Coupled dynamic analysis for wave interaction with a truss spar and its mooring line /riser system in time domain

Coupled dynamic analysis for wave action on a tension leg-type submerged floating tunnel in time domain

Comparison of linear spring and nonlinear FEM methods in dynamic coupled analysis of floating structure and mooring system

The oil and gas exploration has moved from shallow water to much deeper water far off the continental shelf. Spar platforms under deep water conditions are found to be the most economical and efficient type of offshore platform. Several Spar platforms installed in the Gulf of Mexico and North Sea proves its suitability for deep water exploration. Accurate prediction of motions of a Spar hull is very important for the integrity and associated costs of the riser/mooring line. The most common approach for solving the dynamics of Spar platform is to employ a decoupled quasi-static method, which ignores all or part of the interaction effects between the platform, mooring lines and risers. Coupled analysis, which includes the mooring lines, risers and platform in a single model, is the only way to capture the damping from mooring lines and risers in a consistent manner. The present coupling is capable in matching the forces, displacement, velocities and acceleration for mooring line with Spar hull at the fairlead position and riser with Spar hull at the riser keel connection. It can handle possible significant nonlinearities. The output from such analyses will be platform motions as well as a detailed mooring line and riser responses. In actual field problems hydrodynamic and aerodynamic loads act simultaneously on Spar platform, mooring lines and risers. In finite element model, the entire structure acts as a continuum. This model can handle all nonlinearities, loading and boundary conditions. The selected configuration of Spar platform is analysed under wave force together with wind loading and its structural response behaviour in steady state is studied. An automatic Newmark-β time incremental approach in ABAQUS/AQUA environment has been implemented to conduct the analysis in time domain. The wind force acting on the exposed part of the platform encompasses mean and fluctuating wind components. The frontal region includes the topside assembly and the spar hull portion above the sea level. High degree of nonlinearities makes the solutions convergence sensitive and it requires large number of iterations, at each time station. Spar responses in surge, heave and pitch along with top tension in moorings are computed. The coupled Spar experiences significant lateral shift along wave direction due to wind loading. Increase in standard deviation shows the participation of wind loading giving higher fluctuations. The CML tension increases for wind loading but the extent of the tension fluctuations under wind loading is not much due to high pretension of mooring line.

Dynamic analysis of offshore structures: C. A. Brebbia and S. Walker Newnes-Butterworth, London, £25.00, 323 pp, 1979

Ambient and forced vibration tests were recently conducted on an offshore steel template platform in the Ekofisk complex of the North Sea. Valuable information about the dynamic characteristics of the structure was obtained. This paper focuses on the measurement and analysis of damping data for the platform. Modal damping values are very important in the dynamic analysis of offshore structures. Correct values of modal damping need to be utilized for the dynamic analysis to accurately predict the response of the structure. Little experimentally verified information of the modal damping characteristics of the fundamental and higher order modes of offshore structures is currently available. Damping values were calculated for the fundamental and higher order jacket modes which included natural frequencies up to approximately 5 Hz. Damping values are compared for forced and ambient vibration data, for bandwidth and response techniques, and for different excitation levels. The general results indicated that the fundamental modes important in fatigue had damping values of 1 to 3 percent of critical and the higher order modes had damping values of 2 to 3 percent of critical. Emphasis is placed on the apparent merits and limitations of the data presented. Response comparisons are also presented using an analytic model of the platform with various values of critical damping. INTRODUCTION Accurately predicting the dynamic behavior of offshore oil and gas production platforms to withstand the effects of earthquakes and environmental fatigue due to wind and wave loading is an essential part of the design of such structures. Research and development work currently being undertaken to apply integrity monitoring techniques to these platforms also requires an accurate interpretation of the structural dynamic behavior. These applications of vibration technology translate directly into cost effective design and increased reliability and safety of offshore structures. The complexity of these structures requires a high level of theoretical analysis in developing models which duplicate the dynamic behavior under design loading conditions. Current state-of-the-art modeling methods usually apply finite element techniques which discretize the structure into an assemblage of mass and stiffness matrices which mathematically define the structure. The complexity of these structures also implies the need for confirmation and improvement of the structural models by experimental techniques. Using system identification methods, both analytic and experimental results can be used to accurately define the dynamic behavior of structures. Structural damping is one of the least understood yet most vital design characteristics of dynamically loaded structures. An analytical model of a structure in which low critical damping values are used will respond much differently to environmental loads than one in which higher damping values are used. This paper presents values of critical damping obtained experimentally on an offshore platform and comparisons are made to an analytic model of the structure. The vibration tests were conducted on an offshore steel template platform in the Ekofisk complex of the North Sea. Both ambient and forced vibration test procedures were utilized.

