Engineering Forecasts and Monitoring for Offshore Construction Operations

Transcription

1 Engineering Forecasts and Monitoring for Offshore Construction Operations Peter Lai 1 (M), Anton Slozkin 1 (AM) 1. Saipem Limited, Engineering Department A successful offshore construction operation such as platform installation, decommissioning and subsea pipeline construction relies on good engineering and planning before the operation. Good planning is helped by having the ability to use the first available weather window to carry out the operation at the field offshore. Therefore, the ability to forecast the governing engineering criteria during the operation becomes an essential component. This paper presents the developed methodology to forecast events and a summary of the status of the development, including its limitations and applications. It is focused in three areas: forecast and monitor the dynamic motion characteristics of the operation, pile stick-up motion for jacket installation, and prediction of sagbend bending moment during subsea pipeline construction. It is concluded by presenting the proposed future development and advantages. KEY WORDS: Forecast, Monitor, Motion, Wave Energy, Marine Operation, Platform, Installation, Decommissioning and Pipelaying. NOMENCLATURE Design criteria ratio which is multiplied by the design limiting sea state to get the allowable sea state for operation DAF Dynamic Amplification Factor DP Dynamic Positioning LRET Lloyd s Register Educational Trust MRU Motion Reference Unit P-Delta Large Deflection Effect RAO Response Amplitude Operator RINA Royal Institution of Naval Architects T 2 Significant period which is based on the zeroth and second order spectral moments INTRODUCTION Good engineering and careful planning are essential components to make an offshore construction operation successful. When we are offshore, we also rely on the ability to use the first available window for the operation to perform the operation safely and efficiently, and to minimize the downtime. Traditionally, the limiting sea states for a marine operation such as platform installation, decommissioning and subsea pipeline construction are defined in the engineering phase using theoretical spectra. During the offshore operations, mariners and engineers make reference to the weather forecast and compare to the defined limiting sea state to make the key decision for the operation (i.e. go or no-go). The usual weather forecast comprises wind speed, significant wave height and significant period (T 2) of the sea state. It also provides swell height, period and direction information. However, this forecast does not fully describe how the energy is distributed in terms of direction and periods. For example, if there is more than one swell component, the forecast swell would be for the mean of swell components. In the marine construction industry, we are working in calm sea states compared to other offshore design industries, who deal with scenarios such as 50 or even 100 year storms. In these calm installation sea states, the usual condition comprises multi-wave components from multi-directions. Mariners would call them Confused Seas. However, each component may have different effects on our operation. In the engineering phase of the project, analysis has been based on single (e.g. Jonswap) or double peaked wave spectrum (Ochi-Hubble Spectrum/ Torset Haugen), theoretical spectrum to study the dynamic behaviour. However, these theoretical scenarios can be quite different to the actual installation sea states. In real operation, the sea state usually comprises of more than one component. The components can occur from different directions with difference spreading, Tp and Hs and; most important of all; have different effects on the measured motion. Figure 1 shows an example of the analysed and actual seaspectra.

