Fpso Model Tests And Analytical Correlation

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.

Until recently, most of the turret mooring systems have been designed using the so-called quasi-static approach. There is an increasing awareness that this approach may be deficient for certain applications, and that dynamic analyses are needed. To this end, this paper presents a rational dynamic analysis procedure for designing turret mooring systems. Specifically, practical methods for computing the low frequency motions and dynamic line tensions are presented, based on the latest analytical developments, model tests, and design experience. Model test data are presented for three turret mooring system in water depths of 70 meters, 75 meters, and 370 meters, respectively, to substantiate the significance of drag force on mooring lines on the low frequency motions, the modified Rayleigh distribution and the RINA method for interpolating the statistical low frequency responses, and line dynamic behavior. Finally, this procedure is illustrated using the turret mooring system of a 230K DWT tanker in 370-meter water depth as an example case. The results of the dynamic analyses are compared with model test data and with the results using a quasi-static analysis approach. INTRODUCTION Motion Response Characteristics The motions of a turret moored vessel can be described by three components: mean offset, low frequency motions and wave frequency motions. The mean offset is caused by the mean environmental force and is constant for the duration of interest. Low frequency motions refer to the resonant surge, sway, and yaw motions due to low frequency cyclic loads associated with wave groups or wind dynamics. These have periods varying from a low of 60 to 90 seconds to a high of a few hundred seconds. Wave frequency motions refer to the vessel motions excited directly by ocean waves. The periods vary from 3 to 25 seconds. The consensus is that these three types of motion can be considered as decoupled physical processes, and hence, they can be simulated individually and combined for the maximum offset. Computing the mean offset and the wave frequency motions is straightforward. The former involves only static catenary calculations. The latter can be accomplished using the highly developed frequency and time domain analysis techniques. The simulation of the low frequency motions remains a relatively complex task. The principal difficulties lie with obtaining quality data for wave drift force coefficients, estimates of low frequency damping, and low frequency wind spectra. While practical analytical tools exist, the prediction of low frequency motions is still in a research stage, Reference 1. Line Tension Response Characteristics A mooring line may respond to fairlead motions in three ways:change in the catenary configuration (catenary stiffness),stretch {elastic stiffness), andchange in the catenary configuration and stretch at the same time. If the fairlead moves slowly (low frequency motions) the line will move toward the new static-equilibrium position. The line will essentially be in static equilibrium at all times. The tension perturbation can be calculated using a static catenary equation.

Ship-shaped Floating Production Storage Offloading platforms (FPSO) are commonly used in the production of oil and gas in offshore deepwater regions. The vessel is held in place by mooring lines anchored to the seabed during operation, either in spread or turret mooring arrangement. When designing such systems, water depth is a main factor that needs to be considered. At greater depths, the hydrodynamic properties of mooring lines become important and may not be accurately predicted through traditional experiments or numerical quasi-static models. Numerical simulation using coupled dynamic analysis is thus recommended, as the hull-mooring behaviour is analysed simultaneously, and the damping and added mass properties of the entire mooring line system is taken into account. This paper investigates the motions and mooring line tensions of a turret-moored FPSO at various water depths ranging from 1000 m to 2000 m. The analysis focuses on numerical simulations in the fully coupled dynamic time domain. The study utilizes the commercial software AQWA, with the FPSO model subjected to a unidirectional random wave condition. The hull hydrodynamics is first solved using the 3D radiation/diffraction panel method, and the hull response equation is then coupled with the mooring line equation. The dynamic motions and mooring line tensions results are presented in terms of statistical parameters as well as response spectrum. The results highlight the significance of greater water depths on low frequency responses in surge motions and mooring line tensions, and provides insight on the increasing and decreasing trend of these responses.

To investigate the dynamics of gravity cage, the finite element program Aqua-FE™, was applied to gravity cages with two different meshes, square-mesh net and diamond-mesh net. Two case studies are chosen to compare the dynamics of cages based on the numerical modeling techniques. The numerical models were developed in Aqua-FE™ to simulate the effects of waves and current. They were validated by comparison with water tank results. The comparison showed good agreement. In both case studies, we consider several loading conditions consisting of different uniform currents and monochromatic waves. Assuming that the system can be modeled as a linear system, we investigated the motion response (heave, surge and pitch) characteristics of the fish cages with square-mesh net and diamond-mesh net, their deformation and the resultant tension in mooring lines. For different wave conditions, we study the storm response of the two cages based on the response amplitude operators (RAOs). In particular, the length of the grid line has significantly effect on the motion of the cage and the tension in mooring lines in wave. In additional, the effect of the mesh shape on the motion of the cage and tension in mooring lines is also analyzed. The comparison results show that the surge motion of the fish cage with square-mesh net is significantly larger than that with diamond-mesh net.

