OTC 17172

Offshore Monitoring; Real World Data for Design, Engineering and Operation H. van den Boom, J. Koning, and P. Aalberts, MARIN

Copyright 2005, Offshore Technology Conference This paper was prepared for presentation at the 2005 Offshore Technology Conference held in Houston, TX, U.S.A., 2–5 May 2005. This paper was selected for presentation by an OTC Program Committee following review of information contained in a proposal 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, its officers, or members. Papers presented at OTC are subject to publication review by Sponsor Society Committees of the Offshore Technology Conference. 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 a proposal of not more than 300 words; illustrations may not be copied. The proposal must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, OTC, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract The challenging development of new platform concepts and their installation in deeper water in more remote areas and more severe weather conditions require a direct feedback from offshore experience to design and engineering. Moreover the platform operation itself can be enhanced by utilizing the actual behavior of the platform and its environmental conditions. Present day sensor technology, data acquisition and transmission systems enable continuous monitoring of the dynamic response of structures in relation to the actual environmental conditions. Motions, loads, structural response as well as the detailed wind, wave and current conditions at the platform can be recorded to derive the environmental loading, the platform response characteristics and special phenomena such as VIV. Obviously adequate analysis and presentation of the measured results is crucial to meet the objectives of a monitoring campaign. This has proven to be a powerful combination not only to provide feedback to design and engineering but also to give ‘feed forward’ to the operation itself as well as to inspection and maintenance procedures. Design and engineering are based on limited input such as metocean data which is used for computational methods and guidelines with obvious limitations. Full scale data provide the actual input data and results to validate and develop design and engineering methods. In other cases the actual operation benefits from monitoring as decision making can be based on solid data. Finally monitoring can contribute to record the history of loading and response of platforms which is important for long-term purposes such as structural health monitoring.

Introduction Experience has always been a driver for improvement. Some people have argued that experience is mainly based on mistakes. Others have stated that there is no such thing as a ‘bad experience’ as we always learn from them. In offshore technology, experience has been built up from the early days when oil had to be produced inshore in shallow water by means of ‘marinized’ rigs located in sheltered areas, to the present deep water oil and gas production in deep, harsh and remote locations around the world. Many innovations have been achieved by combining practical experience with a thorough understanding of physics. Experiments and laboratory tests have greatly contributed to this development and they still do. For each major offshore platform concept scale model wave basin tests are still being performed. Not only to check design values but also to observe whether certain phenomena have not been overlooked in the design process and numerical analysis. Sophisticated analytical and later numerical models have been developed (often with the aid of model basins) to analyze the dynamic behavior of platforms in further detail and they enabled the evaluation of various concepts in an early design stage. Computational and physical scale models share the fact that they have been constructed by people with a specific purpose and therefore they do not necessarily resemble reality in all its aspects and detail. Relevance, applicability and accuracy of these models have to depend on each aspect under investigation. Mutual comparisons of various models are obviously helpful but for the final verification and validation only the Real World counts. For this purpose visual observations or incidental measurements as conducted in the past do normally not suffice. Most interesting phenomena in the behavior of platforms such as motions or loads are related to the environmental conditions i.e. the wind, wave and current which have a stochastic nature. For this reason continuous automated monitoring over a longer period is required. Present day sensor technology, computer capability, data net work and storage facilities, enable the extensive and detailed monitoring of the behavior of offshore platforms in many aspects. Over the last decade the development of these systems and the demand of the industry have resulted in a permanent or a temporary monitoring system on many new platforms. The data provided by these systems is not only used for design

