Flexible Riser

A riser system is essentially conductor pipes connecting floaters on the surface and the wellheads at the seabed. There are essentially two kinds of risers, namely rigid risers and flexible risers. A hybrid riser is the combination of these two. The riser system must be arranged so that the external loading is kept within acceptable limits with regard to:
  • Stress and sectional forces
  • VIV and suppression
  • Wave fatigue
  • Interference
The riser should be as short as possible in order to reduce material and installation costs, but it
must have sufficient flexibility to allow for large excursions of the floater.

Flexible riser
Source: http://fyrvik.com/img/projects/visund_flexible_reiser.PNG
The riser system of a production unit is to perform multiple functions, both in the drilling and
  • production phases. The functions performed by a riser system include:Production/injection
  • Export/import or circulate fluids
  • Drilling
  • Completion & workover
A typical riser system is mainly composed of:
  • Conduit (riser body)
  • Interface with floater and wellhead
  • Components
  • Auxiliary 
Flexible Riser Global Configuration 

Flexible risers can withstand both vertical and horizontal movement, making them ideal for use with floating facilities. This flexible pipe was originally used to connect production equipment aboard a floating facility to production and export risers, but now it is found as a primary riser solution as well. There are a number of configurations for flexible risers, including the steep S and lazy S that utilize anchored buoyancy modules, as well as the steep wave and lazy wave that incorporates buoyancy modules.  

Flexible riser
Source: http://3.bp.blogspot.com/-J0VioFdF86I/U-tmkGLswAI/AAAAAAAAAkQ/eSwM9fJsN14/s1600/imgaf001.png

Flexible risers can be installed in a number of different configurations. Riser configuration
design shall be performed according to the production requirement and site-specific
environmental conditions. Static analysis shall be carried out to determine the configuration.
The following basis shall be taken into account while determining the riser configuration:
  • Global behavior and geometry
  • Structural integrity, rigidity and continuity
  • Cross sectional properties
  • Means of support
  • Material
  • Costs
The six main configurations for flexible risers are shown in figure below. Configuration design
drivers include a number of factors such as water depth, host vessel access / hang-off location,
field layout such as number and type of risers and mooring layout, and in particular
environmental data and the host vessel motion characteristics.

Flexible riser configuration
Source: Subsea Pipelines and Risers - Yong Bai (2005)
  •  Free Hanging Catenary
This is the simplest configuration for a flexible riser. It is also the cheapest to install because
it requires minimal subsea infrastructure, and ease of installation. However a free hanging
catenary is exposed to severe loading due to vessel motions. The riser is simply lifted off or
lowered down on the seabed. A free hanging catenary under high vessel motions is likely to
suffer from compression buckling at the riser touch down point and tensile armor wire
'birdcaging'. In deeper water the top tension is large due to the long riser length supported.

  • Lazy wave and steep wave
In the wave type, buoyancy and weight are added along a longer length of the riser, to
decouple the vessel motions from the touch down point of the riser. Lazy waves are preferred
to steep waves because they require minimal subsea infrastructure. However lazy waves are
prone to configuration alterations if the internal pipe fluid density changes during the riser
lifetime. On the other hand, steep wave risers require a subsea base and subsea bend stififener,
and yet are able to maintain their configuration even if the riser fluid density changes.

Buoyancy modules are made of syntactic foam which has the desirable property of low water
absorption. The buoyancy modules need to be clamped tightly to the riser to avoid any
slippage which could alter the riser configuration and induce high stress in the armor wires.
On the other hand the clamping arrangement should not cause any significant damage to the
external sheath of the riser as this might cause water ingress into the annulus. Buoyancy
modules tend to lose buoyancy over time, and wave configurations are inherently designed to
accommodate up to a 10% loss of buoyancy.

  • Lazy S and steep S
In the lazy S and steep S riser configuration there is a subsea buoy, either a fixed buoy, which
is fixed to a structure at the seabed or a buoyant buoy, which is positioned by e.g. chains. The
addition of the buoy removes the problem with the TDP, as described above. The subsea buoy
absorbs the tension variation induced by the floater and the TDP has only small variation in
tension if any.

'S' configurations are considered only if catenary and wave configurations are not suitable for
a particular field. This is primarily due to the complex installation required. A lazy-S
configuration requires a mid-water arch, tether and tether base, while a steep-S requires a buoy
and subsea bend stiffener. The riser response is driven by the buoy hydrodynamics and
complex modeling is required due to the large inertial forces in action. In case of large vessel
motions a lazy-S might still result in compression problems at the riser touchdown, leaving a
steep-S as a possible alternative.
  •  Pliant wave
The pliant wave configuration is almost like the steep wave configuration where a subsea
anchor controls the TDP, i.e. the tension in the riser is transferred to the anchor and not to the
TDP. The pliant wave has the additional benefit that it is tied back to the well located beneath
the floater. This makes well intervention possible without an additional vessel.
This configuration is able to accommodate a wide range of bore fluid densities and vessel
motions without causing any significant change in configuration and inducing high stress in
the pipe structure. Due to the complex subsea installation that is required, it would be required
only if a simple catenary, lazy wave or steep wave configurations are not viable.

Subsea Pipelines and Risers - Yong Bai (2005)

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