Nowadays, offshore pipelines have a significant
role in development of oil and gas industry in different parts of the world.
This crucial industry is laid on seabed by various methods either embedded in a
trench (buried method) or laid on uneven seabed (unburied method). Construction
of unburied pipeline is the most common method for its rapid and economic
performance. In this method, however, the pipelines are subjected to various
lengths of free spanning throughout the route during its life time, which may
threaten the pipelines safety. Free spanning in offshore pipelines mainly
occurs as a consequence of uneven seabed and local scouring due to flow
turbulence and instability; hence, with no doubt, free spanning occurrences for
unburied pipelines are completely inevitable.
Fredsoe and Sumer (1997) assessed the role of
free spans in unburied offshore pipelines. They acknowledged the previous
studies and mentioned that resonance is the main problem for offshore pipelines
laid on the free spanning. Pipelines resonance happens when the external load
frequency as a result of vortex shedding becomes equal to the pipe Natural
Frequency. This phenomenon may burst the pipe coating and may lead to develop
more fatigue on the pipelines. Different design guidelines, e.g. DNV (1998) and
ABS (2001), have accepted a less stringent approach and recommend the free
spanning to be reduced to the allowable length to avoid fatigue damage. These
guidelines proposed a simple formulation to calculate the first Natural
Frequency based on the pipelines specifications and seabed conditions; however,
all of the guidelines encourages using modal analysis at the final phase of
design.
Choi (2000) studied the effect of axial forces
on free spanning of offshore pipelines. The results indicated that the axial
force has a significant influence on the first Natural Frequency of the pipe.
In this research, the different seabed condition has been broken down into
three main types and the general beam equation for the boundary conditions was
analytically solved. He also compared his result with Lloyd’s approximate
formula, which estimates the first Natural Frequency of the beam considering
axial load effect. Xu et al. (1999) applied the modal analysis to incorporate
the real seabed condition to assess pipelines fatigue and Natural Frequency
(NF). Later, Bai (2001) approved Xu et al. (1999) approach and emphasis on
applying the modal analysis to determine the allowable length of free span for
offshore pipelines.
In practice, a considerable amount of works
have been applied to determine the allowable free span length, however, there
is still lack of knowledge in assessing the role of all effective parameters in
determination of allowable free span length. The objective of this paper is two
folds: (i) to assess the role of main effective parameters on Natural
Frequency; and (ii) to present a simple formula for allowable free span length
with accounting for the seabed condition. To do so, first the approaches of DNV
(1998) and ABS guidelines are discussed and then the modal analysis is outlined
to have a useful tool to assess the role of all involved parameters. Finally, a
case study on the Qeshem pipelines is performed to evaluate the free span
allowable length.
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