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verification and validation of numerical models and engineering methods, but also for establishing better design criteria, forensic research and for development of new concepts. Moreover monitoring systems contribute to the safety and the economical operation of platforms by providing the operators on-line information on weather conditions, platform motions, riser loads and consumed platform or component life time. Operation as well as inspection and maintenance can benefit significantly from dedicated monitoring. Obviously the sensor and computer technology involved is essential for this work, however crucial to successful monitoring is the physical understanding of the dynamic behavior aspects as this understanding is key to the system design and the derivation and presentation of the crucial results from the abundance of data collected. In this paper several aspects of dynamic behavior of offshore platforms are discussed in relation to monitoring. Examples of monitoring systems are presented for offshore transports, FPSO’s, offloading buoys and deepwater production platforms. Dynamic behavior of offshore platforms Motions An important feature of offshore platforms and in particular offshore vessels and floaters such as TLP’s, Spars, Semi-submersibles and FPSO’s, is their motion characteristic. These motions are basically in 6 degrees of freedom and described by 3 translations (surge, sway and heave) and 3 rotations (roll, pitch and yaw). For free floating units mean deviations only exist in the vertical modes heave, roll and pitch) and originate from hydro-statics, weight and other mean forces such as the mean wind force. First order wave forces result in motions with the same periods as the waves (typically 1-20 seconds) and with amplitudes linear with the wave heights but strongly dependent on the geometry and mass of the structure. Moored floaters feature low frequency motions in the horizontal modes due to the spring action of the mooring and the mass of the floater. The natural period is normally large (50 to 500 seconds) and at these periods the damping of water is limited to small viscous effects. Therefore low magnitude excitation such as originating from second order wave forces or from wind speed variations [1] will result in resonant motions with large amplitudes. Such motions may also be observed in the vertical modes (heave, pitch, and roll) on platforms with large mass and small hydrostatic reaction forces i.e. spars and semi-submersibles [2]. Ships, barges and FPSO’s are known for their resonant roll behavior. The current FPSO Roll Jip is aiming at reducing such roll motions (www.marin.nl) Monitoring of motions is nowadays common on board most floaters and offshore vessels. Compact sensor units based on 3 accelerometers, 3 solid state rate gyros or on Fiber Optic Gyros (FOGs) provide continuous and accurate records of

motions in all 6 modes. The value of motion recording is increased significantly by utilizing this information to derive the operation critical data such as the motion, acceleration or velocities at specific locations, relative motions between floaters or the fatigue loads in the e.g. a jacket transports on towed barges. A next step in heavy lift transports is to minimize critical motion events by weather routing. For this purpose Marin developed an On Board Advisory System OBAS based on the Safetrans JIP-results. The OBAS system which has been developed with Dockwise and has been installed on their “Blue Marlin” heavy lift vessel, comprises detailed weather forecasts, motion prediction, wave and motion recording and derivation of motion related behavior (Figure 1).

Dapps System Outline “Zeevast” project

Dockwise Blue Marlin

DAPS / OBAScomputer system

IO blocks IO blocks

Waveguide radar

RS485 fieldbus

RS 232/422

Inside accommodation close to cable access outer deck

Outside on deck / cargo Various hermetically sealed

weldable straingauges

Anemometer

Bridge nav dataGPS, gyro, etc

6 dof motion sensorMARIN MQK

DAPS system hw comprises

• PCI extension board 8 x RS232/422

• PCI profibus interface card

Figure 1 - On Board Advisory System on ‘Blue Marlin’ Circular and bluff platforms such as spars and loading columns sometimes feature large amplitude motions in constant current or wind. This behavior is discussed below under ‘risers and tendons’.

Loads External loads on offshore platforms often originate from the environmental conditions i.e. wind, wave and current. These flow fields exert their loads through dynamic pressures on the hull (mainly wave and current) and on the topsides (wind). For this reason these exciting loads can not be measured directly but should be derived from pressure measurements, from the counteracting forces delivered by the mooring or from the structural internal loads. These internal structural loads can sometimes be measured directly e.g. when a sub structure is connected to the main structure by means of a clear mechanical interface such as topsides connected to the hull or anchor winches. In the hull itself the internal loads can only be derived by measuring structural elastic deflection (normally referred to as ‘strains’). The local strain however is the summing of various contributions such as weight/buoyancy, global bending, local panel loading e.g. due to cargo or ballast pressures but also from un-equal heating by the sun and residual strain from welding and vibrations in the hull. On a platform moving in

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waves the strains are furthermore affected by the motions due to the inertia forces in the structure and possibly in the tank volumes. To derive the hull loading from the recorded strains requires therefore a good modeling of all these individual contributions. A good example of such a procedure is the monitoring of the fatigue loads on FPSO “Glas Dowr”. As part of the FPSO Integrity JIP (Figure 2) , “Glas Dowr” was equipped during her conversion from a trading double hull tanker into an FPSO with a system to record all data relevant for fatigue loads [3,4].

