OTC FPSO Integrity; Structural Monitoring of Glas Dowr Henk van den Boom (MARIN), Max Krekel (Bluewater) and Pieter Aalberts (MARIN)

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1 OTC FPSO Integrity; Structural Monitoring of Glas Dowr Henk van den Boom (MARIN), Max Krekel (Bluewater) and Pieter Aalberts (MARIN) Copyright 2000, Offshore Technology Conference This paper was prepared for presentation at the 2000 Offshore Technology Conference held in Houston, Texas, 1 4 May This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Abstract The objective of the Joint Industry project FPSO Integrity is to provide insight in the fatigue loading of FPSO s and the accuracy and validity of computational models. The results should contribute to reliable life time prediction and site assessment studies for this relative new concept of production. Within the frame work of this JIP, Glas Dowr, a new build double hull FPSO operated by Bluewater for Amerada Hess at the Durward and Dauntless fields in the North Sea, has been instrumented extensively. The objective of the instrumentation was to measure sufficient signals to derive the fatigue loading on the vessel. The dedicated monitoring system comprised the instrumentation of two cross sections of the vessel with strain gauges, pressure transducers and wave height radar s. Also the deck and the turret were equipped with strain gauges. Both the wave induced motions and the low frequency positions and orientation of the vessel was recorded. For the analysis of the measured signals the wind, wave and current conditions have been recorded as well as the vessel parameters such as draft, trim and tank loads. Data from the Glas Dowr has been collected from October 1, 1997 to 1 August The data has been subjected to long term statistical analysis and for selected periods to more detailed spectral and statistical analysis as well as rain flow counts. The results have been used for validation of existing fatigue load calculation methods used by five classification societies and for the development of new fatigue load prediction software. INTRODUCTION For design, engineering and site assessment of Floating Production Storage and Offloading (FPSO) units, a reliable fatigue life prediction is essential. A computational model of the fatigue life of an FPSO comprises a description of the environment, a model of the fatigue loads and a model of the fatigue response. As the operation of an FPSO is completely different from a trading tanker, in 1996 a Joint Industry project named FPSO Integrity was initiated to provide insight in the fatigue loading of FPSO s and the accuracy and validity of computational models. The background, objectives and structure of the project are described in further detail in ref [1]. The FPSO Integrity JIP comprised four major tasks: 1. Structural monitoring of FPSO Glas Dowr operated by Bluewater for Amerada Hess on the Dauntless and Durward field in the North Sea. 2. Analysis of the measured data to derive the fatigue loads on the vessel. 3. Development of a fatigue load assessment method, which can be used for a fatigue lifetime prediction. 4. Comparative Study on the existing fatigue analysis methods currently used by the five Classification Societies involved in the project. As the ultimate goal of the project was not only development of new calculations procedures but also comparison against existing methods and real life data, the project demanded a strong input from all stakeholders in the FPSO industry. The JIP formally started on September 1, 1996 with a 3-year programme supported by the Thermie Programme of the European Community, the Dutch Ministry of Economic Affairs and sponsored amongst others: American Bureau of Shipping, Amerada Hess, Astano, Bluewater, Bureau Veritas, Chevron, Conoco, Det Norske Veritas, Germanischer Lloyd, Health & Safety Executive, Hyundai Heavy Industries, Lloyd s Register, Marin, Mitsubishi Heavy Industries, Nevesbu, Petrobras, Shell, Statoil, Texaco. In 1998 the JIP was extended with a one year programme supported by the above companies and Daewoo Heavy Industries, Maersk Contractors, Kawasaki Heavy Industries, and Samsung Heavy Industries.

