learn more at · structural damping test whilst including any lessons learnt from the pre-scale...

Drilling Riser Structural Damping Test Peter Padelopoulos*, Michael Ritchie*, Mike Tognarelli**, Jim Chitwood***

*2H Offshore Inc, **BP, *** DeepStar® Houston, TX, USA

ABSTRACT Existing drilling riser design and vortex induced vibration (VIV) analysis methodology rely on critical assumptions such as damping coefficient to represent the complex dynamic behavior. Comparing field measurements with analytical software results show a fatigue damage bias and hence over conservatism in riser design. One possible explanation for this is the structural damping inherent in drilling riser systems with choke and kill line and buoyancy. Full scale testing in air was initiated by DeepStar® to quantify and compare structural damping of bare and buoyant drilling riser joints. The objective was to develop guidelines on structural damping value to be utilized in drilling riser design, particularly VIV. This paper presents the results of full scale testing performed on a number of drilling riser pipe configurations to determine the effect that length, mode, frequency, choke and kill lines and buoyancy modules have on structural damping. Results from the full scale testing indicated that there was more structural damping than first expected. KEY WORDS: DeepStar, Drilling riser, riser joints, damping, buoyant, testing, frequency, mode. INTRODUCTION This paper documents the results of drilling riser structural damping tests funded by the DeepStar® Joint Industry Project. In an effort to more effectively understand and manage vortex-induced vibration (VIV) fatigue integrity of its drilling risers, BP has instrumented several of them on a number of mobile offshore drilling units (MODUs) and offshore production platforms worldwide. As part of this work, the field measured VIV has been compared to calculations using the VIV software package SHEAR7. The comparisons have shown that SHEAR7 is overly conservative when typical design assumptions are used. On average, the fatigue damage may be over estimated by a factor of 30 [1]. One of the critical input assumptions is the structural damping. This is typically assumed to be 0.3% for plain pipe. However, the effect of the interaction between the buoyancy and auxiliary lines may increase the

damping. Higher damping will help reduce the over conservatism in the SHEAR7 calculations. In an effort to determine the structural damping inherent in drilling riser systems with choke and kill line and buoyancy, full scale drilling riser damping tests were conducted at ExPert riser solutions facility in Fouchon, Louisiana in November 2010 and May 2011. The objectives of the structural damping tests were: Quantify and compare the structural damping of buoyant and bare

pipe drilling riser joints; Utilize test results to develop guidance on appropriate structural

damping values for use in drilling riser design and analysis. Bare joints are riser joints which have neither choke and kill lines nor buoyancy modules installed. Buoyant joints have both choke and kill lines and buoyancy modules installed. PRE-SCALE TEST Objective Prior to performing the full scale structural damping test a 1/16 scale test was conducted to verify the proposed test set-up exhibits low damping (< 0.3%) and validate the analytical methodology. In addition, the pre-scale test function tested the equipment including the hydraulic shaker and instrumentation that would be utilized for the full scale structural damping test whilst including any lessons learnt from the pre-scale test that would be beneficial for the full scale structural damping test. Background The test set up was comprised of a 12 foot (3.7 m) in length of ½ inch (1.3 cm) nominal diameter schedule 40 line pipe (i.e. 0.84 inch OD (3.12 cm), 0.109 inch (0.28 cm) wall thickness). Finite element analysis (FEA) was conducted in Flexcom 3D to determine the pre-scale test natural frequency and the maximum allowable pipe displacement and acceleration based on a target stress amplitude of 50 MPa (7.25 ksi). The results of which are given in, Table 1 and Table 2.

