The never-ending battle to extract more reserves from mature provinces such as the UK sector of the North Sea has seen new precedents set for temperature and pressure requirements in rigid riser systems.

The need to transfer multiphase products increasingly from HP/HT wells back to shore makes this challenge even more demanding. In a recent project, for example, riser materials were exposed to extreme temperatures ranging from 180°C to -30°C (356°F to -22°F) in certain well conditions.

Historically, industry consensus maintained that pipe-in-pipe systems able to withstand environmental challenges such as corrosion, structural integrity, and thermal management would be too costly and complex to apply to riser systems. However, companies now work closely with supply partners to engineer, procure, and construct innovative pipe-in-pipe assemblies as a cost-effective solution to flow assurance issues.

Pipe-in-pipe bends, while challenging technologically, can lead to simplification of overall pipeline design and can give better pipeline performance in times of operation and shutdown.

Why insulate risers and bends?

HP/HT fields are technically more complex to develop because of the inherently higher energy in the well fluid and its multiphase composition. Managing the extreme pressure and operating temperature must be based on criteria such as corrosion, maintaining structural integrity, and thermal management.

One particular challenge is managing pipeline shutdown. Less expensive solutions for managing the insulation of bends, such as wet coatings, compromise overall shut-down times due to reduced thermal efficiency.

However, solutions such as “self-draining” spools present a significant design challenge that can be mitigated by the inclusion of pipe-in-pipe bends, enabling the same thermal integrity to be maintained in the whole line.

Tata Steel previously implemented a solution for pipe-in-pipe bends for a North Sea development. But new insulation techniques have since been developed that give far superior insulation properties.

It is important that pipe bends have a straight portion on the end to enable efficient welding to the next section. This can present the insertion of one bend into another. (Images courtesy of Tata Steel)

Risers, spools, and bends

The main challenge with the construction of pipe-in-pipe bends is how to pass the inner flowline bend into the outer casing pipe. It is important that pipe bends have a straight portion on the end to enable efficient welding to the next pipe section, and this can present the insertion of one bend into the other.

The second construction challenge is efficient insulation. Wrapping or sheathing is simply not practical during this operation because the insulation would occupy the annulus of the assembly and prevent integration.

Insulation

A system developed by Tata Steel overcomes these problems by deploying granular Nanogel insulation into the annulus of the pipe-in-pipe system.

Nanogel is made by first forming a silica gel then expelling the water from the silica matrix. The resulting material is granular with trapped nanopores of air, inhibiting heat transfer by conduction, convection, and radiation (with the inclusion of an opacifier).

The deployment of a novel polymeric bulkhead, cast directly into the annulus, provides a solid barrier to retain insulation, allowing for the relative movement of the inner and outer bends. The polymer is a “syntactic” material, silicone rubber with glass microspheres dispersed through the matrix with high strength, flexibility, and thermal efficiency. The tangent ends of the inner and outer bends are held rigidly, ensuring the assembly tolerances achieved at manufacture are retained when the unit is transferred to the welding contractor for incorporation into the pipeline spool or riser.

For the insulation to be effectively deployed, providing consistent thermal performance, the annular gap throughout the assembly must be uniform. In this instance, it is important that the manufacturing tolerances of the pipe and bends are controlled closely.

Steel pipe and bend manufacture

Together with Tata Steel, Eisenbau Kr?mer (EBK) and the pipe bending plant of Salzgitter Mannesmann Grobblech (SMGB) have developed a series of controls, including a process and measurement system, to ensure all bend dimensions are controlled closely and mating bends can be produced, matched, and paired so the most accurate assembly is produced.

In respect to the process-related thinning in the extrados of the hot induction bends, the wall thickness for the inner and outer mother pipes was increased accordingly. To match precisely, the mother pipes have been manufactured with the same ID as the riser pipes.

Two 16-in. clad bends are illuminated in the quenching tank after austenitization at a pipe bending mill.

EBK supplies Tata Steel with mother pipe, which has material properties that allow formation through hot induction bending. The main material challenges are to ensure the mechanical properties are suitable after bending. Therefore SMGB is taking responsibility for the chemical design of the prematerial. This also involves the consideration of a series of heat treatment and forming processes. Eisenbau Kr?mer uses a multipass welding process and steel plate from premium mills in Europe.

The manufacturing process at EBK generates pipe of the closest dimensional control through a series of cold-forming and sizing operations such as external calibration.

At the SMGB pipe bending plant, these special mother pipes are bent by hot induction bending. Heat is applied through electrical induction to the mother pipe materials, and the pipe is slowly formed, giving the correct geometry. In most pipeline applications, the critical dimensions are the positions and attitudes of the ends of the bends (center-to-end dimension) maintaining the overall geometry of the pipeline.

However, with pipe-in-pipe bends, it is important that the bend radius is accurately controlled to ensure the two bends can be integrated. The precise dimensions after bending also need to be maintained following heat treatment. For the inner clad bends, a full body quench and temper (QT) heat treatment is applied at the SMGB bending mill to guarantee homogenized material properties for the bends, fulfilling mechanical and corrosion requirements.

HP/HT material properties for pipe-in-pipe

Additional material complexities also have to be overcome. Generally in HP/HT lines, there are challenges because of corrosion, low temperature toughness, and strength. These parameters require careful material selection to maintain the balance of properties from the outset through to bend production.

Thermal stresses need to be managed as the loads are shared between inner and outer pipe. In addition, insulation can lead to extreme temperature being retained in the pipe materials during operation and shutdown that can form challenging conditions for conventional steel products.

HP/HT well environments present some of the most challenging and technologically demanding conditions for field developments, not least because the properties in each reserve present significant challenges in terms of material selection and design.

Tata Steel and its supply partners have expanded capabilities further with the design and creation of cost-effective insulated pipe-in-pipe bends for risers and spools – once considered too difficult to accomplish.

Comments