2 2 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC The objective of the measurement campaign was to collect real life data to be used for the understanding of fatigue loads on FPSO s, for the development of computational models and for the validation of new and existing fatigue load prediction models such as operated by the five classification societies involved in the JIP. For this purpose FPSO Glas Dowr, converted from a new built double hull tanker was equipped with a dedicated structural monitoring system. Extensive data sets were derived from the system during the operation of the Glas Dowr on the Durward and Dauntless field in the North Sea from October 1997 to July Glas Dowr is the fifth FPSO owned and operated by Bluewater, and its second operating on the UKCS. A general arrangement is shown in Fig. 1. She produced the Durward and Dauntless fields, in UK blocks 21/11 and 21/16, on behalf of Amerada Hess Limited from august 1997 to august The fields are located some 150 nm east of Aberdeen. The local water depth is about 75 m. A more detailed description of the FPSO can be found in ref [1]. The present paper describes the measurement campaign on board FPSO Glas Dowr and the analysis of the collected data (task 1 & 2 in the JIP), whereas the work performed by the classification societies and the comparison of their results with those from the measurements is described in ref. [2]. MONITORING SYSTEM General Aiming at the fatigue loads on FPSO s, measurement of actual loads and responses on an FPSO in service was considered essential for the study from the very start. The objectives of the measurements were to provide a better insight in the physical phenomena, to support mathematical modelling and to produce benchmark data for validation of computational models, engineering methods. Fatigue loads on a vessel can not be measured directly. In general fatigue originates from various internal and external loads such as wave, cargo, wind, current and mooring loads. The occurrence of these loads their magnitude and frequency region all vary in time. The forces may introduce overall fatigue loads (hogging and sagging of the hull) as well as local hull loads. By measuring the response of the hull e.g. strains in combination with the governing parameters of the excitation sources such as the wave elevation, one may be able to distinguish the various fatigue loading components. To evaluate global and local response and load components, however, the instrumentation should cover a fair share of the structure as well as the details of the environmental conditions. Both low cycle large amplitude loads and high cycle low amplitude loads contribute to fatigue damage. For this reason not only severe storm conditions are relevant but also milder weather is relevant for fatigue monitoring. Another important aspect of FPSO s in service is that although they are permanently moored, their loading and response can not be considered as a stationary process. Due to production, storage, ballast and offloading the tanker condition changes continuously. For the evaluation of the results the status of the vessel (e.g. draft, trim tank loads) should be known at all times. As the response has to be related to the input the wave, wind and current condition are extremely important and have to be recorded continuously. Weather varies from hour to hour not only in magnitude but also in direction. Due to its mooring the vessel not only moves at wave frequencies but also around the natural frequencies of the moored system with large amplitudes. Although the forward positions of the turret enable the FPSO to weathervane; in specific situations waves may encounter the vessel from other directions than head on. Vessel motions and heading are therefore needed in the analysis. Essential for the design of the structural monitoring system is the modelling of fatigue loading; selection of measurements, analysis of the measured signals and coding of a fatigue load prediction software all depend on a basic model describing the fatigue load processes. Starting at existing computational models for fatigue loads on ships several areas were identified to which in this measurement campaign additional attention is being paid (see ref. [5]). An example of this is the well known side shell fatigue loading which can not be represented by the linear hydrodynamic model. Based on the general objectives, experiences from similar campaigns, the identified specific areas of interest, fatigue modelling hypothesis and restrictions of the vessel in 1996 a fully automated structural monitoring system for the FPSO Glas Dowr has been designed, assembled and shop tested. Steelwork preparation on the vessel was conducted by the end of 1996 and the actual instrumentation of the vessel was carried out early The structural monitoring system started to deliver results on October 1, Below the instrumentation is presented in further detail. Wave and cargo pressures and structural response. To derive the fatigue loading the structural response of the vessel as well as relevant load variables were measured at two cross sections of the vessel; at half length (frame 66.5) and approximately at a quarter length from the bow (frame 81.5). At these two cross sections the measurements are aiming at the following quantities: global and local hull strains cargo and ballast tank pressures external water pressure relative wave heights