Learn more at www.2hoffshore.com

Table 1. Pipe Data for Pre-Scaled Tests

Pipe Mass Mass Reference Diameter Length Second Natural

Frequency 12 ft, 0.84 inch OD x 0.109 inch thickness

1.3 kg/m 4.6 kg 0.0213 m 3.66 m 9.94 Hz

Table 2. Maximum Excitation Amplitude for Pre-Scale Tests

Pipe Pipe Amplitude Maximum

Acceleration Maximum Curvature

Maximum Stress

12 ft, 0.84 inch OD x 0.109 inch thickness

0.0095 m 3.78 g 5.31 /m 50 MPa

Pre-Scale Test Set-Up The pre-scale test configuration is shown in Figs. 1~2. It is comprised of a ½ inch nominal diameter, schedule 40 pipe supported at five locations in the horizontal position via a fabricated test frame and twelve inch length wire cables. At one end of the pipe, a spring connects the pipe to the piston of a hydraulic oscillator, which is utilized to excite riser motion, Fig. 3. A spring of equal stiffness on the opposite side of the riser circumference is fixed to the test frame. The springs used to transmit force from the oscillator piston and the end of the horizontal pipe in the pre-analysis have stiffness values of 100 N/m (0.57 lb/in). The oscillator moves the piston at the appropriate frequency in order to excite the pipe at its second natural period, resulting in the mode shape shown in Fig. 4. The first mode of the system is actually a pendulum type mode with no pipe bending. After reaching steady state, the oscillator is turned off and data from the accelerometers located along the pipe measure the decay in response which is used to calculate the structural damping of the pipe.

Fig. 1 – Pre-Scale Test Set-up Configuration

Fig. 2 – Pre-Scale Test Set-up

Fig. 3 – Hydraulic Shaker and Spring Interface with Test Pipe

Fig. 4 – Mode 2 Shape Three high resolution, tri-axial accelerometers are utilized to measure the excited and the free vibrating test pipe response per the specifications in Table 3. The tri-axial accelerometers are attached at defined locations via curved magnets as shown in Fig. 5. Note that the sensors were initially mounted on the top of the pipe as shown in Fig. 3. However, this generated a non-concentric load that disrupted the system natural frequency. Hence, the sensors were subsequently mounted on the pipe ends. The weight of the sensor magnets changed the pipe natural frequency and the excitation frequency was varied until it matched the free decay frequency of the pipe and instrumentation. A general purpose force sensor is included to measure the force input of the hydraulic shaker. The sensor signal is transmitted to a terminal equipped with processing software via a data acquisition system with a maximum sampling frequency of 2 kHz with no analogue filter.


Table 3. Tri-axial Accelerometer Specifications

Parameter Value Sensor Direction X, Y, Z Range ± 5.0 g Measure Response 0.5 to 10.0 Hz Sampling Frequency 1.0 kHz Resolution ± 0.001 g rms Accuracy 0.01 g (after calibration) Alignment Error 0.3 deg (between axis)

Fig. 5 – Tri-axial Accelerometer Mounted on End of Test Pipe A number of tests were conducted to evaluate the test sensitivity to various assumptions including the effect of the displacement controller on the hydraulic cylinder input and the spring interface. The following tests were conducted: Hydraulic excitation to steady state followed by piston stop held

with hydraulic controller; Hydraulic excitation to steady state followed by piston stop by

hydraulic system shut-off; Manual excitation at free end with springs removed; Manual excitation at mid-span with springs removed. Pre-Scale Test Results The calculated test natural frequency was 9.94 Hz as given in Table 1. This is based on a plain pipe without instrumentation installed. However, as discussed above, during the pre-scale test the accelerometers were mounted at the pipe ends. Following excitation at 9.94 Hz and subsequent free decay of the test set-up the decay frequency modulated to a frequency of 9.3 Hz which was attributed to the accelerometer and magnet mass at the pipe ends. An example of the response spectra of a pre-scaled test during free decay is shown in Fig. 6. It shows that the decay frequency is close to 9.3 Hz.

Fig. 6 – Measured Response Frequency of 9.3 Hz during Test Free Decay The key findings from the pre-scaled test are summarized as follows: Following shut down of the hydraulic cylinder the damping

determined from the pipe measurements was initially 0.15% and reduced to 0.06 to 0.07% as the vibration amplitude diminished, this is shown in Figs. 7~8;

The measured damping for all four tests was within the target test damping value of 0.3% and hence confirmed the suitability of the test set-up;

The hydraulic shaker and instrumentation functionality was confirmed in the pre-scale test and hence providing confidence for full scale testing.

