Deepwater Mooring System Research Articles

Numerical Investigations on Suspended Power Cable Configurations for Floating Offshore Wind Turbines in Deep Water Powering an FPSO

Abstract Floating offshore wind turbines (FOWTs) are an opportunity for floating production storage and offloading units (FPSOs) to reduce emissions. To avoid long connecting power cables with long transmission distances between a FOWT and an FPSO, the novel concept of a suspended power cable in a large water depth of 1000 m is investigated in this study. The power cable is kept floating between the sea surface and the seabed without touching either of them. A catenary configuration and two configurations with subsea buoys attached at different locations along the cable are investigated. The OC3-Hywind 5 MW reference FOWT is set up with a deepwater mooring system, and a spread-moored FPSO is modeled with characteristics similar to existing FPSOs. Steady-state and dynamic simulations are carried out in the numerical software OrcaFlex. The different configurations are first evaluated in steady-state analyses. The largest tensions are observed for the catenary configuration, whereas it shows the lowest horizontal cable excursions. Buoys attached along the center section of the cable lift it into regions with strong currents. This results in a large horizontal excursion of the cable and large tensions. The suspended configuration with buoys attached evenly over the cable results in significantly lower tensions than the other two configurations. It is studied further with dynamic analyses. The tensions at the floater hang-offs increase by a maximum of 24% compared to steady-state results indicating that dynamic analysis is crucial for the design of suspended cable configurations.

Active truncation model test method of deep-water mooring system: A numerical simulation study on time delay compensation of actuator motion

A numerical simulation platform is established to realize the active truncation model test method of deep-water mooring system, and the time delay is focused in this paper. The Stewart platform is chosen as the actuator aiming to realize the motion control of truncated point in mooring line. The corresponding kinematic model and motion control strategy of actuator are established. The method of cubic spline interpolation is proposed to solve the time delay compensation of actuator motion control, the influences parameters affecting compensation results are compared, and the corresponding compensated effects are verified. Through compensating the input signal of actuator, the real-time motion tracking accuracy of the truncated point of mooring line is effectively improved, which have contribution to the success of following physical experiments.

Motion tracking control of actuator in the active-truncated model test of deep-water mooring system

The motion of truncated point in the active-truncated model test of deep-water mooring system can be simulated using the actuator, and the motion tracking control of actuator determines the validity of test results. Taking the Stewart Platform as actuator, the kinematic model is established. The permanent magnet synchronous motor (PMSM) is selected as the drive motor of actuator, and the transfer function is identified using the white noise signal excitation. The Feedforward control and RBF neural network control methods are designed to compare the motion tracking control effect at the truncated point and the corresponding influences on the top tension at the fairlead. The simulation results show that the feedforward control can reduce the motion tracking error by 50% while the RBF neural network can reduce the tracking error by 75%, which both have obvious effect on the motion tracking control and provide theoretical basis for constructing the active-truncated model test platform of deep-water mooring system.

Multi-mode reliability analysis of mooring system of deep-water floating structures

This paper proposes a comprehensive method for reliability analysis of a deep-water mooring system. The novelty of the newly developed method is that it considers the failure modes of floater offset excess (the function of the intact mooring system), mooring line breakage (safety), and fatigue (persistence) as part of the mooring system's reliability assessment. In addition, the quantitative relation between mooring line breaking strength and cumulative fatigue damage is considered. In this research, the annual maximum floater offset and mooring line tension are calculated based on the wave condition with a 1-year return period. Long-term mooring line tension ranges are obtained from coupled analysis in the time domain for all the sea states in the wave scatter diagram and the fatigue damage is assessed using the Palmgren–Miner rule and T-N curve. Then, the mooring system's reliability is investigated by running Monte Carlo simulations. The results demonstrate that the failure probability considering the three failure modes is much greater than that of any single mode and neglecting the correlation between different failure modes leads to significant underestimation of the mooring system failure probability. Moreover, the sensitivity analysis indicates that the underestimation proportion is obviously affected by the fatigue exponent (m).

Optimal design and hydrodynamic response analysis of deep water mooring system with submerged buoys

ABSTRACTBased on the traditional catenary mooring system, a new optimised mooring system with submerged buoys is proposed. The idea and method in optimal design of mooring system with buoys are introduced first. Then, the governing equations of mooring line and the coupled model of platform with its mooring system are built. Finally, the global responses of a semi-submersible platform with mooring lines attached buoys are numerically simulated in time domain, and the effects of buoy size and position are investigated. The results would be helpful for expanding the design water depth of catenary mooring system.

Dynamic truncation method for mooring lines with tension estimation combined in the time and frequency domains

ABSTRACTAn improved truncation method for a deepwater mooring system based on the dynamic similarity of the mooring line in the time domain is proposed in this paper. The optimisation algorithm NSGA-II (non-dominated sorting genetic algorithm) is combined with the slender rod model to optimise the mooring line parameters in the static and dynamic analysis of the truncated system. A reliable fitness function is created in the frequency domain by transferring the mooring forces from the time domain with FFT, which makes the dynamic similarity between the truncated and full-depth mooring systems available and effective. An floating production storage and offloading unit (FPSO) mooring system working in 1500 m water depth is truncated to 1050 m and 700 m. The harmonic forced motions as well as the global response and mooring forces of the mooring system under irregular waves are consistent with each other across different water depths, thereby validating the proposed truncation method.

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.

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.

Analysis on deepwater mooring system of sandglass-type FDPSO

In order to solve the performance limitations of traditional ship-type and cylindrical FDPSO (floating drilling production, storage and offloading unit), we present a new FDPSO with a sandglass-type floating body. Based on the hydrodynamic performance of the newly developed floating model, a suitable analysis method for its deepwater mooring system has been proposed. Furthermore, by studying the effects of different mooring parameters on the motion performance of FDPSO, we achieve some useful conclusions and principles to design the mooring scheme in deep sea which has been validated to satisfy the Det Norshke Veritas (DNV) requirements. The study of this paper is expected to provide a reasonable and feasible reference and design solution for the deep sea positions of the new sandglass-type FDPSO.

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.

Development of study on the dynamic characteristics of deep water mooring system

To meet the needs of those exploiting deepwater resources, TLP and SPAR platforms are used in some areas and are considered excellent platforms in deep water. However, many problems remain to be resolved. The design of mooring systems is a key issue for deep water platforms. Environmental loads in deep water effect the physical characteristics of mooring line materials. The configuration and analysis of mooring systems involve nonlinearity due to this fluid-solid coupling, nonlinear hydrodynamic forces, and their effects on stability of motion. In this paper, some pivotal theories and technical questions are presented, including modeling of mooring lines, the theory and method of coupled dynamics analysis on the mooring system, and the development of methodologies for the study of nonlinear dynamics of mooring systems. Further study on mooring systems in deep water are recommended based on current knowledge, particularly dynamic parameters of different materials and cable configuration, interactions between seabed and cable, mechanisms of mooring system response induced by taut/slack mooring cables, discontinuous stiffness due to system materials, mooring construction, and motion instability, etc.

A Combined Riser Mooring System for Deepwater Applications

For deepwater developments, riser and mooring system requirements become a significant factor in the cost of the overall field development. Therefore, methods for reducing the riser and mooring design requirements and minimizing the cost of these systems become increasingly important. This paper presents the design methodology for a combined riser mooring (CRM) system and demonstrates the feasibility of the system for a test case application in a deepwater development offshore West of Africa. The CRM system offers significant benefits over the independent riser and mooring systems, namely reduced riser dynamics, reduced vessel offsets, a smaller seafloor footprint and system installation prior to the arrival of the FPSO.