E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-1506

EVALUATION OF HYDRODYNAMIC LOADS ON OFFSHORE TRUSS STRUCTURE
USING LARGE EDDY SIMULATION METHODAKIHIKO NAKAYAMA1, ZAFARULLAH NIZAMANI2, YUSUKE UCHIYAMA3, CHENG YEE NG4, MONTASIROSMAN5(1)

Universiti Tunku Abdul Rahman, Kampar, Malaysia
akihiko@utar.edu.my(2)

Universiti Tunku Abdul Rahman, Kampar, Malaysia
zafarullah@utar.edu.my(3)

Kobe University, Kobe, Japan,uchiyama@habor.kobe-u.ac.jp(4)

Universiti Teknologi Petronas, Seri Iskandar, Malaysia,(5)

Universiti Teknologi Petronas, Seri Iskandar, Malaysia,

chengyee.ng@utp.edu.mymontasir.ahmedali@utp.edu.my

ABSTRACTA Large-eddy Simulation (LES) has been conducted of the flow around typical oil-production jacket platform in a sea ofrelatively shallow water depth. The LES solves for a three-dimensional flow with moving free surface using appropriate
sub-grid turbulence and wall models. The structure consists of various slender tubular members in truss configuration. Itis assumed rigid and the pressure distribution obtained by the LES is integrated over individual members and over theentire structure to evaluate the loads. The flow approaching the jacket platform is assumed known but is possible to beobtained from the larger-scale simulation reflecting the details of the bathymetry of the surrounding sea bed and themetocean conditions. Although the results need to be examined in detail and cases with different current speed, direction
and wave heights but the present results are reasonable and promising first step for further calculation..

Keywords: Offshore jacket platform, LES, hydrodynamic loads

1INTRODUCTION
The hydrodynamic loads on realistic offshore structures have been traditionally computed by semiempirical formula such as Morrison formula (Chakrabarti, 1987). Recent development of numerical simulation
methods of free surface flows is making realistic simulation of the flow and the loads on solid or elasticstructures possible. The Large-Eddy Simulation (LES) methods combined with accurate representation of thefree surface movement are promising technique to apply to obtain hydrodynamic loads on offshore structuresor realistic scales. Difficulties in applying these numerical methods are due to limited computer capacity torepresent turbulent flows with wide range of scales of fluctuations past structures of complex geometry. Auniform flow past a truss structure of relatively simple configuration has been computed by a large eddy
simulation method (Nakayama et al., 2010). The computation indicated that in order to represent the overall
flow field disturbed by the structure, the computational region must be at least a few times larger than theoverall dimension of the structure, while to represent the turbulent motion with significant energy that
contributes to the pressure and the shear forces on the structure, the smallest members need to be resolved
by at least several meshes. These opposite requirement indicate that the number of grids needed in thecalculation becomes large. However, many easily accessible computers nowadays allow tens of millions ofgrid points and simulation past complex truss structures consisted of many slender members is becoming arealistic alternative.
In the present work, a large eddy simulation method that is applicable to turbulent flows with free surface(Nakayama, 2012) is used to simulate the flow past a jacket platform that is deployed in shallow sea. The flow
approaching the platform is obtained by letting the bottom boundary layer to develop to the full depth of thesea and used as the initial condition. The details of the flow and the loads on the whole structure andrepresentative members are obtained from the simulation.

2LARGE EDDY SIMULATION OF FLOW PAST A TRUSS STRUCTURE
In the simulation of natural streams and those in coastal areas, we observe that the flow regions are veryirregular in shape and change in time. Second, the scales of the flow, both spatial and time-wise are very
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E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

large and the spacing of grid points and the time step that can be accommodated by a readily available
computer system are nowhere enough to resolve dissipation scales. Yet with a reasonable number of grid
points like a few million points, which can divide the total flow region into cubes of orders-of-magnitude smallerin length, most of the large-scale flow and a good fraction of the turbulent fluctuations can be resolved fromwhich not only the mean flow but turbulent stresses can be evaluated.
A LES method developed by one of the present authors can be used for this purpose. It is a finite
difference code constructed on a rectangular grid system. The turbulence model for the sub-grid scales used
the Smagorinsky model with a wall model based on smooth or rough surface wall similarity. The free surface
is represented by the depth function which is solved by a method similar to the VOF method. Detailed
description of the method is given in Nakayama (2012). The main feature is that the wall turbulence thatcannot be resolved by the grid is modeled by the standard wall model for smooth and rough boundaries. Itallows reasonable simulation of flows of very large Reynolds numbers.