A new approach to predict dynamic mooring tension using LSTM neural network based on responses of floating structure

On dynamic coupling effects between a spar and its mooring lines

The Turret Mooring System (TMS) for the Shell Prelude FLNG (Floating Liquefied Natural Gas) facility consists of a Top Mounted Internal Turret (TMIT) integrated at the bow of the FLNG hull. Permanently connected to the seabed through a dedicated mooring system, this Turret allows the FLNG facility to passively weathervane in response to prevailing weather conditions and enables transfer of fluids and utilities (in both directions) between the subsea wells and FLNG topsides. This paper describes the challenges faced during the engineering and construction of the most challenging TMS ever executed to date. Prior to Prelude, SBM Offshore (SBM) had designed for Shell a TMS suitable for a large range of gas compositions from a portfolio of remote offshore gas fields, including North-West Australia, in water depths ranging from 165m to 1,350m, with the generic FEED vision "engineer one, build many". The Prelude TMS was finally developed with a detailed design to meet the specifics of the 250m field water depth and to remain safely on station for 25 years in cyclonic weather conditions. To achieve the robust design target, field proven solutions were revalidated for application at a greater scale, for a longer design life, harsher environment and the high reliability demands of the project. The execution strategy followed the requirements of TMS contractor direct and end Clients. The TMS, located very close to the bow, moors the 488m long, 74m wide Prelude FLNG hull. The system will ultimately accommodate 8 production risers, although only 4 are presently installed. The mooring system design is governed by the 10,000-year storm, resulting in a weathervaning system comprising 40 bogies, 40 radial wheels and 32 lower pads transferring the load to the hull. The mooring arrangement is composed of 4 bundles of 4 lines each, with 175mm R4 chains at top and bottom, and a mid-water section of 162mm sheathed diameter steel wire rope. Several challenges have resulted from the requirement to comply with a complex set of codes and standards, the severity of the cyclonic conditions, the record size of the FLNG hull, and from the required capacity of mooring components pushing beyond the existing industry experience base. Lessons learned from a selection of key challenges encountered during the engineering and construction of this one of a kind TMS will be presented with a view to facilitating future developments offshore North West Australia.

Deep-sea aquaculture can alleviate the spatial and environmental pressure of near-shore aquaculture and produce higher quality aquatic products, which is the main development direction of global aquaculture. The coastline of China is relatively flat, with aquaculture operations typically operating in sea areas with water depths of approximately 30–50 m. However, with frequent typhoons and poor sea conditions, the design of mooring system has always been a difficult problem. This paper investigated the multiple cages, considering two layouts of 1 × 4 and 2 × 2, and proposed three different mooring system design schemes. The mooring line tension of the mooring systems under the self-storage condition was compared, and it was observed whether the mooring line accumulation and the contact between the mooring line and the steel structure occurred on the leeward side. Additionally, flexible net models were compared with rigid net models to evaluate the impact of net deformation on cage movement and mooring line tension. Finally, based on the optimal mooring design, the dynamic response of the mooring system under irregular wave conditions was analyzed and studied, providing important reference for the safety and economic design of the mooring system of multiple fish cages.

FPSO is widely used during the deep‐sea oil and gas exploration operations, for which it is an effective way to keep their position by means of positioning mooring (PM) technology to ensure the long‐term reliability of operations, even in extreme seas. Here, a kind of dynamic positioning (DP) controller in terms of structural reliability is presented for the single‐point turret‐moored FPSOs. Firstly, the mathematical model of the moored FPSO in terms of kinematics and dynamics is established. Secondly, the catenary method is applied to analyze the mooring line dynamics, and mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, mathematical model for the whole turret mooring system is established. Thirdly, a structural reliability index is defined to evaluate the breaking strength of each mooring line. At the same time, control constraints are also considered to design a state feedback controller using the backstepping technique. Finally, a series of simulation tests are carried out for a certain turret‐moored FPSO with eight mooring lines. It is shown in the simulation results that the moored FPSO can keep its position well in extreme seas. Besides, the FPSO mooring line tension is reduced effectively to ensure mooring lines safety to a large extent in harsh sea environment.

With the increasing demand for key technologies in deepwater oil and gas exploration, the mooring system, a critical component for the positioning of deepwater floating platforms, plays a vital role in ensuring the stable operation of offshore oil and gas platforms under harsh working conditions. In this study, we propose an optimization method for mooring systems based on Enhanced Multi-task Genetic Optimization (EMGO). Taking a semi-submersible platform mooring engineering project in the South China Sea as an example, a multi-body coupled model of the semi-submersible platform and its mooring system was established in compliance with mooring system design specifications. A custom-developed program was utilized to generate 7,751 sets of dynamic mooring system data. Subsequently, an enhanced multi-task learning (MTL) neural network was designed to predict platform offsets and mooring line axial tensions under various working conditions with high accuracy. The optimal performance of the MTL model achieved a mean squared error (MSE) of approximately 0.044 and R-Square (R²) of 0.96. Combining the enhanced MTL model with a genetic algorithm, iterative optimization was performed to converge on an optimal mooring design parameter set tailored for the semi-submersible platform. The proposed method achieved a reduction of over 20% in maximum axial tension in mooring lines and a reduction of over 10% in maximum platform offset. The EMGO algorithm demonstrated excellent capability in high-dimensional nonlinear fitting and generalization. Validation of the optimized design was performed on taut mooring systems of three additional semi-submersible platforms, confirming the feasibility and effectiveness of the optimization method under extreme environmental conditions in the South China Sea. This study introduces an innovative intelligent design method for deepwater taut mooring systems, which significantly improves the safety and reliability of ultra-deepwater platforms. It provides valuable guidance for intelligent design in offshore and marine engineering fields.

