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Ye, Q.; Jin, W.-L., and Bai, Y., 2020. Time-domain structure analysis of a typical semisubmersible drilling rig. In: Liu, X. and Zhao, L. (eds.), Today's Modern Coastal Society: Technical and Sociological Aspects of Coastal Research. Journal of Coastal Research, Special Issue No. 111, pp. 118–123. Coconut Creek (Florida), ISSN 0749-0208. This paper evaluates time-domain structure analysis approaches of a semisubmersible drilling rig using a nonlinear finite element method. Hydrodynamic random wave loads are calculated by WVTDUT, which was developed by a research group from the Dalian University of Technology. Time-domain structure analysis of a semisubmersible platform is very time consuming; simplified time-domain analysis approaches are therefore proposed to improve efficiency using equivalent time series. Validity of the approach is tested by a concrete example and simulation results are discussed in detail. The work of this paper can lead to a better understanding of structure behavior of a typical semisubmersible platform under characteristic wave loads. Performing structure analyses in time domain increases the accuracy and reliability of structure performance and the present study may be a benchmark study on time-domain structure analysis of a semisubmersible drilling rig.

In this paper, the multi-body coupled dynamic characteristics of a semisubmersible platform and an HYSY 229 barge were investigated. First, coupled hydrodynamic analysis of the HYSY 229 barge and the semisubmersible platform was performed. Relevant hydrodynamic parameters were obtained using the retardation function method of three-dimensional frequency-domain potential flow theory. The results of the hydrodynamic analysis were highly consistent with the test findings, verifying the accuracy of the multifloating hydrodynamic coupling analysis, and key hydrodynamic parameters were solved for different water depths and the coupling effect. According to the obtained results, the hydrodynamic influence was the largest in shallow waters when the coupling effect was considered. Furthermore, the coupled motion equation combined with viscous damping, fender system, and mooring system was established, and the hydrodynamics, floating body motion, and dynamic response of the fender system were analyzed. Motion analysis revealed good agreement among the surge, sway, and yaw motions of the two floating bodies. However, when the wave period reached 10 s, the motion of the two floating bodies showed severe shock, and a relative motion was also observed. Therefore, excessive constraints should be added between the two floating bodies during construction to ensure construction safety. The numerical analysis and model test results of the semisubmersible platform and HYSY 229 barge at a water depth of 42 m and sea conditions of 0°, 45°, and 90° were in good agreement, and the error was less than 5%. The maximum movement of the HYSY 229 barge reached 2.61 m in the sway direction, whereas that of the semisubmersible platform was 2.11 m. During construction, excessive constraints should be added between the two floating bodies to limit their relative movement and ensure construction safety.

During the towing of semisubmersible platforms, waves impact and superpose in front of the platform to form a ridge shaped “water ridge”, which protrudes near the platform and produces a large slamming pressure. The water ridges occur frequently in the towing conditions of semisubmersible platforms. The wave–slamming on the braces and columns of platform is aggravated due to the water ridges, particularly in rough sea conditions. The effect of water ridges is usually ignored in slamming pressure analysis, which is used to check the structural strengths of the braces and columns. In this paper, the characteristics of the water ridge at the braces of a semisubmersible platform are studied by experimental tests and numerical simulations. In addition, the sensitivity of the water ridge to the wave height and period is studied. The numerical simulations are conducted by a Computational Fluid Dynamics (CFD) method, and their accuracy is validated based on experimental tests. The characteristics of the water ridge and slamming pressure on the braces and columns are studied in different wave conditions based on the validated numerical model. It is found that the wave extrusion is the main reason of water ridge. The wave–slamming pressure caused by the water ridge has an approximately linear increase with the wave height and is sensitive to the wave period. With the increase of the wave period, the wave–slamming pressure on the brace and column of the platform increases first and then decreases. The maximum wave–slamming pressure is found when the wave period is 10 s and the slamming pressure reduces rapidly with an increase of wave period.

An innovative Bayesian motion-based wave inference method is derived and assessed in this work. The evaluation of the accuracy of the proposed prior distribution has been carried out using the results obtained during a dedicated experimental campaign with a scale model an Oil and Gas (O&G) semisubmersible platform. As for the Bayesian statistical inference approaches, the features of the proposed novel prior distribution, as well as the hypotheses adopted, are discussed. It has been found that significant improvements can be obtained if the new approach is adopted to estimate the sea conditions from measured vessel motions. Finally, it is possible to highlight a substantial reduction of the computing time when the sea conditions are estimated by means of the improved Bayesian method, if compared with the conventional approaches for motion-based wave inference.