The presence of the magnet mounted accelerometers had an effect on the test natural frequency due to their high mass relative to the pipe mass. However, the instrumentation was not expected to influence the full scale test.

Fig. 7 – Pre-Scale Test Pipe Measured Acceleration Response at Excitation End


Fig. 8 – Acceleration Decay Curve with 0.15% Damping Curve Fit FULL SCALE TEST Objectives Full scale structural damping tests were conducted utilizing Cameron CIW RD 21 inch, ⅝ inch wall thickness drilling riser joints to quantify and compare the structural damping of buoyant and bare pipe drilling riser joints. The objectives of the structural damping tests were: Quantify and compare the structural damping of buoyant and bare

pipe drilling riser joints; Utilize test results to develop guidance on appropriate structural

damping values for use in drilling riser design and analysis. The full scale structural damping tests were conducted at the ExPert riser solutions facility, Fouchon, Louisiana in November 2010 and May 2011. Background Four (4) Cameron CIW RD 21 inch, 50 ft length drilling riser joints, were utilized for the full scale test program details of which are given in Table 4. Choke and kill line data for the 21 inch drilling riser joints is given in Table 5. The four choke and kill lines are equally spaced around the circumference of the riser pipe and are offset approximately 45° from the lifting pad-eyes as shown in Fig. 9. Table 4. Cameron CIW RD 21 inch Riser Pipe Data

Riser Length

Type of Joint OD WT

Pipe Material

Weight in air

50 ft (15.24 m)

Bare Pipe 21.0 in

0.625 in

X-80 Steel

9,500 lbs Slick 14,600 lbs Buoyant 21,900 lbs

Table 5. Cameron CIW RD 21 inch Riser Choke and Kill Line Data

Parameter Value Number of Lines 4

OD 4.00 in WT 0.75 in

Distance from Center Line to Center Line of Riser Pipe 17.56 in

Fig. 9 – Riser Joints at ExPert Yard Prior to conducting the full scale test program, FEA was conducted to determine the expected full scale test natural frequency and the maximum allowable pipe displacement and acceleration. This was based on a not to exceed target stress amplitude of 50 MPa (7.25 ksi) which was selected to avoid excessive fatigue damage accumulation in the drilling riser joints. The full scale test FEA results are summarized in Table 6. Bare joints are riser joints which have neither choke and kill lines nor buoyancy modules installed. Buoyant joints have both choke and kill lines and buoyancy modules installed. Pipe displacements are extracted for the target stress amplitude of 50 MPa (7.25 ksi). Table 6. Full-scale Pre-test Analysis Results Summary

Pipe Configuration Mode #

Natural Frequency Pipe Displacement

100 ft Bare 2 3.273 Hz 1.75” (4.45 cm) 150 ft Bare 2 1.671 Hz 4.09” (10.39 cm) 200 ft Bare 2 0.967 Hz 7.44” (18.89 cm) 100 ft Bare 3 9.377 Hz 0.66” (1.68 cm) 150 ft Bare 3 4.262 Hz 1.48” (3.75 cm) 200 ft Bare 3 2.450 Hz 2.71” (6.88 cm)

100 ft Buoyant 2 3.302 Hz 1.78” (4.51 cm) 150 ft Buoyant 2 1.483 Hz 3.97” (10.08 cm) 200 ft Buoyant 2 0.870 Hz 7.25” (18.42 cm) 100 ft Buoyant 3 8.681 Hz 0.72” (1.83 cm) 150 ft Buoyant 3 4.035 Hz 1.53” (3.88 cm) 200 ft Buoyant 3 2.256 Hz 2.71” (6.89 cm)

Full Scale Set-up The horizontal full scale structural damping test configuration is shown in Figs. 10~11. The Cameron CIW RD 21 inch drilling riser joints are suspended from fabricated test frames with cables attached to lifting


pad-eyes located near the flanges. At one end of the pipe, a spring connects the pipe to the piston of a hydraulic oscillator, which is utilized to excite riser motion at defined frequencies. A spring of equal stiffness on the opposite side of the riser circumference is fixed to the test frame as shown in Fig. 12. The springs used to transmit force from the oscillator piston and the end of the horizontal pipe have approximate stiffness values from 9,982 N/m (57 lb/in) to 22,416 N/m (128 lb/in) depending on the test conducted. Higher stiffness springs were utilized to increase the test amplitudes. The oscillator moves the piston at the appropriate frequency in order to excite the pipe at its second natural period, resulting in the mode shape shown in Figure 3. After reaching steady state, the oscillator is turned off and data from the accelerometers located along the pipe measure the decay in response which is used to calculate the structural damping of the pipe.

