Challenges in estimating the vessel station-keeping performance

Challenges in estimating the vessel station-keeping performance Luca Pivano, Brede Børhaug and Øyvind Smogeli Marine Cybernetics, Trondheim, Norway E-mail: lp@marinecyb.com One of the challenges in the offshore industry is to make sure that a vessel or floating structure is able to perform its tasks in a safe and efficient manner. Many critical operations such as drilling and diving require high station-keeping accuracy regardless of the environmental conditions. For estimating a vessel s station-keeping capability, the traditional quasi-static DP Capability Analysis (DPCap) is often employed as specified in IMCA M140. This paper presents an experimental validation of the next level capability analysis tool based on time-domain simulations called DynCap: Dynamic Capability. Experimental results show that by considering the complete vessel dynamics, environmental forces, and control system dynamics, the DynCap analysis is closer to reality. Keywords: Dynamic Positioning; DP Capability; Dynamic Station-keeping Capability 1 Introduction In the last 10 years the number of dynamic positioning (DP) units has increased dramatically driven by an increased offshore activity. At the same time the operational safety as well as fuel consumption and emission reduction has gained more attention. One of the challenges in the offshore industry is to make sure that a vessel or floating structure is able to perform its tasks in a safe and efficient manner, according to given procedures, rules and regulations. For planning of marine and offshore operations, especially in harsh environments, it is essential to determine the weather operational window where the vessel can maintain its position and heading, typically also after a single failure. Many critical operations such as drilling, diving, and offloading require high station-keeping accuracy regardless of the environmental conditions. It is clear that with the increase in safety critical operations performed by DP units, the DP capability has become even more important than before. It is therefore fundamental to possess accurate tools for analyzing the vessel DP capability for different vessel configurations and weather conditions. The availability of such tools in the market today is very limited as they require advanced models of the vessel and its equipment. This represents a challenge due to the complexity of the system and the equipment to be modeled. Today the DP capability of a floating offshore structure is recommended performed by ISO 19901-7 and IMCA M140, where the latter is considered as the current industrial standard. DP capability analyses and plots are normally required by class. The DP system class notations however are not related to the results of these analyses but focus on the vessel redundancy concept. The idea behind the IMCA M140 specification is to enable a direct comparison of individual vessel s performance and provide an indication of station-keeping capability in a common and understandable format. However, there are open issues often addressed by operators, oil companies and ship yards: How much can we trust the results from the traditional capability analysis? Do they convey a realistic picture of a vessel s station-keeping capability in dynamic operating conditions? Are they conservative or non-conservative? 1

The traditional analysis based on the IMCA M140 specification, here defined as a DPCap analysis, is performed by statically balancing the maximum obtainable thruster force against a resultant mean environmental force due to wind, wave drift, current, and possible other loads. This is done for the full angle-of-attack envelope (0 360 deg). The results of such analyses are presented in form of polar plot (wind envelope) where the maximum wind at which the vessel can maintain position and heading is plotted for each angle of attack, typically given with 15 deg spacing. In addition, results may also be presented as thrust envelope showing the thruster utilization under given design conditions. A DPCap analysis is inherently quasi-static, meaning that several important assumptions and simplifications must be done to facilitate the analysis. Due to this, the results can be significantly different from the actual vessel capability. To obtain results closer to reality, model based experiments may be carried out, but due to the high cost involved these are rarely done. Another solution would be to run time-domain simulations using advanced models built upon vessel specific data. A DP Capability analysis tool based on time domain simulations was presented in Lauvdal (2000) but details on the modeling of the dynamic systems and environmental loads were not provided. In order to obtain results comparable to the actual vessel performance, modeling of the vessel dynamics and environmental loads must be accurate. An example of this challenge is provided in Muddusetti and Phillips (2006) where the measured DP capability was dramatically different from the computed one due to large interaction between the thrusters and the rig structure. In Pivano and Smogeli (2012) the next level capability analysis tool coined DynCap was introduced. DynCap is based on systematic time-domain simulations with a complete 6 DOF vessel model, including dynamic wind and current loads, 1st and 2nd order wave loads including slowly-varying wave drift, a complete propulsion system including thrust losses, a power system, sensors, and a DP control system with observer, DP controller, and thrust allocation. This tool can provide a variety of results other than vessel DP capability, from fuel consumption and power and thruster utilization, to vessel operability during the year. In this paper the methodology presented in Pivano and Smogeli (2012) is validated with experimental results showing that by considering the complete vessel, environmental force, and control system dynamics, results are much closer to reality. The outline of the paper is as follows: In section 3 we take a closer look at today s standard for DP capability, followed by a description of the DynCap methodology in section 4. In section 5, a comparison between the capability plots obtained with the traditional DPCap and DynCap, computed for a PSV design, is provided. Section 6 and 7 present the DynCap experimental validation and a discussion of the results, respectively. Conclusions are provided in section 8. 2