Figure 2 - FPSO Integrity JIP As shown in Figure 3, two cross-sections of the FPSO were instrumented with pressure gauges to record wave, crude and water ballast pressures at 3 levels. Hull girder strains were measured in the ballast tanks at the same locations at both the outer and inner shell. At the same cross sections the relative wave height on Port and Starboard side were measured by level gauge radars. Similar radars were located on the stern quarters and on the bow. The global bending of the hull was measured by 4 long base strain gauges fitted on deck. The wave induced first order motions were measured by means of accelerometers and rate gyros and the mean position and low frequency motions were recorded by means of enhanced Differential GPS. The heading of the FPSO was derived from a dedicated compass. To complement the instrumentation the

strains in the turret were measured as well as the slamming pressures underneath the bulbous bow, and on the turret deckhouse. To record the loading condition, crude and ballast tank levels the monitoring system was interfaced with the onboard DCS-network. A directional wave rider buoy was deployed 1 nm from the FPSO to record the wave energy and direction. In total 168 signals were recorded at a sampling rate of 10Hz.

Figure 3 - Instrumentation FPSO Glas Dowr The monitoring campaign for the FPSO Integrity JIP comprised 22 months in 1998 and 1999 when the FPSO was producing the Durward and Dauntless fields in the North Sea some 150 nm East of Aberdeen. Although the campaign was aiming at fatigue loading, interesting extreme events were encountered such as the April 3, 1998 storm where 10 m (33 ft) significant wave height was recorded by the monitoring system. Recorded bending moments have been compared with the design values as shown in Figure 4.

Figure 4 - Recorded Bending moments compared with design

values Data was analyzed on-line as a quality control measure. Half hour statistics of each signal were presented as time series of periods of one month. As illustrated by Figure 5 these ‘long term analysis’ provided an excellent overview of the results and assisted in understanding the operations on board such as

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offloading and to select periods of stable conditions for more detailed statistical and spectral analysis. The fatigue analysis included rain flow counts using the total number of measured cycles [5].

Figure 5 - Long term analysis The results have been compared with the results of fatigue analysis conducted by the 5 participating Classification Societies (Figure 6). The FPSO Integrity campaign has significantly contributed to the development of FPSO fatigue design and engineering as utilized by the 24 participants (oil companies, operators, designers, yards and class societies) in this JIP [5].

Figure 6 Comparison of fatigue life calculations Dynamic Positioning Station keeping by computer controlled thrusters is becoming more and more important now that the offshore development is moving to deeper and deeper water and pipeline infra structure on the seafloor often prohibits anchoring. Although Dynamic Positioning has been introduced in the 70’s by deepwater drill ships, the effectiveness and power consumption of DP vessels, often offer room for improvement. Thruster arrangements and DP-controls are often not integrated in the vessel. Flow interactions between thruster mutually, thrusters/hull and current often lead to significant performance degradations. Monitoring of the performance of the vessel in combination with the DP-controls, the actual power consumption and the environmental conditions provides solid data for improving thruster arrangements and controls. Accurate monitoring of the main propulsion, the thrusters or the podded propulsors to obtain the delivered thrust, the shaft torque and the consumed power are essential in these campaigns.

Another approach to improve DP operations is to make better use of the knowledge of the exciting forces. Traditionally the DP control algorithms are based on a feed back loop where position deviations are used as input for thrust control. Feed forward is only applied for wind, where the measured wind speed and direction is used to predict the wind load, is counteracted by thrust prior to the actual position offset. In the DP-JIP this feed forward approach has also been applied to waves. By measuring the relative wave height around the vessel with 6 level gauge radars, the wave drift forces on the Shiehallion shuttle tanker Loch Rannoch have been proven to be estimated correctly (Figure 7).