Fig. 10 – 200 ft Full Scale Test Set-up Configuration

Fig. 11 – Full Scale Test Set-up with Buoyant Joints

Fig. 12 – Full Scale Test Shaker Springs The full scale tests utilized the same instrumentation as the pre-scale test, hence three high resolution, tri-axial accelerometers were used to measure the excited and the free vibrating test pipe response The tri-axial accelerometers were attached at defined locations via curved magnets, Fig. 13.

Fig. 13 – Full Scale Test Instrumentation Mounted on Pipe Test Methodology A number of drilling riser pipe configurations were tested in order to determine the effect that total length, mode, frequency, choke and kill lines and buoyancy modules have on structural damping. The test matrix is given in Table 7 below. The test approach involved hydraulic excitation to steady state followed by piston stop. The measured response decay was then used to determine the pipe damping. Table 7. Full Scale Structural Damping Test Matrix

Pipe Type Pipe Length

100 ft 150 ft 200 ft 21 in Bare Joints X X X 21 in Buoyant Joints X X X


The test sequence adopted to obtain the test excitation frequency is given below:

i) Commence test at calculated natural frequency and small initial amplitude;

ii) Incrementally increase the test amplitude until maximum shaker amplitude or vibration amplitude as per Table 6 is reached;

iii) Determine decay frequency from test. If the decay frequency modulates such that it differs from the excitation frequency steps (i) and (ii) are repeated using decay frequency until excitation frequency equals the test frequency.

The approach utilized to determine the structural damping as a function of the test amplitude is given below:

i) Filter through band-pass filter; ii) Calculate maximum acceleration direction of each time step; iii) Determine maxima of response peaks; iv) Truncate Array of Time and Maxima to include only those

during decay period; v) Divide the maxima into five peak segments; vi) Use damping equations below to calculate average Damping

Ratio for each segment; vii) Record the greatest acceleration in each segment, which

occurs at the start. The maximum acceleration at each time step is determined by Eq. 1 using theta (θ) in five degree increments.

)cos(*)(_)cos(*)(_max(max_ tyacceltxaccelAccel (1) Where, t = time and accel_x and accel_y are horizontal acceleration measurements. The damping ratio is calculated by Eqs. 2~3.

1

ln(log_n

n

AmplitudeAmplitudedecrementd

(2)

))_/()*2(1(1)_(

22 ratiodampingratiodampingz

(3) The displacement amplitude is determined from acceleration as follows:

i) Determine the decay frequency from each segment; ii) Calculate angular velocity from the segment decay

frequency; iii) Calculate amplitude using both angular velocity and segment

maximum acceleration. Angular velocity omega is determined by Eq. 4.

frequency**2 (4) Amplitude from acceleration is determined by Eq. 5.

2

onAcceleratiAmplitude (5)

Full Scale Test Results The bare pipe test response frequencies are given in Table 8. The bare pipe damping test results are summarized in Figs. 14~15 and show the structural damping ratio as compared against the vibration amplitude and maximum pipe curvature for the three test lengths with mode 2 and also the 200 ft test length in mode 3. Table 8. Bare Pipe Test Response Frequencies

Length Mode # Frequency 100 ft 2 2.93 Hz 150 ft 2 1.48 Hz 200 ft 2 0.868 Hz 200 ft 3 2.18 Hz

0

0.001

0.002

0.003

0.004

0.005

0.006

0 0.05 0.1 0.15 0.2 0.25 0.3

A/D

Dam

ping

Rat

io

Bare 100ft Bare 150 ft Bare 200 ft Mode 2 Bare 200 ft Mode 3 Fig. 14 – Bare Pipe Damping vs Amplitude