2 Abbreviations CFD Computational fluid DynCap Dynamic Capability dynamic DGPS Differential Global H s Significant wave height Positioning System DOF Degrees of freedom IMCA International Marine Contractors Association DP Dynamic Positioning PMS Power Management System DPCap Traditional DP Capability PSV Platform Support Vessel 3 Today s DP Capability Standard The IMCA M140 specification which the traditional DPCap is based upon is quite basic allowing the analysis to be computed with environmental forces from non-vessel-specific coefficients, thruster forces from generic rules-of-thumb and without giving specifications on DP control system and thrust allocation. It is possible to extend the IMCA M140 specification with more realistic assumptions and models, but this is not standardized. This can be done for example by using actual vessel model data such as wind, current, and wave-drift coefficients, realistic thruster models, and realistic static thrust allocation including e.g. forbidden zones and thrust loss effects based on actual allocated thrust. The fact that the IMCA M140 specification presents relaxed requirements and that the analysis can be computed employing disparate methods makes the comparison of the DP capability between different vessels difficult. Furthermore it is not straightforward to assess how realistic the results from the analysis are compared to the actual performance. One of the strongest assumptions in the traditional analysis is that the vessel is considered at rest. Assuming the vessel at rest when trying to estimate the limiting weather at which the vessel can stay in position is a paradox as the largest vessel motion will occur at those sea states. Due to this restriction it is not possible to include the dynamic loads from waves, wind and current, and the corresponding dynamic response of the vessel with its DP system. Hence, the DPCap analysis can only balance the static (mean) environmental forces with the mean thruster forces, meaning that a certain (assumed) amount of thrust must be reserved to counteract the unknown dynamic forces and vessel motion. Typically 15%-20% of the thrust is reserved for dynamic loads. This is often referred to as dynamic allowance. Furthermore, the 6DOF vessel motion and the related thrust losses, as well as all other dynamic effects in the propulsion system like rate limits cannot be included in the computation. Moreover, due to the quasi-static nature of the DPCap analysis it is also not possible to account for the transient conditions during a failure and recovery after a failure. For early design verification and concept development such an analysis may be adequate, since it is fast and relatively simple, and requires limited model knowledge. For a detailed study of a vessel s station-keeping capability, vessel operability, fuel consumption and emissions in realistic dynamic conditions, however, the traditional DPCap analysis is not adequate. 3

Next section presents DynCap, a dynamic capability analysis tool developed to remove the limiting assumptions in the traditional capability analysis with the final goal of getting results comparable to reality. 4 DynCap DynCap is based on systematic time-domain simulations with a complete 6 DOF vessel model. A block diagram describing the vessel simulator is shown in Figure 1. By allowing the vessel to move, the strongest assumption considered for the traditional DPCap is removed. This allows the employment of dynamic wind and current loads, 1st and 2nd order wave loads including slowly-varying wave drift, as well as the dynamics of the propulsion system and power system. A model of the PMS is also included to simulate relevant functionalities for DP operations such as black-out prevention, load limitation and sharing, and auto-start and autostop of generators. A model of the full DP control system model is also included with observer (Kalman filter), DP controller and thrust allocation, sensors, and position reference systems. The complete propulsion system model includes actuator rate limits and computation of dynamic thrust loss effects such as the interaction between thrusters, interaction between thrusters and hull, ventilation, out-of-water effects, and transversal losses. Figure 1: Time-domain simulator block diagram By considering the vessel, environmental loads and DP system dynamics, it is not necessary to assume a certain amount of thrust reserved for dynamic loads as for the traditional DPCap. DynCap utilizes all the available thrust capacity like the vessel would do in real life. In addition, the DP system model includes functions that can be found in the majority of DP 4

