Abstract 20 Chapter 1IntroductionSubmarine pipelines are essential mainstream


Abstract The stability pipeline structures are very important for offshore engineering to transfer power energy through remote area. Thus, long pipelines or cables are one of the most popular to construct. In order to ensure that pipelines are stability on seabed, these pipelines depend on the hydrodynamic forces acting on pipeline against self-weight and line tension.  This study aims to use computational fluid dynamics (CFD) to examine the oscillatory wave flow involved hydrodynamic forces on pipeline located on fixed solid boundary. In addition, the focus is relatively on small diameter of cylinders and high Keulegan-Carpenter flows. Moreover, in case of unsteady oscillatory flow, the time dependent of turbulent or laminar are role important in boundary layer.Furthermore, the ANSYS Fluent software package will be provided for user to be familiar and studying. The numerical simulations will be compared with experimental data from the Aberdeen Oscillatory flow Tunnel (AOFT) and also develop the numerical model to investigate the oscillatory flow induced forces on cylinder located on fixed bed.            Contents                                                                                                                                                 PageAbstract                                                                                                                          11)    Introduction                                                                                                             3          1.1  ) Technical Background                                                                                  4-51.2  ) The wall proximity effect to seabed                                                             5-61.3  ) The condition of hydrodynamic forces with wave boundary layer        61.4  ) Hydrodynamic forces acting on fixed cylinders on seabed                    71.5  ) Forces on a cylinder in oscillatory flow                                                       71.5.1)   In line force in oscillatory flow                                                                        7-91.5.2)   The vertical line force in oscillatory flow                                           91.5.3)   Recommended calculations for industry (DNV-RP-F109)                        9 2)    Literature review                                                                                                    102.1  ) Validation of force coefficients on cylinder                                                            102.2  ) Numerical Simulations                                                                                  112.2.1)   Hydrodynamic forces and oscillatory flow                                        11-122.3  ) The relation of gap flow in oscillatory flow and steady current                          13-14  conditions         2.4  ) Oscillatory flow interaction in close wall proximity                                                15-16 3)    Experimental Analysis                                                                                          173.1 ) Numerical Simulation                                                                                                17 4)    Conclusion                                                                                                              185)    Appendices                                                                                                                         196)    References                                                                                                              20     Chapter 1IntroductionSubmarine pipelines are essential mainstream of fluid and power transportation due to highly demand using of human increasing. The subsea pipelines and cables are used to transport products from one location to another, however these pipelines and cables are often construction onto the seabed without protection. In order to make stability of pipelines, hydrodynamic forces are important roles and are induced to consider by waves and currents. Sarpkaya and Rajabi who are among first studies to measure lift, drag and inertia coefficients acting on cylinder placed at varied distance on the bottom of seabed in oscillatory flow. These force coefficients are involved in the function of Reynolds number (Re), Keulegan-Carpenter (KC), the thickness of boundary layer and the gap to diameter ratio. In deep water, the wave motion has a small effect to pipeline and the pipelines are strongly affected by the steady current. In shallow water, on the other hand, the wave-induced oscillatory motion has more heavily affected. The purpose of this study is to investigate the stability of pipelines in seabed by using the basic of Computational Fluid Dynamics (CFD) and familiarization with the ANSYS in order to analyze and simulate the oscillatory flow interaction around a cylinder on solid boundary. The other aim is focus on high Keulegan-Carpenter flows such relatively small diameter cylinders. In additional, the numerical simulations will be involved to compare validated against force measurements that were very recently obtained in the Aberdeen Oscillatory Flow Tunnel (AOFT) as part of an ongoing research project.          Background Theory1.1)       Technical Background            When the pipeline is resting on the seabed, the basic analysis form to consider about the stability design is shown below in the Figure 1. The sketches illustrate the pipeline under seabed with the symbols direction to make cylinder stability. The hydrodynamic force components are defined as  and  where array in the horizontal and vertical directions respectively,  is defined as the friction coefficient,  is the submerged weight of the pipe and D is the diameter of the cylinder.            The assured stability of pipeline resting on the seabed intended by purpose need to have1)    The horizontal seabed resistance for pipeline on the lateral movement to pipeline on the seabed is always greater than the horizontal hydrodynamic force on the pipeline.

