Flexible Pipe

Flexible pipes have been used in the oil industry since 1972, when Coflexip was awarded a patent to build a high pressure, flexible steel pipe. The first application was used in drilling as a 15,000 psi Kill and Choke line. Since then, flexible pipe designs have improved to produce the flowlines and risers that are now used in the offshore oil industry.

For deepwater, the flexible pipes are used mainly for dynamic risers from a subsea pipeline end manifold (PLEM) or riser tower to a floating production system such as an FSO, FPSO, and TLPs. The other uses are static risers, static flowlines, subsea jumpers, topside jumpers, and expansion joints. Flexible pipes are used for versatile offshore oil and gas applications including production, gas lift, gas injection, water injection, and various ancillary lines including potable water and liquid chemical lines.

Offshore flexible pipe
Source: https://www.sintef.no/globalassets/marintek/projects/flexiblepipes_4subsea.png

The main advantages of flexible pipelines are:
  • Ease and speed of installation
  • No large spans because it follows the contours of the seabed
  • Almost no maintenance for life of the project
  • Good insulation properties are inherent
  • Excellent corrosion properties
  • No field joints because the pipe is of continuous manufacture
  • No need of expansion loops
  • Can be made with enhanced flow characteristics
  • Sufficient submerged weight for lateral stability
  • Accommodates misalignments during installation and tie-in operations
  • Diverless installation is possible—no metrology necessary
  • Load-out and installation is safer, faster, and cheaper than any other pipe application
  • Retrievability and reusability for alternative application, thus enhancing overall field development economics and preserving the environment
  • Fatigue life longer than steel pipe
The codes that are used for the design of flexible pipe are:
  • API SPEC RP 17B—‘‘Recommended Practice for Flexible Pipe’’
  • API SPEC RP 17J—‘‘Specification for Unbonded Flexible Pipe’’ 
  • API SPEC RP 17K—‘‘Specification for Bonded Flexible Pipe’’
  • ISO 10420—‘‘Flexible Pipe Systems for Subsea and Marine Riser Applications’’
  • API Spec RP 2RD—‘‘Design of Risers for Floating Production Systems (FPSs) and Tension-Leg Platforms (TLPs)’’
Since there are only three manufacturers (NKT Flexibles, Wellstream, and Technip formerly Coflexip), and the manufacturing of flexible pipe requires wrapping of many intertwining layers of high strength stainless steel carcass and special polymers, the material price of a flexible line is hundreds of times more expensive than an equivalent high strength carbon steel pipe. Consequently, general use is limited to special applications and in small quantities compared to use of high strength carbon steel pipe.

Ultra-deepwater use of flexible pipe is limited, due to the inability of these pipes to withstand high external hydrostatic pressure. Presently, the maximum depth at which flexible pipes have been used is 2000m. The material make-up of each layer of flexible pipeline is described below.

Flexible pipe layers
Source: http://www.nov.com/fps_landing/images/02_dynamic-flexible-risers_pipe.jpg

  • Layer 1: Carcass. The carcass is a spirally wound interlocking structure manufactured from a metallic strip. The carcass prevents collapse of the inner liner and provides mechanical protection against pigging tools and abrasive particles. 
  • Layer 2: Inner liner. This is an extruded polymer layer that confines the internal fluid integrity.
  • Layer 3: Pressure armor. This consists of a number of structural layers comprised of helically wound C-shaped metallic wires and/or metallic strips. The pressure armor layers provide resistance to radial loads.
  • Layer 4: Tensile armor. The tensile armor layers provide resistance to axial tension loads. This is made up of a number of structural layers consisting of helically wound flat metallic wires. The layers are counter wound in pairs.
  • Layer 5: Outer sheath. The outer sheath is an extruded polymer layer. Its function is to shield the pipe’s structural elements from the outer environment and to give mechanical protection.
These are the primary layers. Some of the other layers that are not shown are the antiwear layers and insulation layers. The anti-wear layers are non-metallic layers that are inserted between the structural elements to prevent wear and tear between the structural elements. Additional layers of material with low thermal conductivity can be applied in order to obtain specific thermal insulation properties of the pipe.

All the flexible pipes have the same fundamental concept. Some variation may occur in choice of materials in case of special operating environments such as high pressures, high temperatures, sour service (high H2S and/or CO2 content), deep water, etc.

The end fitting of the flexible pipe is extremely important as it seals the different layers preventing any water ingress and also allows it to be connected to other pipeline appurtenances. The common end fittings that are used are:
  • Flanges
  • Grayloc connectors
  • Hydraulic subsea connectors
Another device that is used at the end of the flexible pipes is the bend restrictor. This is used to prevent excessive bending because most flexible pipes have a minimum allowable bend radius. Any bending beyond this would comprise the integrity of the flexible pipe.

Offshore pipelines - Dr. Boyun Guo (2005)


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