control systems available today, such as black-out prevention and load limitation. If the required power for maintaining position and heading exceeds a preset limit, the thruster loads are limited such that those limits are not passed. During DP operation, the vessel position and heading motion is characterized by two components: The DP motion, which is what is displayed on a DP screen, and is checked towards positioning limits in the DP (watch circles). This is a filtered, low-frequency motion, which is due to the mean wave drift, thruster, wind and current forces, and lowfrequency external forces from mooring lines and offloading hose/hawser for example. In literature, this is also referred to as the low-frequency (LF) motion. The harmonic motion due to first-order wave loads, which is oscillating about the low-frequency DP motion. In literature, this is also referred to as the wave frequency (WF) motion. The actual motion of the vessel is the sum of these two components. Depending on the requirements to the operation, either the DP vessel motion or the total vessel motion (DP plus wave frequency motion) can be used to check if the position acceptance criteria are satisfied in the DynCap analysis. Another advantage of the DynCap analysis, compared to the traditional DPCap study, is that the limiting environment can be computed by employing a wider set of acceptance criteria. The position and heading excursion limits can be set to allow a wide or narrow footprint, or the acceptance criteria can be based on other vessel performance criteria such as sea keeping, motion of a crane tip or other critical points, or dynamic power load. In this way the acceptance criteria can be tailored to the requirements for each vessel and operation. An example of position and heading acceptance criteria is shown in Figure 2. In this case, the wind envelope plot is found by increasing the wind speed and checking whether the vessel footprint stays within the predefined position and heading limits. Another benefit of employing the complete vessel dynamics is the ability of identifying temporary position and/or heading excursions due to dynamic and transient effects. One of the solutions for reducing the fuel consumption and emission is to run the engines close to the optimal shaft speed. When the power demand is reduced, for example in calm weather, it is more convenient to run only few engines to power the ship or rig. In order to do this, the power plant has to be set-up as a ring-bus where all the high-voltage switchboards are connected, see for example Settemsdal and Radan (2012). To avoid power black-outs in case of a failure, the operations rely on advanced protection systems and quick start-up of stand-by generators in case of sudden increase of power demand. In this case it is important to check that the load increase does not induce any power reduction to the thrusters that could cause a temporary excursion outside the acceptance limits. This excursion may put at risk the safety of the operations and cannot be identified with the traditional DPCap analysis. 5

Figure 2: Example of position and heading acceptance limits The next section will present a comparison between the capability plots obtained with the traditional DPCap and DynCap, computed for a PSV type vessel. 5 Comparison of DPCap and DynCap analysis results The comparison between the traditional DPCap and DynCap is carried out employing a PSV type vessel design. The design is the same as that of the model-scale vessel CyberShip III (Børhaug 2001) used at the Norwegian University of Science and Technology. The vessel is equipped with two electrically driven stern azimuth thrusters, and one electrical driven bow azimuth thruster. The vessel s main particulars are presented in Table 1. Table 1: CyberShip III main particulars Description Unit Value Loading condition name - Loaded SG=0.94, Departure Length overall m 68.28 Length between perpendiculars m 59.13 Breadth m 13.11 Draught m 4.59 Mass displacement tons 2003.4 To be able to compare the results, the vessel model used for DPCap and DynCap are configured with the same data where possible. The wind and current load coefficients are provided by a CFD analysis. The thruster model is configured based on data from experimental trials. The traditional analysis includes models of static thrust loss while DynCap includes also dynamic thrust loss effects as explained in Section 4. In the DynCap analysis the DP control gains is set to high (typical available gains on industrial DP systems are low, medium and high). For DynCap analysis it is assumed that the position reference systems are represented by two DGPSs. The thrust allocation is configured with forbidden 6

