The motion response of a moored semi-submersible-type single module (SMOD) of a Very Large Floating Structure (VLFS) is significantly influenced by the seabed topography and shallow water effects. To investigate the motion response of a moored SMOD that is located near islands, both numerical and experimental studies have been conducted. The hydrodynamic parameters of the SMOD were acquired by the use of the panel method. A finite-element model was adopted to calculate the tension forces of the mooring lines. It was found that the moored SMOD exhibited low-frequency characteristics in shallow water. As the seabed became more inclined, the roll motion became larger, while the sway and heave motions hardly changed. As the water depth became shallower, the heave and roll motions were mitigated; however, the sway motion was aggravated.
As a key technology, the study of the positioning capacity of a Very Large Floating Structure (VLFS) is always a research focus in the field of ocean engineering, and many studies have been carried out and reported (Ohmatsu, 2005). Normally VLFSs can be classified into two types according to their geometry: pontoons and semi-submersibles (Lamas-Pardo et al., 2015). The pontoon-type VLFS is a very large floating pontoon structure. Mooring systems are often equipped for this kind of VLFS to keep the floating structures on site. The Mega-Float program in Japan is a typical pontoon-type VLFS research program, and many relevant studies have been addressed (Ohmatsu, 2005; Watanabe et al., 2004; Wang and Tay, 2011). The semi-submersible-type VLFS consists of an upper deck, some columns, and submerged pontoons. Because of the small waterline-area geometry, the semisubmersible- type VLFS is suitable for deep water in a harsh marine environment. The Dynamic Positioning (DP) system is thus regarded as being more appropriate for this kind of VLFS due to the requirement of mobility as well as the ability to resist the harsh sea condition. The Mobile Offshore Base (MOB) concept proposed by the U.S. Navy is a typical semi-submersible VLFS project that is still under research (Palo, 2005).
This paper presents a comprehensive dynamic analysis of a marine spar platform with various mooring system configurations. From a practical viewpoint, the mooring system configuration is managed by reel-motor devices that change cable lengths while keeping all cables under tension. The spar platform is anchored to the seabed by twelve mooring cables (in six cable bundle arrangements), and the domain that the cable-driven spar platform can be within is called the platform effective area. The analysis is based on a global frame of reference at the seabed and a local frame of reference at the platform center of gravity. Under the context of rigid body dynamics, the averaged values of the mooring cable tension are calculated through the use of a second norm measure. The platform dynamic response under unidirectional harmonic water waves and changeable submerged depths is investigated over the entire spar platform effective area. The minimum platform natural frequency at each location within the effective area is used as a measure of the platform degree of rigidity.
Spar floating marine platforms are often used for offshore operations such as oil and gas exploration and production and wind energy harvesting. A spar platform consists of a floating structure that is connected to a heavyweight spar and anchored to the seabed by a cable-based mooring system. The first spar platform in the oil industry was installed in the North Sea in the 1970s and used for oil storage and offloading (Bax and de Werk, 1974; Van Santen and de Werk, 1976). While the floating platform is exposed to the environmental loads, the mooring system has the objective of retaining the floating structures’ location. Commonly used mooring systems consist of three cables (Karimirad and Moan, 2012; Jeon et al., 2013; Muliawan, Karimirad and Moan, 2013; Muliawan et al., 2013; Si et al., 2014; Kim et al., 2014; Yu et al., 2015), four cables (Downie et al., 2000; Chen et al., 2001; Sethuraman and Venugopal, 2013), nine cables (Zhang et al., 2007; Zhang et al., 2008; Montasir and Kurian, 2011; Montasir et al., 2015), and twelve cables (Wang et al., 2008; Yang et al., 2012).
A number of researchers analyzed the dynamic response of spar platforms through the use of different numerical and experimental techniques. The spar platform motion was investigated by Ran et al. (1996) through the use of a higher-order boundary element method, and they compared their numerical results with the measurement data, showing good agreement. Jha et al. (1997) obtained an analytical prediction for wave drift damping and viscous forces that influence the dynamic response of spar platforms. The effect of nonlinear sea waves on the dynamic response of a spar platform was investigated by Anam and Roesset (2002) through the use of the hybrid wave, stretching, and extrapolation models. Using Morison’s equation, Anam et al. (2003) studied the differences between the time domain analysis and frequency domain analysis in predicting the spar platform slow drift response.
