WAVE IMPACTS EXCITATION ON SHIP-TYPE OFFSHORE STRUCTURES IN STEEP FRONTED WAVES. Arjan Voogt and Bas Buchner MARIN, The Netherlands

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1 Proceedings of OMAE-FPSO 2004 OMAE Speciality Symposium on FPSO Integrity 2004, Houston, USA OMAE-FPSO' WAVE IMPACTS EXCITATION ON SHIP-TYPE OFFSHORE STRUCTURES IN STEEP FRONTED WAVES Arjan Voogt and Bas Buchner MARIN, The Netherlands Joaquín Lopez-Cortijo Garcia IZAR FENE Shipyards, Spain ABSTRACT Wave impacts in steep fronted waves can induce extremely large impact pressures on the hull of moored ship-type offshore structures. As part of the SAFE-FLOW Joint Industry Project these loads were investigated with a dedicated series of model tests. Based on the results a prediction method is developed, which can be applied in the early design of the bow structure. The paper first summarizes the test philosophy and the developed prediction method. Then the practical design considerations related to bow slamming are discussed in a study focusing on the new DP-FPSO concept for a single environment. INTRODUCTION Several incidents in recent years have revealed that wave impact loading is an important issue for moored floating production systems. Wave impact damage has been experienced by both the Foinaven and Schiehallion FPSOs. The present paper was developed within the SAFE-FLOW project (SAFE-Floating LOading by Waves), which is initiated in response to encouragement by the UK Health and Safety Executive (HSE) following earlier joint research, such as the JIP F(P)SO Green Water Loading, completed in 1997 (Buchner, 2002). Its main objective was to investigate water impact loading hazards in detail. Based on model tests carried out for both a free floating FPSO and a fixed schematic bow a prediction tool (BowLab) is developed which will be discussed in this paper. EXPERIMENTS As part of the project MARIN performed 2 series of model tests at scale 1:60 in deep water. First tests were carried out on a free floating Schiehallion FPSO model (Figure 1). Figure 1: Bow impact event on free floating model In these tests in irregular seas the incident wave data, vessel motions and resulting relative motions, bow pressures and structural response are measured. The tests showed that more detailed load measurements were necessary and that an investigation was needed into the relation between the incoming waves and these loads. This resulted in the second model test series on a highly instrumented fixed simplified bow. The simplified bow was instrumented with a large array of pressure transducers and 3 force panels. The test program, also making use of extensive video recordings, was designed such that it was possible to determine the correlation between undisturbed wave shape and the impact pressure time traces. 1 Copyright 2004 by ASME

2 From these tests irregular sea incident wave data and bow pressure results are available on a fixed schematic bow structure with varying rake and plan angles. It was found that the magnitude of the wave impacts at the front of the bow is dominated by the wave characteristics (namely the local wave steepness), rather than by the motions of the ship relative to the waves (relative wave motions). Further the maximum pressures are measured close to the crest of the incoming waves. An example of a steep wave front reaching the bow structure is shown in Figure 2. Sea Stat motion RAO non lin wave WF ship motions empirical relations location im pac structural properties Figure 3: Load response prediction method bow slam properties structural response To predict the probability with which structural responses occur within one selected sea state, BowLab uses the different building blocks described in the sections below. Non Linear Wave The local wave steepness (dζ/dx) can be determined from measurements of the wave elevations in an array of probes. An example is shown in Figure 4, which shows the spatial wave profile for successive steps in time. The time step between the different lines is 0.31 seconds and the distance between the probes 6 metres allowing for an accurate derivation of the local wave steepness. Figure 2: Steep wave approaching the bow PREDICTION METHOD To calculate the probability of wave impacts on any given vessel structure, at any given elevation, a prediction method for the simulation of the water surface and the occurrence of wave impacts was created. Evaluating the available results, it was decided to develop a method that split up the problem in two main problems, see Figure 3: The relation between the local wave characteristics and the magnitude (and other characteristics) of the wave impact, the lower line in the figure. The position of the impact based on the related ship motions, the upper line in the figure. The combination results in a localised impact with specific properties, resulting in the structural response of a local structure with its specific structural properties. p 6 m MWL R1 R2 R3 R4 R5 R6 Figure 4: Visualisation of local wave steepness The combined spatial and temporal information of the sea state needed to derive the local wave steepness is not generally available (in full scale data and model tests). Therefore the vertical free surface velocity is preferred as input to the model. The vertical free surface velocity (dζ/dt) is related to the local wave steepness (dζ/dx) by the wave celerity. In steep waves that cause the bow impact, linear theory clearly under predicts the wave steepness. The most suitable method of simulating the water surface to give a reasonable probability of vertical free surface velocity was found to be second order wave theory, as described by Sharma and Dean (1981) for instance. Applying second order wave theory results in an improved prediction of dζ/dt, as shown in Figures 5 and 6 for the basin waves applied. 2 Copyright 2004 by ASME

