Time domain viv analysis of a free standing hybrid riser

Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii, USA

Deift University of Technology

Ship Hydromechanics Laboratory

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TIME DOMAIN VIV ANALYSIS OF A FREE STANDING HYBRID RISER

Nicole Liu Yongming Cheng Jaap de Wilde21 Roger Burke11 Kostas F. Lambrakos

1Technip, Houston, Texas 77079, USA 2)

Maritime Research Institute Netherlands. Haagsteeg 2, Netherlands

ABSTRACT

A free standing hybrid riser (FSHR) is a proven solution for deepwater floating production systems. In this system, a

submerged buoyancy can supports and tensions a vertical riser.

The riser top is connected to a floating production system

through a flexible jumper. The FSHR has been used in West

Africa and Brazil and will be put into application in theGulf

of

Mexico. Experimental and analytical efforts are continuing to

better understand the vortex-induced vibration (VIV) response of this riser system.

This paper presents a comparison of experimental and

numerical resultsofthe VIV responseofa FSHR. The analysis

is performed with a time domain VIV code ABAVIV which

uses the finite element software package ABAQUS to calculate

the response from the VIV forcing. Unlike frequency domain

VIV codes, ABAVIV captures structural nonlinearities and the transient nature of the VIV phenomenon. Comparisons between

numerical and experimental results for buoyancy can VIV response and loading at the bottom of the riser are presented

and show generally good agreement. The relative contributions

ofbuoyancy can and riser VTV to the overall system response

are investigated. The paper will also include calculated VIV

response from frequency domain methods.

1. INTRODUCTION

A FSHR is a vertical pipe supported by a submerged buoyancy

can. The top of the riser is connected to a floating structure

through a flexible jumper. FSHRs are finding increasing

application in deepwater fields. Further understanding of the WV behavior of FSHR systems will improve their design. In

2006, Jaap de Wilde at Marin ([I]) conducted tow tank

experiments to study the VIV response ofa FSHR. A 1:68.75 scale model ofa FSHR was subjected to uniform currents of

OMAE2009-7951 O

This paper presents the results of a study to compute the VIV

behavior of the Marin FSHR using a time domain analysis

method that has been benchmarked by field measurements and model test data for top tensioned risers and SCRs ([2-7]). This is the first application of the method to a FSHR. Comparisons between numerical and experimental results, as documented in [11, are presented. In addition, results from frequency domain analysis are also presented.

Other models and analysis tools are also being used within Technip for the analysis of VIV and VIM behavior of FSHR

systems (e.g. OrcaViV and others).). In addition, work is

ongoing where complex CFD analyses tools that include fluid-structure interaction effects are used with more usual

engineering tools. These different approaches are used for

commercial projects and for internal research and development and will be the subject of future publications.

2. ANALYSIS METHODOLOGY

ABAVIV is a VIV time domain program that uses Morison's equation to calculate hydrodynamic forcing due to cross flow VIV. The formulation, described in detail in [21, accounts for

the relative motion between the riser and water. The

hydrodynamic forces from VIV are resolved into components in

line and transverse to the current andlor riser motion causing

the VIV. The VIV force, which is a component of the transverse force, is a Morison type force proportional to the square of the

in-line

velocity component, and to a time dependent

lift

coefficient which is harmonic in time [2].

ABAVÌV accounts for all geometrically nonlinear aspects of the riser modeling, and the unsteadiness in the VIV phenomenon.

Predicts

riser VIV due to

current and/or waves. accounting for the relative motion between the riser

and the flow.

Treats uniform or

sheared currents

that vary

in

direction with depth (accounts for current

directionality).

Predicts VIV for straked risers accounting for strake

VIV suppression efficiency, and damping.

Predicts riser intermittent VIV caused by supporting

vessel motions [3, 6].

The riser is modeled using variable length elements that are

refined at locations of high curvature.

3. NUMERICAL MODELING

A full scale model of the Marin FSHR, shown schematically in Figure 1, was constructed using ABAQUS. The buoyancy can properties are

Length = 34 m,

OD=5.5 m,

Dry weight = 2,833 kN, Upward thrust = 5,167 kN, and E = 30E6 psi.

The vertical riser properties are

Length = 344 m,

OD = 550 mm, Thickness = 69 mm, Dry weight = 1,047 kN Wet weight = 4.41 kN, and E = 30E6 psi.

