Truncated Mooring System Research Articles

Investigation of Low-Frequency Pitch Motion Characteristics for KRISO Standard Offshore Structure (K-Semi) Moored with a Truncation Mooring System

In this study, a model test was carried out to evaluate the station-keeping performance of Korea Research Institute of the Ships and Ocean Engineering (KRISO) standard semi-submersible, K-Semi in the Deep Ocean Engineering Basin (DOEB). The test was performed using a 1/50 scaled model with 12 mooring lines. The water depth was set to 3.2 m using a moveable bottom structure and truncated mooring lines. The dynamic behavior of the K-Semi was examined using a free decay test and regular wave as well as irregular wave tests of 1800. Large surge, heave and pitch due to drift motion from second-order effect were observed. The reason for the excessive low-frequency pitch motion is attributed to the restoring force and moment related to the mooring system as excessive surge occurred. The numerical model of K-Semi with the truncated mooring system was tuned and calibrated using model test results such as free decay and regular wave tests. Through numerical analysis, the motion characteristics of the K-Semi were compared with irregular model test results. In the case of a semi-submersible using a mooring system, it is observed that excessive pitch motion due to vertical restoring force and moment may occur when large horizontal displacement occurs. The effect of low-frequency pitch motion related to surge motion is an important factor in upwell estimation as well as second-order motion in referred DNV-GL OTG13.

Data-driven model and key features based on supervised learning for truncation design of mooring and riser system

A truncated mooring and riser system is the foundation of hybrid model testing of deepwater floating platforms. Existing truncation design methods are mainly based on multi-objective optimization algorithms, which make them complicated and time-consuming. This paper presents an empirical truncation design method, which can instantaneously determine the properties of truncated mooring systems and risers using data-driven models generated based on supervised learning with an artificial neural network. Features of the input parameters are constructed specifically for the truncation design and filtered according to their rank of importance to improve the performance of the data-driven models. Moreover, the relationship between the truncated properties and overall parameters, such as the truncated depth, is investigated to propose methods for expanding the application range of the data-driven models. The accuracies of the data-driven models are evaluated statistically using test samples, in comparison with the accuracies of the optimization algorithm. Finally, the significant features determining the truncated properties are discussed to reveal the regulations of the truncation design. The proposed empirical method can serve as a more efficient tool for the applications of hybrid model testing and the reveal of key features is helpful for understanding the truncation process.

Numerical study on the optimised design of mooring truncated systems for different ocean basins

ABSTRACT Experimental tests are an important source of information for the design of floating production systems. Worldwide, there are few ocean basins with capabilities to reproduce ultra-deep-water scenarios, and they are characterised by having large dimensions. Even so, an equivalent truncated mooring/riser system has to be used. It is a technological challenge to reproduce with a truncated mooring system the coupled static responses for a semisubmersible considering the complete offset range. In this work, a methodology and computational tool based on a genetic algorithm is applied to optimise the design of a truncated equivalent mooring system in terms of the static responses. The prototype system is a semisubmersible platform for 2000 m water depth. The ocean basins considered in this study are OTRC, MARIN, MARINTEK and LABOCEANO. Results are satisfactory for every ocean basin studied which means that the methodology and computational tool applied is robust and efficient for different deep-water experimental design scenario.

Study on Vortex-Induced Motions of A New Type of Deep Draft Multi-Columns FDPSO

A numerical simulation and an experimental study on vortex-induced motion (VIM) of a new type of deep draft multi-columns floating drilling production, storage and offloading (FDPSO) are presented in this paper. The main dimension, the special variable cross-section column and the cabin arrangement of the octagonal pontoon are introduced based on the result. The numerical simulation is adapted to study the effects of current incidence angles and reduced velocities on this platform’s sway motion response. The 300 m water depth equivalent truncated mooring system is adopted for the model tests. The model tests are carried out to check the reliability of numerical simulation. The results consist of surge, sway and yaw motions, as well as motion trajectories. The maximum sway amplitudes for different types of offshore platform is also studied. The main results show that the peak frequencies of sway motion under different current incidence angles and reduced velocities vary around the natural frequency. The analysis result of flow field indicates that the change of distribution of vortex in vertical presents significant influences on the VIM of platform. The trend of sway amplitude ratio curve of this new type FDPSO differs from the other types of platform. Under 45° current incidence angle, the sway amplitude of this new type of FDPSO is much smaller than those of other types of offshore platform at 4.4 ≤ Vr ≤ 8.9. The typical ‘8’ shape trajectory does not appear in the platform’s motion trajectories.

