FULL SCALE MEASUREMENT OF LH11-1 FPS HULL AND MOORING SYSTEM

FULL SCALE MEASUREMENT OF LH11-1 FPS HULL AND MOORING SYSTEM Qu Yan Research Institute of CNOOC Beijing, China Shi Zhongmin Research Institute of CNOOC Beijing, China ABSTRACT Nanhai Tiaozhan FPS of LH11-1 oil field is the only production SEMI of CNOOC used in South China Sea at the present state. From May 2011 to Oct 2011 the platform has been intensively instrumented with full scale measurement system. During that period several typhoons have passed by the South China Sea. Both the metocean data and response of the SEMI have been recorded during the measurement. The paper presents full scale measured results and numerical analysis of the FPS in one typical typhoon event. The comparison shows that the hull/mooring system coupled analysis result agrees well with the measured result in low frequency domain and different in the wave frequency domain. INTRODUCTION Full scale measurements have been considered as an important part of the integral management of the offshore structures [1]. It contributes to assessing and verifying global performance of the offshore structures, operating facilities in a safe manner and minimizing down time and lost production et al [2]. Many offshore structures have been instrumented with comprehensive measurement system to obtain response of the structure in the extreme condition such as hurricane[3-4]. Igor Prislin and John Halkyard et al (1999)[5], Igor Prislin and Himanshu Gupta et al (2000) [6]presents results of the full-scale measurements of the Oryx Neptune Production Spar Platform, some comparisons were also made with a time domain analytical tool TDSIM. Full scale data measured at Horn Mountain Spar in hurricane events has also been analyzed and compared with time and frequency domain numerical analysis results [7-9]. Steve Perryman and John Chappell et al (2009) [10] studied Holstein Spar motions in hurricane Rita and Ike. Per Tigen and Sverre Haver (1999) presents comparisons between full scale measurements and numerical predictions for the Heidrun TLP [11]. R.N.Perego, G Li, P.A. Beynet et al (2005) compared measured data to the design level responses and full coupled frequency domain simulation result of the Marlin TLP in hurricane IVAN [12]. The Marco Polo TLP had also been monitored intensively from 2004 to 2009, results of the measurements and analysis were summarized by P.J. Aalberts (2010) [13]. All the work listed above give an insight to the deepwater floaters performance in the sea and provide important verification database to analysis tools. In order to know more about floater's behavior in South China sea, deepwater engineering Key lab of CNOOC research institute decided to conduct a full scale measurements on NaiHai Tiaozhan FPS in LH11-1 oil filed. The measurement system was installed on the platform in May 2011 and recalled in Oct 2011 before the SEMI was towed back to shipyard for a heavy maintenance. During the measurements, several typhoon storms passed the South China Sea. Metocean condition and response of the SEMI were recorded during those typhoon events. The full scale data and coupled analysis result of the SEMI in typhoon events were presented in this paper. THE NANHAI TIAOZHAN SEMI SUBMERSIBLE LiuHua 11-1 oil field is located in South China Sea, approximately 215 km from Hong Kong and 240 km from Shenzhen with water depth 260~300m. The oil field is developed by one FPSO, one FPS and one subsea central manifold as shown in figure 1. 1

Figure 1 LH11-1 Oil Field development The semisubmersible used in LH11-1 oil field as shown in figure 2 was named as Nanhai Tiaozhan, formerly known as the West Stadrill, which is a Sedco 700 class semisubmersible. The semi-submersible drilling vessel was purchased in September 1993 and converted into the FPS and moved onto location in June 1995. Figure 2 Nanhai Tiaozhan Semisubmersible at LH11-1 Oil Field The FPS was modified with necessary systems to drill, complete and work-over horizontal subsea wells. Additionally, the FPS installs and operates subsea manifold systems and houses electrical generation and distribution equipment to provide power to the ESPs. (John R. Frase, Xiao Qi Liang, Lindsay E. Clark, 1996). The Nanhai Tiaozhan SEMI was a twin pontoon SEMI with 8 columns. Horizontal and diagonal braces were used to increase the structure stiffness. Principal dimensions of the FPS was presented in table 1. 2

