Dynamic Analysis of Semi-Submersible Production Platform Under the Failure of Mooring Lines

This paper quantitatively studies the transient dynamic response of a semi-submersible production platform with the loss of one or several positioning mooring lines.A semi-submersible platform,production risers,and positioning mooring lines are all included in the numerical simulation.Increased motion of the semi-submersible platform,tension variation of the remaining mooring lines/risers and the risk of mooring line or riser clashing are all investigated through fully coupled time-domain analysis.Combined environmental loads are selected from irregular waves and the steady current varying from very rough to extreme sea conditions.Three dimension radiation/diffraction theories and Morison’s equation are applied to calculate first-order wave force and second-order mean drift force of floating semi-submersible platform.Nonlinear time-domain finite element methods are employed to analyze the behavior of mooring lines and risers.Results show that the failure of mooring lines seriously reduce the platform’s stability performance.The tension of the rest lines is also increased accordingly.Remaining lines which are closer to the failed lines will have larger tension increase to compensate.Line-Line distance provides practical information for the risk of clashing investigation.

Probability Distribution of the Hull Motion and Mooring Line Tension of Two Floating Systems

The statistical and distribution characteristics of the responses of a floater and its mooring lines are essential in designing floating/mooring systems.In general,the dynamic responses of offshore structures obey a Gaussian distribution,assuming that the structural system,and sea loads are linear or weakly nonlinear.However,mooring systems and wave loads are considerably nonlinear,and the dynamic responses of hull/mooring systems are non-Gaussian.In this study,the dynamic responses of two types of floaters,semi-submersible and spar platforms,and their mooring lines are computed using coupled dynamic analysis in the time domain.Herein,the statistical characteristics and distributions of the hull motion and mooring line tension are discussed and compared.The statistical distributions of the dynamic responses have strong non-Gaussianity and are unreasonably fitted by a Gaussian distribution for the two floating and mooring systems.Then,the effects of water depth,wave parameters,and low-frequency and wave-frequency components on the non-Gaussianity of the hull motion,and mooring line tension are investigated and discussed.A comparison of the statistical distributions of the responses with various probability density functions,including the Gamma,Gaussian,General Extreme Value,Weibull,and Gaussian Mixture Model(GMM)distributions,shows that the GMM distribution is better than the others for characterizing the statistical distributions of the hull motion,and mooring line tension responses.Furthermore,the GMM distribution has the best accuracy of response prediction.

Comparison Study and Forensic Analysis between Experiment and Coupled Dynamics Simulation for Submerged Floating Tunnel Segment with Free Ends under Wave Excitations

This paper presents dynamic-behavior comparisons and related forensic analyses of a submerged floating tunnel(SFT)between numerical simulation and physical experiment under regular and irregular waves.The experiments are conducted in the 3Dwave tank with 1:33.3 scale,and the corresponding coupled time-domain simulation tool is devised for comparison.The entire SFT systemconsists of a long concrete tunnel and 12 tubular aluminummooring lines.Two numerical simulation models,the Cummins equation with 3D potential theory including second-order wave-body interaction effects and the much simpler Morison-equation-based formula with the lumped-massbased line model,are designed and compared.Forensic analyses for mooring-line adjustments in the simulation are carried out in view of the best representation of the physical system.After that,the measured pre-tension distribution and systemstiffness of twelvemooring lines arewell reproduced in the numericalmodel.Subsequently,the dynamic responses and mooring tensions of the SFT are compared under regular and irregular waves.The measured and simulated results coincide reasonably well for both regular-and irregular-wave conditions.

The Effect of Key Design Parameters on the Global Performance of Submerged Floating Tunnel under Target Wave and Earthquake Excitations

This study presents a practical design strategy for a large-size Submerged Floating Tunnel(SFT)under different target environments through global-performance simulations.A coupled time-domain simulation model for SFT is established to check hydro-elastic behaviors under the design random wave and earthquake excitations.The tunnel and mooring lines are modeled with a finite-element line model based on a series of lumped masses connected by axial,bending,and torsional springs,and thus the dynamic/structural deformability of the entire SFT is fully considered.The dummy-connection-mass method and constraint boundary conditions are employed to connect the tunnel and mooring lines in a convenient manner.Wave-and earthquake-induced hydrodynamic forces are evaluated by the Morison equation at instantaneous node positions.Several wave and earthquake conditions are selected to evaluate its global performance and sensitivity at different system parameters.Different BuoyancyWeight Ratios(BWRs),submergence depths,and tunnel lengths(and mooring intervals)are chosen to establish a design strategy for reducing the maximum mooring tension.Both static and dynamic tensions are critical to find an acceptable design depending on the given target environmental condition.BWR plays a crucial role in preventing snap loading,and the corresponding static tension is a primary factor if the environmental condition is mild.The tunnel length can significantly be extended by reducing BWR when environmental force is not that substantial.Dynamic tension becomes important in harsh environmental conditions,for which high BWR and short mooring interval are required.It is underscored that the wet natural frequencies with mooring are located away from the spectral peaks of design waves or earthquakes.

Motion and Dynamic Responses of A Semisubmersible in Freak Waves

The present research aims at clarifying the effects of freak wave on the motion and dynamic responses of a semisubmersible. To reveal the effects of mooring stiffness, two mooring systems were employed in the model tests and time-domain simulations. The 6-DOF motion responses and mooring tensions have been measured and the 3- DOF motions of fairleads were calculated as well. From the time series, trajectories and statistics information, the interactions between the freak wave and the semisubmersible have been demonstrated and the effects of mooring stiffness have been identified. The shortage of numerical simulations based on 3D potential flow theory is presented. Results show that the freak wave is likely to cause large horizontal motions for soft mooring system and to result in extremely large mooring tensions for tight mooring system. Therefore, the freak wave is a real threat for the marine structure, which needs to be carefully considered at design stage.

Hydrodynamic Analysis of a Semi-submersible Wind-Tidal Combined Power Generation Device

Energy shortages and environmental pollution are becoming increasingly severe globally. The exploitation and utilization of renewable energy have become an effective way to alleviate these problems. To improve power production capacity, power output quality, and cost effectiveness, comprehensive marine energy utilization has become an inevitable trend in marine energy development. Based on a semi-submersible wind-tidal combined power generation device,a three-dimensional frequency domain potential flow theory is used to study the hydrodynamic performance of such a device. For this study, the RAOs and hydrodynamic coefficients of the floating carrier platform to the regular wave were obtained. The influence of the tidal turbine on the platform in terms of frequency domain was considered as added mass and damping. The direct load of the tidal turbine was obtained by CFX.FORTRAN software was used for the second development of adaptive query workload aware software, which can include the external force. The motion response of the platform to the irregular wave and the tension of the mooring line were calculated under the limiting condition(one mooring line breakage). The results showed that the motion response of the carrier to the surge and sway direction is more intense, but the swing amplitude is within the acceptable range. Even in the worst case scenario, the balance position of the platform was still in the positioning range, which met the requirements of the working sea area. The safety factor of the mooring line tension also complied with the requirements of the design specification. Therefore, it was found that the hydrodynamic performance and motion responses of a semi-submersible wind-tidal combined power generation device can meet the power generation requirements under all design conditions, and the device presents a reliable power generation system.