Bai, Yanbin (COTEC Offshore Engineering Services) | Lv, Haining (Shanghai Jiao Tong University) | Wang, Jim (COTEC Offshore Engineering Services) | Cheng, John (COTEC Offshore Engineering Services) | Luo, Yong (COTEC Offshore Engineering Services)
A light weight Semi-submersible platform for deepwater production named as QX-Semi has been developed. Attributed to innovative designs such as triple columns and simplified structural configurations, the QX-Semi achieves a significant CAPEX reduction, 14% for hull structure and mooring system compared with conventional production Semi-submersibles.
This paper addresses the global performance of QX-Semi in a water depth of 300m. To verify the mooring design and investigate the global performance, motion responses of QX-Semi were conducted using both the frequency-domain and the time-domain fully coupled approach adopting viscous damping coefficients obtained from wave basin model test. Extreme motion characteristics were subsequently verified by wave basin model test adopting combined wind, wave and current so as to simulate the severe meta-ocean environments. Satisfactory agreements between the numerical and experimental investigations have been achieved, such as extreme WF roll, pith and heave motion and horizontal motion responses. VIM is conducted using a numerical towing tank model, which reveals comparable motion amplitude compared to recent developed Semi-submersible FPS. Moreover, these comparative studies prove the feasibility and survivability of QX-Semi in the operation condition and in the extreme/survival condition respectively. The results show that the light weight QX-Semi has excellent global performance characteristics for deepwater development.
Considering the low oil price in recent years and the continuing suppressed market in the foreseeable future, offshore oil & gas industry has accumulated incentives to implement cost-effective technology to retrieve hydrocarbon in mid-deep (300~1000m) and deepwater (>1000m) water, in particular for marginal fields. Upstream companies are confronting overwhelming challenges on reducing CAPEX and OPEX of FPS so as to enhance the competence of offshore production. Besides FPSO, Semi-submersible FPS is receiving more and more attention as an affordable solution for deepwater E&P. Recently, COTEC Inc. developed a new light-weight Semi FPS as shown in Fig.1, under financial support of CNOOC. It is named as QX Semi and is targeted for oil & gas fields in South China Sea with range of water depth from 300 to 1000 meters, such as LH, LS and LW fields. Attributed to unique configuration and smart structural design, QX Semi saves 12% on hull fabrication and 21% on procurement and installation of mooring systems compared to a conventional four column Semi FPS using the same design basis such as met-ocean data, water depth, topside function requirements, design draft and operating displacement.
As an alternative design to circular FPSO (Floating Production Storage and Offloading), a concrete FPSO with tangerine transverse cross section is investigated. As a two-level structure, the bottom has larger tangerine transverse cross section, and two different top structures are considered: circular shape and tangerine shape. In frequency domain, characteristics of hydrodynamic coefficients are investigated along comparison between the two cases, and Response Amplitude Operator (RAO) is obtained for the linear system. Structural strength is also examined around the transverse cross sections. Following the frequency domain, optimum mooring system is designed and global performance is analyzed using time-domain nonlinear simulation program, CHARM3D/HARP (Kim 2006), which couples floating body dynamics, random seas, and mooring-riser dynamics. In the time domain, the FPSO with tangerine transverse cross sections for both of the top and bottom structures is considered. A mooring system is designed to have maximum offset less than 5% of the water depth as well as maximum tension less than 60% of Minimum Breaking Load (MBL) in survival condition (RP2SK 2005). Similarly to damping structures of existing circular FPSOs, a set of damping plates in heave direction are tested and influences of the heave damping plates on the global performance are discussed.
As an alternative to ultra-deep water offshore platforms, a tangerineshaped FPSO is investigated for global performance. GS E&C suggested two difference concrete hull shapes in a tangerine form that can result in capacity expansion of the production and storage. Contrary to the circular hull shape (Srinivasan et al. 2008, Beck and Vandenworm 2011), the tangerine transverse cross section can be more endurable against environmental loads. Considering active hydrodynamic interactions around the free surface, two designs with different cross sections of the top structures in Fig. 1 are compared in terms of hydrodynamic coefficients and responses.
