Construction of unburied pipeline is the most common method in
offshore pipeline system. Unburied pipeline should be designed appropriately
due to the bathymetry condition. And it is inevitable founding the existence of
free span. Free spanning in offshore pipelines mainly occurs as a consequence
of uneven seabed and local scouring due to flow turbulence. An illustration of
free span is showed by the figure below:
Freespan Definition
Source : http://www.neo.no/img/freespan_fig.gif
According to Fredso and Sumer (1997), resonance is the main
problem for offshore pipelines laid on the free spanning. Resonance happens
when the environment’s frequency becomes equal to the pipe natural frequency.
Resonance may lead to develop more fatigue on pipelines. In order to reduce the
risk caused by free spanning, a maximum allowable length of free span should be
determined. Span length is described with the following image:
Span Length
Source : https://nonerieska.files.wordpress.com/2013/01/post8-7.jpg
An allowable length of free span can be calculated by the
following formula (DNV 1998 & ABS 2001) :
in which E = modulus of elasticity; I = bending moment of inertia
pipeline; C = coefficient of seabed condition; Vr = reduced velocity (Fredso
and Sumer, 1997).
Vr defined as:
where U = streamwise flow velocity; D = outer diameter of pipe; me
= effective mass (including structural mass, mass of content and added mass);
fn = natural frequency of the pipe free span.
Natural frequency of free span pipe defined as:
In practice, the use of these formula for estimation of maximum
free span length is not very applicable since there is difficulties in
determining the exact seabed conditions.Therefore, different approaches usually
adopted. One of the method is modal analysis.
Modal Analysis
Natural frequency of pipelines can be obtained using the
Euler-Bernoulli beam equation which is defined as (Xu et al, 1999 and Bai,
2000):
with y = in-line displacement of pipe; x = position along the pipe
span; t = time; C = total damping ratio; T = axial force of pipe (positive
under tension); and F(t,u,y) = total external forces.
External forces and damping ratio only influence the resonance
amplitude, so it can be ignored and the pipe free vibration equation is
expressed in the following equation:
There are several codes that can be used as reference containing
free spanning on offshore pipeline, like DnV RP F105 (Pipeline Free Spanning)
and API RP 11 11, 1999.
Bakhtiary, Abbas Yeganeh et al. Analysis of Offshore Pipeline
Allowable Free Span Length. Iran University of Science and
Technology. Iran. 2007.
Dega Damara Aditramulyadi
Student ID : 15512046
Course : KL4220 Subsea Pipeline
Lecturer : Prof. Ir. Ricky Lukman Tawekal, MSE, Ph. D.
Eko Charnius Ilman, ST, MT
Ocean Engineering Program, Institut Teknologi Bandung
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