The numerical procedures for dynamic analysis of mooring lines in the time domain and frequency domain were developed in this work. The lumped mass method was used to model the mooring lines. In the time domain dynamic analysis, the modified Euler method was used to solve the motion equation of mooring lines. The dynamic analyses of mooring lines under horizontal, vertical, and combined harmonic excitations were carried out. The cases of single-component and multicomponent mooring lines under these excitations were studied, respectively. The case considering the seabed contact was also included. The program was validated by comparing with the results from commercial software, Orcaflex. For the frequency domain dynamic analysis, an improved frame invariant stochastic linearization method was applied to the nonlinear hydrodynamic drag term. The cases of single-component and multicomponent mooring lines were studied. The comparison of results shows that frequency domain results agree well with nonlinear time domain results.

Model order reduction (MOR) of nonlinear systems draws great attention in the past several decades. This paper presents a new MOR method in time domain for nonlinear dynamical systems. The new algorithm is based on the combination of the Taylor series expansion for the state variable x(t) and the trajectory piecewise polynomial technique. Firstly, the nonlinear system is approximated by a piecewise polynomial representation. Then, based on the Taylor series coefficients of x(t), we formulate the projection matrix V for the piecewise polynomial system and the compact model of the piecewise polynomial system is obtained in the following. Besides, error estimation and stability analysis are also presented in this paper. Finally, two nonlinear systems are tested to verify the effectiveness of the algorithm.

A shallow water disconnectable STL turret mooring and riser system has been developed for water depth between 30 and 50 m. This technology is based on APL’s disconnectable STL (Submerged Turret Loading) and STP (Submerged Turret Production) technologies which had been widely applied for water depth between 70 m to 2600 m for FPSOs and LNG offshore terminals. The advantage of disconnectable system is that the mooring and riser system can be designed to a preferred sea state. When the sea state is higher than design sea state (like hurricane), the vessel can be disconnected and sail away. The shallow water STL system consists of STL buoy, mooring lines, riser system and landing pad. The interface with vessel is the same as traditional STL system. The mooring and riser system are connected to the vessel through STL buoy and can be pulled into vessel by using ship winch. Unlike traditional STP and STL buoys, the shallow STL buoy has a net weight and will stay on the landing pad when disconnected from vessel. The landing pad is designed to support the impact load from STL buoy and supply enough friction for the STL buoy to stay in position during 100-year storm. The mooring system design has taken the advantage of directionality of weather when close to the shore by using different mooring line length in different directions. Further an innovative Hold-Back-Wave riser configuration has been developed for shallow water system. The riser configuration has a larger flexibility compared to traditional wave configuration and has proved to be feasible for significant wave height at least 7 m when connected to the vessel and 10+ m when disconnected from the vessel. Model test for the disconnectable shallow water turret mooring and riser system had been performed in MARINTEK, Trondheim with a LNG re-gasification vessel model at 30 m water depth. For connected system, significant wave height Hs = 6 m and 8 m has been tested. The mooring and riser system perform well, as predicted. For disconnected system (when the buoy sitting on the landing pad), significant wave height Hs = 10 m has been tested. The STL buoy is sitting on the landing pad without significant movement and the riser system performs well. SIMO program has been used to calibrate the model test results with numerical simulations. By adjusting surge, sway, yaw damping and 2nd order wave drift force, the calibrated SIMO model agrees well with model test results and can be used for similar development.

Assessment of hydrodynamic performance of the Submerged floating tunnel (SFT) under wave actions is one of the important factors in the design of the SFT. In this paper, an efficient and accurate theoretical hydrodynamic model is developed to analyze the global spatial dynamic response of the SFT under wave actions. The tube is treated as a more accurate BOEF model with three degrees of freedom, and the new calculation formula of cable-constraint reaction on the BOEF model is proposed. A numerical approach based on potential flow theory and a higher-order boundary element method (BEM) was developed to calculate the wave force on the SFT tube. The numerical results are successfully validated by direct comparison against published numerical data and the calculation results of the finite element model.