Figure - 7 Loch Rannoch DP monitoring Mooring Floating production and offloading units in all water depths and locations around the world have to rely permanently on their mooring for station keeping in particular in severe storm conditions. Monitoring of the mooring tension is not only important to adjust the correct pre-tension, but also to assure that all lines are intact. For deepwater moorings, featuring synthetic fiber ropes, monitoring is essential to adjust for the continuous material creep. To monitor the tensions in chain and wire often use is made of pressure gauges or strain gauges in winch, fairlead or chain stopper foundations, however the calibration of such devices is elaborate and friction of lines over fairleads may affect the measurement. [6]. In line tension measurement by instrumented shackles or load pins is more accurate but may be difficult to realize because of the required pre-tensioning through chain stoppers. Underwater power and data transmission is still mostly done by hardwire cable although acoustic data transmission is getting more and more reliable. Local data logging is another viable alternative but requires periodical replacement by divers or ROV. Fiber optic measurement of tension is certainly the future for synthetic fiber ropes where the optical fiber can be installed during the production of the rope and protected by the rope jacket.

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An interesting instrumentation of mooring line tension was designed, fabricated and installed by Marin for the Girassol Buoy. This offloading buoy is located offshore Angola in 1350 m water (see Figure 8).

Figure 8 - Girassol Offloading buoy in 1350 m water offshore Angola

The buoys displaces 715 ton and measures 19 m in diameter. The mooring consists of 9 lines composed of steel chain and polyester fiber rope in a 3x3 spread. After line failures in 2002, it was decided by Total E&P Angola to equip the buoy with new lines and new double articulated arms connecting these lines to the buoy skirt which enable future pre-tensioning of the lines [7, 8]. This pre-tensioning is an important aspect as the pre-tension in the lines is high and polyester fiber exhibit creep. For the adjustment of the pre-tension at installation, tension monitoring during special events and for future pre-tensioning, all lines were instrumented with a dedicated Tension Measurement System. To fulfill the accuracy and robustness requirements Rod Connecting Arms (RCA’s) were instrumented with dedicated amplified, encapsulated strain gauges. To derive the two bending moments and the axial tension, two gauges were spot welded on each side of the arm. The sensor assembly was then protected by dedicated covers which were filled with special resin (Figure 9).

Figure 9 - Sensor assembly on the RCA after coating

Accuracy of the system was verified by a calibration procedure where each RCA was test loaded up to 1000 kN. After the installation of the RCA’s the subsea power and signal cables were connected to the sensor assembly (Figure 10) and Tension Measurement System was used immediately to assist the pre-tensioning of the new lines.

Figure 10 - The RCA’s and tension monitoring system in operation In Figure 11 typical tension recordings for 3 anchor lines in one cluster are presented.

Figure 11 - Tensions measured in 3 mooring legs Risers & Tendons Due to their circular cross section and their slender and elastic properties, risers and tendons in current are vulnerable to Vortex Induced Vibration (VIV). This phenomenon may also occur with complete platforms composed of a circular or bluff hull such as Spars. VIV is caused by flow separation at the trailing end of the structure where the separation point oscillates around the bluff structure causing a Von Karman vortex street in the wake which is responsible for oscillating transverse (lift) forces on the structure. Depending on the mass and elastic support of the structure this may result in resonant oscillations. The amplitudes of these motions can be significant when the damping at the natural

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period is low (Spars or loading columns). VIV may be prevented or reduced by geometry modifications and by adding strakes to the structure which fix location of flow separation how ever the phenomenon is not yet fully understood and both mathematical and physical modeling of VIV are still developing. In case of risers and tendons VIV may cause large fatigue damage in a relative short period. For these reasons the monitoring of the dynamic behavior of risers and tendons on deep water platforms in strong currents, such as the loop current in the GoM, is important to operate the platform safely. Loads in the risers are not only originating from VIV but also from platform motions, wave, current loads and impacts at the Touch Down Point. Riser (and tendon) dynamics include tensions, bending radii (flexible) and moments (SCR), vibrations and full 3-D excursions along the full length [9]. For the top-end of the risers and tendons such instrumentation can be integrated in the connection with the platform such as the flexible riser hang-offs, SCR porches or tendon supports (see Figure 12).

Figure 12 - Instrumentation at riser top

To record the dynamic behavior along the riser in deep water is more of a challenge. Good results have been obtained with self contained sub sea sensors units featuring battery packs and data loggers. The draw back is that for installation and retrieval ROV’s have to be available and that signals are not available ‘on-line’. Acoustic data transmission under water is improving rapidly however reliability (noise sensitive) and power supply are still problematic. For this reason hard wire cabling with multiplex technology is still an option for reliable data transmission. Optical Fiber Technology combines the sensor and data transmission. By means of discriminating laser reflection by specific Bragg gratings, strains can be measured at numerous locations with a single fiber with lengths up to 2000 m. The fiber can be installed on the steel riser and be protected by the riser insulation and coating. Integrating this in the riser construction and installation is straightforward when utilizing reel laying method but more elaborate in the traditional pipe laying mode.