3 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 3 To measure local hull strains, some longitudinals in the ballast tanks were instrumented with strain gauges. The strain gauges were fitted at three levels with special attention to the splashed zone (see Figure 2). Both the plate side and the flange side of the longitudinal web were instrumented. For redundancy for each gauge a spare was fitted and cabled. In April 1998 an increasing number of strain gauges failed to provide reliable stress data. Inspection in the ballast tanks in July confirmed the diagnosis that something was wrong with the coating of the gauges. For this reason all strain gauges in the tanks were replaced by the end of October From then on the gauges have been working reliable and accurate. To derive the global hull strains so-called long base strain gauges were fitted on the main deck of the vessel on port and starboard side longitudinal bulkheads. These gauges actually are high accuracy displacement devices measuring the distance between the two end-points. The LBSG s have performed well throughout the campaign. In the inner hull pressure gauges were fitted to measure the instantaneous pressures due to oil (centre tanks) and ballast water (side tanks). Although sloshing is not expected to contribute significantly to the overall fatigue, the dynamic loading of the cargo and ballast water as moving mass may be important. To measure the local pressures excited by wave action, pressure gauges were fitted in the outside hull at three levels and both sides of the vessel. The relative wave height at the location of the strain and pressure devices is recorded by means of a doppler radar fitted on dedicated platforms outside the hull at deck level. These radar s have proven to measure the wave elevation accurately even in fog, spray and green water conditions. In total seven radar s are installed; on port and starboard side of the two measurement cross sections and in front of the superstructure and one at the bow of the vessel. By subtracting the measured vessel motions from the measured relative wave height the absolute wave height around the vessel can be derived. The bow radar also provides redundant incident wave data. By accounting for the waves diffracted by the vessel also the incident wave spectrum can be derived from this device. The 7 wave radar s around the deck of Glas Dowr have performed very well during the campaign. One radar at the stern quarters was damaged probably by crane operations and the radar at the bow needed to be replaced once due to wave damage. The highest waves; i.e. 10 m. significant wave height were recorded accurately. Motions The motions of the FPSO are of prime importance for the fatigue loading. Motions induced by first order wave forces, result in accelerations and thus in inertial loads due to the cargo and ballast masses, deck loads and the own mass of the steel. Depending on the location on the vessel these accelerations may be dominated by heave, pitch or roll but normally contain contributions from all six modes of motion. Due to its mooring the FPSO also moves in the horizontal plane at low frequencies which are determined by the stiffness of the mooring, the mass of the vessel and low frequency excitations such as second order wave drift forces and wind gusts. As the hydrodynamic damping of the vessel at these low frequencies is small the motions have a resonant character with large amplitudes. The contribution of these motions to the inertia loads in negligible. Combined with the first order motions, however, they are responsible for the dynamic peak loads in the mooring lines and in the turret system. The low frequency yaw motion or heading of the vessel results from the wave, wind and current conditions and the weathervaning characteristics of the vessel. The heading determines the relative wave direction and is therefore considered as a prime parameter in the fatigue loads. As the frequency ranges and accuracy requirements for each of the above motions strongly varies different devices are used for their recordings (ref. [9]). For motions at wave frequencies (typical periods 2 to 20 seconds) use is made of accelerometers and angular rate sensors. These sensors have proven to be reliable and accurate as indicated by Table 1 and Figures 3 and 4. A benefit of the angular velocity sensors for measuring rotations is that a compact sensor unit such as the Marin Quality Kit can be placed anywhere on a vessel to record all six modes of motion. On board Glas Dowr additional accelerometers have been installed for sensing impacts and for redundancy and quality control reasons. Heading of the FPSO is recorded by means of a dedicated flux gate compass For the recording of the low frequency motions of Glas Dowr use is made of an Enhanced Differential GPS-system. This system comprises the normal GPS satellites, as well as a dedicated reference station which for this purpose was located on the Kittywake platform some 10 miles from the Durward and Dauntless fields. By advanced processing the ED-GPS system is capable of recording the position of the vessel within 0.30 m. Figures 3 and 4 present the motions as derived from the accelerometers and from the ED-GPS system. When comparing these two motion recordings we have to keep in mind the frequency range of the systems as well as the magnitude of the motions. The recordings were made on April 3, 1997 when Glas Dowr experienced waves with a significant height of 10 m. Although mooring loads were not considered to contribute significantly to the fatigue damage of the hull, the loads in the

