FLUID STRUCTURE INTERACTION IN OFFSHORE ...irf/Proceedings_IRF2016/data/papers/6258.pdf · Topic_L: Energy and Thermo-Fluid Systems -636- of engineering and scientific fields, such

Proceedings of the 5th International Conference on Integrity-Reliability-Failure, Porto/Portugal 24-28 July 2016

Editors J.F. Silva Gomes and S.A. Meguid

Publ. INEGI/FEUP (2016)

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PAPER REF: 6258

FLUID STRUCTURE INTERACTION IN OFFSHORE ENVIRONMENT

Daniel Oliveira1(*)

, Aldina Santiago2, Constança Rigueiro

3

1,2ISISE -

Department of Civil Engineering, University of Coimbra, Coimbra, Portugal

3ISISE -

School of Engineering, Polytechnic Institute of Castelo Branco (IPCB), Castelo Branco, Portugal

(*)Email: [email protected]

ABSTRACT

The increasing use of oil enlarged its exploration to offshore environmental. The deposits

found appeared very deeply, leading to the need to build stable structures that support its

extraction. Therefore, environmental conditions on offshore structures exhibit dynamic, non-

linear and unpredictable behavior, which makes the analysis of these structures one of the

most demanding tasks of structural engineering. A recent and powerful method to analyze a

fluid-structure interaction is using numerical software with the capability of Computational

Fluid Dynamics (CFD) modelling.

In this study, a one-way coupled Fluid-Structure Interaction (FSI) between hydrodynamic

loads and a jacket offshore platform structure was modelled. This problem was divided in two

parts: a structural model (jacket) and a Volume Of Fluid (VOF) based on hydrodynamics

models. To synchronize data between these two models, an interface was defined. The

structure was modelled in ABAQUS with a 3D Finite Element (FE) model with the aim to

determine the deformation caused by environmental actions, namely hydrodynamic loads

induced by waves and current. The environmental conditions were modelled in STAR-

CCM+, using a Computational Fluid Dynamics (CFD). It was used an implicit time

integration in both models and the data was synchronized explicitly, one iteration per time

step. The main objectives of this study were: i) investigate the recent capabilities of STAR-

CCM+ to model linear and non-linear waves; ii) to understand how to carry out a co-

simulation with ABAQUS, and iii) to demonstrate how it can be used in the structural design

and optimization stages.

Keywords: Fluid structure interaction, offshore platforms, hydrodynamic loads,

computational fluid dynamics, STAR-CCM+, finite elements; ABAQUS.

INTRODUCTION

The increasing use of oil, enlarged its exploration to offshore environmental. The deposits

found appeared very deep, leading to the need to build stable structures that support its

extraction. Therefore, environmental conditions on offshore structures exhibit extremely

dynamic, non-linear and unpredictable behavior. It makes the analysis of these structures one

of the most demanding tasks of structural engineering.

The Fluid-Structure Interaction (FSI) is a field of studies which has as principal objective

analyse the influence of an internal or surrounding fluid flow on one or more solid structures.

The FSI analysis are very complex due to the multidisciplinary nature and strong non-

linearity of both (fluid and structure). This type of interaction can be observed in a wide range

Topic_L: Energy and Thermo-Fluid Systems

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of engineering and scientific fields, such as wind action on buildings (structural engineering)

and the interaction between the blood flow and an artificial heart valve (medicine).

In offshore environment, the interaction between the dynamic loads (waves, current and wind)

and the platforms has been studied with the main objective to help the engineer in the design

and optimization stages. These studies have been done using experimental tests, which

demonstrate to be a good method to predict the structural response of the platforms under

environmental conditions. However, it requires a huge space for the wave tank. So, for

practical reasons it is necessary to use scale models which induce scale effects on the results;

additionally, it is necessary to simplify the proprieties of the structural material which affect

the structural response, and a limited number of measurements is possible, making the

quantification of the fluid behavior in detail very difficult.

Nowadays, these complex analysis can be also studied using numerical methods. It allows: i)

to study the system in real scale, ii) the modelling of the detailed structural response of the

structure, iii) to quantify in detail the fluid and the structural behavior at any point of the

model and iv) to reduce the cost (time and money) at the stage of the construction of the

models but also during the analyses of the results. However, this method cannot predict

unexpected physical phenomes once there are applied only the physics already known. On the

other hand this limitation can be overcome by comparing the numerical results with the

experimental ones.