Applicability analysis of truncated mooring system based on static and damping equivalence

Physical model tests of floating facilities in deep and ultra-deep water with full-depth mooring system present obstacles because no wave basin is sufficiently large to perform model testing in reasonable scale. The truncated mooring system needs to be designed to reproduce the statics and mooring-induced damping exerted by the full-depth mooring system. Numerical simulation is developed in this paper to analyse the applicability of an innovative approach which designs truncated mooring system based on static and damping equivalence. Considering the catenary, semi-taut and taut mooring systems used for a semi-submersible platform of 1500 m water depths, equivalent truncated mooring systems are designed by using the innovative approach, respectively. The motion responses of the platform with full-depth and truncated mooring systems are calculated. The applicability of this innovative approach for deepwater model test is discussed.

Hybrid model testing using pre-offset and asymmetric truncation design for deepwater semi-submersible with highly compliant mooring system

The hybrid model testing technique has been widely used for evaluating the safety of deepwater floating systems. In this technique, a full-depth mooring system is equivalently substituted with a truncated model by employing truncation design. However, for a deepwater platform with a highly compliant mooring system, the large offset resulting from harsh environments makes it difficult to realize truncation design. The conventional symmetrically truncated mooring system starts to lose its accuracy. To achieve sufficient equivalence and enlarge the offset limit of the truncated model, a pre-offset and asymmetric truncation design is proposed in this paper for a deepwater semi-submersible system. The feasibility of the technique was first validated by numerical calculations. Thereafter, hybrid model testing was performed successfully for a semi-submersible with a highly compliant mooring system operating in harsh environments. Numerical reconstruction and extrapolation were performed, and the numerical results showed good agreement with experimental results. However, some remarkable discrepancies were observed in vertical motions because of nonlinear effects such as wave impact and the influence of bracings.

Experimental verification of a semi-submersible platform with truncated mooring system based on static and damping equivalence

ABSTRACTPhysical model experiments of floating facilities in deep and ultra-deepwater with full-depth mooring system present obstacles because no wave basin is sufficiently large to perform model testing in reasonable scale. Truncation of full-depth mooring system is required for model test. Based on both statics and mooring-induced damping equivalence, truncated mooring lines are designed. A model test of a semi-submersible platform with full-depth and truncated mooring system is presented in this paper. In general, good agreements between the motion responses of semi-submersible platform with full-depth and truncated mooring system are found based on the model test results.

Using Design of Experiments and Design Optimization to Determine Statically Equivalent Mooring System on Truncated Water Depth

Several procedures have been proposed and developed to overcome the challenge in ultradeepwaters testing. A realistic alternative approach uses a hybrid passive methodology through equivalent truncated mooring systems. Often, the searching for equivalent systems involves using a trial-and-error. As an alternative, researches on the use of optimization techniques to establish truncated mooring system with the required properties have been attempted in recent years. In the literature, it is available only approaches considering nongradient-based algorithms. These algorithms usually involve several parameters which require appropriate tuning to provide good performance. Our approach involves optimization algorithms based on gradient. We use a calibration method to perform a static adjustment of design variables to optimally fit truncated mooring system to full-depth mooring system, which proved efficient. A further feature of this work is related to the study of the influence of design variables on the response, through a methodology based on design of experiments (DOE), avoiding the use of irrelevant variables. It should be emphasized that to the authors' knowledge this DOE methodology presented was not seen in other works in this field. We will show that the methodology proposed in this work makes easy to find an equivalent mooring system on truncated water depth. We will present and discuss two fictitious cases, one case based on the literature and another case based on a real scenario. The results show a good agreement between truncated mooring system and full-depth mooring system for the static adjustment.