2 The forecast at the requested field location is issued by the Met Office UK twice daily. It includes the tabulated wave energy distribution data for every three hours for a 5 day period in the form of a data file attached to the , which is sent to the requested address. Figure 1 Sea State Analysed and that during Operation The operation, hence, relies on the mariners and engineers experience and capability to link the forecast to our marine operation. In confused seas, it is difficult to estimate the impact on our operation even for an experienced mariner and engineer. The Presentation of the Forecast Wave System The tabulated wave energy distribution data can be difficult to extract for direct interpretation of the wave system. With a view to that, Saipem Limited had contracted Global Maritime, London, to develop a proprietary software application, VRFGM, to read the forecast data and transform the data into information that engineers and mariners can easily apprehend. An example screen shot is presented in Figure 2. In this paper, the wave energy spectrum forecast is described. That gives us a full picture of wave energy distribution in both direction and period and also the spreading of each wave component. Based on this wave energy forecast, a methodology has been developed to forecast the engineering values directly during the marine operation. This paper focuses in the development of the following three areas. 1. Motion forecasting and monitoring of our construction vessel. That also includes the forecast of the dynamic behaviour of the offshore structure which is lifted on the hooks of the crane vessel during installation in the air or in water. 2. Pile deflected motion and bending moment forecast in wave when the pile is stabbed into the pile sleeve before being driven down to fix the jacket structure 3. The sagbend bending moment during pipelaying operation. The application, limitation and further development are also discussed in this paper. WAVE ENERGY FORECAST Forecast Data The details of the wave energy forecast are presented in references; Lai, Hannam, McCarthy and Sovilla (2006) and Lai, Kennard and Harrison (2013). Figure 2 The Wave Forecast Browser The top graph shows the variation of significant wave height for the whole sea state for a 5 day period. In this example, the waves will slowly increase from 1.2m on 00:00 25 th May to 2.0m by 28th May evening and then drop back to 1.3m by the evening of 29 th May. The details of each forecast sea state can be seen by moving a cursor left and right. The corresponding information of the sea state at that time is shown in the lower graphs. The graphs in the lower-right-hand corner present the wave energy distribution in direction and wave periods. The graph in the lower-left-hand corner shows the contour plot of the wave energy distribution. On 28 May evening 18:00, the sea will be dominated by a component from ESE with 8 sec Tp and a weaker component from N/NNE with 8 sec Tp as well. In this example, there is no Hs distribution for each individual component. However, the Met office UK is working on the partitioning of the wave energy distribution and having the corresponding Hs and Tp of each component presented in their newly-developed wave component forecast With the presented information, mariner and engineer can have a better understanding of the sea state that they would be in and hence a better understanding of the behaviour of the operation. Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 2

3 ENGINEERING FORECAST AND MONITORING Forecast The engineering forecast is performed in the Frequency Domain to minimise the computation requirement. However, it is restricted only to the linear cases or cases with linear assumptions to simplify the mathematical model. The dynamic responses per unit regular wave amplitude for a range of wave periods, called Response Amplitude Operators (RAO), are prepared for different wave headings prior the operation. These can be interpreted as measures of response of a specific parameter (i.e. motion, velocity, force and deflection) due to wave excitation forces. For example RAOs can represent the motion at a specific point on the construction vessel; such as the crane tip, the last roller or simply at the centre of gravity. This also can be the magnitude of the bending moment of a pile or a pile structural deflection under wave loading before it will be hammered down. The 3 hour maximum response can be calculated by combining the pre-prepared RAOs and the forecast wave energy spectrum. The program, VRFGM, is also used to perform the forecast calculation with RAOs and wave energy forecast data as input files. The 3 hour maximum responses are stored in ASCII Text files and also presented in Contour Plots and Line Graphs. ASCII Text Files The 3 hour maximum and significant forecast values are output in 24 files corresponding to each vessel heading relative to North, for the full rosette with 15 interval. These files can also be read by the on-board monitoring systems to perform instantaneous correlation between measured and forecast values. Figure 3 The Contour Plot The mariner can arrange the vessel at specified heading in order to obtain the minimum response, such as roll or pitch as an example. Line Graphs In some marine operations, the vessel heading is fixed such as lowering down a topside structure onto the platform, lowering down a subsea/jacket structure onto the sea bed and pipeline construction. For these operations, the line graphs, rather than the contour plots, will be particularly suitable. A typical line graph is presented in Figure 4 which presents the six degrees of freedom of the 3 hour maxima at a fixed vessel heading with reference to North. The maximum values in these files can also be easily rearranged to represent a practical situation for the operation using EXCEL, MathCAD or MatLab, such as the maximum motion among different locations. Contour Plots A typical contour plot is presented in Figure 3. The engineering values are plotted against the vessel heading with reference to North and the time and date. The vessel heading with minimum or maximum response values can be easily identified and presented in the contour plots. As an example, the red dotted line in Figure 3 shows the vessel heading with minimum response. Figure 4 The Line Graph Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 3