Table 1 FPS principal Dimensions Operating Survival Draft [m] 23 23 Displacement [kn] 28248349 28248349 Vessel weight [kn] 26607707 26888934 VCG from BL [m] 22 21 Roll Gyradius [m] 28 29 Pitch Gyradius [m] 29 29 Yaw Gyradius [m] 31 33 GMT [m] 4 5 GML [m] 11 12 Length overall [m] 90 90 Port/stbd Column Span [m] 59 59 Fore/aft Column span [m] 69 69 Mooring system of the FPS was designed as 11-point mooring line arrangement to cope with the directional typhoon storm, which are the governing design weather conditions. 6 of the mooring lines were arranged on the starboard side because the most severe typhoons are expected from the NE to E direction.(liuhua 11-1 development FPS Project Mooring Design Report). Arrangements of the mooring system and heading of the SEMI can be illustrated in figure 3. FULL SCALE MEASUREMENT SYSTEM Figure 3 Mooring system arrangement of Nanhai Tiaozhan FPS Wind Wind speed and direction are main input to evaluate floating system responses. Those data on the SEMI were recorded by two wind sensors. A mechanical propeller type sensor was installed when the measurement started and another ultrasonic sensor was added after it was noticed that mechanical propeller type sensor had broken record in the typhoon on the platform. The wind sensors were mounted at roof of a room on the operation deck. 3

Wave Conditions A wave measure sensor on the upward scanning ADCP and a downward looking air-gap radar sensor were deployed on the FPS to measure the wave data. To avoid wave measurement result been affected by the hull motion, a heave motion compensation device was installed on the SEMI. The Wave parameters including significant wave height, peak frequency of the spectrum and other wave characteristics were all collected by ADCP and radar sensors. Unfortunately, time history of the wave elevation were failed to be recorded due to improper setting of the system. Figure 4 Downward looking wave height radar sensors and the heave motion compensation device. Current Speed and Direction Current data are directly related to mooring and hull system motion. The speed and direction of the current were measured by Acoustic Doppler Current Meters (ADCPs). 2 ADCPs are deployed on the SEMI to measure the current from sea surface to a requested water depth. Vessel Motions The FPS motion in six degrees of freedom are the main parameters of interested in the measurement. The hull motion was measured by a differential GPS (DGPS) and rotation was recorded by the Inertial Navigation System. Those sensors were installed on the same roof where the wind sensor located as shown in figure 5. Mooring/umbilical response Figure 5 DGPS, INS and wind sensor installed on the SEMI. 4

Mooring system of the Naihai Tiaozhan FPS was instrumented with top tension load cells at the chain stoppers to measure the mooring lines top tension. The real-time signal of top tension was transit to the central control room to help marine operation of the FPS. To develope the mooring and riser system monitoring technology, subsea inclinometer and accelerometers were manufactured and installed on two of the mooring lines and one umbilical. The sensor packages were installed by divers at water depth from 20 meters to 50 meters and recalled by ROV. Figure 6 shows photograph of the sensor packages and installation process on the umbilical. Figure 6 Subsea inclinometer and accelerometer sensor packages used in the measurement of the mooring system and subsea installation by divers Video observation of wave action on the hull Video observation of the wave action on the hull was considered to be helpful to study air gap and verify wave height measurements. Four video cameras were installed on the FPS to observe the wave action on the column from different directions. Figure 7 shows the video recorded by one of these cameras in one typhoon process. Figure 7 Pictures of wave act on the FPS captured by the video cameras installed on the SEMI MEASURED DATA IN NOCK-TEN TYPHOON During five months duration of the measurement project, several typhoon storms passed South China Sea. Most paths of the storms were not close to LH11-1 oil field. Therefore, the platform did not impacted by harsh environmental condition this year. The typhoon Nock-Ten around July 28, 2011 was one of the typical storms affected the FPS during the measurements. It was selected for a detailed analysis after the first data package was taken back in September. Figure 8 to Figure 9 shows wind data when the Nock-Ten path close to the platform at July 28, 2011. It shows that wind velocity is about 18 m/s at the FPS location in the typhoon event. Wind direction changed from 70 deg to 130 deg during the typhoon process. It should be noted that 0 Deg of wind direction plotted here is from the real north of the global coordinate system. The fluctuation of the wind direction needs to be taken into consideration in the numerical analysis. 5