The current study case corresponds to a real scale-model of a tubular steel jacket offshore

structure located at Western Gulf of Mexico (95º W) under hydrodynamic actions (waves and

current). This interaction was analyzed by the numerical method using a one-way FSI

coupling method with a Computational Fluid Dynamics (CFD) solver (STAR-CCM+) and a

Computational Solid Mechanics (CSM) solver (ABAQUS). The main objective is to

demonstrate that this coupling method is a powerful tool to predict the system (fluid-

structure) response under these conditions [1].

2. FLUID-STRUCTURE INTERACTION MODEL

Since the collapse of the Tacoma Narrows Bridge (1940) caused by the wind action, the

structural engineers start to pay more attention to the fluid-structure interaction at the design

stage. The powerful numerical solvers have been recently used to study a wide variety of FSI

problems of offshore platforms [2 - 7].

With numerical analyzes, the systems can be modelled by two different approaches:

monolithic approach and partitioned approach. The monolithic approach use a unified

algorithm (a single system equation) for the entire problem. It means that it is an accurate

method but at the same time it is required to develop a specific algorithm for each case [8, 9].

In contrast, with the partitioned approach the fluid and the structure are treated separately in

two different algorithms (CFD and CSM) where the results are shared by an interface.

Using this coupling method, the results of each solver can be shared using two different

schemes: one-way and two-way coupling (Fig. 1). The one-way coupling is used in FSI

problems where the fluid motion is slight affected by the structural response; in these cases

only the pressures calculated by CFD model are transferred to the structural model (structure

is rigid in the CFD model). Both solvers run separately and the data is transferred explicitly

(in 1 iteration at the end of the time-step). In the other hand, the two-way coupling is used

when the fluid flow is significantly affected by the structural response, introducing changes in

the pressure field in the interface. This process can be explicit or implicit (iterative process,

Proceedings of the 5th International Conference on Integrity-Reliability-Failure

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the data is shared during the time-step and a new one is only started after reach the

convergence level of the results required) [8, 9].

As the jacket platforms are design to show a rigid behavior which slight affect the fluid flow,

a one-way FSI coupling scheme was used in the current study.

Fig. 1- Solution algorithm for one-way and two-way coupling schemes

LOADS ON OFFSHORE ENVIRONMENTAL

Introduction

Due to the location and use of offshore platforms, it is extremely important to identify and

understand which type of actions should be considered on the structural analysis. There are

four different types of actions:

- permanent actions: they correspond to the ones which variations in magnitude with time

are negligible in relation to the mean value (self-weight of structures; self-weight of fixed

equipment, etc.);

- variable actions: they are generally caused by the use (ex: machine’s vibration) and

occupancy (self-weight of people and temporary equipment);

- environmental actions: they are distinct from the variable actions because they are

generated due to environmental phenomenon (such as wind, waves and current, marine

growth, ice, temperature, earthquakes, etc.);

- accidental actions: collisions, dropped objects, fires and explosions are the main

accidental actions that are required to consider in the design stage [10].

In this case only the hydrodynamics actions (waves and current) were considered on the

structural analysis of a jacket platform.

Ocean Waves

The waves are distinct in mechanical and electromagnetic waves. The ocean waves are

classified as mechanical waves once they are generated by physical disturb on the

environment. The characteristics of these waves are defined by the wave period (T), the crest

height (hc) and the trough height (ht). The wave height (H = hc + ht) and the water depth (d)

are represented in Fig. 2.

CSM Solver CSM Solver CSM Solver

1 1 (After convergence of steps

2 and 4)


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Fig. 2 - Ocean wave parameters [11]

The most common ocean waves are generated by the wind action. In this case, the wind acts

on the free surface of ocean disturbing the water equilibrium. Once it is a dynamic and

nonlinear action, it generates waves with different characteristics (wave height and period)

that make the ocean state irregular and random. In the design of offshore platforms, the single

wave method is used to simplify these characteristics - a single extreme regular wave is used

with the objective to maximize the structural response. The characteristics and propagation of

these regular waves are modelled by the wave theory that represent it (Airy wave theory,

Stokes wave theories, Cnoidal wave theory, Solitary wave theory or Stream Function theory).

For other analyses the usage of the fully developed sea state is required, which can be

described by the power spectral density obtain in the field measurements. In offshore field the

Pierson-Moskowitz and the JONSWAP spectrums are the most used [12].