4 MOTION FORECAST AND MONITORING Forecast The motion at the crane tip of the construction vessel and the cargo barge motion response are the usual values that are forecast for our platform installation and decommissioning operations. With the help of the contour plot, the barge will be able to line up with the waves for minimum dynamic response such as roll response as example. In general, that would be the best heading to lift a heavy offshore structure off the cargo barge to ensure a maximum limiting sea state. Although the methodology is restricted to linear assumption, it does provide capability to forecast multi-body dynamic behaviour; such as offshore structures on the hook in the air or subsea structures fully submerged before they touch down on the sea bed. Figure 5 shows a typical installation of a flare tower. platforms at the top of deck modules or the welding station at the J-lay tower, which can be well above water level. Hence their safety can be considered. Monitoring A motion monitoring system is fitted on Saipem 7000, which is a semi-submersible crane vessel with up to 14,000t lifting capability, to monitor the crane tip motion and acceleration. The motion is measured by the Motion Reference Units (MRUs) of the on board DP system and transferred to the crane tip with the rigid body motion assumption. Details are presented in Lai, Hannam, McCarthy and Sovilla (2006). This monitoring system is linked to the forecasting computer. The measured crane tip accelerations are presented together with the maximum forecast values on the screen of the monitoring system and provide an instantaneous correlation between the forecast acceleration and measured values. A correlation screen shot is shown in Figure 6. Forecast Measured Figure 6 The Monitoring System - Correlation Flare Pin Figure 5 A Flare Tower Installation Installing a tall structure, which is not heavy, can be a challenge. Even a weak swell would excite the pendulum mode of the structure. With the presented methodology, the effect of all the wave components can be considered. The motion at the flare pin can be forecast and the operation can be planned accordingly to avoid large motion response. In addition, it is also possible to forecast the motion on the vessel where our workers will be working, such as rigging Figure 6 presents the crane tip acceleration for a module lift at pre-hook up condition. Both cranes were in the Lift-off positions and were rigging up the slings. The correlation can build up confidence in the forecast values and will increase the design criteria ratio () (B700 Section 4 of DNV, 2011) which the warranty surveyor applies to the design limiting sea state, with a view to covering the uncertainty in actual and forecast environment condition. The allowable operating sea state will be times the design limiting sea state. The 2007 RINA and LRET Ship Safety Award was given to Saipem Limited for the work on the motion forecasting and monitoring system for Marine Operation which was based on the system fitted on the Saipem 7000 (RINA 2008). PILE STICK-UP MOTION AND BENDING MOMENT FORECAST FOR JACKET INSTALLATION Background The pile is a steel tubular structure which fixes the jacket platform on the sea bed. Figure 7 presents the typical Jacket structures which are fixed with piles (the dark red tubes). Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 4

5 hand, it may be non-conservative compared to the actual sea state. In this case, the operation would be unsafe and could cause damage to the pile/hammer/jacket structure. This will directly and indirectly affect the safety of the operation and our profit. The application of the engineering forecast methodology for the pile installation is a new approach in the offshore industry. The main purpose of it is to improve planning of the piling operations and determine the operational window. The forecast predicts the actual stresses in the pile s structure as well as its amplitude and direction of motion relative to the Jacket. This methodology provides a good support and improves safety of the pile stick-up and driving operations. Figure 7 Typical Jacket Structures fixed by Piles The piles can be of various lengths depending on the soil properties and type of the Jacket structure installed. It can be long, slender and also flexible. When the pile is stabbed-into the pile sleeve before hammered down, it can be water surface piercing and the clearance between the pile and jacket structure can also be very small, as shown in Figure 8. Top Shim Pile Stick-Up Bending Moment and Deflection Analysis Structural Dynamic Effects In the calculation, the structural dynamic effect and large deflection effect are considered separately and combined it afterwards. The base shear forces of the slender pile under regular waves with unit wave amplitude and a range of wave periods are calculated with and without structural dynamics considered. In the computation model, the effect of the gap between the shim in the pile sleeve and the pile is ignored. In addition, the current load is considered as a steady side load to the pile which pushes the pile to one side with initial inclination. The corresponding Dynamic Amplification Factor (DAF) is established by comparing both results. A linear check is performed by comparing the DAF for different wave amplitudes at each calculated wave period. The DAFs near the natural period (typically ±0.5 sec) are non-linear while those outside the natural period zone are linear. The non-linear viscous damping effect plays an essential part in the resonance zone. Figure 8 Pile Stick-Up The pile is stressed and deflected under wave excitation and also experiences other forces like static force due to the weight of the underwater hammer sitting on top of the pile (hammer weight can be even higher than the weight of the pile itself). The risk of damaging the pile, hammer and jacket structure can be very high. The limiting sea states for piling operations are defined in the engineering phase using a two-dimensional theoretical sea state which is quite different to the actual installation sea state as stated in previous section. It may be conservative compared with the actual sea state, causing unnecessary downtime. On the other Figure 9 Dynamic Force (or DAF) for the Period Range Analysed Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 5