We first investigate differences between the two shapes in terms of hydrodynamics and response amplitude operators in freely floating condition at each wave frequency. Using the coupled time-domain analysis, optimum mooring system is designed for an area of interest, Walker Ridge in the Gulf of Mexico where the water depth is about 2,950 m. Subsequently, global performance analysis is carried out with the optimum mooring system by combining 3D panel diffraction and radiation method for the hull and nonlinear finite rod element methods for the mooring system. The fully coupled dynamics is solved, respectively, using WAMIT and CHARM3D/HARP.
During the course of the Offshore Code Comparison Collaboration, Continued, with Correlation (OC5) project, which focused on the validation of numerical methods through comparison against tank test data, the authors created a numerical FAST model of the 1:50-scale DeepCwind semisubmersible system that was tested at the Maritime Research Institute Netherlands ocean basin in 2013. This paper discusses several model calibration studies that were conducted to identify model adjustments that improve the agreement between the numerical simulations and the experimental test data. These calibration studies cover wind-field-specific parameters (coherence, turbulence), hydrodynamic and aerodynamic modeling approaches, as well as rotor model (blade-pitch and blade-mass imbalances) and tower model (structural tower damping coefficient) adjustments. These calibration studies were conducted based on relatively simple calibration load cases (wave only/wind only). The agreement between the final FAST model and experimental measurements is then assessed based on more- complex combined wind and wave validation cases.
In 2013, a 1:50-scale model of the DeepCwind semisubmersible floating offshore wind turbine was tested at the Maritime Research Institute Netherlands (MARIN) ocean basin under the direction of the University of Maine. The data from this test campaign was then used in 2015/2016 within the framework of Phase II of the International Energy Agency (IEA) Wind Task 30 Project, also known as OC5 (Offshore Code Comparison Collaboration, Continued, with Correlation). The National Renewable Energy Laboratory (NREL), both led and participated in the OC5 project, which included conducting a series of model calibration studies to improve the match between their numerical model and the wave tank test data. Several of these studies and their key findings are presented in this paper. The authors modeled the DeepCwind system using NREL's open-source wind turbine simulation software FAST version 8 (NREL, 2015).
The key properties of the numerical and the physical model as tested in the wave tank are summarized below.
A suitable mooring system significantly influences the motion response of the Very Large Floating Structure (VLFS). The motion response of a single module (SMOD) of a semi-submersible VLFS with tension leg mooring system near island was investigated by numerical study. A model test was also conducted to validate the results of time domain simulation. The numerical calculation coincides the experimental results well. It has been demonstrated that the heave and roll motion decrease as the water gets shallow, while the motions increase in extremely low frequency.
The concept of very large floating structure (VLFS) such as mat-likes and semi-submersibles has attracted many researchers’ interest in the last few decades (Wang and Wang, 2015; Lamas-Pardo et al., 2015). The mat-like VLFS is a structure similar to pontoon-like ship hull, while the semi-submersible VLFS is similar to the semi-submersible platform, consisting of several modules interconnected. The mat-like VLFS is designed to located near the shore using the breakwaters to reduce the wave impacts. Because of the small waterline-area and good mobility, the semi-submersible VLFS is more suitable for deep sea.
It's important to estimate the hydroelastic of the VLFS accurately and effectively, especially for mat-like VLFS. Mode-expansion method (Kim and Ertekin, 1998; Ohmatsu, 2000) and mesh method (Yago and Endo, 1996; Motohiko et al., 1999) is two representative methods for calculating the mat-like VLFS's hydroelastic response. The B-spline Galerkin scheme developed by Kashiwagi (Kashiwagi, 1998a) is a typical mode-expansion method. It uses B-spline function to represent the pressure and Gakerkin scheme is applied for meeting the boundary conditions. This method reduces computational time well and the accuracy of the results is acceptable for practical use. The mesh method uses the boundary element method (BEM) and finite element method (FEM) to acquire the hydroelastic response of mat-like VLFS. This method consumes huge computational time and needs quantities of computer memory.