3SIMULATION OF FLOW PAST JACKET PLATFORM

We consider a model jacket platform that is very close to the real one deployed in shallow sea area off
Malaysian coast. Figure 1 shows a sketch of the platform and the flow past it. The jacket consists of fourcylindrical legs of diameter 1.5m and about 38m below the sea surface braced with horizontal and diagonal
members. Since the structure above the water does not contribute to the hydrodynamic characteristics it isabbreviated.

Figure 1. Model Jacket platform in shallow sea.
For the present simulation of the flow, the ocean current is assumed to be a fully developed flow in thedirection perpendicular to the plane of the truss structure as shown in this figure. The average velocity of thecurrent is taken to be 0.7m/s which is a typical current used for design purposes of offshore structures in thisregion. The flow is computed in a rectangular domain including the structure from 50m upstream to 40m
downstream of the structure and y=-25m to 25m, where x is the coordinate in the direction of the current withthe origin at the center of the jacket and y is the horizontal coordinate perpendicular to the direction of thecurrent. The grid of 155 x 125 x 138 is used and is shown in Figure 2. The grid spacing is taken so that thecross section of the thinnest member is resolved at least by 2 x 2 grids.

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E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

Figure 2. Computational grid. The spacing in the vertical direction is constant 0.3m, the smallest horizontal
spacing is 0.25m and the largest spacing is 2m.

4

SIMULATION RESULTS AND DISCUSSION

An overall view of the simulated flow at an instant is shown in Figure 3. It shows the free surface
elevation, the flow velocity vectors on it and in a vertical cross section along the current at the center of thejacket structure. The color of the solid surfaces indicates the pressure and the color of the velocity vector in
the cross section indicates the velocity magnitude.

Figure 3. Example of simulated flow when a uniform current approaches. The color on the structure surface
indicates the pressure and the color of the arrows indicate the velocity magnitude.

The time variation of the hydrodynamic force in the direction of the current on the entire structure isshown in Figure 4. The fluctuation is large and about 1/3 to 1/2 of its magnitude. Different time scales of thetime variations are seen. First a small time scale of less than 100th of a second is a small scale turbulent
fluctuations. The fluctuation period of about 0.05 sec appears seen from t=3.5sec. It is the fluctuation causedby the small cylindrical members which has a diameter of 0.5m. The slower undulation of period al large as 1
sec is also seen. This can be from the large scale turbulence of the boundary layer with the depth of 38m.

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E-proceedings of the 38th IAHR World Congress
September 1-6, 2019, Panama City, Panama

Figure 4. Hydrodynamic force in the horizontal direction of the current.
Figure 5 further shows the instantaneous velocity profiles along selected vertical lines. These arebasically the velocity profiles in a turbulent boundary layer and those at places downstream of the structure
are the wake flow disturbed by the structure. It is seen that even at x=0 and 10m which are within thestructure and immediately downstream, respectively the disturbances are at most of the same magnitude as
the turbulent fluctuation.

Figure 4. Velocity distribution at the center plane y=0 at three w the figure and within figure/illustration width.

5CONCLUSIONS

The flow past a model of complex offshore jacket structure has been simulated using LES technique.
The results are preliminary but appear to represent the complex flow with reasonable accuracy. The mainfeature is that the fluctuation of the hydrodynamic forces is large and the fluctuations are consisted of a few
different time scales. They are considered to be due to different scales of the structure. The small scales are
the thickness of the small members and the large scale is the overall size of the structure and the depth of thesea. The quantitative evaluation still needs to be done and comparison with existing methods of analysis
should be made.
ACKNOWLEDGEMENT
This work was conducted with a support of Fundamental Research Grant Scheme (FRGS) of Malaysian
Ministry of Higher Education.
REFERENCES
Chakrabarti, S.K. (1987). Hydrodynamics of Offshore Structures, WIT Press, Southampton, UK.

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