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Evaluation Study of Free Spanning Subjected to
Hydrodynamic Loads.
Mohammed Jabbar Mawat
Engineering College, University of Basra.
mohammed.mawat@gmail.com
Ahmed Sagban Khudier, Sattar J. Hashim,
Engineering College, University of Basra. Engineering College, University of Basra.
Ah_civil1978@yahoo.com sattarhas@gmail.com
Abstract:
Suspended spans generally occur in subsea pipelines as a result of the irregularity of
seabed. Additionally the suspended spans mostly result from the scouring phenomena
around the installed non-buried pipeline. So as to discuss the hydrodynamic
surrounding the pipeline and determining the significant deflections and associated
stresses of the subsea pipeline in unsupported part, therefore, it’s very necessary to
study the hydrodynamic surrounding the pipeline in detail. A two main aims have
been done in this study, first assess the stresses at free span section and the second one
was the effect of soil characteristics in contact area between pipeline and the seabed
soil. A combined model of stresses/lateral displacement has been made. An ANSIS
model has been built on the offshore pipelines as a consequence of the combined
hydrodynamic loads such as wave/current effects. The calculations have been
computed by using the finite element method for the free span to describe the
surrounding environment in more accuracy. The pipeline stresses intensity increases
with closing to free span center. This is attributed to the fact that UY and UZ have
more maximum values at these region.
Keywords: Free span, subsea pipelines, Finite element method, Hydrodynamic load.
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1. Introduction
One of the serious problems, during the operational state, for the structural shelter of
pipelines is uneven areas in the seafloor, as they enhance the development of free
spans. The unsupported parts of pipeline that are touching the seabed at his ends, may
form because of the maladjustment of seabed or artificial supports like rock, beams,
another pipeline (DNV-RP-F105, 2006), or the scouring of underlying soil,
(Yaghoobi, 2012), in construction of on-bottom (unburied) method which presents a
common construction method in offshore pipelines systems, since this method results
to reduce of construction time and associated costs (Georgiadou, 2014). Consequently,
in the on-bottom offshore pipeline, single and/or multiple free spans (L) along its
length are formed (Fig. 1). When a pipeline is in a free span, fluid flow caused by
waves or currents or both will cause vortices to be formed and shed in the wake of the
flow which can lead to fatigue damage in the pipe (Carl M. Larsen, 2002).
Under this framework, numerous researchers have developed and utilized various
numerical models for the examining of different aspects in the dynamic behavior of
free span pipelines.
In (T. Elsayed, 2012) the outhor proposed an approach based on the developing of a
nonlinear finite element model for the viewing of subsea pipelines for free spanning.
Combined stresses/lateral displacement is functioning on subsea pipelines as a result
of combined hydrodynamic loads, particularly wave/current effects that are calculated
making use of the finite element model for free spans.
(Project Consulting Services INC.,1997) the study establishs a method to evaluate and
analyze the pipeline of free spans, based on the information generating from the
research.The information that concluded from the work is used to outline preventative
and steps which are corrective for the subsea pipeline free spans.
Figure 1. span performed on seabed
2. Numerical Model
The composite of hydrodynamic loads and pipe-soil interface considers most
challenges that have difficult in the submarine pipeline model (Kristian, 2008). A
nonlinear finite element model (FEM) is applied to model the hydrodynamic forces
and the interaction between pipeline and soil of seabed in free spanning analysis using
general package of ANSYS, Inc. Release 16.1 program. The analysis model includes
Friction forces and soil stiffness representation and it contains two elements, the first
is PIPE288 model a total length of the pipeline. PIPE288 has two-nodes with six
degrees of freedom at each node (displacement in the x, y, and z directions and
rotations about the x, y, and z directions), Fig. (2-a), and the second one is
COMBIN14 that used to represent pipe-soil interaction at side spans of pipeline (the
shoulders). The element COMBIN14 has two-nodes with three degrees of freedom at
each node: displacements in the x, y, and z directions of option longitudinal spring
damper. Fig. (2-b)(ANSYS Help). Figure 3 shows geometric configuration of pipeline
in ANSYS software with global coordinate system. Where mid span length was
divided into 50 element of PIPE288 with element length equals to 0.24m, and side
span was divided into 20 element of PIPE288 for each side (10 elements through 4.5
m starting from shoulder beginning and 10 elements through 1.5m at shoulder
ending). Same arrangements have been made for shoulder model with element
COMBIN14.