6 In the engineering phase, the Dynamic-Base-Shears are calculated in regular waves for different wave amplitudes with the same wave period near the natural period. The non-linear curve is then curve-fitted to a 2nd order polynomial which is a function of wave amplitude. For the theoretical spectrum, the amplitude at the specified wave period is known at a given sea states. The corresponding Dynamic-Base-Shear is hence calculated. Finally the corresponding Dynamic Base-Shear spectrum is calculated as well as the 3 hour maximum values. Therefore the limiting sea states are defined with the nonlinearity properly considered. The wave amplitude at the specified period (e.g. natural period) for the spectrum of the limiting sea states can be identified and apply a straight line (linearised) through to the corresponding wave amplitude. The results take into both irregular wave spectrum and regular waves. It is not a perfect way to handle it but it is a practical way to handle it with the forecast wave energy information. The large deflection theory is mainly to handle the deflection caused by the heavy hammer. The deflection is non-linear proportional to the hammer weight. The deflection caused by wave excitation is linear. A linear check was performed and confirms that large deflection effect due to waves is linear with the gap between the shim and pile ignored. That matches with the expectation. The corresponding RAO is hence created. Bending Moment and Deflection RAOs The RAO calculated from the large deflection theory are multiplied with the corresponding DAF RAO. The resulting RAO is the RAO used for forecasting, as shown in Figure 11. A linear assumption is applied to those within the natural period zone which would give a reliable estimation at the limiting sea state. The linear assumption would underestimate the DAFs in small waves and overestimate the DAFs in big waves, as shown in Figure 10. However, it would give a good estimation around limiting sea states. Figure 11 Calculation of the Bending Moment RAOs Figure 12 schematically presents large deflection and structural dynamic loading problems. Figure 10 Linearisation of the Dynamic Force A DAF RAO is hence developed. Large Deflection Effects Since the pile can be slender and flexible, the large deflection theory is used to calculate the corresponding maximum bending moment at the top of the shim within the pile sleeve of the jacket and deflection at the top of the pile. As with the DAF calculation, the bending moment and deflection is calculated with 1.0m regular wave amplitude for a range of wave periods. Figure 12 Pile Structural Behavior from P-Delta and combined P-Delta + Structural Dynamic Loading Analyses Forecast Results As with the motion forecast, the bending moments and deflections are forecast by combining the RAOs and the wave energy forecast spectrum. This approach is valid for a site with low or moderate current. The vortex-induced-vibrations (VIV) are not considered. The RAO is specially arranged to show the direction of the pile deflection with maximum surge. An example is shown in Figure 13. Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 6

7 The bending moment can also be forecast and compared to the allowable, as shown in Figure 16 as an example. Pile Bending Moment with MHU1700 DrivingCondition Bending Moment (MNm) Forecast 100% 80% 70% Figure 13 Pile Motion Contour Plots Pile motion direction According to the forecast in this example, the pile will move North-South on the day 11/07, while the pile will move ESE- WNW on 12/07 evening. The forecast calculates deflection and direction of the dominant pile motion due to the various wave components of the wave spectrum. It is concluded that the pile generally has two components of motion, i.e. main longitudinal (which is in-line with dominant wave component) and transverse (due to the wave spreading and effect of other wave components and asymmetrical loading in the pile). Hence the motion footprint draws an elongated figure of eight. As presented in Figure 14, the forecast software detects the dominant motion component, records its direction and reports in the forecast Sep-11 Time (hours) 00:00:00 Figure 16 The Forecast Pile Bending Moment The allowable bending moment with different driving efficiencies are also presented. The periods with values higher than the allowable will be the downtime and the operation can be planned accordingly. Application in the Offshore Industry Project Background The methodology has been used for one of the Saipem s installation projects. The project involved installation of the two jackets and twelve piles. In this case, the piles were extremely slender with significant concentrated load on the top from the hammer. There also were very tight clearances from the jacket structure (shown in Figure 17), hence apart from maximum forces, it was critical to predict pile motion amplitude and especially its direction. Figure 14 Determination of the Dominant Pile Motion Figure 15 shows the corresponding resultant deflection at two locations of the piles (at the top and 21m above water level) as an example. Pile Deflection JRP Pile No Hammer 0.4m Penetration Figure 17 Pile at the jacket Leg with Small Clearance Deflection (m) Jul-12 00:00:00 Time (hours) Figure 15 The Forecast Pile Deflects TOP 21.0m Forecast Validation A forecast validation was performed during this operation. The piles were monitored when they were in the stick-up condition. The monitoring was performed for the period between 0.5 and 1.5 hours and it was believed that it allowed us to capture the maximum motion. The validation procedure is summarised on the flowchart below in Figure 18. Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 7

