Page 184 - Pipelines and Risers

Trawl Impact. Pullover and Hooking Loads 157

- fatigue strength: the required fatigue life is that no fatigue failure should occur before the
subsequent inspection in which possible dent damage can be detected and repaired. Based
on information from an American Gas Association (AGA) study Fowler et al. (1992), it
can be documented that a dent depth of 5% OD might be acceptable from the point of
view of fatigue due to cyclic internal pressure.

- bucklingkollapse: the collapse pressure will be reduced because of dents. The allowable
strain is reduced from the viewpoint of Strain-based Design Criteria.

Internal pressure can reduce the dent depth. However, the reduction of dent depth due to
internal pressure is neglected.

Strictly, it is necessary to check the local stress and strain to ensure that no leak occurs during
the impact process. Since pullover loads are much higher than impact loads, such leak check
is to be done only for pullover loads.
11.3.2 Acceptance Criteria for Pullover Response Analyses
In the pullover response analyses, ‘no leak’ means satisfaction of the strength requirements to
local buckling and fracture/plastic as discussed in Chapter 4. Especially, girth weld fracture
shall be a governing failure mode because local buckling strain is considered to be large.

According to STATOIL (1996), free-spans are generally permitted in areas where trawling
occurs, provided that the above criteria are satisfied.

11.4 Impact Response Analysis

11.4.1 General
The impact analysis is carried out in order to define the impact energy that must be absorbed
by the coating and the testing requirements for the coating.

For concrete coated pipelines, the impact energy is generally assumed protected by the
coating and no further analysis is required by STATOIL (1996).

11.4.2 Methodology for Impact Response Analysis
The analysis will be carried out following the procedure recommended in the document
STATOIL (1996). The finite element model recommended in this design guide is similar to
that proposed by Bai and Pedersen (1993).

This kind of detailed analysis is carried out because the traditional impact analysis, assuming
the impact energy will be totally absorbed by the steel and insulation coating as deformation
energies, is too conservative. Kinetic energies absorbed by the trawl board and the pipe can be