We present a three-dimensional (3-D) endoscopic optical coherence tomography (OCT) system based on a dual axis microelectromechanical system (MEMS) mirror. The diameter of MEMS mirror was 1.2 mm and both axes were capable of scanning up to 20° (optical) at greater than 1 kHz with excellent linearity. The MEMS mirror was packaged in a machined acrylic endoscopic housing which provided mechanical protection and electrical interconnects as well as optical alignment of the MEMS device to a focusing GRIN lens. The endoscopic MEMS probe was integrated and tested with both a fiber-based time domain (TD) OCT system and Fourier domain (FD) OCT system. Combining the 2-axis lateral scanning of the MEMS device with an axial scan allowed 3-D volume images to be obtained at a rate of 3 frames/s for the TD system and 7 frames/s for the FD system. In the initial investigations, in vivo 3-D OCT images of a human finger as well as images of animal tissue such as healthy rabbit trachea, normal and cancerous regions of hamster cheek pouch tissue were obtained. These images allowed real-time diagnosis of diseased tissue and also clearly delineated important features and tissue structures.

This paper presents model test and analytical correlation results of a single point moored Floating Production, Storage and Offloading tanker based system (FPSO). Both Permanent and Disconnectable turret mooring system are considered. Analytical calculation of green water effects, vessel motions, turret and mooring line loads are compared with model test results. Frequency and time domain analysis techniques of calculating line tensions are demonstrated and results are compared to the model tests measured time histories. The analysis procedure discussed in this paper is aimed toward estimating I)motions at bow relative to the water surface, 2)mooring line tensions and 3)turret design loads for a turret moored FPSO with a reasonable degree of accuracy. it is demonstrated that mooring line tensions can be calculated accurately with a finite element time domain computer simulation of the line dynamics provided the input fairlead motions are correct. Thus the method of computing time histories of mooring line tensions based on free-floating vessel motions is applicable for other spread moored floating systems in the same range of water depth. CONCLUSIONS AND RECOMMENDATIONS The model test results confirm the feasibility of both types of turret moored systems and validate the analytical tools and procedures. Results derived from the time domain line dynamic analysis agree well with the model tests. The dynamics of slack line are over estimated by using the frequency domain approach. Two methods of API mooring line fatigue calculation are compared. This paper demonstrates that a frequency domain quasistatic approach provides an efficient tool for initial sizing of mooring components. The maximum global mooring line and turret loads may be estimated by combining the frequency domain line dynamic results with the quasistatic approach. A time domain line dynamic approach can confirm and/or predict the maximum line tension with a high level of confidence, provided the vessel motion at the line top can be prescribed accurately. INTRODUCTION A turret moored 140,000 DWT tanker system is being designed for 1,000 feet of water in the South China Sea. The turret moored tanker is an attractive concept for both production facilities and offshore storage. Design of the weather vaning turret mooring system and riser system connected to the seafloor are two key system components for FPSO development. In the design of a turret mooring system the prediction of the extreme motion responses of the moored vessel is of the utmost importance. A moored tanker exposed to severe environmental conditions experiences motion responses in two distinct frequency ranges:Wave frequency responses around the wave period range (12-15 seconds).Low frequency responses around the tanker natural period range (100 seconds); The wave frequency response is generally well predictable, whereas estimates on the low frequency responses are more difficult and may vary by an order of magnitude depending on analytical tools and design procedures. Often a well executed model test program in parallel with analytical approach is imperative to ensure a reliable design of the turret mooring system.

