Page 415 - Pipelines and Risers

382 Chnpter 20

In the lazy S and steep S riser configuration there is added a subsea buoy, either a fixed buoy,
which is fixed to a structure at the seabed or an buoyant buoy, which is positioned by e.g.
chains. The addition of the buoy does that the problem with the TDP is omitted, 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. The subsea buoy has the additional function, by
reducing the length of riser supported by the toptensioner the requirements to the toptensioner
is reduced proportionally.

- Lazy wave and steep wave
The lazy and steep wave configurations are in shape and function the same as for the lazy and
steep S configurations. In the wave type there is not added a single buoy, instead there is
added buoyancy and weight along a longer length of the riser where it is beneficial. With this
distributed weight and buoyancy it is easy to make the riser shape desired.

- Pliantwave
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.

Riser configuration design shall be performed according to the production requirement and
site-specified. Static analysis shall be carried out to determine the configuration. The
following basis can 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 riser system must be arranged so that the external loading is kept within acceptable limits
with regard to:
- Tension
- Bending
- Torsion
- Compression
- Interference