Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)27

Establishment of Evaluation Method , and Extension ofKnowledge for Vortex Induced Vibration of Riser

SHUNICHI IKESUE*1

YOSHITO ANDO*1

HIROYUKI YAMAMOTO*2

AKIO USAMI*3

HIDEYUKI SUZUKI*4

MASANORI KYO*5

It is believed that the use and development of marine resources will be promoted in the futurein Japan and also needs of drilling risers will grow. One of the concerns for riser is Vortex InducedVibration (hereinafter referred to as VIV). The assumed usage area of a riser tube includes seas
where the marine/tidal current is strong. In this case, the Reynolds number is higher, which differsfrom the hydrodynamic force characteristics considered in conventional VIV design. Additionally ,much of conventional researches focused on cross flow directional vibration, and the effect of
inline directional vibration was not considered. This paper describes Mitsubishi Heavy Industries,
Ltd.’s (MHI) efforts in these issues.

|1. Introduction
Risers are roughly classified into the following two groups: drilling risers used for subsea
drilling for oil development or scientific surveys of the earth, and production risers used for liftingup crude oil collected in offshore oil fields to the sea surface. In Japan, the drilling riser equippedon the D/V Chikyu is well-known. Both for drilling risers and production risers, breakage of theriser causes significant economic loss and impacts on the environment. Therefore this is one of theimportant themes in major international marine engineering institutes.
Because a riser string is very long and thin, its main breaking mode is local buckling.
Therefore there is a need to design a riser so that it is always tensioned in order to prevent such
local buckling.
In recent years, VIV (Vortex Induced Vibration), which is induced by vortexes discharged by
the riser, has been attracting attention in line with the increasing use of risers under conditions
where the tidal current is strong. Because a riser is very long, even vibration of high order mode
may occur. If vortex shedding at a frequency of 1 to a several Hz causes resonance of a riser, safety
problems such as fatigue damage may arise from VIV. VIV has recently been attracting attention,and major international oil companies such as Shell (Netherlands), BP (UK), Chevron (U.S.), andPetrobras (Brazil), as well as classification societies such as DNV (Norway) and academic
institutions, have been cooperatively promoting studies. MHI has also been working on theestablishment of riser VIV evaluation techniques in a joint study with Nippon Kaiji Kyokai (NK),the University of Tokyo and the Japan Agency for Marine-Earth Science and Technology. This
document describes a summary of the study.

*1Nagasaki Research & Development Center, Technology & Innovation Headquarters*2

Ship & Ocean Business Department, Shipbuilding & Ocean Development*3

Natural Resources and Energy Department, Nippon Kaiji Kyokai*4Professor, Graduate School of Frontier Sciences, The University of Tokyo*5Group Leader, The Center for Deep Earth Exploration, Japan Agency for Marine-Earth Science and Technology

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)28|2. Efforts in establishment of riser VIV evaluation techniques
2.1

Issues for riser VIV evaluation techniques

VIV phenomena are very complicated and scientifically interesting. In many countries,
research on VIV is being studied vigorously. However, methods to accurately estimate the behaviorof VIV and allow it to be reflected in design are undeveloped. It is expected that VIV estimationand design techniques of a riser will be developed in the future.
Estimation methods of riser VIV behavior developed in the past are roughly classified into
the following three categories in terms of the handling of fluid:[1] Preparing a database of hydrodynamic force characteristics and performing structural
calculation in reference to the data.
[2] Performing 2D-CFD at each cross section in response to structural behavior and coupling itwith structural calculation.
[3] Coupling CFD with structural calculation in a three-dimensional model.
In regard to [2], Yiannis Constantinides et al.(1) examined a method that coupled
two-dimensional CFD with FEM. In regard to [3], C. Le Cunff et al.(2) examined a method that
coupled three-dimensional CFD with FEM (modal method). These methods, which consider theflow field around the riser and the structural behavior of the riser directly, are very important for anunderstanding of detailed phenomena. However, they are not suitable for design because
computation time may be very long. On the other hand, [1] is suitable for design becauseparametric study for flow rate, flow rate distribution, the diameter of a riser tube (rigidity), etc. canbe performed in a short time, although preparation of the database requires time. For example,
Nozawa et al.(3) published the original LINE-3D_VIV code based on method [1]. A database usedfor this method can be established based on results obtained from water tank test. However, theestablishment of a database has the following issues:(1) For an experiment assuming a high Reynolds number in seas where the marine/tidal current
is strong, a large model and a high-speed towing unit are needed. There are a limited
facilities where hydrodynamic force can be obtained under such conditions. In particular, awater tank test itself is impossible for Reynolds numbers in the millions.
(2) In a VIV phenomenon, not only the cross flow directional vibration, which has been
generally considered, but also inline directional vibration may have an effect onhydrodynamic force. However, there is little knowledge of hydrodynamic force inconsideration of this effect.
Therefore in this study, a hydrodynamic force database for conditions with higher Reynolds
numbers assuming seas where the marine/tidal current is strong and in consideration of effect of
inline directional vibration was established with the support of hydrodynamic force evaluation
using CFD in addition to a water bath experiment.