Mooring lines systems for point absorber, wave energy converters (WECs) devices, must be engineered to withstand the cyclical loads and motions generated by the interaction of the wave load, as well as the WEC's motions in the random elevation of the sea surface. A simulation model was developed and validated concerning the simulated of point absorber motioned in a previous study; this study compares tension methodologies for point absorber with moorings lines using simulation approach. The purpose of this paper is to propose a mooring lines simulation approach that is suited for a one-body and two-body point absorber thorough examination of the heave motion response and, tension analysis of mooring lines coupled on the device and on seabed. The case study is a cylindrical, floating point absorber with one, two, three and four distributed mooring lines with catenary and taut configurations. The dynamics of the point absorber were simulated and analysed in the time domain using different configurations of mooring lines using Computational Fluid Dynamic (CFD) simulation approaches were compared. A thorough response analysis is undertaken using a Malaysia low wave condition in shallow water. The results were compared to the assessed for both one and two-body point absorber methods. The simulation process employing a two-body with four mooring lines recommended as the preferred method for capturing the interaction between the WEC system's components, which has been demonstrated to be critical for heaving response evaluation. The average maximum tension on four mooring lines with catenary configuration is preferred to have less tension with high heaving response of the device compared to others. The findings showed that a two-body point absorber with four catenary mooring lines average maximum tension of 700 N is more efficient at absorbing low wave heights than a single body.

Floating unit, mooring lines and risers comprise an integrated dynamic system that responds to environmental loading due to wind, waves and currents in a complex way. The sea current forces acting on the moorings/risers and the resulting damping effects can be an important contribution for the floating unit response. For the integrity assessment of the riser system, one of the traditional methodologies employed is to consider wave frequency (WF) floater motion as dynamic excitation through the vessel Response Amplitude Operator (RAO), while low frequency (LF) floater motions are accounted for by an additional static offset. In deep waters, especially with a large number of slender structures supported by the vessel, it becomes increasingly important to consider the coupling effect between the floating unit, mooring lines and risers, since this can affect the motions of the platform and consequently the riser system response. In this paper, the coupled vs. de-coupled approaches for assessment of the riser system are evaluated by case studies of a deep water, spread-moored Floating, Production, Storage and Offloading unit (FPSO) operating in offshore Brazil. Analyses for floating unit response coupled with moorings/risers and the top connection response of the slender structures are presented and the two approaches are compared.

Summary form only given. We present a frequency-domain system used for characterizing the scatter from arbitrary samples in the frequency range of 220-500 GHz. This system uses a vector network analyzer along with quasi-optical beam shaping components and frequency converters to illuminate and capture scatter radiation providing nominal dynamic range of 90 dB over the 220-325 GHz and 70 dB over the 325-500 GHz band. High-precision motion control of source and receiver, as well as, precision optical alignment techniques allow detailed characterization of scattered electric field and phase from samples. Precision motion control and carefully designed sample mounting allow for a large part of the four-dimensional bi-directional reflectance distribution function (BRDF) of a sample to be accessed. Measurements over a bi-static angle ranging from 205° to 24° with an angular resolution of 0.002° can be made, including mono-static scattering. Out-of-plane angle can be adjusted over the range of +/-5° at 0.5° increments to increase sample BRDF coverage. Data collected from this system is used to generate a zoology of scattering phenomenology from real world and canonical samples. Samples with designed deterministic as well as non-deterministic surface topology statistics were used to both validate this system and to further develop scattering models at millimeter and THz frequencies. The results of an inter-comparison with an analogous time-domain (TD) system developed by NIST in Gaithersburg will be presented. System design and performance details as well as sample data which are compared to both scattering models and the TD system in Gaithersburg will be presented. Examples of different applications for such a system are also presented.

The lazy-wave riser is an input and output riser for a flexible development system, which is widely used in all the riser and pipeline systems. Because of the influence of various factors, its configuration description, control and motion which have a strong nonlinear character are complex during the running process of the lazy-wave riser. Reference to the specific structure and environmental parameters of a certain lazy-wave risers system with a 300 thousand tons FPSO, with the basis of the specific process of the flexible riser system at work, the lazy-wave risers were discretized into lumped mass models, combined with AQWA, the simplified dynamic model of the whole system at the depth of 2100m has been established by the large hydrodynamic analysis software OrcaFlex. The dynamic response characteristics of the lazy-wave risers have been given by using time domain coupling method. With and without the consideration of the 2nd wave drift load in the simulation process, the effects of the 2nd wave drift load on the results are obtained. The simulation results reveal the difficulty of simulation convergence caused by a large number of risers and flexible components. The 2nd order wave drift loads have a significant effect on the riser system, resulting in the increasement of the effective tension at each end of each riser. To counteract the magnitude of the FPSO response caused by such loads, the number of mooring lines needed to be increased or combined with dynamic positioning techniques to optimize the design.