An improved quasi-static model for mooring-induced damping estimation using in the truncation design of mooring system

The horizontal motions of floating oil and gas structures by a mooring system can be modeled as a resonant oscillator whose natural frequencies may be much lower than wave frequencies. Thus, the low frequency (LF) excitation caused by the irregular wave results in resonant motion responses in the horizontal plane. For instance, mooring lines may provide 80% of the total LF surge motion damping of a moored tanker in water depth of 200m, thus reducing the floater resonant excursion significantly. Therefore, when designing truncated mooring system for model testing, mooring damping equivalence should be considered. Thus, simple but effective method for predicting mooring line damping is required for designing truncated mooring system. Different with other quasi-static methods for estimating mooing line damping, besides normal drag forces, tangential drag forces and seabed friction are also considered in the improved quasi-static model presented in this paper. The analytical results based on the improved method are compared to other quasi-static and time domain results for a chain mooring line and a wire line. In addition, model tests of a 1:70 scaled catenary mooring line are conducted. Then, the improved quasi-static results, the AQWA results and the experimental results are compared.

Four-level screening method for multi-variable truncation design of deepwater mooring system

The truncation design of a mooring system is the foundation of hybrid model testing techniques used in experimental research on deepwater floating platforms owing to the limited size of wave basin. Screening multi-variables for multi-objectives leads to difficulties in truncation design aimed at achieving acceptable equivalence, especially for an asymmetric mooring system. This paper proposes an efficient four-level screening method for the truncation design. The selected multi-variables of each mooring line are screened individually to generate feasible regions according to pretensions and initial hang-off angles. Subsequently, the feasible regions are concentrated by the second- and third-level screening: design truncated mooring lines and truncated mooring systems according to static equivalence. Finally, numerical forced oscillation is employed as the fourth-level screening to fulfil dynamic equivalence. Case studies of the truncation design of symmetric and asymmetric mooring systems have verified the proposed method.

Dynamic design optimization of an equivalent truncated mooring system

An important part in the development and hydrodynamic verification of new floating production systems involves carrying out testing on ocean basin laboratories. However, there is no ocean laboratory able to perform the testing of ultra-deepwater floating structure at reasonable model scale. Several methods have been proposed and developed to solve this problem. The hybrid passive method is organized in steps, where the first step is responsible for setting the truncated design. If this step is not performed satisfactorily, it may hamper the success of the testing. Thus, in order to minimize this issue, we propose in this paper the use of design optimization techniques to find the ideal truncated full-scale design considering the dynamic effects. We apply a calibration method to adjust design variables to optimally fit truncated mooring system to full-depth mooring system, in order to minimize the differences in motion and tension responses from the two systems. Furthermore, we check the truncated optimal design for many wave conditions using dynamic simulations. Due to the large computational cost involved in this check, we use high-performance computing to quicken this process. We will present and discuss two cases. In both cases, the results show that the truncated design found is equivalent to the full-depth design.

Truncation Design and Model Testing of a Deepwater FPSO Mooring System

Limitation of offshore basin dimensions is a great challenge for model tests of deepwater mooring system. The mooring system cannot be modeled entirely in the basin with a reasonable model scale. A classical solution is based on hybrid model tests for the truncated mooring system. An efficient truncation method is proposed in this paper taking advantage of the mechanical characteristics of catenary mooring system. Truncation procedures are presented both in vertical and horizontal directions. A turret moored floating production storage and offloading (FPSO) is analyzed, and its mooring system is truncated from the original 914 m water depth to 736 m and 460 m, respectively. Numerical simulations are performed based on catenary theory and lumped mass model to these three systems, including the original untruncated system and two truncated systems. The static characteristics and dynamic response are investigated, and the results are compared between the truncated and untruncated system, and good agreements are obtained, verifying the preliminary truncation design. Model tests are conducted to the three mooring system configurations in the deepwater basin of the Harbin Engineering University. The static and dynamic properties are found to be mostly consistent between the untruncated system and two truncated systems, except for some discrepancy in 460 m system. It indicates that the truncation design is successful when the truncation factor is large, while difference still exists when the truncation factor is small. Numerical reconstruction to the model test in 460 m and extrapolation to 914 m are also implemented. The results are found to be consistent with those in 914 m, verifying the robustness and necessity of the hybrid model testing, especially for the mooring system with large truncation.