2)    The submerged weight must be higher than the lift force on pipeline.The main focus on the wave boundary layer effect is to consider only lateral stability of the pipeline. Thus, the absolute stability equation, recommended by DNV-RP-F109, can be written as As Morison’s equations illustrate that the lift and drag force are proportional to the square of fluid velocity particle and the inertia force is directly proportional to fluid acceleration particle. The lift force performs vertically upwards from the cylinder on seabed, whilst the drag force acts in the same way of fluid flow. The inertia force can act in the direction of flow or against flow depending on the flow in that situation is accelerating or decelerating.Chapter 2 Literature review2.1) Validation of force coefficients on cylinderThe force coefficients such as the drag force, inertia force and lift force depend mainly on KC number and Reynolds number.         Most of experiments cases consider a gap ratio of a cylinder proportional to the bed in the range of 0 e/D. From the Figure 3 represents regular and irregular wave actions which e/D = 0.25 in order to compare the rage of the gap ratio (e/D) with other published in literature. As we can see from Figure 3 that drag force ( and lift force  decrease with an increase of KC number, however inertia force increases with an increase of KC number. In case of literature of Sarpkaya (1976) conducted a U-shaped tunnel to examine the effect of force coefficients  and  of a cylinder with regular oscillatory flow for e/D = 0.22 with the range of KC number from 1 to 34 and the range of Reynolds number from  to . In addition, Bryndum et al. (1992) presents the experimental results for force coefficients  and  of a cylinder for   e/D = 0 with KC number ranging from 3 to 160 and Reynolds number ranging from  to .   The comparison of Figure 3 can be indicated that the results of e/D = 0.25 are better agreement with those gap ratio e/D = 0.22 of Sarpkaya (1976) than those gap ratio e/D = 0 of Bryndum et al. (1992). This might be implied that for e/D = 0.25 is reasonable and reliable, however in our case will focus on very close proximity to                the bed (e/D ?0)                 2.2) Numerical Simulations2.2.1) Hydrodynamic Forces and Oscillatory flow            This part is about the hydrodynamic forces acting on cylinder near bed and also oscillatory flow induced vortices wave around cylinder . simulated the experimental set up the oscillatory flow around the cylinder with the diameter, the gapto diameter ratio, the Keulegan-Carpenter number and pipe Reynolds number are 9 cm, 0.4, 20 and respectively. The pressure around a cylinder is measured immediately with oscillatory motion in water to consider the time variation of lift force acting on the cylinder. As the Figure 4 below shows about the vorticity contours with six instants in a half period of oscillatory motion. The clockwise and anti-clockwise of vortice rotating are defined as negative (dash lines) and positve (solid lines) respectively.To begin with this period
motion, a positive vortex was evoked over the cylinder in previous period (Figure
4 a,b). The mean flow of two negative vortices in left side of pipe move toward
to cylinder (Figure 4 a,b). When the flow velocity increases, the cylinder is
generated by two vortices at the upper and lower parts (Figure 4 b-d). At the
same time, the positive vortex in gap side between cylinder and wall are grows
and generated (Figure 4 b-d). Meanwhile, the plane boundary is rolled up when a
negative vortex grows close to the surface. When the flow velocity decreases in
(Figure 4 d-f), the positive vortex splits into two part the lower part of cylinder. One part of positive vortex is rolled up,
while the other one is convected far away from cylinder.                      The chart above represents about time
variation of lift force acting on the pipe records the vortex shedding and
vortex motion phenomena around the pipe. As mentioned previously, the lift
coefficient is calculated as. There
are some differences between the simulated and measured lift coefficient as
Figure 4.1 shown above. This is because applied turbulence model might not be
able to simulate the size and magnitude of the vortices in cases with high
adverse gradient pressure (Menter, 1994). On the other hand, the results are in
reasonably good agreement with the experimental data of 2.3)
The relation of gap flow in oscillatory flow and steady current conditions In this section is to study the gap flow
velocities for different gap ratios in oscillatory flow and in steady current
conditions. In order to simulate the oscillatory flow conditions, the pipe
diameter, oscillation period flow, maximum orbital velocity, Reynolds number in
pipe and Keulegan Carpenter number are 5 cm, 1.78 s, 0.197 m/s, 9500 and 7. In
part of steady flow conditions, the pipe diameter, free stream velocity and
Reynolds number in pipe are 5 cm, 0.197 m/s and 9500 respectively. The
simulations are investigated in different gap ratios where are 0.1, 0.2, 0.3,
0.5 and 0.7 respectively.            Figure 5a represents the variation of the maximum gap flow
velocity against different gap ratios in both flows oscillatory flow and steady
current flow conditions.        To begin
with oscillatory flow conditions, the maximum gap flow velocity decreases when
the gap increases. On the other hand, in case of steady current condition the
maximum gap flow velocity slightly rises up as the gap increases also. We can
notice that the maximum gap flow velocity of oscillatory flow is higher than
steady current flows. Sumer and Fredse (2002) also indicated that the water
discharge in oscillatory flow is much larger than in steady currents. This may
be because the large pressure gradient that attribute from incoming flow.            Figure 5b
illustrates the mean gap flow velocity against the different gap ratios. This
can be clearly seen that the oscillatory flow condition is also higher than
steady current flow. In addition, Figure 5b shows that the oscillatory flow in
both mean and maximum gap flow velocities exceed the amplitude of free stream
velocity. So, this can be pointed out that in case of small KC number the
inertia is dominated while the KC number become larger, the drag force is
dominated because the flow pattern is similar to the steady flow conditions.
Thus, we can mention that if KC number increases, the small range of the gap
ratios the  
decreases.                     2.4) Oscillatory flow interaction in close wall
proximity             The purpose of
this section is to examine the effect of wall proximity on flow regimes around
a cylinder. In addition,   who conclude the visualization study of the
flow around a cylinder on fixed bed for the half of the wave period in Figure 6
. In
each sequence is involved variety of KC and Re for the flow regime around the
cylinder where the flow can be separated to five different regimes as shown on
the table 2 below.