a- Pipe288 element b- Combin14 element
Figure 2: elements used in the Ansys Model
Figure 3. Modeling pipeline in ANSYS software
3. Boundary Conditions
The displacements in X, Y and Z directions for the node of the element combin14 that
symbolized by "α", see Figure 4, should be fixed. The nodes are located at ends of the
pipeline, "β", treated as no displacements and rotation in all directions. The other
nodes of the pipeline are leaved to be free and symbolized by "γ ".
Figure 4. Boundary conditions in the ANSYS Model
4. Loadings
Figure 5 explains submarine pipeline with proposal loads that exerted on the free span
part. The loads can be easily sorted into two groups: (a) static loads that is resulting
from weight, buoyancy, internal pressure and steady current, (b) dynamic loads that
appears from the motion of water around the pipeline free span which is generated by
current and waves as shown in table 1. The acting of hydrodynamic loads, on the free
span, which are divided into two groups: 1) drag, lift, and inertia forces, and 2) flow
induced vortex shedding on free span, the effect of VIV in the present paper is not
taken into consideration.
Figure 5: Typical exerted loads on the free-span of a submarine
Pipeline (Kristain, 2008).
Table 1: Loads Acting on the Offshore Pipeline Free Span
α
α
α
β
γ
γ
Load Analysis Type
Self Weight Static
Buoyancy Static
Internal pressure Static
Hydrostatic Pressure Static
Current Drag Force Static
Steady Lift Force Static
Wave Drag Force Dynamic
Inertia Force Dynamic
5. Analysis Procedure
The evaluation and assessment of free spans must consider the number of
variables that can be classified into the following categories:
Pipeline materials properties at the free span.
Pipeline contents properties at the free span.
Pipeline supports and the behavior of the pipeline free span geometrically on
the bed of sea.
Environmental properties around free span.
The data of these categories are listed in table 2.
First, a static analysis is achieved that includes the calculation of the static response of
the pipeline due to the static loads which is included in Table 1. Afterward the
dynamic analysis is conducted by applying wave action in normal direction on the
free span. The ocean loads are input globally by using ocean commands which is
involve the current and/or waves effect, drag, lift and buoyancy.
The following are the input groups of the ocean-loading which are available (ANSYS
help, 2016):
Basic (required for any ocean loading)
Current (optional, for applying drift current)
Wave (optional, for applying a wave state)
Zone (optional, for applying local ocean effects)
The wave is input along with Airy wave theory (often known as linear wave theory).
In the fluid dynamics, the Airy wave theory gives a description that is certainly
linearized for the propagation of gravity waves on the surface layer of a homogeneous
fluid. The theory supposes that the layer of the fluid has a uniform mean depth,
furthermore the fluid flow is inviscid, irrotational and incompressible.
Table 2: Properties of Free Span
Pipeline Outside Diameter(m) 0.3227
Pipe Wall Thickness(m) 0.0127
·Young’s Modulus(Pa) 21 * 1010
Poisson’s Ratio 0.3
Density of Steel(Kg/m3) 7850
Internal pressure(Pa) 21 * 105
Water depth(m) 37.0
Density of Sea Water(Kg/m3) 1025
Sea Current Velocity(m/s) 0.41
Boundary conditions Fixed-Fixed
Span Length(m) 12.0
Shoulder Length(m) 6.0
Wave high(m) 9.5
Wave period(s) 8.5
CDy, CDz, CM0.5, 0.5, 2
Sea bed soil type Loose sand
6. Results and Discussion
Figs. 6a-6b show the time series of displacements in y-direction (UY) and in z-
direction (UZ) respectively only at node 27, which is located at the center of the
unsupported pipeline. It must be mentioned that the effect of different load conditions
on the pipeline’s displacements is more obvious in the case of UZcompared to UY,
where the maximum absolute value of UZ is larger than value of UY
(a)
(b)
Figure 6. Time Domain of UY (a) and UZ (b) at node 27
Figs. 7a~7b illustrate the time domain of ϬbY and ϬbZ at node 27. The effect of weather
conditions is more significant in the case of ϬbZ (Fig. 7b), in which a larger increase of
the peak values and the amplitudes of ϬbZ is observed compared to ϬbY (Fig. 7a). It
could be seen the effects clearly in Figs. 8a~8b which shows the hydrodynamic force
in y-direction(FY) and in z-direction(FZ) respectively.
