

The wall thickness can be reduced by making the pipes from steel with a higher yield strength.

For example, using a 110-ksi steel would mean having a wall thickness of about 30–40 mm.

Unfortunately, welding such high-strength steels successfully is also extremely challenging, not least in terms of controlling the hardness of the final weld.

The whole problem surrounding the welding of this ultra-high pressure riser pipe has been resolved by using a shrink-fit process to attach the flange connectors to the pipes – ultimately made from 80 ksi steel with a wall thickness in excess of 50 mm.

On the face of it, the process is very straightforward. The flange pieces with their short stubs are heated using induction coils and, in their expanded state, carefully mated with the main pipe sections. Subsequently allowing the assembly to cool completes the process and generates a high-quality structural connection and a gas-tight seal.

The simplicity of the process belies the amount of effort that has gone into developing it for this application – and also the high level of attention that has to be paid to the details of the process. For example, the profiles and surface finishes on the two components are critical; the heating process needs to be very carefully controlled; and it is important to ensure the precise alignment of the two components during the mating process. (A purpose-designed construction jig is used to bring the flange and the pipe together).

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