3 measured 2 nd order linear dζ/dt linear 6 2 nd order 10 measured 12 Figure 5: Measured, first and second order waves Vertical free surface velocity Figure 7: Empirical relation for local impulse The relation is independent of the sea state (within the range tested) and holds for a schematic flat plate bow. Within the design method the mean fit is used as a maximum that can occur. For more realistic curved bow shapes the loads are reduced as described in the next sections. The spreading around this mean can be used as input to the derivation of the load factors in a first principles reliability approach. Figure 6: Probability vertical free surface velocity It is clear that the second order theory is not capable of describing the asymmetry in the measured non-linear wave. However within the accuracy of the present design tool this is not considered a critical aspect and the distribution of the vertical free surface velocities do match the measured nonlinear distribution reasonably well. Empirical Relations To estimate the probability of an impact at a certain vertical free surface velocity, the percentage of impacts occurring were counted for bins of vertical free surface velocities (Guedes Soares, et al. 2004). It is observed that up to a certain vertical free surface velocity no impacts occur. Above this threshold level, the probability increases linearly with the vertical free surface velocity up to the probability of 100%. Beside the slam probability, the slam magnitude is of vital importance. After analysis of all data, it was decided to relate the slam impulse (I), the area under the load time trace, to vertical free surface velocity (dζ/dt). Figure 7 shows the measured impulses versus the corresponding vertical free surface velocities. For different velocity bins the mean and standard deviation of the occurring impulses is added to the figure, resulting in straight lines. The comparison with the wave heights of the incoming undisturbed waves show that most slam loads occur relatively close to the wave crest. This is consistent with the observation that the maximum velocity occurs close to the crest front of the incoming wave. WF Ship Motions The test results showed that the slam centre occur close to the crest of the undisturbed non-linear wave. However for the design of a bow panel in a floating structure, also the ship motions should be taken into account to determine the location of the impact. To check the assumption that these motions in steep waves can still be described with linear diffraction theory, the model tests reported with a floating model (Voogt, 2001) were analysed further. An iterative procedure has been developed that fits a linear wave and its second-order contributions to a measured wave train. The procedure is described in Voogt and Buchner (2004) where a good comparison between measurements and linear calculations is found, which is confirmed in the time traces in Figure 8. 3 Copyright 2004 by ASME

4 and stiffeners in the bow area this is not always sufficient to determine an accurate response with resulting stresses. The monitoring on the Schiehallion bow (Hodgson, 2003) allows for this comparison between pressures and stresses. Based on these results the pressure duration, immersion velocity and spatial extent are considered critical (Figure 10). Data already described Impulse Magnitude Location of Slam Pressur Duration Structur Geometry Load- Response Prediction Immersion Velocit Figure 8: Measured and calculated vessel motions Impact Location Even for these large differences between the linear wave and the measured wave, the linear wave is still capable of describing the major part of the ship response. Therefore the following procedure is implemented to determine the location of the impact: For a time trace of the linear wave the motions of the vessel are calculated. With this linear wave non-linear wave components can be calculated and added to the time traces. Combining the time trace of the non-linear wave and the linear ship motions gives a relative wave motion in front of the vessel. From these relative motions the slam centre can be determined if a slam occurs. Figure 9 visualizes the procedure. The yellow ship is moving on the blue linear seas. While the red line indicates the second order wave which determines the impact size and location. Impact & location Target Bow Shape Structural Response Equivalent Pressure Figure 10: Bow impact characteristics Slam Spatial Extent For some impacts the pressure front travels upward over the plate, while others occur more instantaneously. If we determine the time between the impacts over the height of the flat plate differences of 0.0 to 0.3 seconds can be found for two adjacent transducers. This corresponds to a velocity of 10 m/s and higher. Up to immersion velocities of 100 m/s the pressure time trace of the slam show the traditional shape (progressive slams), above this velocity only short sharp (instantaneous) slams occur. Different immersion velocities tend to occur at comparable values of vertical free surface velocity. Therefore the probability distribution of the immersion velocities is determined. This distribution is independent of the slam size. The average distribution is shown in Figure 11. This figure shows two types of slams: The blue bars on the left result in slams that progress over the bow. The duration of the resulting load time traces on a bow segment are direct related to these velocities. For velocities above 100 m/s (the red bar) an instantaneous slam occurs. The duration of the resulting load time trace is independent off the target size and thus independent of the immersion velocity. 2nd order wave Figure 9: Wave impact location Bow Slam Properties So far the method focussed on the local impact pressures on the bow and on its position. However, to design the plating Figure 11: Probability of immersion velocities 4 Copyright 2004 by ASME