In this thematic, it is also relevant the variation with time of the pressure field. The

propagation of the energy introduced by the wind (wave propagation) occurs longitudinal and

vertically by the contact of particles. These ones describe an orbital movement where the

direction and magnitude of the velocity vector are constantly changing with time (Fig. 3). It

means that the direction and magnitude of the wave load on the structure are constantly

changing too [13].

Fig. 3 - (a) e (b) Orbital movement of a particle at the free surface, c) velocity of wave particles [13]

Ocean Currents

Ocean currents can affect the wave propagation mainly in the surface particles, changing the

surface shape, the magnitude and the direction of the particle velocities and the wave period.

There are two types of currents: surface currents (caused by wind) and deep currents (caused

by differences of density). The deep currents are larger and slower than the surface currents

which present maximum speed at the surface and zero for depth higher than 100 m.

d

c


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NUMERICAL ANALYSIS

Structural model

In the current case, the offshore jacket platform designed as Merluza-1 was modelled to study

the capabilities of FSI coupling method. The structure was divided in three parts: foundations

(pile system), jacket (space-frame structure) and deck (where the equipment are installed). For

this study only the jacket part was used. This jacket is a welded tubular space frame structure

composed by A 500 steel tubular members (Fig. 4):

- Pile Sleeves (dark green)

- Corner Legs (white green)

- Inside Legs (red)

- Bracing System (yellow)

- Other structural elements (blue)

The structure has 150 m height, 60 m long and 60 m wide (at the base). The CAD 3D model

was constructed in the Autodesk INVENTOR in real scale.

Fig. 4 - Jacket structural geometry [1]

Numerical modelling of the fluid behaviour

This jacket was considered at Western Gulf of Mexico (95º W), where the mean water free

surface was 134 m high and the singular wave method was applied. Based in the data of API

2INT-MET [14], the waves were modelled with a significant wave height of 23 m and a mean

wave period of 13.6 s using the 5th

order Stokes wave theory. It was used a water speed at the


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surface of 1.50 m/s. The fluid behaviour was modelled in the commercial CFD code, STAR-

CCM+, which uses the Reynolds-average Navier-Stokes (RANS) equations to solve the

governing equations of continuity and momentum. A Volume Of Fluid (VOF) multiphase

model was choose to simulate water and air flow where the K-Epsilon turbulence model was

integrated. Both fluids were considered with constant properties and incompressible. The

water properties were defined as: density of 977.551 kg/m3 and dynamic viscosity of 8.887 x

10-4

Pa.s. For the air properties, it was used 1.18415 kg/m3 to density and 1.855.8 x 10

-5 Pa.s

to dynamic viscosity.

An implicit time integration with second-order discretization was used with a time-step of

0.06 s with a maximum of 5 iterations per time-step was imposed.

Taking into account the study of Santo and co-authors [6], a rectangular 3D domain

representing a wave tank was here defined with similar geometry: 2000 m long (y-direction),

450 m wide (x-direction) and 300 m high (z-direction), where the main objective of this

solution was to avoid the effect of reflected waves (at the boundaries) in the surrounding area

to the structure.

The waves were considered as unidirectional in y-direction. The faces of the domain were

defined as boundaries: inlet (front and top faces), outlet (back face), symmetric planes (lateral

faces) and the bottom as a wall which represented the sea bed. The structure was imported

from INVENTOR and it was fixed at 450 m from the inlet. Its surface was defined as

interface for the co-simulation. Also the wave damping option was activated in the area near

the outlet (last 1000 m in y-direction) to reduce wave oscillations and false reflections.

For the domain, a Finite Volume Method (FVM) was generated with hexahedral shaped cells

(trimmed mesh). Volume and surface controls were used to refine the mesh near the free

surface region, around the jacket and on the jacket’s surface. The final mesh solution was

generated with 8.1 million cells (Fig. 5).

Fig. 5 - CFD model FVM mesh of the numerical wave tank (STAR-CCM+): a) outside view; b) position of the

jacket; c) detailed view on the mesh of the control volume around the jacket; d) detailed view on the surface

mesh of the jacket interface [1]


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The CFD model was solved alone during 7.0 s to obtain a stabilized initial solution for the co-

simulation. After that, the time was reset.

Numerical modelling of the structural behaviour

The jacket model was imported from INVENTOR to ABAQUS with real scale, Figure 6. The

structure was considered as fixed at the base [15]. It has been meshed with Finite Element

Method (FEM) and it was generated 359.827 3D shell elements - S4R elements (linear

hexahedral elements) and 359.103 nodes (Fig. 6). The material of the jacket structure is Steel

A500 with elastic behaviour: Elastic Modulus: 205 GPa, Poisson’s ratio: 0.3, Density: 7850

kg/m3. For the structural simulation, it was choose a dynamic implicit analyse, which was

defined to run with a time-step of 0.06 s, as in the CFD model.