2.2

Conditions for obtaining a hydrodynamic force database covered in this study

This section describes the conditions covered in this study. Figure 1 shows an image of thevortex shedding around a cylindrical column. The periodical vortex shedding around a cylindricalcolumn generates inline and cross flow directional periodically fluctuating hydrodynamic force.
Because the vortex induced vibration of a riser is induced by this periodically fluctuatinghydrodynamic force, it is important to understand the relationship between the vibration of acylindrical column and periodically fluctuating hydrodynamic force.
Here, we examine the conditions for obtaining a hydrodynamic force database based on an
example of the D/V Chikyu. The riser outer diameter is approximately 1.2 m including thebuoyancymodule. When strong marine current of 0.5 m/s (deep water) to 3.0 m/s (surface water) isassumed, the Reynolds number ranges from approximately 600,000 to 3,600,000.
When a Reynolds number is within this range, the Strouhal number of the cylindrical column(frequency of vortex shedding x diameter of cylindrical column / flow rate) is approximately 0.2 to0.5, and the hydrodynamic force characteristics seem to differ from the conventionally andinternationally studied range where the Reynolds number is 300,000 or less (the Strouhal number isapproximately 0.2). In addition to cross flow directional vibration, inline directional vibration also
occurs in actual vibration and causes changes in the characteristics of the vortexes occurring aroundMitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)29

a cylindrical column, and thus the hydrodynamic force characteristics of vortex induced vibration
are affected.
In this study, high Reynolds numbers of 500,000 and 3,000,000 are selected as conditions for
obtaining a hydrodynamic force database in consideration of inline directional vibration in additionto cross flow directional vibration.

Figure 1 Image of vortexes shedding around cylindrical column (Example of CFD underconditions with higher Reynolds numbers)

2.3

Establishment of hydrodynamic force databaseIn this study, hydrodynamic force at a Reynolds number of 500,000 was obtained through awater tank test at MHI, and hydrodynamic force at a Reynolds number of 3,000,000, an experimentthat is difficult to conduct, was obtained through CFD. In the water tank test and CFD, cross flow
directional and inline directional vibration was applied to a cylindrical column element in aconstant flow field in the following manner to obtain time history data of hydrodynamic force.

(y = Ay sin(2πf y ⋅ t )x = Ax sin 2 ⋅ 2πf y ⋅ t + γ)

(1)
(2)

Where x and y are the inline directional displacement and cross flow directional
displacement of the cylindrical column element respectively, Ax and Ay are the applied amplitudeof the inline directional excitation and the cross flow directional vibrationexcitation respectively,and γ is the phase difference between inline directional and cross flow directional excitation
applied. Because the frequency of inline directional hydrodynamic force is twice as large as that of
the cross flow directional hydrodynamic force due to the alternating occurrence of vortexes,vibration was applied so that the frequency of inline directional vibration (2fy) would be twice aslarge as that of the cross flow directional vibration (fy).
Figure 2 presents an overview of the water tank test equipment created for this study. In thisequipment, a relative flow field was generated by towing a cylindrical column model. A exciter thatcould apply forced excitation in two directions, both vertical and horizontal, was developed toobtain time history of hydrodynamic force acting on the cylindrical column.

Figure 2 Overview of water water tank test equipment

For CFD, Figure 3 shows an example of a flow field under forced excitation. A CFD method
for a forced excitation in two directions to a cylindrical column element in a constant flow field
was developed to obtain time history of hydrodynamic force acting on the cylindrical column.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)30

Figure 3

CFD case example (Flow rate vector diagram)In this study, F0x and F0y, hydrodynamic force amplitude in each direction of vibration
application, and φx and φy, phase against vibration application displacement, were obtained by sine
wave fitting of time history of hydrodynamic force obtained by the experiment and CFD. Based on
the result, hydrodynamic force was divided into a component proportional to the acceleration of theforced excitation and a component proportional to the velocity of the forced excitation, and theadded mass coefficient and the attenuation coefficient were calculated as shown in Figure 4 andarranged in a database.