sectors for the stern azimuth thrusters such that they are not able to direct the propeller race towards each other as this creates a large thrust loss effect. The environmental loads were set to be collinear (current, wind and waves from the same direction). The results shown in this paper are obtained with no ocean current. The wind to wave relationship is taken from the IMCA M140 specification, valid for the North Sea. Further details on the analysis setup and the computations of the DPCap and DynCap can be found in Børhaug (2012). Figure 3 shows the wind-envelope plot obtained from the DPCap and DynCap. The outermost curve is obtained with DPCap with no thrust reserved for dynamic loads. Figure 3: Wind envelope Comparison between DPCap and DynCap Considering a typical station-keeping operation where the vessel would try to minimize the environmental forces by placing the bow against the environmental loads, the plot shows that the vessel would be able to maintain position with wind of about 26.5 m/s (0 deg of angle of attack) according to traditional methods of calculating the DPCap (Case 1). This limiting wind speed corresponds to a significant wave height (H s ) of 10.55 m. With the environmental loads coming from 90 deg, the plot shows that the limiting wind speed is reduced to 14 m/s (H s of 4.7 m). This is as expected as for this angle of attack the vessel environmental loads are larger than for head sea due to much larger cross-sectional vessel area against the weather. The red line (case 2) in the plot is obtained with DPCap with 20% of the available thrust reserved for dynamic loads. The limiting wind speed results are, as expected, smaller than the case with no dynamic allowance. 7

DynCap results are shown in the same figure with a black line and are obtained considering the vessel total motion (DP plus wave frequency motion) employing position and heading acceptance criteria of 5 m (radius of the position acceptance circle) and 3 deg (heading stays between -3 and 3 deg). These acceptance criteria are typically employed in most of the DP operations, see for example DNV (2011). By looking at the time-domain series from the DynCap analysis it was observed that the limiting factor is the heading controllability. Due to the motor dynamics and azimuth angle rate limits, the thrusters cannot produce force immediately in the desired direction, thus limiting the vessel heading controllability. The limiting wind values for 0 and 90 deg angles of attack are given in Table 2. 6 Experimental validation In order to validate the proposed DynCap method, a model scale experiment was conducted at the Marine Cybernetics Laboratory (MCLab) located at Marintek, Trondheim. Experimental setup The MCLab is a basin 6.45 m wide, 40 m long and 1.4 m deep. In addition the laboratory is equipped with a movable bridge, and a towing carriage, which is capable of speeds up to 2 m/s. The MCLab is also equipped with a wave maker, which is a 6 meter with single paddle and is operated with an electrical servo actuator and fans to generate wind. The wave maker is able to produce regular and irregular waves, up to 0.3 meter high. Vessel positioning is provided by an infrared camera system, which can determine the position and orientation of a floating body equipped with passive infrared markers. The camera system is capable of providing real-time position data, at a rate of 10Hz, which is similar to a normal GPS system send rate. The vessel used for the experiments, CyberShip III shown in Figure 4, is a 1:30 model scale of a PSV. For additional details on the experiment consult Børhaug (2012). In order to compare the results with the model scale experiments, the analyses are carried out with the scaled vessel model. The results are then scaled back to full-size to ease the discussion and interpretation. Figure 4: CyberShip III scale model 8

Results In order to obtain the station-keeping capability of CyberShip III, a line search is conducted, where the environmental loads are varied according wind to wave relationship used for the DynCap analysis in section 5. The same acceptance criteria as for the DynCap analysis are employed for the experiments, 5 m and 3 degrees, considering the necessary scaling. The experiment resulted in 23.5 hours of data logged, and over 10 million data points. Details on the result logs can be found in Børhaug (2012). Figure 5 shows the wind envelope for CyberShip III, where the results from DPCap, DynCap and experiments are included. Case 2 and Case 3 are the same curves as presented in section 5. In particular it is interesting to see that both plots obtained from DynCap and from the experiments have a dip at the 30, 60 and 90 degree angle. The fact that both the simulations, and the experimental results have these unique characteristics, is a strong indication of the accuracy of the DynCap analysis and the vessel model. Table 2: Maximum wind for 0 and 90 deg angle of attack including experimental data Case 0 deg 90 deg 1 DPCap with no thrust reserved for dynamics 26.5 m/s 14 m/s 2 DPCap with 20% reserved for dynamics 25 m/s 13.5 m/s 3 DynCap with 5 m/ 3 deg limits 9.5 m/s 4.5 m/s 4 Experimental DynCap 5m/ deg limit 8 m/s 4 m/s Investigating the differences between the DynCap results and the experimental results, we find that the relative difference between the plots is in average approximately 10%. Comparing DPCap with dynamics allowance and the experimental results, we find that the relative difference is much larger, approximately 60%. The limiting wind values for 0 and 90 deg angles of attack are given in Table 2. Investigating the logs of the experiments, it was found that for all headings, it is the heading deviation that is the limiting factor. Due to the motor dynamics and azimuth rate limits, the thrusters cannot produce force immediately in the wanted direction, thus limiting the vessel heading controllability. As discussed in the previous sections, the same behavior was observed from the logs of the DynCap analysis. 9