25 140 130 20 120 wind speed(m/s) 15 10 wind direction(deg) 110 100 90 80 5 70 0 0 6 12 18 24 Time(h) Figure 8 Wind velocity of Nock-Ten typhoon measured on the Nanhai Tiaozhan FPS at July 28, 2011. 60 0 6 12 18 24 Time(h) Figure 9 Wind direction of Nock-Ten typhoon measured on the Nanhai Tiaozhan FPS at July 28, 2011. Wave measured in Nock-Ten typhoon is shown in figure 10 to 12. The results show that significant wave height fluctuated around 3 m in the afternoon and increased to 3.5 m in the evening. The peak frequency period of the wave is fluctuated between 7 to 9 seconds. These statistical values of the parameters are mean result over 20 minutes. Hm0 H10 Hmax Wave parameters Tp Tm02 Wave parameters 5.5 9.0 5.0 8.5 4.5 8.0 4.0 Height (m) 3.5 Period (s) 7.5 3.0 7.0 2.5 6.5 2.0 1.5 6.0 3:00 6:00 9:00 12:00 15:00 18:00 21:00 3:00 6:00 9:00 12:00 15:00 18:00 21:00 Thu 28 Jul 2011 Time Thu 28 Jul 2011 Time Figure 10 Wave height of Nock-Ten measured on the Figure 11 Wave peak frequency on the SEMI SEMI Direction of the wave is mainly about 100 Deg from north in the whole day. Wave parameters DirTp SprTp MeanDir 120 110 100 Direction (deg) 90 80 70 60 50 3:00 6:00 9:00 12:00 15:00 18:00 21:00 Thu 28 Jul 2011 Time Figure 12 Wave direction of Nock-Ten typhoon measured on the Nanhai Tiaozhan SEMI at July 28, 2011. 6

Because the SEMI was not on the path of Nock-Ten, the influence of typhoon on structure was slight. Figure 13 to 16 show measured motion result. Driven by the wave and wind, the SEMI offsets about 4 meters at surge and sway directions of the hull coordinate in the afternoon. After that the hull moved towards south west along with the wind direction change. 6 4 Surge Sway 2 Motion(m) 0-2 -4-6 -8 0 5 10 15 20 25 Time(s) Figure 13 Surge and Sway of Nanhai Tiaozhan FPS on July 28, 2011. Due to limitation of the DGPS accuracy in the heave measurements, heave of the semi was not properly recorded in the typhoon event. The signal shown in figure 14 was mainly included by tide change. Therefore, the heave result will not be made detailed comparison with the numerical analysis result. A set of low frequency accelerometers will be added into the measurement system in the next phase of project started from 2012. 2 1.5 1 0.5 Heave(m) 0-0.5-1 -1.5-2 0 5 10 15 20 25 Time(h) Figure 14 Heave of Nanhai Tiaozhan SEMI on July 28, 2011. The INS system showed a good performance in the measurement of the SEMI angular motion. Roll and pitch motion shown in figure 14 implies that maximum roll and pitch angel of the hull was 0.5 degree in the morning at July 28, 2011. The pitch motion increased to minus 1.5 degree with a static tilt angle of -0.5 degree due to wind direction change from afternoon to evening as shown in figure 9. 7

1.5 1 Roll(deg) Pitch(Deg) 0.5 Rotation(Deg) 0-0.5-1 -1.5-2 -2.5 0 5 10 15 20 25 Time(h) Figure 15 Roll and pitch of Nanhai Tiaozhan SEMI on July 28, 2011. Yaw motion measured results show heading change of the FPS in the typhoon due to the environmental actions. Results shown in figure 15 imply that heading of the hull fluctuated slightly with directional change of the environmental actions. In the afternoon the heading rotated about 0.75 degree because wind direction change. 309.5 309 Yaw 308.5 308 Heading(Deg) 307.5 307 306.5 306 305.5 305 0 5 10 15 20 25 Time(h) Figure 16 Yaw of Nanhai Tiaozhan FPS on July 28, 2011. NUMERICAL ANALYSIS OF THE SEMI IN TYPHOON EVENT Metocean and structure response data obtained from the full scale measurement provide an important database for the verification of floating structure motion analysis. Those data help verifying design result of the structure, accuracy of the analysis tools, evaluating validity of the design standard. Wind, wave and current in a typhoon process change with time both in magnitude and direction. At present state most analysis tools are not able to model the non-stationary environmental actions. Therefore, numerical simulation to the whole typhoon process must be separated into several sections. Considering stationary of the environmental actions in Nock-Ten typhoon process, the physical process between 10:00 to 15:00 in July 28, 2011 was selected for the simulation and comparison. Table 3 listed the environmental action parameters during this period used for the analysis. The fully coupled Hull and Riser/mooring analysis program HARP was adopted for the analysis of the SEMI in the typhoon event. The software is a suite of integrated hydrodynamic and structural analysis modules for offshore engineering applications. Its fully coupled analysis program Charm3D along with the wave radiation/diffraction panel program WAMIT, form a system to perform global analysis of offshore floating platform motions and structural analysis of risers and moorings [16]. 8