Wind, Wave and Current As the dynamic behavior of platforms originate from wind, wave and current, accurate recording of these environmental conditions is essential for the success of monitoring campaigns with permanent systems. The wind, wave and current conditions are the drivers of the design and engineering of platforms as they provide important design criteria, which are normally based on historical statistics. Long term monitoring of the conditions have already shown that these criteria often have to be reviewed and up-dated. The recent NTL of MMS for deepwater platforms in the GoM to monitor the current over the full depth of the water column is an example thereof. Accurate information on the conditions is also of importance for the operational and safety on board. Wind speed and direction are essential for helicopter operations. Wave and current information for supply boats, divers and maintenance jobs. The interpretation of the recorded behavior of the platform also requires detailed wind, wave and current information. By utilizing this information motions and loads can be derived in dimensionless form such as response amplitude operators and load transfer functions which are required to verify design data and methods whereas the full time traces are required to do a deterministic time domain simulation. It should be noted that also the duration, sampling and processing of the environmental conditions should be related to the aspects under investigation. In particular low frequency excitation of motions requires special processing [1]. Wind speed and direction is traditionally measured by means of an anemometer with vane. In hurricane conditions these sensors are often damaged and for this reason solid state acoustic sensors which measure the wind speed in 3 components are favored. Acoustic measurements are also the state of the art in deepwater current measurement. Acoustic Doppler Current Profilers (ADCP) have proven their accuracy and robustness for measuring the current speed and direction to a depth of 1000m below the surface (Figure 13). A 38 kHz ADCP is able to measure the velocity components in 60 so-called bins of 16 m (53 feet). Distortions by reflections may be caused by neighboring structures, water surface and the seafloor. An upward looking ADCP can be mounted in a sub-sea buoy moored just above the sea floor to record the near bottom current and to complement the total water column. Horizontal ADCP’s are used for measuring the surface layer currents as well as the waves. An accurate and practical means to measure the wave profile is the use of down looking level gauge radars. The relative wave height measured by this device has to be compensated with the local vertical motion of the radar to derive the absolute wave height. Combing the signals of 3 distributed wave radars the full 3-D directional wave spectrum can be derived (Figure 14).

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Figure 13 - ADCP set up and close up This 3-D wave spectrum can also be derived from X-band (navigation) radar by filtering the wave scatter clutter of the scanner signal. Wave direction and period have proven to be accurate from these measurements however the wave height is still approximated as the backscatter intensity is no unique measure for wave height.

Figure 14 - Wave radar onboard Marco Polo

Work is underway to measure the wave profiles from floaters some 1000 m ahead. Such remote sensing of waves enables the deterministic estimation of floater motions which will enhance the safety of motion critical operations. Integrated monitoring systems As outlined earlier the value of platform monitoring is strongly enhanced by integrating the sensor data into a single system where the envisaged critical aspects are derived form the available data and presented to the crew in such away that they can use it for the safe and efficient operation of the platform and to the designers and engineers so that they can verify, validate and develop their methods and obtain important input for future designs. An example of such an integrated monitoring system is the system Marin designed and installed on the Marco Polo TLP on behalf of GulfTerra and Anadarko (Figure 15) . The TLP payload is 11,500 tonnes at an displacement of 27,412 tonnes and it supports 6 x 13 3/8”dual casing top tensioned risers, one 12” oil export SCR and one 18”gas export SCR with options to accommodate future flow line SCR’s.

Figure 15 - Marco Polo platform

The monitoring system comprises the measurement of wave induced motions by a compact sensor unit with accelerometers and rate gyros, low frequency motions by means of Differential GPS, structural strains in the tendon support structures by means of protected and encapsulated amplified strain gauges, riser and tendon tensions and vibrations. Wind speed and direction is measured by solid state acoustic sensors at two locations (flare tower, and heli deck), wave elevation and direction by 3 level gauge radars and the current profile by means of ADCP. All sensors are hardwire cabled to the central main cabinet which is equipped with a UPS systems and a power supply from the emergency generator.