In this paper, an optimal semisubmersible platform is sought considering two key geometry variables: the diameter of the outer cylinders and their radial distance from the platform centre. The goal is to identify a platform configuration able to most efficiently contrast the combined wind-wave action, keeping the platform dimensions as small as possible. The amplitude of the Response Amplitude Operator (RAO) peaks and the integral area of the RAOs in a range of excited frequencies for the selected degrees of freedom are chosen as targets to be minimised. Through an efficient frequency domain simulation approach, we show that upscaling techniques proposed in the literature may lead to overdesigned platforms and that smaller and more performing platforms can be identified. In particular, the optimised platform shows a reduction of about 51% in parked and 54% in power production of the heave RAO peak, and a reduction of about 37% in parked and 50% in power production of the pitch RAO.

Recently, semisubmersible floating offshore wind turbine technologies have received considerable attention. For the coupled simulation of semisubmersible floating offshore wind energy, the platform is usually considered a rigid model, which could affect the calculation accuracy of the dynamic responses. The dynamic responses of a TripleSpar floating offshore wind turbine equipped with a 10 MW offshore wind turbine are discussed herein. The simulation of a floating offshore wind turbine under regular waves, white noise waves, and combined wind-wave conditions is conducted. The effects of the tower and platform flexibility on the motion and force responses of the TripleSpar semisubmersible floating offshore wind turbine are investigated. The results show that the flexibility of the tower and platform can influence the dynamic responses of a TripleSpar semisubmersible floating offshore wind turbine. Considering the flexibility of the tower and platform, the tower and platform pitch motions markedly increased compared with the fully rigid model. Moreover, the force responses, particularly for tower base loads, are considerably influenced by the flexibility of the tower and platform. Thus, the flexibility of the tower and platform for the coupled simulation of floating offshore wind turbines must be appropriately examined.

A neural-network-based prediction of motion responses of a semi-submersible is presented here. The fully connected neural networks and the long–short-term memory networks were employed to establish the neural networks for motion predictions. The effects of network architectures and time steps were investigated in depth. The predicted results were compared with the measurements and the predictions based on other conventional methods. The results demonstrate that the neural-network-based approach could offer a fast and accurate prediction of heave, roll, and pitch responses of a semi-submersible. This provides a promising alternative for evaluations of hydrodynamic performances of new designs and monitoring of the dynamic behavior of in-service floating structures.

A new energy transfer path analysis (TPA) method that aims to analyze the transfer path of components with characteristic frequencies in semisubmersible platform structures is proposed in this paper. Due to the complexity of semisubmersible platforms, traditional TPA methods based on measurements are no longer applicable. In the proposed method, the structure is considered to be a “source-path-receiver” system and the vibration signals from the source-points are analyzed first to determine the characteristic components. Then, the signals from the source-points and the receiver-points are decomposed by introducing a state–space model. Through correlation functions, the characteristic components can be extracted, and the transfer path can be obtained by calculating the transmissibility functions. By using transmissibility functions, the proposed method only relies on the output responses and avoids the measurement of force and transfer functions. Three examples, one numerical example containing a two-degree-of-freedom (2-DOF) model and a 5-DOF model, one experiment implemented on a barge, and the data from a dynamic positioning (DP) cabin of a semisubmersible platform were used to investigate the performance of the proposed method. The results show that the proposed method can be used to assess the transfer path quantitatively and has potential value of application in engineering.

A novel semi-submersible platform is proposed for 5 MW wind turbines. This concept focuses on an integrated system formed by combining porous shells with a semi-submersible platform. A coupled aerodynamic–hydrodynamic–mooring analysis of the new system is performed. The motion responses of the novel platform system and the traditional platform are compared. The differences in hydrodynamic performance between the two platforms are also evaluated. The influence of the geometric parameters (porosity, diameter, and wall thickness) of porous shells on the motion response behavior of the new system is studied. Overall, the new semi-submersible platform exhibits superior stability in terms of pitch and heave degrees of freedom, demonstrating minimal effects on the motion response in the surge degree of freedom.

Recently, more wind turbine systems have been installed in deep waters far from the coast. Several concepts of floating wind turbine systems (FWTS) have been developed, among which, the semi-submersible platform—due to its applicability in different water depths, good hydrodynamic performance, and facility in the installation process—constitutes the most explored technology compared to the others. However, a significant obstacle to the industrialization of this technology is the design of a cost-effective FWTS, which can be achieved by optimizing the geometry, size, and weight of the floating platform, together with the mooring system. This is only possible by selecting a method capable of accurately analyzing the FWTS-coupled hydro–aero–structural dynamics at each design stage. Accordingly, this paper provides a detailed overview of the most commonly coupled numerical and physical methods—including their basic assumptions, formulations, limitations, and costs used for analyzing the dynamics of FWTS, mainly those supported by a semi-submersible—to assist in the choice of the most suitable method at each design phase of the FWTS. Finally, this article discusses possible future research directions to address the challenges in modeling FWTS dynamics that persist to date.