Fig. 6 - CSM model FEM mesh (ABAQUS) [1]

Numerical modelling of the one-way coupling

The one-way coupling was led by STAR-CCM+, what means that all the coupling parameters

where defined in this software. The same time-step of ABAQUS and STAR-CCM+ was

choose for the coupling time-step (0.06 s). Both simulations were coupled to each other

explicitly, the pressure distribution was transferred to the structural model at each co-

simulation time-step.

During this process it was used a stabilization technic that is called pressure ramping. This

technic is used in the simulations to slowly raise the loading toward the full value. In the

current case it was used during the first 0.5 s. The coupled simulation was solved for 7.20 s

(approximately half wave period).

RESULTS

As mentioned before, the main objective of this study was to understand, explore and verify

the potential of a coupled simulation of offshore FSI problems. Detailed analyse of the

structural results were out of the scope of this document.


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In the CFD model it was analysed the variation with time of the water velocity at the free

surface and in a horizontal plane at 34 m depth from the mean water level. Based on the

results showed in Fig. 7, it is possible to verify that the particles of the free surface showed

the highest speed, which was maximum at the wave crest (≈8 m/s) and minimum at the wave

trough (≈1 m/s). Also, as expected, phenomena such as diffraction and reflection of the waves

were verified during the wave impact on the structure (Fig.7.b).

Fig. 7 - Instantaneous velocity (m/s) of water particles (t = 3.6 s): a) at free surface and b) at a horizontal plane

34 m below mean water level [1]

The analysis of the results was made comparing the results from both models at the same

time-step. The pressure distribution and the wall shear stress solved at the CFD model were

compared with the Von Mises stresses distribution and the deformations of the elements

obtained at the structural model. Results at the time step 2.1 s, 4.2 s and 7.2 s, represented in

Fig. 8 and Fig. 9, show the importance of choosing this type of analyses to understand the

structural response when a dynamic load is acting. The wall shear stress distribution appeared

at the surrounding area of the free surface (with a maximum of 170 Pa at 2.1 s), where the

water speed and the turbulence are maximums, introducing the biggest deformations

registered at the top of the jacket.

From von Mises stress in Fig. 9.a, it is also possible to verify that when the structure was in

the wave crest zone (t = 2.1 s), the stresses at the top were higher (139.5 MPa) than the ones

observed when the structure was in the through zone (18.57 MPa at t = 7.2 s). This relation is

explained by the decreasing of the wave speed and of the water depth (lower pressure).

Looking for the global results of Von Mises stresses, it is verified a maximum stress of 160

MPa (approximately) in some joints at the base of the jacket.

Looking for the deformations presented in Fig. 9.b, an oscillatory movement around the

vertical axe is observed, what means that the direction of the load on the structure changed, as

mentioned in section 3.2. At the top of the structure, a maximum deformation of 18 cm in the

wave direction at t = 2.1 s and a minimum of 4 cm in the opposite direction at t=7.2 s is

observed. Also, some local deformations are detected, which can be neglected for this study

once some simplifications were made in the structural model.

a) b)


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Fig. 8 - CFD model results at the instants 2.1 s, 4.2 s, 7.2 s: a) wave elevation, b) wall shear stress (Pa) and c)

pressure (Pa) [1]


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Fig. 9 - CSM model results at the instants 2.1 s, 4.2 s and 7.2 s: a) Von Mises Stresses (Pa) and b)

deformation in y-direction (m) [1]

CONCLUSIONS

A one-way coupling was used in this paper to study a FSI problem of offshore environment.

The results of this study confirm the potential of this method when the main objective is to

analyse in detail the dynamic response of the offshore platforms under dynamic loads. It

t= 2.1 s t= 4.2 s t= 7.2 s


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shows that it can be useful to help the engineers not only at the design and optimization stages

but also to understand events recorded in already installed structures.

Analysing the global response of the structure (Von Mises stresses and deformations), it is

possible to verify that the jacket is able to withstand the hydrodynamic loads induced by

waves and current.

ACKNOWLEDGMENTS

This work is financed by FEDER funds through the Competitivity Factors Operational

Programme - COMPETE and by national funds through FCT – Foundation for Science and

Technology within the scope of the project POCI-01-0145-FEDER-007633

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