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Chapter 20

Design of Deepwater Risers

20.1 General

Metallic Catenary Risers (MCR), Flexible risers and other riser concepts will be widely used
in deepwater drilling and production. In this chapter the MCR will be outlined and project
application to Statfjord C will be given. Then several types of risers are introduced such as
flexible risers, drilling and workover risers. The uses of risers in large offshore platforms in
the Norwegian North Sea are summarized in a table. Codes and guidelines as well as vortex-
induced vibrations and fatigue are presented in detail in subsequent chapters (see Chapter 22.8
for example).

20.2 Descriptions of Riser System

20.2.1 General
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 riser and
flexible riser. A hybrid riser is the combination of these two. There are a variety of possible
configurations for marine risers, such as free hanging catenary riser, top tensioned production
riser, lazy S riser, steep S riser, lazy wave riser, steep wave riser and pliant wave riser, see
Figure 20.1. Due to the requirement of deepwater production, new configurations are also
available, such as Compliant Vertical Access Riser (CVAR), (multibore) hybrid riser.

- Catenary
The free hanging catenary riser is widely used in deep water. This configuration does not need
heave compensation equipment, when the riser is moved up and down together with the
floater, the riser is simply lifted off or lowered down on the seabed. In deeper water the top
tension is large due to the long riser length supported, to reduce the size of the top tensioner
buoyancy modules could be clamped to the top end of the riser. The surface motion is directly
transferred to the Touch Down Point (TDP), this means that the failure mode could be
overbend or compression at the TDP. The most severe motion is heave from the first order
vessel motion.

- Lazy S and steep S