Page 228 - Pipelines and Risers

Installation Design 201

@/t=36) where the twist emerges much more slowly. In a model of the pipelay process, the
initiation of the lay with plastic strain over the stinger shows that the twist occurs even more
slowly. After a few kilometers of pipelay, the twist in one joint has a constant angular velocity
as the joint leaves the stinger and descends toward the seabed. The nature of the twist
phenomenon is thus, in general, not an instability.

A last observation deals with the modeling of pipelay process by means of beam elements in a
general-purpose finite-element program. These elements represent an elastic line and
therefore they have zero section radii. As a consequence, they cannot represent the coupling
between transverse motion and twist as discussed above. Special-purpose elements have to be
used.

The examples illustrate elastic twist behavior that reduces the overbend pipe's potential
energy when subject to out-of-plane loads due to current or lateral displacements.

12.4.10 Installation Behaviour of Pipe with Residual Curvature
Pipelay vessels have gradually adapted to the technical challenges of deepwater projects by
increasing tension capacity and stinger length. The larger lay vessels have reached physical
limitations where further increase in their capacity would, in principle, be too costly for a low
oil price scenario. Increasing the utilization of the pipe strength capacity by curving the
stinger more sharply to obtain steeper departure angles is a cost-effective alternative. Since
the tension required to install the pipe will be lower, it brings the added benefit of reducing
the seabed intervention needed for freespan support. See Damsleth et al. (1999).

Today's larger S-lay vessels are fitted with total tension capacity of 300 to 500 tonnes. The
stingers are 60 to 100 m long to cope with installing pipelines in 300m to 700m water depths.
But the present 45' to 55" stinger departure angles result in about half the lay tension
remaining with the pipe on the seabed. In areas where the seabed is uneven, the high residual
tension develops both larger and more frequent freespans. In order to obtain the lowest
residual tension, the stinger must provide as steep departure angle as possible.

The stingers of most of the larger pipelay vessels have already been extended to install
increasing pipe sizes in deeper water. Extending them further would make them more
vulnerable to environmental loads and increase weather downtime. To install large diameter
pipe in very deep water (1500m to 2500m) with the present tension capacity requires stingers
with up to 90-degree departure angles. The present stinger arc lengths can be maintained
while the curvature is increased. Depending on the D/t of a given pipe size, a permanent
curvature in the overbend may develop causing eventual pipe rotation.

While the controlled curvature of the stinger permits the use of strain criteria, deeper water
installation demands stinger curvature leading to greater plastic deformation of the pipe in the
overbend. Detailed structural analysis can be used to develop project-specific strain criteria
for installation (Bai et al. 1999) that allows plastic strain in the overbend. However, it has
been demonstrated that permanent curvature of the pipe can potentially lead to unacceptable