5 For the schematic bow with the large number of pressure transducers the spatial extent is derived by integrating the bow pressures downward from the maximum slam over increasing areas, with steps of 1.5 metre. This resulted in two typical types of slams: Instantaneous slam: The whole pressure time trace decreases with the integral over the area. This type of impulse is recognized as an instantaneous slam. The decrease of the pressure depends on the size of the pressure front and drops down fast during integration. Progressive slam: The rise time increases while the pressure impulse remains relatively constant. This second type of impulse progresses along the structure. Due to the pressure integration the rise time increases with the size of the area and the impulse remains approximately the same. In the prediction method, the time traces for instantaneous and progressive slams are scaled to represent the correct impulse and immersion velocity for each calculated impact. With the resulting curves and the location of the slam centre the equivalent static pressure on a specified panel can be determined. Structural properties The width and curvature of the panel can result in a reduction of the equivalent static pressure. For very small panel width the results can be considered two dimensional and multiplication of the pressure with the width of the panel gives the design load. For larger panels the curvature of the bow becomes important. Even for a flat plate the average pressure reduces with the width of the panel due to local wave effects. An empirical formula is fitted through the measurements performed in Glasgow and documented in Barltrop and Xu (2004). Since one would expect the curved bow factor to be always less than the flat plate value, the pressure for a curved plate is limited to the flat plate value. The resulting factor between the local pressure and the pressure average over the width of the plate is shown in Figure 12. characteristic of bow impact loads, particularly instantaneous slams, that pressure loads are applied very quickly. Dynamic response of the structure is therefore likely, requiring that dynamic amplification be included in any design calculations. The structural evaluation of bow slam loading and structural response is in principle a coupled hydro elastic problem. These coupled effects are likely to be most important when the natural period is longer than twice the duration of the slam. The full coupled problem has not been investigated in the present project. Instead a simplified approach has been used. With the rise and decay time (and their characteristics) of the slam and the natural frequency of the structure the dynamic response can be calculated with a single degree of freedom mass spring system. Excluding hydro-elastic effects, the structural response can be described with the equation of motion in which a constant damping, added mass and mass are assumed. For a typical large local slam the rise and decay times are about 10 ms and for a panel natural period of about 1/10 sec the DAF will be 0.96 (Figure 13). This DAF smaller than 1 occurs if the structure has a long period and does not have time to respond to the impact load before it starts to reduce. DAF 0.96 Rise time 0.01 s Decay half time 0.01 s Structural freq. 10 Hz Pressure Pmax Pmax/2 Figure 12: Pressure reduction for curved panels Tr T½d Time Structural response The previous sections focussed on the bow impact loading. However, for the evaluation of ship-type offshore structures, the structural response under this impact loading determines whether a structure is able to survive a certain event. It is a Figure 13: Example Dynamic Amplification Factor 5 Copyright 2004 by ASME