The chain properties are

Length= 1168m,

Dry weight = 96 kN, and Wet weight = 67 kN, E = 30E6 psi.

The riser was pinned at the bottom.

WV forcing was computed using full scale currents. Table

gives both the model and full scale currents.

Table i Model and Full Scale Currents

All numerical results are presented in full scale. The simulation time was 1600 seconds.

4. RESULTS

4.1 Measurements

Figure 2 presents the measured time histories of cross flow shear loads at the bottom of the riser for three different current speeds. The time histories showtwodistinguishing periods. The longer

period corresponds to vortex shedding of the buoyancy can, while the shorter period corresponds to vortex shedding of the riser. Table 2 gives thesetwo periods for each current speed.

The periods were estimated from the time histories.

Table 2 Measured Periods at Bottom of Riser

The ratio of the two periods is different from the ratio of the

buoyancy can OD to riser OD (which is ten), suggesting that the

Strouhal numbers of the buoyancy can and riser are not the

same. However, the ratio

of any

two long periods is approximately inversely proportional to the ratio

of the

corresponding current speeds, suggesting that the Strouhal number of the buoyancy can is the same for a large range of

current speeds.

Two peak responses are shown in the spectrum of the measured

riser loads, as shown in Figure 3. The

l peak response is

related to the buoyancy can oscillation, while the 2

peak

response is related to the higher modes of riser oscillations. The magnitude of the peak is greatest for a current speed equal to

0.83 mIs. This peak occurs at a frequency close to 0.0209 Hz, which corresponds closely to the l natural frequency of the FSHR, indicating that buoyancy can WV was the dominant

cause of FSHR response. Hence, the value of Strouhal number was determined at 0.13 and was used for both buoyancy can and

riser. The time domain program ABAVIV does not currently

have the capability of applying variable Strouhal numbers. The

measurements also show that the magnitude of the 2r peak

becomes more significant as the current speed increases.

4.2 ABAVIV Results and Comparison

Modal data is not a required input for the time domain ABAVIV

program. However, to demonstrate the structural similarity of

the numerical and experimental FSHR, natural frequencies and

mode shapes of the numerical model were calculated. Table 3

lists the first eight natural frequencies.

Current Long period Short period Ratio

(m/s) (s) (s)

0.50 75.5 5.74 13.15

0.83 52.3 2.50 20.92

1.82 24.9 1.31 19.01

Model Scale (mis) Full Scale (mis)

0.06 0.50

0.10 0.83

0.16 1.33

0.20 1.66

Table 3 Comønted FSHR Natural Frequencies

Figure 4 shows the corresponding niode shapes. The ist natural period of the numerical model is 58.45 seconds, as compared to 54 seconds observed in the experiments [1].

Using ABAVIV, cross-flow shear loads at the bottom of the riser were computed and compared to the measurements, as shown in Figure 5. The computed loads show two dominant frequencies, corresponding to buoyancy can WV (first peak), and riser WV (second peak). Table 4 compares the computed and measured magnitudes and frequencies for the first peak. Similar to the trends observed in the experimental results, the calculated buoyancy can response is greatest at 0.83 mIs, and

compares well with the experimental response. Except at current 0.5 mIs, the frequency band width of the I dominant response of the simulation agrees well with the measured data.

Table 4 Riser Bottom Response duc to Buoyancy Can VIV

Table 5 shows the magnitude and frequencies of the 2 peak

response. Compared to the first peak, the 2utd

peak response is

well defined. It

occurs close to the riser vortex shedding

frequencies. However, the calculated peak frequencies are shifted systematically from the measured data, indicating that the appropriate Strouhal number of the riser differs from the

Strouhal number of the buoyancy can. Using a different Strouhal number of 0.2 for the riser will shift the 2T peak frequency and match the measurements.

Table 5 Riser Bottom Resnonse (lue to Riser VIV

As shown in Figure 6, the computed time histories of cross flow shear load at the riser bottom clearly displays both buoyancy can and riser VIV response. The computed time histories show good

qualitative comparison with the measured time histories in

Figure 2.

Buoyancy can motions were calculated and compared to the

measured data, as shown in Figure 7. The calculated motion at

current speed = 0.50 m/s is much lower than the measured response. At higher current speeds, calculated buoyancy can VIV amplitude agrees with measurements. These trends are

shown more clearly in Figure 8, which presents computed and

measured buoyancy can response A/D (A = amplitude of

buoyancy can motion, and D = buoyancy can OD) against

current speed.