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All sensor systems, main cabinet and cabling were pre-installed on the hull and the topside in the outfitting phase at KOS in Ingleside within 4 months from order. Final connections, commissioning of the system and training of the crew were conducted in May 2004 after installation of the TLP in 1320 m (4300 ft) water at Green Canyon block 608. The measured data is subjected to quality control and to processing to derive required signals such as combined wave and low frequency motions. The Graphical User Interface (GUI) enables the inspection and control of the system, the review of actual and extreme values and the display of the signal histories (Figure 16). All measured and derived data is stored at a sampling of 5 Hz.

Figure 16 - Marco Polo GUI display The data-acquisition, process, display and storage software is based OPC (Ole for Process Control) server protocol which enables a modular connection of all sensor units and a seamless interfacing with other systems. In this way the system can also be interfaced with the platform data network and Internet so that results can be accessed continuously both offshore and onshore (Figure 17).

Figure 17 - World wide monitoring

The system has been in operation since its commissioning in June 2004. In September 2004 the passage of Ivan the Terrible was recorded successfully. This hurricane which passed at 120 nm distance resulted in maximum wave heights of 15 m at Marco Polo (Figure 18).

Figure 18 - Ivan the Terrible passing Marco Polo The monitored data at Marco Polo is utilized in a Joint Industry Project aiming for the verification of TLP design and engineering models and methods for future ultra deep TLP’s. To record the dynamic behavior along the riser in deep water is more of a challenge. Good results have been obtained with self contained sub sea sensors units featuring battery packs and data loggers. The draw back is that for installation and retrieval ROV’s have to be available and that signals are not available ‘on-line’. Acoustic data transmission under water is improving rapidly however reliability (noise sensitive) and power supply are still problematic. For this reason hard wire cabling with multiplex technology is still an option for reliable data transmission. Optical Fiber Technology combines the sensor and data transmission. By means of discriminating laser reflection by specific Bragg gratings, strains can be measured at numerous locations with a single fiber with lengths up to 2000 m. The fiber can be installed on the steel riser and be protected by the riser insulation and coating. Integrating this in the riser construction and installation is straightforward when utilizing reel laying method but more elaborate in the traditional pipe laying mode. Conclusions With new concept platforms moving into deeper and more remote and exposed conditions, design & engineering methods need to be developed and validated. Operations of platforms including inspection and maintenance can benefit from On-line display of actual met-ocean conditions and platform responses. Monitoring systems can provide this information and are being installed more and more on such platforms. Crucial for the success of monitoring campaigns and permanent monitoring systems is the understanding of the physical phenomena involved. This is not only relevant for the design of the system and the selection of sensors but also for

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an effective and economic processing of data including the derivation and presentation of the critical aspects. Without this approach the abundance of collected data will lead to exhaustive and time consuming ‘data mining’. Smart processing of data actually may include the prediction of platform behavior in the future. An example of this is the on-line fatigue damage monitoring which assists offshore inspection and maintenance. In On Board Advisory Systems this approach can be taken even one step further by taking into account the weather forecasts in the behavior predictions in such a way that the actual operation can be optimized to enhance production and safety. “Monitoring is accelerating experience without having to

make mistakes”. REFERENCES 1. Boom, H. van den, Kuipers R.J.P.E.

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OTC Houston, May 2000. 5. Cammen, J. Van der, Francois, M., Kaminski, M.L.

“Is Uncertainty of Wave Induced Fatiguing Loading on FPSO’s Uncertain? OMAE Specialty Conference on FPSO Integrity,” Houston, August, 2004.

6. Boom, H.J.J. van den;

“Dynamic Forces in Tow Lines” RINA Spring Meeting, London, April 1986.

7. l’Hostis, D., Kerouedan, J.;

“Girassol Buoy & FPSO Tandem Offshore Operational Feedback 14 th FPSO Research Forum (www.fpsoforum.com)” Paris, October 27, 2004.

8. Martin G. Brown,

“Floating production Mooring Integrity JIP” OTC 17499, Houston, May 2005

9. Boef, W.J.C. et al.;

“Analysis of Flexible Riser Systems” 5th Int. Conference on Floating Production Systems, London, December 1989