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Regime
 

KC range

Reynold number
(Re)

1

< 2 2 2 3 4 4 10 5 15      Note that when KC is very small, the flow separation behind the cylinder might not occur.1)    In the first regime, the flow separations don't occur but the flow particles behind the cylinder are similar to elliptical movement.2)    In the second regime, the flow is first time occuring with small votices under downstream of the cylinder3)    In the third regime, the flow is stronger than the the second regime and  two small votices occur behind a cylinder while other large two occur downstream.4)    In the fourth regime, there are two different flow behaviours are illustrated. The first flow separation is symmetric vortices. The second flow represents that the larger vortex shedding happening at the downstream of a cylinder.5)    The last regime, the votex shedding is no more occuring but there is the large vortex form at the upper area downstream of a cylinder.                                   Chapter 33) Experimental analysis 3.1) Numerical Simulation            The chart above represents the simulation about the vertex maximum of X velocity against flow time as in this example is approximately 3,000 time-step running, which is around 30 flow cycles. In addition, the oscillatory flow and orbital amplitude of this model simulation set up are 5 second and 0.1 meter respectively. As we can see that after 1200 time-steps, the flow becomes repeatable. Thus, this can be implied that it has converged and can stop the simulation.  The chart showing above is using the data from CFD simulation and applies to Matlab code on appendices (1).               4.Conclusion            In this report is to analyze the oscillatory flow interaction by the Morison's equations are relative to the pipeline by hydrodynamic forces including with lift, drag and inertia forces. These equations represents that the drag and lift forces are proportional to x-direction and y-direction respectively, and the inertia force is directly relative to fluid particle acceleration. In order to ensure that pipeline is stable, the submerged weight must be higher than the lift force on pipeline. Meanwhile, the frictional force needs to exceed the drag and inertia forces combining. In addition, the hydrodynamic coefficients in oscillatory flow depend on Reynolds number (Re), Keulegan-Carpenter number (KC) and the position of the pipe. Furthermore, Most of experiments cases consider a gap ratio of a cylinder proportional to the bed in the range of 0 e/D. In addition, if KC number increases, the small range of the gap ration will decrease.                   This report will continue in Computational Fluid Dynamics (CFD) by using Aberdeen Oscillatory Flow Tunnel (AOFT) experimental data to analyze and ANSYS Fluent software package will be used more in the spring term also. This report is not affected any risk assessment because I use computer from University of Aberdeen and VMware Horizon to get access at home.           

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