In the present study, the Finite-Analytic Navier-Stokes (FANS) code is coupled with an in-house finite-element code for time-domain simulation of the hydrodynamic response of Catenary Anchor Leg Mooring (CALM) buoy system. In the FANS code, the fluid domain is decomposed into multi-block overset grids and the Large Eddy Simulation (LES) is used to provide accurate prediction of vortexinduced motion of the buoy. The mooring system is simulated with a nonlinear finite element code, MOORING3D. An interface module is established to facilitate interactive coupling between the buoy and mooring lines. The coupled code was calibrated first for free-decay case and compared with model test data. The coupled code was then employed for the simulation of two degree-of-freedom vortex-induced motion of a CALM buoy in uniform currents to illustrate the capability of the present CFD approach for coupling mooring analysis of offshore structures. With the study it can be verified that the coupled method is able to provide an accurate simulation of the hydrodynamic behavior of the CALM buoy system.
The CALM system is widely used as an efficient and economic single point mooring system in offshore engineering applications. Compared to other floating structures like Floating Production Storage and Offloading (FPSOs) or Tensioned Leg Platform (TLP), CALM buoy is more sensitive to the response of mooring lines and oil offloading lines due to its considerably smaller inertia, damping and hydrostatic stiffness. These features for buoy can result in dangerous motions causing fatigue damage in mooring and flowlines systems. Therefore, it is essential to develop advanced numerical methods for accurate estimate of dynamic motion for CALM buoys.
Several numerical investigations have been performed for dynamic analysis of the CALM buoy system. Most of the numerical models use empirical coefficients for lift, drag and added mass in their simulation of the CALM buoy systems, such as those presented by Ryu et al. (2005) and Sagrilo et al. (2002). Berhault et al. (2004) performed CFD simulations of forced oscillations and forced heave / pitch motions of the CALM buoy model to provide more accurate evaluation of the hydrodynamic damping coefficients.
Dynamic analysis of mooring line systems under various environmental conditions including waves, currents and winds requires computationally expensive time domain analysis. In this paper, Nonlinear Autoregressive with Exogenous input (NARX) model was used to predict the time histories of a mooring line top-tension. The dataset for network training was obtained from the nonlinear analysis of the mooring system under random ocean waves of different sea states. To check the validity of the proposed method, the top-tension of a mooring line under the variety of different ocean wave environment with different significant wave heights and modal periods was predicted using the proposed method and compared with the nonlinear time domain analysis results. The predicted time series of the top-tension of a mooring line in different sea states has good correlation compared with the direct analysis results This method can be used to predict the top-tension of a mooring line without the computationally demanding nonlinear time domain analysis.
Due to the increasing demand in energy, exploitation of oil and gas in deep waters has been gained more importance. Exploration of oil in deep waters needs floating production systems connected to mooring lines and risers such as FPSO (Floating Production, Storage and Offloading) and semi-submersible platforms. Mooring lines that keep a floating offshore structure in position are required to comply with safety limits such as tensions. Engineers use nonlinear time domain finite element analysis to obtain the tensions in different sea states. Short term dynamic analysis which is approximately 3 hours for each sea state takes too much time because of considering the hundreds of wave spectra. To have meaningful solutions in rapid mode with high accuracy, researchers develop prediction method for time domain analysis. Sagrilo et al. (2000) focused on the development of a high effective practical approach to assess the short-term extreme response statistics of flexible risers excited by the first-order heave motion of a floating unit. The analysis of three different flexible risers configurations illustrated the accuracy and the robustness of this approach to calculate the extreme response statistics. Billings and Wei (2005) proposed a new class of wavelet networks for nonlinear system identification. In the new networks, the model structure for a high-dimensional system was chosen to be a superimposition of a number of functions with fewer variables. Mazaheri and Downie (2005) used a response-based method as a reliable alternative approach. In order to perform the calculations faster using large databases of sea states, ANN was designed and employed. The response based method was applied to a 200000 dwt FPSO and obtained results were discussed in the case study. Mahfouz (2007) described a new method to predict the Capability-Polar-Plots for offshore platforms using the combination of the artificial neural networks and the Capability Polar Plots Program (CPPP). The estimated results from a case study for a scientific drilling vessel were presented in his study. Guarize et al. (2008) indicated a very efficient hybrid ANN-Finite Element Method (FEM) procedure to perform a nonlinear mapping of the current and past system excitations (inputs) to produce subsequent system response (output) for the random dynamic analysis of mooring lines and risers. Yasseri et al. (2010) proposed a predictive method for identifying the range of sea-states considered safe for the installation of offshore structure. They made finite element analysis for different sea-states with characterization of significant wave height and mean zero-up crossing wave periods. They identified the range of sea-states suitable for safe pile-driving operation as a case study. Pina et al. (2013) presented a new surrogate model based on artificial neural networks (ANN) to evaluate the response of mooring lines and risers expeditiously. Their aim was to get result as accurate as finite element method based dynamic analysis in shorter time. Queau et al. (2014) aimed to test the robustness of their previous researches and extend the ranges of the input parameters for steel catenary riser systems under static loading, by means of numerical simula-tions. An approximation using a series of neural networks was presented; it successfully approxi-mates over 99% of the cases of the database with an accuracy of ±5%. Jacob et al. (2014) proposed an approach based on Wavelet Networks (WN) - a combination of the feed-forward neural network architecture with the wavelet transform. The goal is to obtain dramatic re-ductions in processing time, while providing results nearly as good as those from non-linear dynamic fi-nite element methods Kim (2015) predicted the dynamic response of a slender marine structures under an irregular wave with NARX based quadratic Volterra series. He compared the predicted time series of the response of structure with quadratic Volterra series and nonlinear time domain simulation results. Volterra series results coincided with simulation results.
