Page 417 - Pipelines and Risers

384 Chapter 20

Initial riser configuration can be developed for the minimum wall thickness based on
suspended lengths and a given top angle. To obtain an optimum riser configuration, the top
angle shall be varied by adjusting the floater position with respect to the. fixed riser end.
Variations in floater position of approximately 10% in each direction from the normal
position in the plane of the riser can be used. Initial configuration development can be
conducted using dynamic analyses with extreme storm.

In order to meet harsh environments of deepwater, optimization theory can be applied to
obtain an optimized riser configuration since the significance of the design requirements will
vary along the riser. This indicates that the wall thickness of an optimized configuration may
vary along the entire riser length. The total length of a riser is also a design variable. The riser
should be as short as possible in order to reduce costs, but the riser must accommodate
sufficient flexibility to allow for large excursions of the floater.

20.2.2 System Descriptions
The riser system of a production unit is to perform multitude of functions, both in the drilling
and production phases. The functions performed by a riser system include:
- Productiodinjection
- Drilling
- Exporthmport or circulate fluids
- Completion
- Workover
A typical riser system is mainly composed of
- Conduit (riser body)
- Interface with floater and wellhead
- Components
- Auxiliary

20.2.3 Component Descriptions
Apart from the basic pipe structures there is a considerable amount of auxiliary equipment
used in a riser system. The riser design must give attention to these items as, in many
instances, these could turn out to be critical areas for the design point of view. The
components of a riser system must be strong enough to withstand high tension and bending
moments, and have enough flexibility to resist fatigue, yet be as light as practicable to
minimize tensioning and floatation requirements. Figure 20.2 shows some of the components
for a catenary riser and a top tensioned riser.
Some detailed descriptions of riser components are given below.

- Riser joints
A riser joint is constructed of seamless pipe with mechanical connectors welded on the ends.
KilVchoke lines are attached to the riser by extended flanges of the connector. The riser can
be run in a manner similar to drill pipes by stabbing one stalk at a time into the string and
tightening the connector.