| dc.description.abstract | Subsea pipelines are likely to be exposed to fishing activity, which may results in trawl
gear interacting with a pipeline. The interaction is classified into impact, pull-over and a
special case called hooking. The trawl load is considered to be an important design load in
pipeline design. In the recent developments of the subsea pipelines, the pipe-in-pipe (PIP)
system is a solution for high-pressure/high-temperature requirements. Previous research and
findings mainly focus on trawl gear interaction with a single pipe wall pipeline. There is
limited research on trawl gear-PIP interaction. The main objective of this thesis work is to
simulate and investigate the impact and pull-over responses of a pipe-in-pipe system during
the interference with an otter trawl board.
The numerical study were carried out based on nonlinear finite element (FE) method by
means of the computer software SIMLA. Based on the previous models for the single pipe
wall pipe, modifications on the models were made to account for PIP. An advanced impact
model was enabled to study the impact response of PIP. Later, a detailed clump weight pullover
model was modified and studied by using the new contact element (cont153) in SIMLA.
Finally, a detailed trawl board pull-over model (with simplified geometry) was modified with
the cont153 element to study the PIP response under pull-over loads. More details are
described as follows.
Firstly, a study was carried out to investigate the impact response of a single pipe wall pipe
and a PIP system. The impact model was established according to the Recommended
Practice DNV-RP-F111 (RP) by using an advanced impact calculation method. Various
pipeline parameters like pipe wall thickness, content density, concrete coating, specified
minimum yield strength (SMYS), different trawl gears, and position of centralisers for PIP
were considered. The purpose is to check how these parameters influence the impact
response.
For pull-over analysis, to gain more understanding of the cont153 element, the clump weight
model from Maalø’s work was tested. As a result of this study, a contact stiffness with good
contact behaviour was obtained and then used in further study. It is also found that the
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friction coefficient has important influence on the results. The new contact element was then
used in a trawl board model with the stiffness defined in the clump weight case. The warp
line tension results are compared with previous model test results. The comparisons show
that for lower span heights (0.5 m and 1.0 m), good agreements were achieved, but noticeable
deviations were found for higher span heights (5.0 m).
Finally, the detailed trawl board model was used to investigate the pull-over responses
(displacement, bending moment, strain, etc.) of a PIP at low span height (up to 1.0 m).
The pull-over responses from the detailed model were compared with those from RP load.
The main finding is that the responses increases as the span height increases, and the
responses from the detailed model are in general lower than the RP case. This finding
indicates the possibility to further optimise PIP design in the view point of trawl board
pull-over loads at low span heights. | nb_NO |
