2 Executive Summary

The different riser concepts available for deepwater field development can be categorised as follows:

  1. Flexible risers (either monobore or IPB technology)

    • Free hanging, e.g. the flexible riser runs in a catenary shape from the floater connection point and directly down to the seabed.

    • Wave or ‘S’ shapes, e.g. buoyancy devices are used to give the flexible risers additional flexibility, mainly in shallow water condition.

  2. Steel risers

    • riser installed on side or indide Shallow Water platerform such as Jacket or concrete plateform.

    • Top-tensioned risers, e.g. vertical steel riser using tensioning system (at floater level) composed either of hydraulic cylinders or buoyancy cans.

    • Steel Catenary risers (SCRs), i.e. free hanging steel risers.

    • Wave shaped risers, i.e. buoyancy devices are fitted at the lower part of the riser to obtain a multiple catenary shape.

  3. Hybrid risers

    • Single Hybrid Riser (SHR) and Riser towers/bundles, e.g. a steel (single or bundle) riser in straight ‘tensioned’ configuration by means of a buoyancy can (e.g. at 50-200m water depth level) and is connected to the floating production facilities by flexible jumper(s).

    • Sub-surface buoy systems, e.g. SCRs are supported by a buoyant mid-water arch connected to seabed foundations by tethers and flexible jumpers allow connection to the surface floater.

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In general terms, for moderate water depths (from 200 m up to 800 m) the flexible pipe is the preferred riser solution, as more riser flexibility is required to accommodate the FPS motions and offsets.

In deeper water depths (e.g. > 800m) steel riser in catenary shape would be more cost-effective alternative. However in ultra-deepwater (e.g. beyond 1500m) higher SCR load tensions are to be expected at the FPS hang-off arrangement and Hybrid riser systems would be a preferred technical solution.

The advantages and disadvantages of above deepwater riser concept are summarised in the following table:

Configuration

Advantages

Disadvantages

Flexible

  • Flexibility of the riser system, fit-for-purpose in shallow water depths and/or harsh environmental conditions.

  • Ease of installation

  • Possibility to actively heat the riser (IPB technology)

  • Most Stringent passive insulation requirements could be difficult to fulfil (IPB technology / active heating could be required in some cases). Flexible risers typically limited to 2-3W/°C.m².

  • Water depth / diameter limitations exist (end cap effect)

  • Low local content (depending on field location)

  • High cost riser system with water depth

Steel Riser

  • Relatively low cost riser system

  • Steel based system features well-known design (except for VIV phenomenon)

  • Can be used for high temperature and high pressure case

  • Utilisation restricted to deep waters (to allow for riser system sufficient flexibility)

  • Better adapted to low motion floaters (e.g. Spar, TLP)

  • High loads exerted on the production floater at riser hang-off

  • Criticality of the touch-down zone (fatigue, embedment, etc.)

  • Difficult welding criteria

  • Special pipe tolerances

  • Flex-joint integrity is difficult to assess

  • Wet insulation coating system capability in very deep waters

  • VIV fatigue is still an issue (no fully reliable design tool)

  • Long Engineering process regarding fatigue criteria

Riser towerand SHR

  • Floater motion decoupling versus vertical steel riser fatigues by means of flexible jumpers

  • Combines advantages of rigid and flexible risers (i.e. compliance to floater motions and optimised cost for a deepwater system)

  • Upper flexible lines individually retrievable

  • Well adapted for congested area

  • Reduced deck space and payload on floating unit

  • Schedule flexibility by permitting the installation of maximum components prior the FPSO arrival

  • Flexibility of field layout by permitting the routing flowlines independent of the approach angle of the riser hang-off, thus reducing flowline lengths and minimising routing constraints.

  • High cost of the connection assembly between the riser tower and the air can.

  • Potential for flow induced vibrations particularly for multi-line applications

  • Loss of one riser tower will induce closure of several systems like production, water injection, gas lift, etc.

  • Complex installation process for SHR which might also requires use of different construction vessels.

  • Installation of towers by towing and upending is inducing complexity and risks.

  • Onshore fabrication of towers is inducing logistic constrains.

Sub-surface buoy system

  • Floater motion decoupling versus vertical steel riser fatigues by means of flexible jumpers

    • Combines advantages of rigid and flexible lines

    • Low riser hang-off loads, thus improving topside payload and reducing the specification level of riser pull-in equipment.

    • Local fabrication and content

    • Installation flexibility: all Flowlines/Risers and umbilicals can be installed and pre-commissioned prior to the FPU arrival at site.

    • Reduces the time to ‘first oil’ and eliminates the need to mobilise additional equipment onto the FPU for Flowline flooding, gauging, pressure tests, de-watering, etc.

    • ‘Open’ Riser system where each Riser element can be inspected (e.g. visual survey, NDT) and a built-in monitoring device allows permanent logging of the buoy motions and tether tensions.

    • Flowlines/Risers and umbilicals can be recovered to surface for repair or replacement with minimum disturbance of the field operations.

    • Riser replacement and field decommissioning facilitated by way of reversible procedures.

  • Complex System

  • In case of one main tether failure, the replacing tethers will have a different stiffness due to its different loading history. This would require all tethers to be replaced.

  • Complex installation process (several separate installation phases and utilisation of different construction vessels).

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