Figure 17 shows hydrodynamic model for the SEMI hull used for WAMIT. The wet surface of column, pontoon and brace were modeled in ANSYS and mesh was transform to the WAMIT format. Table 3. Environmental condition of Nock-Ten Wind Velocity 18.5 m/s Dir 90 Deg Wave Hs 3.0 m Tp 8.0 S Spectrum 3 Shape Dir 100 Deg Current Depth Velocity [m/s] 0 0.55 m/s 10 0.50 m/s 20 0.50 m/s 30 0.30 m/s 50 0.20 m/s 100 0.10 m/s 200 0.10 m/s Figure 17 WAMIT Hydrodynamic model of the Nanhai Tiaozhan FPS The hull/ mooring model for Charm3D is shown in figure 18. The mooring lines are modeled as beam elements. The drag and added mass coefficients for mooring lines are listed in Table 4 Table 4 Hydrodynamic Coefficients for Mooring line Drag Coef. Added Mass Chain 2.45 1 Wire 1.2 1 9

Figure 18 Hull and mooring coupled analysis model of the Nanhai Tiaozhan SEMI For the selected typhoon event of July 28, 2001, a 3-hr realization was run for the sea state from 10:00 to 15:00 in the Charm3D. Figure 19 to 22 show the power spectrum density of the measured and calculated results. Considering accuracy of the heave motion is not sufficient for the analysis and yaw motion is affected by direction change of the environmental action, the heave and yaw results are not shown here for comparison. The comparison shows that the coupled analysis succeeded to capture peaks of the motion in frequency domain. But the energy distribution of the calculated and measured results is different to each other especially in surge and roll direction. The difference may induced by many factors such as insufficiency modeling of the environmental loads, further change of the FPS mass properties. Further detailed analysis will be carried out in the following phase of the project. Figure 19 Power spectrum density of measured and calculated surge Figure 20 Power spectrum density of measured and calculated sway 10

Figure 21 Power spectrum density of measured and calculated roll Figure 22 Power spectrum density of measured and calculated pitch Figure 23 to 26 show comparison of statistical results between measured and calculated hull motion. It should be noted that in the statistical analysis the measured results have been filtered using a Zero-phase forward and reverse digital filtering to get rid of the higher frequency component in the signal which is considered not caused by the hull motion. The comparison shows that the numerical analysis gives different analysis results to the measured results at different direction. It seems that the difference was mainly induced by the insufficient accuracy of modeling the direction of random environmental actions. Figure 23 Statistical results of measured and calculated surge Figure 24 Statistical results of measured and calculated sway Figure 25 Statistical results of measured and calculated roll. Figure 26 Statistical results of measured and calculated pitch. 11