(a)
(b)
Figure 7. Time Domain of Bending stresses ϬbY (a) and ϬbZ (b) at element 26
(a)
(b)
Figure 8. Time Domain of hydrodynamic forcesFY (a) and FZ (b) at element26
Fig.9 and Fig.10, show the displacement and bending stress configurations in y-
direction and z-direction along the total length of the pipeline (free span (Lf) plus part
of pipeline's shoulders (Ls)) are shown respectively.
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Figure 9. Displacement (UY) and Bending stress (ϬbY) configurations along total
length of pipeline
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Figure 10. Displacement (UZ) and Bending stress (ϬbZ) configurations along total
length of pipeline
As in Figures 9 and 10 display the pipeline stresses intensity which is increases when
closing to the free span center. This result is attributed to the fact that UY andUZ have
more maximum values at these region.
7. Conclusions
The dynamic behavior of a single free span offshore pipeline is analyzed in this
work and the effect of various factors/parameters (different design conditions, wave
and current characteristics, soil characteristics and boundary conditions at the ends of
the pipeline) on its dynamic behavior is established.
It should be mentioned that the effect of different load conditions on the
pipeline’s displacements is more obvious in the case of UZ compared to UY,
where the maximum absolute value of UZ is larger than value of UY.
The effect of weather conditions is more significant in the case of ϬbZ (stresses
in z-direction). Where a larger increase of the peak values and the amplitudes
of ϬbZ are observed compared to ϬbY.
The pipeline stresses intensity increases with closing to free span center. This
is attributed to the fact that UY and UZ are affected more maximum values at
these region.
References:
ANSYS help of Release 16.1, 2016.
Carl M. Larsen, Kamran Koushan and Elizabeth Passano, 2002. "Frequency and
Time Domain Analysis Of Vortex Induced Vibrations For Free Span Pipelines".
The 21st International Conference on Offshore Mechanics and Arctic
Engineering, Oslo, Norway.
Kristian Ruby, 2008 ''Free-span Analyses of an Offshore Pipeline''. Department of
civil engineering, Aalborg University, master project
Project Consulting Services, INC. 1997. ''Analysis And Assessment Of Unsupported
Subsea Pipeline Spans''. United States Department Of The Interiorminerals
Managment Service.
Det Norske Veritas DNV-RP-F105, 2006. "Free Spanning Piplines". Recommended
Practice.
Sofia Georgiadou, Eva Loukogeorgaki and Demos C. Angelides, 2014 ''Dynamic
Analysis of a Free Span Offshore Pipeline.'' Proceedings of the Twenty-fourth
(2014) International Ocean and Polar Engineering ConferenceBusan, Korea.
T. Elsayed, M. Fahmy and R. Samir, 2012 '' A Finite Element Model for Subsea
Pipeline Stability andFree Span Screening.'' Canadian Journal on Mechanical
Sciences & Engineering Vol. 3 No. 1.
Yaghoobi, Mehdi; Mazaheri, Said; Jabbari, Ebrahim, 2012 '' Determining Natural
Frequency of Free Spanning Offshore Pipelines by Considering the Seabed Soil
Characteristics''. Journal of the Persian Gulf (Marine Science)/Vol. 3/No. 8/25-
34.