8 Validation Case 2 Figure 20 presents the pile which was monitored for the Validation Case 2. In this case there was a discrepancy between forecast and measured Hs, hence the forecast deflection could be scaled down (following a linear assumption) in order to obtain the deflection which would correspond to actual wave conditions. The final value became closer to the actual value but the discrepancy is still high. Further correlation is needed in order to identify the source of discrepancy. Figure 18 Forecast Validation Flowchart The pile motions were recorded using camcorder and then analysed by re-playing the video and examined on the computer screen. This method provided a very quick set-up. It was very flexible, very easy to perform and did not cause any delays to the operations. The main disadvantage was that the accuracy is highly dependent on the view angle which is required to be perpendicular to the direction of the pile motion. Pile Under Observation The wave conditions were continuously collected from the wave rider buoy. It was providing such information as wave direction, significant wave height and period. The data was recorded and then compared with forecast data. The following two sub-sections present some of the validation cases. Validation Case 1 Figure 19 presents the pile which was monitored and its location relative to the Jacket. The view angle was acceptable, as the waves (and the pile) where moving transversely. Also the forecast and actual Hs gave a good correlation. Figure 20 Pile Monitored for Validation Case 2 PIPE LINE SAGBEND BENDING MOMENT FORECAST AND ASSESSMENT Sagbend in Pipelaying During a S-lay pipelaying operation, the pipe line hanging between the last roller and the sea bed is called the sagbend, as shown in Figure 21. As can be seen from the result, the measured maximum deflection is very close to forecast one. Figure 21 Pipelay Operation Pile Under Observation Figure 19 Pile Motion Monitoring Validation Case 1 When a big, heavy and stiff pipeline is being laid in shallow water, the sagbend bending stress of the pipeline needs to be carefully monitored to avoid buckling the pipeline. All the existing methods would only detect the buckling of the pipeline after it happened but not before. A methodology is presented here to forecast the sagbend bending moment and can be used to avoid buckling the pipe during lay. Lai, Kennard and Harrison (2013) shows the details. Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 8

