Page 197 - Pipelines and Risers

170 Chapter 1 I

Ideally conditional load factors should have been defined for pullover response of pipelines
for different slenderness (as Euler beam). This is an area of future research.

The following main assumptions in the pullover analysis have been made:

Dents and ovalisation are not accounted for in the pipe elements, Le. the pipe cross-section
is always circular during deformation.

An equivalent pipe wall density is used to obtain the correct submerged weight,
accounting for the effect of concrete, corrosion coating and buoyancy.

11.7 Case Study

11.7.1 General
It has been common industrial practice in the North Sea, to trench or cover all pipelines less
than 16” in order to protect them from fishing gear interference. To trench a pipeline is costly
and may lead to an additional requirement to cover it with backfill plus rockdumping in order
to restrain it from buckling out of the trench.
A 3-D non-linear transient Finite Element model has been developed to investigate the
structural response of pipelines subjected to pullover loads. A realistic 3-D model of an
uneven seabed is simulated by importing survey data directly into the model, see Tornes et al.
(1998).
Through a case study it will be shown how a 10” High Pressure - High Temperature flowline
was found to be able to withstand pullover loads when left exposed on an uneven seabed. See
Tomes et al. (1998) for full documentation.

11.7.2 Trawl Pull-Over For Pipelines on an Uneven Seabed
Pipelines installed in areas with uneven seabed will have a number of free spans along the
pipeline route. Furthermore, a HPA-lT pipeline laid on an uneven seabed may have undergone
global buckling prior to being exposed to trawl pullover loads. To assess the structural
response of the line under these circumstances, it is necessary to apply the pullover loads on a
3D in-place model for a given load case. In the following examples, pullover simulations have
been applied to a small diameter HP/HT flowline that has undergone global buckling prior to
being exposed to the trawl load. Intermittent rock berms have been applied to control the
thermal buckling behaviour and the model is therefore limited to the section of the flowline
between two adjacent rock berms.
The vertical and horizontal configuration of the flowline in its as-laid condition, at its
maximum pressure (370 barg) and temperature (135 deg), are shown in Figure 11.4. A large
horizontal buckle has formed across the large span at KP 4.250 and a further span has formed
at KP3.700. In this particular design case, the approach has been to use intermittent rock
dumping as a means of controlling the buckling behaviou. The extent of the model has
therefore been limited to approximately 1500 m, i.e. the distance between two rock berms.
The effective axial compressive force prior to pullover has been reduced to about 5 tonnes