AbstractA valuable property of a turret moored FPSO is its ability to weathervane or self adjust its heading to the varying metocean environment. Still, heading control with thrusters is necessary when a different heading than the natural weathervaning heading is wanted for operational reasons. In particular, in non-colinear metocean conditions it may be desirable to keep the bow of the FPSO against the waves to reduce rolling. It may happen that the weathervaning becomes unstable. In this case, the vessel will go into fishtailing motion, which is a phenomenon that is governed mainly by interaction between sway and yaw. Fishtailing is weather-dependent. When it happens, heading control is needed to stabilize it.One important objective of the study has been to investigate if satisfactory heading control can be obtained using thrusters aft only, or if thrusters forward are necessary.For a given FPSO design, we study weathervaning stability by formulating a linear model in sway and yaw. Based on this model with damping neglected, we develop three simplified criteria for stability. Using heuristic argumentation, we accept the criteria as conditions for sufficiency. The system's eigenvalues give sufficient and necessary conditions for stability. Using metocean hindcast data covering a time span of 56 years, the simple criteria are tested against the eigenvalues of the sway-yaw model. We found good agreement.The heading controller used is essentially a PID controller. It is found that with aft thrusters only, good heading control can be achieved for all the metocean states in the 56 years span, provided the controller gain is moderate. With high controller gain, the thrusters may excite resonance in the turret mooring. Using additional thrusters forward gives the freedom to reduce turret resonance. Time domain simulation with an accurate 6-degree-of-freedom model shows that thrusters aft and forward gives slightly better control than control using aft thrusters only. Still, using only aft thrusters appears to give satisfactory heading control.
Hadley, C. (Shell Global Solutions (US) Inc.) | Bradford, K. (Shell Global Solutions (US) Inc.) | Young, A. (Geoscience Earth and Marine Services Inc.) | Trandafir, A. C. (Fugro GeoConsulting Inc.) | Bruce, B. (Fugro GeoConsulting Inc.)
AbstractShell's Stones field is located below the Sigsbee escarpment on the continental rise in the Walker Ridge area of the Gulf of Mexico, in a water depth of approximately 9,500-ft. The seafloor is irregular due to the presence of furrows, which have been eroded by bottom currents. These features, combined with positioning uncertainties associated with surveys in this extreme water depth, posed a unique challenge to characterization of the seabed and to siting of infrastructure.Design soil properties were selected by integrating Autonomous Underwater Vehicle survey data with data from two geotechnical investigations. These data sets are typically correlated by using the seafloor as the vertical reference datum. This was not possible in the Stones field, so the data was instead correlated to a marker horizon, which was common to both the shallow seismic and the geotechnical data sets. Additionally, seafloor bathymetry data were screened using pre-determined criteria to optimize FPSO mooring anchor locations.This paper presents sample geophysical and geotechnical data from the Stones field and describes the process used to integrate the data and develop design information. The selection of FPSO mooring anchor locations within the variable bathymetry is also described. Finally, installation records from the FPSO mooring system are presented, which demonstrate the value of the integrated approach to site characterization. The process for data integration at Stones can serve as a model for future developments, with complex seafloors.