6 DP FPSO The proposed method was now applied, as a case study, to the DP FPSO concept, developed by IZAR FENE Shipyards in co-operation with FMC Sofec, DNV and MARIN, Cortijo et al (2003) and see Buchner and Cortijo (2003). The hull of the DP- FPSO has been designed taking the following key aspects into consideration: a) Environmental forces induced by waves, wind and current should be minimised in order to keep the CAPEX/OPEX of the Unit within reasonable limits, as well as the emissions produced by the dual (diesel/gas) generators, which must be continuously running in a fully DP vessel. b) The hull configuration must allow for an adequate arrangement of the thrusters at the fore and aft ends, to avoid thruster-thruster and thruster hull/turret interactions. Moreover, the layout of the machinery spaces and the ease of thruster overhauling is significantly affected by the selected vessel hull forms. c) In heavy storms (hurricanes, squall events, etc.), the vessel behaviour against green waters and slamming occurrences must be acceptable, insuring adequate safety of the people onboard and sensitive equipment (fire fighting, lifesaving equipment, etc.). d) Layout requirements, such as: Accommodation forward (for navigation in sail away condition after disconnection), turret amidships (to minimise riser system dynamics and vessel motions for turret buoy (dis)connection operations), and offloading equipment at the stern for tandem offloading, have to be catered for in the design. e) The hull design must also account for constructability issues, i.e. a simpler structure is easier to built, inspect and maintain. In order to accomplish the above mentioned design requirements of the DP-FPSO, the vessel hull has been provided with the following features: - Hull forms typical for new-built FPSOs, with prismatic mid-body in way of the cargo tanks, sloped flat transom and triangular bow. Rounded bilge of adequate radius to accommodate bilge keels, as well as camber of suitable height. - The fore and aft ends have been designed to properly accommodate the envisaged number and size of thrusters (three forward and three aft). The triangular bow allows a suitable arrangement for the three forward thrusters, minimises the wind, current, wave drift forces and forward resistance, and yields a simpler construction. In order to provide room for installation of the offloading equipment and flare, the hull shape aft at main deck level will be square (full beam) in a cantilever structure of rounded shape to prevent slamming. - A forecastle deck of suitable height is provided to insure acceptable green water loads in extreme design conditions. A poop deck is provided as well for similar reasons. Bulwark of reduced height (to avoid large induced loads on the stanchions and supporting deck) is fitted forward and aft as required. A bow flare angle of 30 degrees has been provided above the maximum draft. - A moonpool of diameter as specified by turret designer is located amidships. In Figures 14 and 15 the artist impression and body plan of the vessel are shown. The vessel characteristics are shown in Table 1. Figure 14: Body plan of the DP FPSO Table 1: Main particulars of the DP FPSO Parameter Value Unit Displacement (fully loaded) 214,757 Metric tonnes Length between perpendiculars 260 m Breadth 46 m Depth 28 m Draft (fully loaded) 20.5 m Figure 15: Artist impression of the DP FPSO 6 Copyright 2004 by ASME

7 WAVE IMPACT ANALYSIS The prediction method can be used to estimate a suitable long-term distribution of slam response pressures on any relevant structure and this in turn can be used as input to a reliability assessment of the design. From this assessment load factors can be derived which needs to be applied to asses the design pressure on the construction as described by Fyfe (2004). Although in a real design the most critical weather conditions should be determined for a specific vessel, the present paper focuses on design methodology rather than the design for a specific area. Therefore, the present paper makes use of the 100-year Hurricane for the Gulf of Mexico, see Table 2. Table 2: 100-year Hurricane for the Gulf of Mexico 4 m 5m Parameter 100-year wave Unit Significant wave height 12.5 Metre Spectral peak period 13.0 Seconds JONSWAP gamma The structural configuration is checked both for plating and stiffener design at different elevations on the bow and for the supporting transverse frames. The characteristic dimensions are derived from the drawings in Figure 16. The right hand side drawing shows the vertical distance of 5 metres between the transverse frames. Four stiffeners are located between these frames, resulting in a horizontal stiffener spacing of 1 metres. The figure also shows the stiffener design from elevation with 4 metre web spacing. As each plate is supported by a stiffener at the top and bottom the typical panel dimensions for the calculation of the loads are the equivalent. For the present design a panel of 4 metre wide and 1 metre height is used. For the transverse frame design a panel height of 5 metre is used which is the distance between the decks. Figure 16: Structural Configuration As mentioned before the width and curvature of the panel can result in a reduction of the equivalent static pressure. For very small panel width the results can be considered two dimensional and multiplication of the pressure with the width of the panel gives the design load. For larger panels the curvature of the bow becomes important. This curvature is described for the present design with the local panel radius derived from a plan view of the deck as shown in Figure m Figure 17: Panel diameter For this typical panel the design loads and response are calculated with the prediction method described above. The results are shown in Table 3 for different elevations of the panel above the mean surface level. The table shows both the static and dynamic pressure. In the calculation of the dynamic pressure the panel stiffness and added mass are used to calculate the period of the response (30 Hz). 7 Copyright 2004 by ASME