0

0.001

0.002

0.003

0.004

0.005

0.006

0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007

Curvature (1/m)

Dam

ping

Rat

io

Bare 200 ft Mode 2 Bare 150 ft Bare 100 ft Bare 200 ft Mode 3 Fig. 15 – Bare Pipe Damping vs Curvature The bare pipe test results are summarized as follows: Depending on the vibration amplitude and test length the

measured damping ratio is in the range of 0.003 to 0.005, or 0.3% to 0.5%;

As a generalization damping increases with increasing amplitude. Although the damping results indicate a reduction in damping at the maximum test amplitudes;


The maximum damping response for different pipe lengths varies from 0.5% for the 100 ft test to 0.4% for the 200 ft length. The source of these differences is not known. One possible explanation is friction damping in the test support shackles. The number of shackles per unit length reduces as the test length increases. This may also explain the reduction in damping at higher test amplitudes as the friction damping is overcome for larger shackle rotations/test amplitudes.

The buoyant pipe test response frequencies are given in Table 9. The buoyant pipe damping test results are summarized in Figs. 16~17 and show the structural damping ratio as compared against the vibration amplitude and maximum pipe curvature for the three test lengths with mode 2 and the 200 ft test length in mode 3. Table 8. Bare Pipe Test Response Frequencies

Length Mode # Frequency 100 ft 2 3.1 Hz 150 ft 2 1.3 Hz 200 ft 2 0.85 Hz 200 ft 3 2.1 Hz

y = 0.0044Ln(x) + 0.0407R2 = 0.8165

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

A/D

Dam

ping

Rat

ios

Buoyant 100 ft Buoyant 150 ft Buoyant 200 ft Mode 2Buoyant 200 ft Mode 3 All Log. (All)

Fig. 16 – Buoyant Pipe Damping vs Amplitude

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.00002 0.00004 0.00006 0.00008 0.0001 0.00012

Curvature (1/m)

Dam

ping

Rat

io

Buoyant 100 ft Buoyant 150 ft Buoyant 200 ft Mode 2 Buoyant 200 ft Mode 3 Fig. 17 – Buoyant Pipe Damping vs Curvature The buoyant joint test results are summarized as follows: Damping increases with increasing amplitude; Damping values from 2.5% to 3% are measured at the larger test

amplitudes > 0.02 A/D;

The larger test amplitude results are the most relevant for design as the smaller A/D values do not drive the drilling riser fatigue response;

Unlike the bare pipe tests the damping response is consistent for the 3 pipe lengths considered.

The buoyant joint testing was performed in November 2010 and May 2011. The May 2011 testing was conducted with the objective to collect test data at higher amplitudes. The difference between the two tests is the use of higher stiffness springs in May 2011 to drive higher amplitudes and the use of additional test frame reinforcement to mitigate test frame movement that was observed in the November 2010 tests. The buoyant joint test results for November 2010 and May 2011 are compared to determine if the results are consistent, Fig. 18. The comparisons show that the damping vs. A/D trends are consistent between the two test dates. However, there does appear to be additional damping (0.2 to 0.4%) in the November 2010 data at all test amplitudes. This may have been due to the test frame movement which was minimized in the May 2011 tests.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

A/D

Dam

ping

Rat

io

May 2011 Nov 2010 Fig. 18 – Buoyant 200 ft Length Comparisons between November 2010 and May 2011 Tests Pendulum tests were also conducted with the objective of determining the level of damping in the test set-up due to shackle friction and/or test frame movement. The bare and buoyant joint test set ups were excited manually without the spring interface connected in the ‘pendulum’ mode. The resulting pendulum test data was very noisy. Efforts were made to filter the measured response to isolate the pendulum response frequency. However, there were multiple closely spaced response peaks probably due to varying cable lengths and the excitation method (manual). The multiple closely spaced frequencies resulted in amplitude modulation in the time series which could not be used for determination of damping. BENEFIT TO FATIGUE DESIGN The buoyant joint damping test results showed structural damping ranging from 2.5% to 3%. To determine the benefit that this level of damping may have on drilling riser fatigue design a simplified case study was conducted. Wave fatigue analysis was conducted for a single fatigue critical seastate on a Gulf of Mexico drilling riser with a Spar using zero structural damping as is typical for design. The fatigue life with no damping was compared against the fatigue life