Design of Truncated Mooring Line Model in KRISO's Deepwater Ocean Engineering Basin

The present work was an attempt to investigate the applicability of truncated mooring systems to KRISO's deep ocean engineering basin (DOEB) with ratios of 1:100, 1:60, and 1:50. The depth of the DOEB is 15 m. Therefore, the corresponding truncated depths for this study were equal to 1500 m, 900 m, and 750 m. The investigation focused on both the static and dynamic characteristics of the mooring system. It was shown, in a static pull-out test, that the restoring force of a FPSO vessel could be modified to a good level of agreement for all three truncation cases. However, when the radius of the mooring site was reduced according to the truncation factor, the surge motion response during a free-decay test showed a significant difference from the full-depth model. However, the reduction of this discrepancy was achieved by increasing the radius up to its maximum possible value while considering the size of the DOEB. Especially, in terms of the time period, the difference was reduced from 24.0 to 5.3 s for a truncation ratio of 1:100, 54.1 to 8.6 s for a truncation ratio of 1:60, and 31.7 to 3.9 s for a truncation ratio of 1:50. As a result, the study verified the applicability of the truncated mooring system to the DOEB, and therefore it could represent the full-depth mooring system relatively well in terms of the static and dynamic conditions.

Design Optimization for a Truncated Catenary Mooring System for Scale Model Test

One of the main aspects when testing floating offshore platforms is the scaled mooring system, particularly with the increased depths where such platforms are intended. The paper proposes the use of truncated mooring systems to emulate the real mooring system by solving an optimization problem. This approach could be an interesting option when the existing testing facilities do not have enough available space. As part of the development of a new spar platform made of concrete for Floating Offshore Wind Turbines (FOWTs), called Windcrete, a station keeping system with catenary shaped lines was selected. The test facility available for the planned experiments had an important width constraint. Then, an algorithm to optimize the design of the scaled truncated mooring system using different weights of lines was developed. The optimization process adjusts the quasi-static behavior of the scaled mooring system as much as possible to the real mooring system within its expected maximum displacement range, where the catenary line provides the restoring forces by its suspended line length.

Verification of a hybrid model test method for a deep water floating system with large truncation factor

The hybrid model test method has been regarded as the most reliable method to investigate the dynamics of deepwater floating systems when the wave basin is not large enough to conduct the full water depth model tests at reasonable model scales. In order to get the reliable experimental results, the truncation factor should be less than 5. However, as the offshore oil and gas industry moves towards deeper waters, the model tests of floating structures carried out at the wave basin with reasonable model scales need to be revamped. The objective of the present study is to investigate the feasibility of largely truncated hybrid model test for the deep water floating systems. The hybrid model test techniques and the dynamic behaviors of the truncated mooring lines are also studied. In order to achieve the static and dynamic similarities easily in the largely-truncated model tests, an optimization design method of the truncated mooring system is proposed, and the similarities of platform motion responses as well as the mooring lines dynamics are investigated in the largely-truncated model tests. To verify the present method, a deepwater semisubmersible system operated in 1500m and 1000m water depth is truncated to 200m water depth based on the static equivalent principle (the truncation factors are 7.5 and 5 respectively), and the dynamically similar design is implemented to the model tests which are carried out at China Ship Scientific Research Center wave basin. The motion responses of the platform and the mooring line dynamics are obtained. These experimental results are used to compare with the numerical simulation, which is based on the full time domain coupled dynamic analysis method. The comparisons show that the numerical results are in good agreement with the model measurements.

FPSO 안정성 평가를 위한 절단계류선 모델링 수치 연구

In this paper, a numerical analysis for an internal turret moored vessel located at a 400-m water depth is conducted. The target vessel has an internal turret that is located at the 0.2 Lpp position from the fore-side, with 3 × 4 complex mooring lines installed around the turret circumference. To investigate the motion response of the vessel and the structural reliability of the lines, model tests were conducted. The KRISO ocean basin has a water depth of 3.2 m, which represents 192m using a scaling of 1:60. In order to precisely represent the real-scale condition, equivalent mooring lines needed to be designed. Truncated mooring lines were designed to supplement the restriction of the flume's water depth and increase the reliability of the model testing. These truncated mooring lines were composed of two different chains in order to match the pre-tension, simultaneously restoring the curve and variation in the effecti ve line tension. The static similarities were compared using a static pull-out test and free decaying test, and the dynamic similarities were matched via a regular wave test and combined environments test. Consequently, the designed truncated mooring system could represent the prototype mooring system relatively well in the aspects of kinematics and dynamics.