4 4 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC mooring system is important for the fatigue behaviour of the mooring it self. Furthermore recording of the mooring loads enables the validation of computational models and engineering methods for prediction of motions, mooring line tensions and turret loads. For these reasons the turret was equipped with long base strain gauges. At two levels of the turret 6 gauges were fitted around the structure. The mooring winches are equipped with inclinometers so these signals could also be logged. The signals of the long base strain gauges were put on the DCS together with the inclinometer signals in order to get them to the data acquisition system. Slamming and Green Water Recent experiences with FPSO s and FSU s in the North Sea and North Atlantic have shown that green water may cause severe damage to the top sides, in particular the turret house and forecastle deck. To obtain full scale data on bow slamming and green water impact, Glas Dowr was equipped with two pressure gauges underneath the bulbous bow and several accelerometers and strain gauges in the bow structure and the turret housing above deck. Together with the wave radar in front of the bow, slamming and green water events can be distinguished from the recorded pressures, accelerations and strains. Environmental conditions Loads on the vessel originate for a large part from the environment i.e. the waves, current and wind. For structural loads also the temperature (and the orientation with respect to the sun) is of importance. To analyse the measured data and to derive useful information it is therefore essential to record these environmental conditions accurately. In the case of Glas Dowr much attention has been paid to collect the environmental conditions. Besides the 7 wave radar systems on board the vessel, a dedicated directional wave rider buoy was moored approx. 1 nm. West of the vessel. This buoy transmitted the undisturbed wave record, spectrum and directionality to the vessel. During the campaign the wave rider buoy was disabled certain periods. The lack on wave data from the buoy was resolved by deriving data from various other sources such as: - relative wave height measured around the vessel by means of radar s - wave radar on the Shell Kittywake platform some 10 nm away - directional wave rider buoy at Shell s FPSO Anasuria on the Teal field - wave hindcasting. By utilizing these additional sources the wave conditions encountered during the measurement campaign could be derived with sufficient quality. Wind direction and velocity is measured on the Glas Dowr and is available from DCS. Operational conditions For the analysis and interpretation of the measured data the actual condition of the vessel is of prime importance. For this reason the structural monitoring system was connected to the on-board DCS which provided valuable vessel conditions such as draft, trim, cargo and ballast tank loads. The on-board DCS was also used to transport data from the turret to the main monitoring computer. DATA COLLECTED The monitoring system and the sensors used are described in the previous section. An overview of the type of data is given in Table 2. The monitoring campaign on board Glas Dowr started on October 1, 1997 and was completed in July In these 20 months the automated monitoring system has been connected to DCS, fully commissioned, tested and inspected with regular intervals. Maintenance, repair and replacement of components have been conducted when needed and when possible in view of the production operation and travel schemes. Major problems encountered during the campaign concerned: - The vessels DCS-system which came into operation late and was linked in April The straingauges in the ballast tanks suffering from corrosion (replaced September 1998) - The bad receipt of reference signal for the Enhanced capability of the Differential GPS - The wave rider buoy (lost in May 1998 and December 1998) - The sub sea current meter which could not be recovered in October An overview of the availability of the signals i.e. the actual data collected over the complete measurement campaign is given in Table 3. Despite the above problems, the data collection over the full campaign of 20 months covered 74 per cent of the time with an average of 90 per cent of the desired signals. Encountered wave conditions The wave conditions encountered during the measurement campaign from October 1, 1997 to August 1, 1999, are represented in the wave scatter diagram in Table 4. As illustrated by the diagram the campaign covered the complete range of wave conditions representative for the Central North Sea. The highest waves were encountered during the storm on April 3, 1998 where significant wave heights up to 10 meter