Further work is necessary to understand the difference

in

buoyancy can response at 0.5 rn/sec. It is worth mentioning that the buoyancy can response indicated by the underwater video is significantly less (about 44%) compared to the response derived from the accelerometers.

The measured and computed results for buoyancy can motion

and load at the bottom of the riser indicate the following:

At low current speed, 0.5 m/s, the amplitude of the computed buoyancy can motion is small because the

buoyancy can vortex shedding frequency, 0.0118 Hz, is

much smaller than

the FSHR

i mode natural frequency, 0.0171

Hz. However, the

riser vortex

shedding frequency, 0.1176 Hz, is close to the FSHR

2 mode frequency, 0.1175. Hence, there is a

significant contribution of riser WV to the system

response.

At intermediate current speed, 0.83 mIs, the buoyancy can vortex shedding frequency, 0.0 196 Hz, is close to

the FSHR iSt mode natural frequency, 0.0171 Hz,

resulting in a large buoyancy can response. The riser vortex shedding frequency, 0.196 Hz, is between the 2' and 3rd mode of the FSHR. Hence, there is a small

Current (nils) 201 Peak Magnitude (kN x 10g) 2nd_Peak Frequency (Hz) Riser Vortex Shedding Freq. (Hz) M casured 0.50 0.83 1.82 1.45 2.76 10.2 0.162 0.278 0.581 Computed 0.50 0.83 1.82 5.14 0.69 15.49 0.115 0.162 0.396 0.11$ 0.196 0.43! Mode Period (s) Fre,uency (Hz

58.45 0.017 2 8.51 0.118 3 6.02 0.166 4 3.70 0.270 5 2.45 0.408 6 1.80 0.554 7 1.40 0.716 8 1.12 0.890 Current 15t Peak Magnitude 15t Dominant Frequency BC Vortex Shedding Freq. (nils)

(kN x 10)

(Hz) (Hz) Measured 0.50 12.0 0.010 0.016 0.83 16.7 0.017 0.023 1.82 IS.! 0.037 0.042 Computed 0.50 5.6

0.0!00.0!2

0.0118 0.83 20.! 0.013 0.020 0.0196 1.82 4.1 0.037-.0.045 0.0431

greater than the FSHR 1e mode natural frequency, resulting in lower buoyancy can response. However, the riser vortex shedding frequencies are close to the

higher mode natural frequencies of the FSHR, resulting in high mode vibrations.

lt should be mentioned that the model tests were carried out for uniform current over the full water depth of 481 ni. In reality,

however, FSHRs will be used in deeper water with sheared

current.

4.3 Shear 7 Results and Comparison

The FSHR response was also computed using Shear 7 v4.5, a frequency domain V_IV analysis program. Following are the

parameters used in the Shear7 analysis:

St = 0.13, same Strouhal number as used in the time

domain analysis,

Lift coefficient table: type 1, a conservative option, Reduced velocity bandwidth: 0.4,

Power cut off: 0.05 - 0.3, and

Damping coefficients: C1 = 0.2, C2 0.18, C 0.2.

Analysis was performed for the same current speeds as used in the ABAVIV analysis. Shear 7 results show that at each current speed a single mode of the FSHR system was excited: 0.5 rn/s

mode 2, 0.83 rn/s mode 1, 1.33 m/s _ mode 4, 1.66 ni/s -mode 5. ABAVIV excited the same -modes, but in addition

computed the non lock-in buoyancy can response. In Figure 9, riser VIV response from Shear 7 is compared to high frequency components from ABAVIV, computed by filtering out the first

mode response. The comparison shows that Shear 7 predicts

greater higher mode riser VIV response than ABAVIV. Figure

10 shows the 1st mode response computed by Shear 7 and ABAVIV. Shear 7 only computed buoyancy can response for

0.83 rn/s current speed. The magnitude of the response is

smaller than that computed by ABAVIV.

5. CONCLUSIONS AND COMMENTS

s Measurements show that buoyancy can and riser both contribute to the system VIV response. Buoyancy can

VIV response dominates at all current speeds.

However, the contribution of riser VIV response to the system increases with current speed.

ABAVIV can treat the system (buoyancy can and riser), and shows promise as a design tool. However, the predictions for both buoyancy can motion and

loading associated with buoyancy can motion are

under-predicted, and additional calibration needs to

take place.