Dynamics of large-truncated mooring systems coupled with a catenary moored semi-submersible

With the floating structures pushing their activities to the ultra-deep water, model tests have presented a challenge due to the limitation of the existing wave basins. Therefore, the concept of truncated mooring system is implemented to replace the full depth mooring system in the model tests, which aims to have the same dynamic responses as the full depth system. The truncated mooring system plays such a significant role that extra attention should be paid to the mooring systems with large truncation factor. Three different types of large truncation factor mooring system are being employed in the simulations, including the homogenously truncated mooring system, non-homogenously truncated mooring system and simplified truncated mooring system. A catenary moored semi-submersible operating at 1000 m water depth is presented. In addition, truncated mooring systems are proposed at the truncated water depth of 200 m. In order to explore the applicability of these truncated mooring systems, numerical simulations of the platform’s surge free decay interacting with three different styles of truncated mooring systems are studied in calm water. Furthermore, the mooring-induced damping of the truncated mooring systems is simulated in the regular wave. Finally, the platform motion responses and mooring line dynamics are simulated in irregular wave. All these simulations are implemented by employing full time domain coupled dynamic analysis, and the results are compared with those of the full depth simulations in the same cases. The results show that the mooring-induced damping plays a significant role in platform motion responses, and all truncated mooring systems are suitable for model tests with appropriate truncated mooring line diameters. However, a large diameter is needed for simplified truncated mooring lines. The suggestions are given to the selection of truncated mooring system for different situations as well as to the truncated mooring design criteria.

Optimal design of equivalent water depth truncated mooring system based on baton pattern simulated annealing algorithm

The highest similarity degree of static characteristics including both horizontal and vertical restoring force-displacement characteristics of total mooring system, as well as the tension-displacement characteristics of the representative single mooring line between the truncated and full depth system are obtained by annealing simulation algorithm for hybrid discrete variables (ASFHDV, in short). A “baton” optimization approach is proposed by utilizing ASFHDV. After each baton of optimization, if a few dimensional variables reach the upper or lower limit, the boundary of certain dimensional variables shall be expanded. In consideration of the experimental requirements, the length of the upper mooring line should not be smaller than 8 m, and the diameter of the anchor chain on the bottom should be larger than 0.03 m. A 100000 t turret mooring FPSO in the water depth of 304 m, with the truncated water depth being 76 m, is taken as an example of equivalent water depth truncated mooring system optimal design and calculation, and is performed to obtain the conformation parameters of the truncated mooring system. The numerical results indicate that the present truncated mooring system design is successful and effective.

Innovative approach to design truncated mooring system based on static and damping equivalent

An innovative approach is developed to design equivalent truncated mooring system for hybrid model testing. Considering the gravity, tension, current force and mooring line extension, the piecewise extrapolating method is employed to the static analysis of the multi-component mooring line. In addition, the quasi-static approach and dissipated energy model are used to figure out the mooring-induced damping of mooring system. Employing genetic algorithm, an optimised design program is developed to design equivalent truncated mooring system based on the similarity of both static and damping characteristics. Considering the catenary, semi-taut and taut mooring systems used for some semi-submersible platform of 1500 m water depths, equivalent truncated mooring system is designed by using the design program in this paper, respectively. The results prove that the design program is available for all common types of deep water mooring system and can be used for hybrid model testing.

Investigation on optimization design of an equivalent water depth truncated mooring system based on INSGA-II

At present, equivalent water depth truncated mooring system optimization design is regarded as the priority of hybrid model testing for deep sea platforms, and will replace the full depth system test in the future. Compared with the full depth system, the working depth and span are smaller in the truncated one, and the other characteristics maintain more consistency as well. In this paper, an inner turret moored floating production storage & offloading system (FPSO) which works at a water depth of 320m, was selected to be a research example while the truncated water depth was 80m. Furthermore, an improved non-dominated sorting genetic algorithm (INSGA-II) was selected to optimally calculate the equivalent water depth truncated system, considering the stress condition of the total mooring system in both the horizontal and vertical directions, as well as the static characteristic similarity of the representative single mooring line. The results of numerical calculations indicate that the mathematical model is feasible, and the optimization method is fast and effective.