5 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 5 were measured. In these steep wave condition the monitoring system was fully operational including the directional wave rider buoy and a set of high quality data for the present JIP and future work was obtained. DATA ANALYSIS General To perform a fatigue assessment for a specific ship, it is necessary to know the structure s behaviour under a wide range of environmental and operational conditions. These relations were derived directly from the full-scale data by simultaneously monitoring the environmental conditions, loading conditions and the ship s response. The time records derived from the measurements are processed in three steps. Firstly every month so-called longterm statistics were derived. For each signal mean, average, extreme values and standard deviation are derived and plotted as function of time. These results are used for overall quality checks of the system and for general purposes. Over the complete duration of the projects statistics are being collected. For example a wave scatter diagram of the encountered sea conditions is established. From these monthly records specific periods with a duration of 30 minutes were selected on the basis of stationary behavior, weather etc. For these selected periods detailed so-called short term spectral and statistical analysis have been conducted. Subsequently signals were combined into quantities relevant for fatigue loading such as global and local strains for further evaluation and comparison with computational results. The strain in the stiffeners due to different load components was investigated. Using the strains, measured by the Long Base Strain Gauges, and the strains, measured by the local strain gauges in the stiffeners, the strain due to global hull girder bending, outer pressures and inner pressures has been calculated. Correlations between different signals were investigated. Long Term Analysis The Long Term Analysis consisted of the calculation of the statistics (mean, minimum, maximum and standard deviation) per data file (half hour) for all measured and calculated signals. The measured signals comprise: Ship s Distributed Control System (DCS): Turret strain Wind velocity Wind direction Draft fore and draft aft Level in water ballast tanks Level in cargo oil tanks Anchor angles Structural Monitoring system Relative wave height Accelerations and angular velocities Pressure Temperature Strain Heading vessel Also statistics have been presented for motions in 6 d.o.f. at c.o.g., motions at specific locations and hull girder bending moments. The motions were calculated using the local accelerometers. The vertical bending moments were derived using the Long Base Strain Gauges mounted on deck. Presented wave data, comprise the significant wave height, the mean zero up crossing period and the wave direction. The wave direction has been presented for the wave with the highest spectral energy. The wave direction has been presented both absolute and relative to the vessel, using data of the heading of the vessel. The wind direction has been presented absolute to the vessel. One statistic file was generated for each data tape, which contains data files for one week. After four weeks data, the results of the statistics have been reported. Examples of the long term analysis results are presented in Figure 5. Long term analysis have been conducted over the entire duration of the measurements on a monthly basis. Short Term Analysis Using the performed Long Term Analysis, interesting periods were chosen, which were analysed in more detail. The selection criteria of the data for the Short Term Analysis were: Stable environmental conditions. The measured wave spectra during recording of the data files just before and just after recording of the selected data file should be almost equal to the wave spectra during recording of the selected data file itself. Also the standard deviation for the heading of the vessel during recording of the selected data file should be small (5 degrees) A sea state was chosen, in order to fill a scatter diagram. Storm conditions were preferable.

6 6 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC The Short Term Analysis consists of data analysis per file (half hour data). The motions (see Long Term Analysis), hull girder bending moments and the membrane and bending strain components have been calculated. For these data, including the measured pressures, strains and relative wave heights a standard analysis has been performed and presented in a separate subdocument which comprises (see Figure 6): Time traces (300 seconds) Spectra Transfer functions over the waves Rain-flow counts For each measured or calculated signal some statistical and spectral values have been given. A theoretical rain-flow count distribution has been computed and presented as well, assuming a Raleigh distribution using the total number of measured cycles. Disturbed wave spectra at the radar locations have been calculated using the measured motions. Relations between different signals were investigated. Transfer functions between the relative wave height, measured by the wave radar s and the outer pressures have been computed and compared with a theoretical pressure model. Also transfer functions have been determined between the measured pressures and the bending strain components in the stiffeners. These transfer functions have been compared with calculations using a theoretical beam model. Using symmetric considerations, the membrane strain components and long base strain gauge data was divided into strain due to horizontal and vertical hull girder bending. The results were related to each other and have been compared with the simple beam model. Fatigue damage per half hour according to fatigue Class B, C, D, E, F, F2, G and W for the locations strain gauges have been welded, have been calculated. CONCLUSIONS FPSO Glas Dowr was equipped with an structural monitoring system collecting automatically and continuously all data relevant for fatigue loads on the FPSO as well as her response; i.e. wave, ballast and cargo pressures, motions, global and local strains, ship condition and weather conditions. During 22 months of her production on Durward and Dauntless field in the North Sea, FPSO Glas, data has been collected during 74% of the operational time and an average of 90% of the installed sensors. The measured data sets comprise a large variety of wave conditions including severe storms with significant wave heights up to 10 m (corresponding to maximum wave height. 19 m). Extensive and valuable data sets both for fatigue load analysis and method verification were derived and used within the project (see ref [2]). Same data sets can be used for future R&D on related aspects such as green water analysis. A structural monitoring system as developed for Glas Dowr provides reliable data on the actual extreme fatigue and incident loads on an FPSO and the fatigue damage experienced. Long term analysis on the measured data have been conducted and long term statistics reported on a monthly basis. For selected periods short term analysis have been performed and reported. References 1. Bultema S., Krekel M. and Boom van den H.J.J., FPSO Integrity; Joint Industry project on Fatigue Loads OTC 12142, Houston, May Francois, M., Healy B., Fricke W., Mitchel K. and Mo O., FPSO Integrity; Comparative study on fatigue load calculation methods OTC 12144, Houston, May Kaminski, M.L. and Krekel M.; Reliability analysis of fatigue sensitive joints in FPSO, OMAE 95, Copenhagen, Potthurst R. and Mitchell K.; Verification of FPSO Structural Integrity PRADS 98, The Hague, Shin, Y. C. Lee and Jones D.E.; Integrated Motion, Load and Structural Analysis for Offshore Structures, PRADS 98, The Hague, Payer, H.G. and Fricke W.: Rational Dimensioning and Analysis of Complex Ship Structures, SNAME Annual meeting, New Orleans, Adegeest, L.J.M.; Non-linear Hull Girder Loads in Ships PhD Thesis, Delft, Erb, P.R., Jeffrys E.R., Mercier J.A. and Peters D.J.H.: Lessons from TLP Performance Measurements, Proceedings Marin Jubilee Meeting, ISBN , Wageningen Boom, H.J.J. van den; Mooring and Dynamic Positioning 23 rd WEGEMT, Full Scale Surveys of the Performance of Ships and Platforms, Genova, 1997.

