Page 418 - Pipelines and Risers

Design of Deepwater Risers 385

- Flexible joints
Flexible joints allow limited angular motion of the riser. In some cases, these flexible joints
may be a series of ball joints. Pressure compensated flexible joints should be used to decrease
the torque required to deflect the joint. The forces acting on the joint push the inner ball
against the outer casing, causing the joint to bind. To decrease the required torque hydraulic
fluid is injected to spread apart and lubricate the moving parts. With the large area involved,
relatively small pressure are required.

- Slipjoints
A slip joint comprises two concentric cylinders or barrels that telescope. The outer barrel is
attached to the marine riser, and the riser is held in tension by wire ropes from the outer barrel
to the tensioner.

- Buoyancy modules
Buoyancy modules can be attached to the riser to decrease the tension required at the surface.
These modules may be thin-walled air cans or fabricated syntactic foam modules that are
strapped to the riser. These buoyancy modules require careful design and the material for their
construction needs to be selected appropriately so as to ensure that they have a long-term
resistance to water absorption.

Auxiliary components

- Endfittings
The end fittings provide the important function of ensuring that the riser loads (in tension,
bending and torsion) are satisfactorily resisted whilst ensuring that a comprehensive sealing
system is attached both radialIy and axially. The adequacy of terminations must be
determined through careful detailed design, prototype as well as through in-service
experience.

- Bending stiffener
This is normally located at the bottom and top connections. The purpose is to provide
additional resistance to over-bending of the riser at critical points (such as the ends of the
riser, where the stiffness is increased to infinity).

20.2.4 Catenary and Top Tensioned Risers
In shallow water it has been practice to use top tensioned risers, but as design for larger water
depth is accounted the need for new design practise has increased. See Figure 20.2. The
ordinary Top Tensioned riser is very sensitive to the heave movements due to wave and
current loads this is because the rotation at the top and bottom connection is limited. The
heave movement also requires top tension equipment to compensate for the lack of tension. If
the top tension is reduced it will cause larger bending moment along the riser especially if the
riser is located an environment with strong current, and if the effective tension becomes
negative (i.e. compression) then Euler buckling will occur.