Shear7 predicts comparable riser VIV response as

ABAVIV at high frequencies. For only one velocity (0.83 rn/s), Shear7 computes significant can motion, as it locks in on the first mode.

This study has focused on ABAVIV and Shear7, but other

analysis tools and models are also being studied and

evaluated in connection with this behavior.

6. REFERENCES

I. Jaap de Wilde, "Model Tests on the Vortex Induced

Motions of the Air Can of a Free Standing Riser

System in Current", DOT, October 2007.

L. Finn, K. Lambrakos, J. Maher, "Time Domain

Prediction of Riser VIV",4fh International Conference on Advances in Riser Technologies, 1998.

R. Grant, R. Litton (PMB Engineering) and L. Finn, J. Maher. K. Lambrakos (Technip), "Highly Compliant Rigid Risers: Field Test Benchmarking a Time

Domain VIV Algorithm", OTC 11995, May 2000. J. R. Chaplin, P. W. Bearman, et al. "Blind Predictions

of Laboratory Measurements of Vortex-Induced

Vibrations of a Tension Riser", Workshop on Vortex

Induced Vibrations, Center of Excellence for Ships and Ocean Structures, Trondheim, October 2004

J. R. Chaplin, P. W. Bearman, etc. "Blind Predictions

of Laboratory Measurements

of

Vortex-Induced

Vibrations of a Tension Riser". Journal of Fluids and

Structures, 21(1), 25-40, 2005

Y. Cheng, K. Lambrakos, "Time Domain Computation

of Riser VIV from Vessel Motions", OMAE 92432,

2006.

Y. Cheng, K. Lambrakos, "Time Domain Riser V_TV

Prediction Compared to Filed and Laboratory Test

Buoyancy can

Chain

Riser

Foundation

Figure 1 Schematic of Marin FSHR

25E-05 2.OE+05 5.3504 0.3 5+00 o r r r r Frequency (Hz)

-0.53 rn/s

-0.83rr/s

-1.S2 rn/s

Buoyancy Can Bottom

Figure 3 Spectrum of Measured Cross Flow Loads at Bottom of Riser

Chain Bottom

-05 0

05

Normalized Displacement

Figure 4 Mode Shapes

05 0.7

25E+05 2.OE+05 5.OE+04 00E+00 -o

I

r r

r

r Ui 0.2 0.3 0.50 rn/s (exp) 030 rn/s )cal) 0.83 rn/s (exp) ---0.83 rn/s (cal) 182 rn/s - ---1.82 rn/s (tal) -s. Frequency (Hz) r 0.4 05 0.6 0.7

Figure 5 Spectrum of Computed Cross Flow Loads at Bottom of Riser

tO[

_TTTr

-'-i, (c)

-MAI prin1nt

(a) 4

i

lo

-MARII Experiment lo (b) (d)

-I

5m Sm

t

f

MARIs Experiment m

Figure 7 Buoyancy Can Motion at Different Current Speeds (a) 0.5OmIs, (b) 0.83 mIs, (e) 1.33 m/s, (d) 1.66m/s

.

.

_--numercai

4

measurement

r

.

0.4 0.6 0.8 1.0 1.2

Current Velocity (mis)

Figure 8 Buoyancy Can Response versus Current Speed

-100 -300 -500 C o da -900 w -1100 -1300 -1500

-050m/s

-0.83 rn/s -1.33 rn/S -1.66 rn/s -100 --0.50 rn/S

-0.83m/s

-1.33 rn/s -1.6Gm/s 0 0.2 0.4 0.6 0.8 1 A/D STD (a) _(b)

Figure 9 Comparison of High Frequency Components of AID, (a) Shear 7 (b) ABAVIV (D = Riser OD)

1 .0 0.9 0.8 0.7 0.6 E _0.5 E X 0.4 0.3 0.2 0.1 0.0 0 0.2 0.4 0.6 0.8 i A/D STD 1 .4 1.6 1 .8

A/D STD 0 1 2 3 4 5 o 1 2 3 AID STD

-050m/s

-083 rn/s

-133rn/s

-1.66m/s

4 s -100 -100 -300 300 --500 r 500 -C o Co 9oo -900-Lii _w -1100 -1100

-050m/s

-1300 L rn/S 1300 -rn/s

-i 33

rn/s 166 -1500 -1500 (a) (b)