7 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 7 Figure 1 - General Plan, Webframe Midships and Intermediate Floor Glas Dowr

8 8 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC Figure 2 Location of straingauges and pressure transducers at frame 66.5

9 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 9 Figure 3 Vessel motions measured by means of Enhanced Differential GPS EDGPS; Data file 8d093h Displacement [m] Direction East Direction North Vertical Time [min]

10 10 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC Figure 4 Vessel motions measured by means of accelerometers and rate sensors Amp. Surge [m] Amp. Sway [m] Amp. Heave [m] 2 0 8d093h Time [s]

11 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 11 Figure 5 Long term analysis Vertical bending moment fr Nov Dec. 1998

12 12 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC Figure 6 Short term analysis Vertical bending moment fr 66.5 Nov. 20, :30pm

13 OTC FPSO INTEGRITY; STRUCTURAL MONITORING OF GLAS DOWR 13 Table 1 Accuracy and range of motion sensors (g: gravity acceleration) Sensor Resolution Range Survival X,Y m/s2 1 g 10 g Z m/s2 (1+1) g 10 g Rotations 0.04 degr/s 100 degr/s 100g Table 2 Measured data Area Data Source Ship response Local strains in tanks Strain gauges Global strains at deck LBSG/LDTV Turret strains LBSG/DCS Wave pressures side hull Relative wave height around hull Ballast tank pressures Cargo tank pressures Slamming pressures Green water strains Motions in 6 dof Pressure gauges Wave radar s Pressure gauges Pressure gauges Pressure gauges Strain gauges Accelerometers/angular rate Environment Wind speed and direction Vessel wind set/dcs Incident condition Relative wave height Directional wave rider Relative wave height Kittywake radar Annasuria directional wave rider Wave radar s Current Subsea buoy Vessel condition Drafts DCS Tanks levels DCS Heading vessel Compass Position/low frequency motions E-DGPS Anchor line angles DCS

14 14 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC System Structural Monitoring System Distributed Control System DCS Wave Rider Buoy Sub-sea Buoy Compass EDGPS Kittiwake wave and wind data Week no Week no Week no System Structural Monitoring System Distributed Control System DCS Wave Rider Buoy Sub-sea Buoy 1) Compass EDGPS Kittiwake wave and wind data Anasuria wave and wind data System Structural Monitoring System Distributed Control System DCS Wave Rider Buoy Sub-sea Buoy Compass EDGPS Kittiwake wave and wind data Anasuria wave and wind data Week no Notes 1) Subsea buoy not recovered yet

15 15 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC Table 4 Scatter diagram of wave conditions encountered during the campaign (10 Oct 97-1 Aug 99) Mean Zero Up Crossing Period [s] Significant wave height [m]

16 16 HENK VAN DEN BOOM (MARIN), MAX KREKEL (BLUEWATER) AND PIETER AALBERTS (MARIN) OTC Photo 1 Refit of straingauges in ballast tanks September 20, 1998