7 Discussion Figure 5: DPCap and DynCap analysis compared to experimental data As presented in section 6, there is a substantial difference in the vessel station-keeping capability computed according to the current industry standard and with time domain simulations with advanced vessel models. The previous section presented results indicating not only that there may be a significant difference between the DPCap results and the DynCap results, but that experimental data indicate that the DynCap results are significantly closer to the real station-keeping capability. The results indicate that the traditional DPCap analysis appear to be too optimistic. Based upon the results presented in this paper, we may conclude that if an accurate study of a vessels station-keeping capability is desired, the traditional DPCap is not adequate. The simplifications and assumptions made when calculating the DPCap, according to the IMCA 10

M140 specifications, result in wind-envelopes which do not reflect the station-keeping capability of a vessel in a realistic manner. If the capability plot is to be used for determining the vessel operational window or to select the right vessel for an operation, a more detailed standard based on time-domain simulations should be established. In addition such tools like DynCap enable vessel designers to perform quick design review and optimizing their design to maximize the performance and reduce power utilization and fuel consumption. The increased accuracy comes with a price: modeling of the vessel loads, propulsion, and DP control systems needs to be very accurate as shown in the comparison proposed in Table 3. The computational load is also significantly larger for DynCap but still significantly smaller compared to CFD runs for example. Table 3: DPCap and DynCap comparison table Property DPCap DynCap Balance between environmental and thruster forces Static Dynamic Dynamic environmental loads Statistical considerations may be included Included Vessel position Fixed: No dynamic vessel response Free floating Uses 80-85% of the Thruster capacity thruster capacity - All available thruster dynamic allowance of 15- capacity utilized 20% Thruster and rudder dynamics Not included Included Thruster losses Static losses may be included Dynamic losses included DP system External loads Dynamics of the DP system is not accounted for Static may be included Transient effects Not included Included Computational requirements Low High Model complexity Low to Medium High Flexibility Low High DP controller, DP observer and thrust allocation included Dynamic loads may be included 8 Conclusion This paper has presented a comparison of the tradition DPCap analysis and the new DynCap analysis against experimental data from a model scale test. The results showed that the limiting wind speed computed from the DynCap analysis resulted in an average increase from the experimental data of approximately 10%, while the relative increase of the DPCap limiting wind speed is significantly larger (about approximately 60%). This indicates that the results from DPCap are too optimistic and if used for operational planning could put the safety of the operations at risk. The results based on the IMCA M140 specification are still valid for early design verification and concept development but for accurate study of a vessel s station-keeping 11

capability, vessel operability, fuel consumption and emissions in realistic dynamic conditions, however, a new standard based on time-domain simulations should be established. By employing time-domain simulation tools such as DynCap, the operators, ship owners and oil companies will be able to make decisions based on more accurate vessel performance estimation, which will enhance the safety and efficiency of the operations, and may reduce nonproductive time. References 1. Lauvdal, T. (2000). Optimizing and Evaluating the Performance of Power and Thruster Plant in DP Vessels with an Integrated Vessel Simulator, in Dynamic Positioning Conference, Houston (TX), USA, October 17-18. 2. Muddusetti, S. and Phillips, D. (2006). A Practical Approach to Managing DP Operations, in Dynamic Positioning Conference, Houston (TX), USA, October 17-18. 3. DNV (2011), Dynamic Positioning Systems - Operation Guidance, Recommended Practice DNV-RP-E307, Det Norske Veritas. 4. Børhaug, B (2012). Experimental Validation of Dynamic Stationkeeping. Master thesis, Norwegian University of Science and Technology, Department of Engineering Cybernetics. 5. Pivano, L. and Smogeli, Ø. (2012). DynCap The Next Level Dynamic DP Capability Analysis, in Proceedings from Marine Operations Specialty Symposium (MOSS 2012), Singapore, August 6-8. 6. Settemsdal, S. O. and Radan, D. (2012). DP3 Class Power System Solution for Dynamically Positioned Vessels, in proceedings from Dynamic Positioning Conference, Houston, October 9-10. 12