Figure 4

Image of hydrodynamic force wave

For reference, formulas for calculating cross flow directional added mass and attenuation
coefficients are shown below.
<Added mass coefficient>ρπ
4F0 y cos φ y(D 2 LAy 2πf y)2<Attenuation coefficient>−F0 y sin φ y(1ρDLUAy 2πf y2)(3)

Where ρ is fluid density, U is flow velocity, and D and L are the diameter and length of acylindrical column.

2.4

Knowledge about hydrodynamic force characteristics obtained in this study

In this study, a hydrodynamic force database in consideration of not only conventionallyfocused cross flow direction vibration, but also inline directional vibration, were obtained.
Examples of obtained hydrodynamic force characteristics are presented as the following. Figure 5
shows the difference of hydrodynamic force characteristics with and without inline directional
vibration. When inline directional vibration is taken into consideration, the area with high vibratory
force (red to yellow) is larger in comparison to the case without consideration of inline directional
vibration. This indicates the possibility that cross flow directional vibration is amplified by inline
directional vibration.

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)31

Figure 6 presents the effect of the phase difference of the forced excitation. The distributionof the cross flow directional hydrodynamic force characteristics changes depending on the phasedifference of the vibration application. In Figure 6, the area with high damping force (blue to lightblue) extends when phase difference of vibration application exists. This indicates the possibility
that there exists in an actual phenomenon some phase difference that is likely to amplify VIV andsome that is not.

Figure 5 Cross flow directional hydrodynamic force characteristics (effect of inline
directional vibration)

Figure 6 Cross flow directional hydrodynamic force characteristics (effect of phasedifference between inline directional vibration and cross flow directional vibration)

Figure 7 presents the effect of inline directional vibration amplitude. When the amplitude of
the inline directional vibration increases, the area with high cross flow directional damping force(blue to light blue) increases. This indicates the possibility that moderate amplitude growth of theinline directional vibration leads to a range where the cross flow directional vibration is suppressed.
Only part of the results obtained are described above. These hydrodynamic force data havebeen systematically obtained and arranged in a database.

Figure 7 Cross flow directional hydrodynamic force characteristics (effect of amplitude ofinline directional vibration)

Mitsubishi Heavy Industries Technical Review Vol. 50 No. 2 (June 2013)
32

2.5

Utilization of established hydrodynamic force database

The database established through this study has been incorporated in the VIV analysis code
“LINE-3D_VIV” developed by Suzuki et al.(3), and maintained as a VIV evaluation method thatcan deal with high Reynolds numbers in consideration of the coupling of the inline direction andthe cross flow direction. Figure 8 presents its overview. MHI is willing to further enhance riser
VIV estimation accuracy using this evaluation method and to contribute for evolution of risers
deployed in strong marine/tidal currents and improvement of the safety of risers.

Figure 8Outline of VIV evaluation of riser using LINE3D_VIV|3. Conclusion
Through this study, MHI targeted Reynolds numbers as high as 500,000 to 3,000,000
assuming deployed in strong marine/tidal currents not covered by conventional studies of risers,and obtained hydrodynamic force data in consideration of the coupling of the inline directional
vibration and the cross flow directional vibration. The hydrodynamic force data obtained in thisstudy allowed MHI to acquire new response characteristics of the effects of inline directional
vibration and extend knowledge about the VIV characteristics of risers.
In the future, MHI is willing to utilize the results of this study for the development oftechnologies for the improvement of the safety of risers and to contribute to progress in Japanesemarine science development.
This study was supported by Nippon Kaiji Kyokai through the scheme for “collaborative
study for requests from industry”, and was conducted in collaboration with Nippon Kaiji Kyokai,
the University of Tokyo, and the Japan Agency for Marine-Earth Science and Technology.

References
1. Deep sea Mineral Exploration & Technology Division Metals Mining Technology Department JOGMEC ,The outline of action of JOGMEC for development of seafloor hydrothermal deposits and theinternational condition of mineral resources Mineral Resouces Report(2011), p293
2. C. Le Cunff, F. Biolley, E. Fontaine, S. Étienne and M.L. Facchinetti., Vortex-Induced Vibrations ofRisers:Theoretical, Numerical and Experimental Investigation, Oil & Gas Science and Technology –Rev. IFP, Vol. 57 (2002), No. 1, pp. 59-69
3. Nozawa, T. et.al., Numerical prediction and suppression of VIV of deepwater riser ， Conference
proceedings, the Japan Society of Naval Architects and Ocean Engineers，No.1(2009), pp.85-88 (InJapanese)