Page 200 - Pipelines and Risers

Trawl Impact, Pullover and Hooking Loads 173

Equivalent stress a1 1- O.3sec
I-EOSTREOT,
410
390
- 310
p’
I 350
’E 330
-
,$ 310
u‘
290
pro
250
3

11.6b

Figure 11.6 Equivalent stress distribution as function of time.

Span Acceptance Criteria for Pull-Over Loads For The 10” Flowline
The effective axial force in the line will vary from load case to load case. The 10-inch
flowline are in effective compression during normal operation and pressure test and in tension
in the temporary phases. The feed-in of expansion and resulting buckle amplitude during the
trawl board pull-over will be larger when the pipe is in compression prior to being pulled
over, which affects the stresses in the pipeline.
The location of the hit-point relative to neighboring spans and buckles will also affect the
amount of feed-in into the buckled sections of the pipe during pull-over, and in turn the
flexibility of the pipe.
However, for the 10-inch flowline discussed above, it was demonstrated that span height is
the governing parameter in structural response to trawl board pull-overloads.
In order to establish the critical span height with respect to trawl board pullover, a series of
finite element analyses were performed for the 10-inch flowline. The analyses considered
trawl-board pullover loads applied to the flowline at various free-spans along the route. The
spans analyzed had different heights ranging from 0.1 to 1.2 m.
The critical span heights based on equivalent stresses and axial strains criteria were found to
be as listed in Table 11.3:

Table 11.3 Critical Span Heights for Trawl Pull-Over.

Note: Cool-down means ambient temperature combined with full design temperature. In the
shut-down case, the temperature is ambient with no internal overpressure.