Negotiating the pipe-in-pipe bends

Derek Bish
Thursday, December 1, 2011

Harnessing and transporting multiphase products from often high temperature/high pressure well environments to the processing plant presents many a technological challenge. Tata Steel’s Derek Bish discusses one of them: how to construct and efficiently insulate pipe-in-pipe bends.'

Pipe-in-pipe systems that can withstand testing offshore conditions and diverse concerns such as corrosion, structural integrity and pipeline thermal management are already well established. Although they are widely perceived to be too costly and complex to apply to risers and bends, there are significant benefits that can be gained in maintaining pipeline integrity. Tata Steel has further developed this innovative technology as a cost-effective solution to flow assurance issues.

HP/HT fields are technically more complex to develop because of the inherently higher energy in the well fluid and its multi-phase composition. It is not uncommon to experience pressures of 700bar (10,000psi) and temperatures up to 160°C (320°F), so managing the extreme pressure and operating temperature must be based and evaluated 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 involve applying wet insulation coatings. However, the reduced thermal efficiency can compromise the overall shutdown times. Solutions to date have revolved around ensuring that the spools are ‘selfdraining’ in the case of shutdown. This ensures that pipeline bends are largely free from hydrocarbons when the line is not flowing. This can present a significant design challenge that can be mitigated by including pipe-in-pipe bends that enable the same thermal integrity to be maintained in the whole line.

Tata Steel developed a solution for pipe-in-pipe bends and deployed it as part of the development of the Jade field in the North Sea. However, compared to the latest materials technology, the insulation deployed in this system was relatively inefficient. For future developments in similar fields a step change in technology is required, and new insulation techniques have been developed that give far superior insulation properties.

The main challenge with the construction of pipe-in-pipe bends is how to pass the inner flowline bend into that of 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 challenge in the construction of these assemblies is the challenge of how they can be efficiently insulated. Generally most insulation systems for high temperature applications involve the ‘wrapping’ or ‘sheathing’ of the inner pipe with the insulation materials. This pre-assembly is simply not practical with an insulated bend as the insulation would occupy the annulus of the assembly and prevent the integration.

Insulation

The system developed by Tata Steel overcomes these problems by deploying granular Nanogel insulation into the annulus of the pipe-in-pipe system. Nanogel is an innovative insulation material that is made by first forming a silica gel and then expelling the water from the silica matrix. The resulting material is granular in nature with nanopores of air trapped within, inhibiting heat transfer by conduction, convection and radiation (with the inclusion of an opacifier). Nanogel is also very light, hydrophobic and surprisingly resilient to compression.

Once deployed in the annular gap the next challenge for the bend design is securing the insulation in the annulus while allowing the relative movement of the inner and outer bends as the temperature and pressure in the flowline varies through operation. This was achieved by deploying a novel polymeric bulkhead that is cast directly into the annulus and provides a solid barrier to retain the insulation. The polymer is a ‘syntactic’ material, silicone rubber with glass microspheres dispersed through the matrix. The resulting material has high strength, flexibility and thermal efficiency providing the final component to complete the insulated pipe-in-pipe bend assembly. The tangent ends of the inner and outer bends held rigidly ensuring that 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 and provide the consistent thermal performance around the bend, it is important that the annular gap throughout the assembly is uniform. In this instance it is important that the manufacturing tolerances of the pipe and bends are closely controlled.

Manufacturing

To integrate the bends into a combined insulated assembly, it is important to manufacture the steel pipe materials to the best tolerances achievable.

Tata Steel, with its partners Eisenbau Krämer and Salzgitter Mannesmann pipe benders, has developed a series of material controls that ensure the bend in bend integration can be performed. 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 with the connecting pipe dimension, the mother pipes have been manufactured with the same ID as the riser pipes.

Pipe manufactured at Eisenbau Krämer (EBK) in Germany is used by Tata Steel as mother pipe for forming into bends. Mother pipe for bends have material properties that allow the bend to be formed through hot induction bending. The main material challenges for the mother pipe are to ensure that the mechanical properties are suitable after bending, which involves a series of heat treatment and forming processes. EBK uses a multi-pass welding process and steel plate from the premium mills in Europe to ensure that the material produced is of the highest integrity.

Dimensional control of the pipe produced is also important and the EBK manufacturing process generates pipe of the closest dimensional control through a series of cold forming and sizing operations such as external calibration.

The main requirement for dimensional control arises in the bend manufacturing operations. Pipeline bends are made by hot induction bending. Heat is applied through electrical induction to the mother pipe materials and the pipe is slowly formed to give the correct bend geometry. In most pipeline applications the key critical dimensions are the positions and attitudes of the ends of the bends (centre-to-end dimension) to ensure that the overall geometry of the pipeline is maintained. However, with pipe in pipe bends it is important that the bend radius is also controlled very accurately to ensure that the two bends can be integrated into each other. The precise dimensions after bending need to be maintained also after heat treatment. For the inner clad bends a full body quench and temper (QT) heat treatment was applied in order to guarantee homogenised material properties for the bends to fulfil mechanical and corrosion requirements.

Together with Tata Steel, Salzgitter Mannesmann benders have developed a process and measurement system to ensure that all bend dimensions are closely controlled and mating bends can be matched and paired to ensure the most accurate assembly to be made.

HP/HT properties

In addition to the mechanical challenges of constructing pipe-in-pipe bends, other material challenges have to be overcome. Generally in high pressure and temperature lines there are challenges in terms of corrosion, low temperature toughness and strength. These parameters require careful material selection to maintain the balance of properties from steel make through to bend production. Being part of an insulated pipe in pipe system means that thermal stresses need to be managed as the loads are shared between inner and outer pipe.

In addition the insulation can lead to extremes of 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 offer significant challenges in terms of material selection and design.

Taking its knowledge and unrivalled experience in pipe-in-pipe technology and insulation, the 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 – a feat hitherto considered financially and technically too difficult to undertake.

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.'OE

Derek Bish'joined Tata Steel in 2008 as sales and applications engineer working on innovative pipeline solution developments. Prior to this he gained many years experience working within technical sales across a range of industries and markets. He is a qualified metallurgist and a member of the Institute of Materials, Minerals & Mining.

Categories: Equipment Subsea Pipelines Europe Flow Assurance

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