with 2.5% structural damping and a ‘fatigue improvement factor’ determined along the length. The fatigue improvement factor is equal to the (Fatigue life with damping) / (Fatigue life without damping). The results of the case study are shown in Fig. 19. They show a factor of 3 or more improvement in wave fatigue life with 2.5% structural damping. The benefit of the damping increases with water depth. Similar levels of fatigue improvement have also been observed in VIV fatigue predictions.

Fig. 19 – Benefit of Damping on Fatigue Design Fig. 19 also indicates that the overestimation of fatigue damage predicted in [1] cannot be solely explained by the structural damping value used in design. CONCLUSIONS Full scale testing was performed on a number of drilling riser pipe configurations using Cameron CIW RD 21 inch drilling riser joints. The test determined the effect that length, mode, frequency, choke and kill lines and buoyancy modules have on structural damping. The findings are considered a key input for future drilling riser design and improved reliability. Buoyant joint structural damping coefficients from 2.5% to 3% were measured at the larger test amplitudes > 0.02 A/D, as shown in Fig. 17. Structural damping increases hyperbolically with increasing amplitude. The larger test amplitude results are the most relevant for design as the smaller A/D values do not drive the drilling riser fatigue response. The bare pipe measured damping ratio is in the range of 0.3% to 0.5% depending on the amplitude and test length. For the bare pipe tests there is a difference in the damping response for different pipe lengths. The 100 ft test length gives the highest damping of 0.5% compared to 0.4% for the 200 ft length. One possible explanation is friction damping in the test support shackles. The damping test results indicate that drilling riser joints with buoyancy and choke and kill lines do have high inherent damping. However, it should be noted that the tests are in air and lubrication from the water may reduce the damping. Methods are available to conduct similar tests in water should there be sufficient interest and funding from industry in the future. In addition, manual excitation pendulum tests, which were conducted to determine damping from the test set-up, did not give useful data. Hence, if the damping values reported herein are adopted for design

enhancement it is prudent to consider reducing the damping from those measured to account for some damping from the test set-up. The reduction to use should be at the users discretion based on the results within this paper. Whilst higher damping will help reduce the over conservatism in SHEAR7 design calculations, the structural damping value is not the only factor affecting fatigue life prediction. ACKNOWLEDGEMENTS The authors wish to thank Deepstar® for funding the work, providing technical leadership and oversight, and for permission to publish this paper, Transocean for the kind donation of their drilling riser joints, ExPert Riser Solutions for the use of their facilities to conduct the full scale tests, Pulse Structural Monitoring for instrumentation supply and test support and Hydradyne for hydraulic shaker supply and technical support. REFERENCES Paz, Mario (2004). Structural Dynamics: Theory and Computation,

Springer, pp 38 Slocum, SE. The Theory and Practice of Mechanics, H Holt and

Company, pp 257 Tognarelli, M.A., Taggart, A., and Campbell, M. (2008). “Actual VIV

Fatigue Response of Full Scale Drilling Risers: With and Without Suppression Devices”, Proc. 27th OMAE Conf., Estoril, Portugal, OMAE 2008-57046.

Copyright ©2003-2012 The International Society of Offshore and Polar Engineers. All rights reserved.

GoM Drilling RiserFATIGUE IMPROVEMENT FACTOR DUE TO STRUCTURAL DAMPING

2.5% Structural Damping Vs No Structural DampingFatigue Critical Seastate, Tz=6.4s

0

2

4

6

8

10

12

14

16

-200 0 200 400 600 800 1000 1200 1400

Elevation above Mudline (m)

Fati

gue

Im

pro

vem

ent

Fact

or

(-)

Buoyant Joint Section Bare Joint Section


learn more at · structural damping test whilst including any lessons learnt from the pre-scale...

Documents