Enclosure aquaculture is a healthy and ecological aquaculture pattern developed in recent years to relieve the pressure due to the wild fish stock decline and water pollution. The object of this paper was a floating rope enclosure, which mainly consisted of floaters, mooring lines, sinkers and a net. In order to optimize mooring design factors, the hydrodynamic responses of the floating rope enclosure with different mooring systems in combined wave-current were investigated by experimental and numerical methods. Physical model experiments with a model scale of 1:50 were performed to investigate the hydrodynamic characteristics of a floating rope enclosure with 12 mooring lines. Based on the lumped mass method, the numerical model was established to investigate the effects of mooring design factors on the mooring line tension, force acting on the bottom, and the volume retention of the floating rope enclosure. Through the analysis of numerical and experimental results, it was found that the maximum mooring line tension of the floating rope enclosure occurs on both sides of the windward. Increasing the number of mooring lines on the windward side is helpful to reduce the maximum mooring line tension. Waves and current both have an influence on the mooring line tension; in contrast, currents have a more obvious effect on the mooring line tension than waves. However, the influence of the wave period on the maximum mooring line tension is small. The force endured by the bottom of the floating rope enclosure also changes periodically with the wave period. Yet, the maximum force endured by the bottom of floating rope enclosure occurred at the windward and leeward of the structure. The volume retention of the floating rope enclosure increased with the increasing amount of mooring lines.

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.

Mooring lines are one of the most important parts of spar platforms. In this work, experimental investigation was carried out to obtain the dynamic tension and discover the nonlinear characters while a mooring line transfers from taut to taut–slack. Results show that, when system parameters change, the mooring line system may transform from one steady state to another, accompanied by tension skip that is 5 times the former steady tension and twice the latter steady tension. Such skip tension may result in platform breakage. For the entire cable structure, the effects of system parameters are very important and need further theoretical research. The presented research lays the foundation for further studies on this transforming mechanism and for optimization of engineering designs.

Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

This paper discusses the advantages and disadvantages of semisubmersibles and large tankers (150,000 to 250,000 DWT) when used as floating production and storage systems. Primary technical considerations which are discussed include:Mooring LoadsProduction RisersVessel MotionsCrude Storage and OffloadingSystem ReliabilityField Shut-In ParametersCapital ExpendituresLead Times The data included is based on computer simulation, model tests, and industry successes in meeting similar challenges. INTRODUCTION In the past decade several fields have been produced in the North Sea with semi submersible drilling units which were converted to production units. As a result of the proven suitability of semi submersible production units in hostile environments (Argyle & Buchan Fields), many people believe that only semi submersibles are suitable for those locations. With the increasing use of converted tankers as floating production, storage and offloading units, it is appropriate to evaluate their suitability for more hostile environments. SYSTEMS EVALUATED Five basic configurations which are considered in this paper are:Converted Semi; Shuttle Tanker. This includes a semisubmersible drilling unit (modified with production equipment), with continual offloading to dedicated shuttle tankers.Converted Semi; Storage Tanker. This includes a semi submersible drilling unit (modified with production equipment), a permanently moored storage tanker, and periodic offloading with shuttle tankers.Custom Semi Production and Storage. This includes special design and construction of a large semisubmersible, with periodic offloading with shuttle tankers.Production and Storage Tanker; SPM. This includes a very large crude carrier (VLCC), or a converted tanker (150,000 to 250,000 DWT), permanently moored with an articulated tower or single anchor 1eg mooring (SALM) with a rigid yoke, and periodic offloading with shuttle tankers.Production and Storage Tanker; Turret Mooring. This includes a converter a tanker or VLCC permanently moored with a turret mooring system, and periodic offloading with shuttle tankers. For each case, it was assumed that major pipe1ines are not in place, necessitating the use of shuttle tankers. MOORING LOADS Typical mooring loads (without shuttle tanker attached) are shown on Figure 1. The mooring forces are significantly lower for the tanker based unit which weathervanes than for a semi submersible exposed to beam seas. Therefore, a much greater factor of safety can be maintained with similar mooring systems, or the size of the tanker-based mooring can be reduced in order to minimize capital expenditures. The mooring loads for the tanker may be slightly larger than shown when the wind, waves and currents are not collinear.