Figure 27 and figure 28 show offset and tilt of the measured and calculated hull motion results. The comparison shows that the numerical simulation gives an acceptable accuracy to the measured results. However, it can be understand that difference still exists caused by weight change of the hull compare to the design condition. That implies that more detailed analysis is needed to improve the numerical analysis capacity. Figure 27 Measured and calculated offset result of the hull motion in typhoon event. Figure 28 Measured and calculated tilt result of the hull motion in typhoon event. CONCLUSION AND DISCUSSION 1. A set of full scale monitoring system was installed on the Nanhai Tianzhan FPS of LH11-1 oil field. The system worked properly within most time of the measurement period. Metocean, hull motion, mooring line angle and umbilical vibration data were measured continuously in several typhoon processes. 2. Full coupled analysis was made to simulate the hull and mooring system motion in one of the typhoon events by using HARP. The results show the calculated and measured result are in not in a good agreement and further detailed numerical verification is needed. 3. It can be more accurate and convenient to analyze the offshore structures response in typhoon event if the coupled analysis method can used measured metocean data as input directly. ACKNOWLEDGMENTS The authors are grateful to the financial support of the National Science and Technology Major Projects (2011ZX05026-002) and the supports to the full scale installation and maintenance of the measurement system from CNOOC Shenzhen Ltd. LH11-1 oil field operators. REFERENCES 1) J.F. Geyer, S.R. Perryman, M.B. Irani and J.F. Chappell, Floating Systems Integrity Management of BP Operated GoM Deepwater Production Facilities, Offshore Technology Conference, 4-7 May 2009, Houston, Texas, OTC 20137. 2) Irani, M.B., Perryman, S.R., Geyer J.F., von Aschwege, J.T., Marine Monitoring of Gulf of Mexico Deepwater Floating Systems, 30 April 3 May 2007, OTC 18626. 3) R. Edwards, I. Prislin, and T. Johnson, C. Campman, S. Leverette and J. Halkyard, Review of 17 Real- Time, Environment, Response, and Integrity Monitoring Systems on Floating Production Platforms In The Deep Waters Of The Gulf Of Mexico, Offshore Technology Conference, 2 May-5 May 2005, Houston, Texas, OTC 17650. 4) Craig Campman, Roderick Edwards, William Bud Hennessy, Review of floating production platform real-time integrity monitoring system worldwide, Rio Oil & Gas Expo and Conference 2010 Proceedings. IBP3407_10. 5) Igor Prislin, John Halkyard, DeBord, Jr., J. Ian Collins and Jeffrey M. Lewis, Full-Scale Measurements of the Oryx Neptune Production Spar Platform Performance, Offshore Technology Conference, Houston, TX, 1999, OTC 10952 12

6) Prislin, I., H. Gupta, A. Steen, J. Halkyard, L. Finn (2000): "Analytical Prediction of Motions vs. Full Scale Measurements for Oryx Spar Neptune", OMAE 2000, 19th International Conference on Offshore Mechanics and Arctic Engineering Feb. 14-17, 2000, New Orleans, Louisiana, USA 7) Halkyard, J., Liagre, P. & Tahar, A., Full Scale Data Comparison for the Horn Mountain Spar, Proc. Of OMAE 04, Vancouver, Canada. pp. 1123-1132,OMAE 2004-51629. 8) Tahar, A., Halkyard, J., Irani, M., Comparison of Time and Frequency Domain Analysis with Full Scale Data for the Horn Mountain Spar during Hurricane Isidore, Proceedings 25th International Conference on Offshore Mechanics and Arctic Engineering, Hamburg, Germany, 4-9 June, 2006, OMAE2006-92137. 9) Qi Xu, Stephen Perryman, and Igor Prislin, Assessment of the Horn Mountain Spar in Hurricane Ivan, Proceedings of the 26th International Conference on Offshore Mechanics and Arctic Engineering, OMAE2007, June 10-15, 2007, San Diego, California, USA, OMAE 2007-29354. 10) Steve Perryman, John Chappell, Igor Prislin, Qi Xu, Measurement, Hindcast and Prediction of Holstein Spar Motions in Extreme Seas, Offshore Technology Conference, 4-7 May 2009, Houston, Texas, OTC 20230. 11) Per Teigen, Sverre Haver, The Heidrun TLP: measured versus predicted response, Applied Ocean Research Volume 20, Issues 1-2, February-April 1998, Pages 27-35. 12) R.N. Perego, G. Li, P.A. Beynet, J.F. Chappell, D.L. Garrett and R.B. Gordon, The Marlin TLP: Measured and Predicted Responses During Hurricane Ivan, Offshore Technology Conference, 2 May-5 May 2005, Houston, Texas, OTC 17335. 13) P.J. Aalberts, SUMMARY REPORT MARCO POLO JIP, June, 2010. 14) John R. Frase, Xiao Qi Liang, Lindsay E. Clark, Liuhua 11-1 Development-Design and Fabrication Considerations for the FPS, Offshore Technology Conference, 6 May-9 May 1996, Houston, Texas, OCT8187. 15) Reading & Bates Developments company, Liuhua 11-1 development FPS project-mooring Design Report. 16) Harp user manual 13