9 The Methodology Define the Relationship The pipeline sagbend natural period is well outside the wave frequency for a heavy and stiff pipeline being laid in shallow water. The sagbend bending moment can be governed by the motion response of the lay barge. In order to be able to forecast and monitor the sagbend bending moment, the relationship between the motion response of the lay barge and the sagbend bending moment is examined in detail in the engineering phase of development. The sagbend bending moment is calculated using the standard time domain finite element pipe laying analysis with different sea states and wave headings. Traditionally, the computed sagbend bending moment is presented against the Hs and Tp of the analysed sea state together with the corresponding wave heading. The sagbend bending moment can be plotted against the motion, velocity and acceleration at the centre of gravity of the lay barge and at the last roller of the stinger for a range of sea states. It can be observed that if the studied parameter does not have a direct relationship with sagbend, the scatter will be high. It means the sagbend bending moment is really dependent on other parameters instead. For example, Figure 22 shows the 3 hour maximum sagbend bending moment plotted against the sway accelerations at the last roller for a quartering sea. Bending Moment (knm) Dynamic SagBend Bending Moment Vs Last Roller Sway Acceleration Wave Heading 135 to the Vessel Hs=1.5m Hs=2.0m Hs=2.5m Hs=3.0m All Hs Curve Poly. (All Fit Hs) Bending Moment (knm) Dynamic SagBend Bending Moment Vs Last Roller Heave Motion Wave Heading 135 to the Vessel Heave Motion (m) Hs=1.5m Hs=2.0m Hs=2.5m Hs=3.0m All Hs Poly. Curve (All FitHs) Figure 23 Sagbend Bending Moment Against Heave Motion at the Last Roller The natural period of the sagbend of the pipeline is really outside of the installation wave periods in the presented case with high tension for a heavy and stiff pipeline. The sagbend bending moment depends on the movement of the stringer especially the heave motion. Therefore, a strong correlation between sagbend bending moment and the heave motion at the last roller is expected. At any specified heave motion, we can determine a single sagbend bending moment from the curve. The relationship is hence defined. Forecast and Monitoring the Sagbend Bending Moment As with the motion forecasting, the motion response RAOs at the last roller are prepared. The motion at the last roller can be forecast combining the RAOs together with the forecast wave energy. The sagbend bending moment can then be forecast using the pre-defined relationship. Similarly, motion at the last roller can also be measured by the on board monitoring system and the sagbend bending moment can be back calculated with the pre-defined relationship. Hence, the sagbend bending moment is monitored during lay. PLANNED FUTURE WORKS The following future works are planned to push the methodology to next phase of development Sway Acceleration (m/s^2) Figure 22 Sagbend Bending Moment Against Sway Acceleration at the Last Roller The scatter is high and there are more than one sagbend bending moment for a given sway acceleration. This means that the last roller sway acceleration is not directly related to the sagbend bending moment. This does match with known experience. On the other hand, if the sagbend bending moment is plotted against the heave motion of the last roller on the stinger, the scatter is significantly reduced to nearly a smooth curve, as shown in Figure 23. Include non-linear effects by applying forecast directional wave energy spectra to our analytical model and carrying out Time Domain analysis rather than Frequency Domain analysis. The time domain analysis requires heavy computation and can only concentrate on a few limited cases to confirm the effect of nonlinearity on the operations Further work required to implement directional spectra on slow drift excitation Carry out further correlations between measured and forecast pile stick-up motion and direction Carry out model test to validate the relationship between last roller heave motion and sagbend bending moments to confirm the findings. Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 9

10 CONCLUSIONS The methodology is developed to provide forecast to the engineering governing parameters directly and advise the mariners and engineers what to expect during operation. Hence the operation can be planned accordingly. The engineering forecast offers real advantages to the marine operations. We concluded the following advantages and added values to our marine operations. Loic Faure, Bastien Jacques, Yianni Florentiades and S7000 personnel for their work in Pile stick-up forecast and monitoring Xavier Chevalley for his work in sagbend bending moment forecast Stefano Sovilla and Stefano Meggio for their work in the S7000 Crane-tip Motion Monitoring System. Assist in Key Decision Making Process. Maximise Weather Opportunities for Marine Operation Reduce Operational Risk and Improve Safety Predict Ergonomic Situation Lower Weather Downtime and Increase Profit Forecasting Method can be used for other applications with RAOs in Frequency Domain with Linear Assumption. REFERENCES Det Norske Veritas. October Offshore Standard DNV-OS-H101 Lai, Peter, Hannam, Mark, McCarthy, Vince and Sovilla, Stefano. Motion Forecasting and Monitoring for Offshore Installation and Decommissioning Operation IMCA Annual Seminar, ACKNOWLEDGEMENTS The authors want to acknowledge the contributions from the followings to the present work. The UK Met Office for their work in the wave energy forecast Global Maritime for their work on the program, VRFGM, used here Lai, Peter, Kennard, Mark and Harrison, Richard. Forecasting and Assessing Pipeline Sagbend Response for S-Lay Vessel Motions Offshore Pipeline Technology, Miller, Barry. User Manual for Vessel Response Forecast Program Global Maritime Report, GM , July RINA. RINA Affairs June 2008 Lai Engineering Forecasts and Monitoring for Offshore Construction Operations 10