8 Table 3: Equivalent pressures on plate and stiffeners for different elevations on the bow Elevation above MSL [m] Dynamic Pressure [kpa] Static Pressure [kpa] For this specific structure the dynamic response results in a significant increase of the equivalent pressures. The plate and stiffeners are designed against these dynamic pressures. In the chosen condition the maximum pressures are experienced at 10 metres above the mean surface level which is close to the crests. With the prediction method the sensitivity of these loads against the radius of the panel can easily be checked. Table 4 shows the results for the typical panel located at 10 metres above the MSL. The results show that the pressures increase significantly when a blunt bow is used. Table 4: Equivalent pressures on plate and stiffeners for different curvatures of the bow Local Panel Radius [m] Dynamic Pressure [kpa] Static Pressure [kpa] For the transverse frame design a panel of 4 metres wide and 5 metres height is used. For the increased are the response periods are longer. The prediction method can easily be used to perform a sensitivity study on this period and the elevation of the transverse frame. The calculated pressures are shown in table 5 and are lower than for the plate and stiffeners, especially when accounted for the longer natural periods. For the design loads these pressures needs to be multiplied by the panel area resulting in larger loads on the transverse frames than on the stiffeners as expected. Table 5: Equivalent pressures on transverse frames for different elevations and natural periods Elevation Static Pressure Dynamic Pressure [kpa] [m] [kpa] 30 Hz 20 Hz 10 Hz CONCLUSIONS In the present paper the practical design considerations related to the wave impact problem are discussed. The following can be concluded: Wave impact is a significant problem for floating offshore structures in steep waves and needs to be assessed in the early design of the structure. The developed design evaluation method can assist in this process as it predict the bow slam loading problem from the input (scatter diagram) to the output (predicted load and response levels) based on a clear description of the bow slam physics. The method is based upon second order wave theory describing the wave steepness and an empirical relation between the wave steepness and the local impact. The position of this impact follows from a time domain analysis of the linear ship motions. The method assumes long crested waves which are considered a worst case scenario. Applying this tool shows the strong influence of the local panel radius on the calculated loads. The loads can be minimized by decreasing the bow radius as this minimizes the wave entrapment. The structural response determines the design pressures on the constructions and need to be carefully considered in the design process. ACKNOWLEDGMENTS The SAFE-FLOW project (SAFE-FLOating offshore structures under impact loading of shipped green water and Waves) is funded by the European Community under the Competitive and Sustainable Growth Programme (EU Project No.: GRD ) and a group of 26 industrial participants (oil companies, shipyards, engineering companies, regulating bodies). The participants are acknowledged for their interesting discussion of the results during the project and the permission to publish the present paper. The authors are solely responsible for the present paper and it does not represent the opinion of the European Community. REFERENCES Buchner, B., 2002, Green Water on Ship-type Offshore Structures, PhD-thesis Delft University of Technology Buchner, B. and Cortijo, J.L., 2003, Design Aspects Of Green Water Loading On FPSOs, Proceedings of OMAE, paper no , Cancun. Barltrop, N and Xu, L., 2004, Wave Slap Loading on FPSO Bows, Naval Architecture & Marine Engineering of Glasgow & Strathclyde Universities (NAME) 8 Copyright 2004 by ASME

9 Cortijo, J.L, Lago, F. and Mendez, A., 2002/2003, New FPSO-Based Concepts for the Deep Water Challenge, 17th Annual Conference on Floating Production Systems (December 2002, London) and Petrotech 2003 Conference (January 2003, New Delhi). Fyfe, S and Ballard, E., 2004 A design evaluation methodology for green water and bow impact type problem, Proceedings of OMAE-FPSO 2004, ASME, New York, paper OMAE-FPSO' Guedes Soares, C., Pascoal, R., Antão, E.M, Voogt, A.J. and Buchner B. 2004, An approach to calculate the probability of wave impact on an FPSO bow, Proceedings of the 23 st OMAE Conference, ASME, New York, paper OMAE Hodgson, T and Barltrop N., 2003, BP Schiehallion full scale monitoring, January 2000 to January 2003, Atkins Report No ES-03, Issue 01, Aberdeen, UK. Sharma, J. N. and Dean, R. G., 1981, Second-Order Directional Seas and Associated Wave Forces, J. Soc. Petroleum Engineering, 4, pp Voogt, A.J., 2001, Discussion Problem Identification, SAFE-FLOW project, MARIN Report No OB, Wageningen, The Netherlands. Voogt, A.J. and Buchner,B., 2004, Prediction of Wave Impact Loads on Ship-type Offshore Structures in Steep Fronted Waves, Proceedings of the ISOPE2004, paper no JSC Copyright 2004 by ASME