Turret Mooring Research Articles

Feasibility Study of a Novel Open Ocean Aquaculture Ship Integrating with a Wind Turbine and an Internal Turret Mooring System

Marine aquaculture has been expanded into more remote areas due to the increasing demand for high value-added fish products, bringing more challenges in fuel supply and mooring safety to open ocean aquaculture farms. Therefore, the idea of integrating aquaculture farms with offshore renewable energy has attracted tremendous interest. This study proposes a novel open ocean aquaculture ship integrated with a NREL 5 MW wind turbine. In addition, an internal turret mooring system is installed at the bow of the aquaculture ship, making the wind turbine always face the wind blowing direction due to the weathervaning effect. In this study, a preliminary study of the dynamics and wind power generation of the proposed open ocean aquaculture ship was conducted. The aerodynamic and elastic effects of the wind turbine and the effect of nets under the wave and current behavior on the dynamics of the single-point moored aquaculture ship were investigated. Furthermore, the effects of the dynamics of the aquaculture ship on the power generation of the wind turbine were also studied to investigate the feasibility of the idea to integrate an aquaculture ship with a wind turbine.

Physical model test validation of dynamic similarity truncation method for an internal turret mooring system

The mooring system needs to be truncated in physical model test to adapt to the limited dimensions of offshore basins. Especially, the drag force and inertial force are required to be included in the dynamic mooring truncation methods, and the method should be verified by model test comparison. Here an effective dynamic truncation method is proposed based on slender rod theory and multi-objective cuckoo search optimization algorithm. The line tensions in time domain are transferred into frequency domain to implement the dynamic similarity. The proposed process can promote optimization efficiency in a great extent. In order to demonstrate the accuracy of the proposed method, a horizontal truncation is carried out to an internal turret moored FPSO working in 420 m water depth. The model test in deep-water basin is performed with static tests and irregular wave tests. It can be found that model test and numerical results show consistence on the static stiffness characteristics and dynamic response. The comparison verifies the reliability and accuracy of the proposed truncation method well.

Design and Automated Optimization of an Internal Turret Mooring System in the Frequency and Time Domain

A cost-effective mooring system design has been emphasized for traditional offshore industry applications and in the design of floating offshore wind turbines. The industry consensus regarding mooring system design is mainly inhibited by previous project experience. The design of the mooring system also requires a significant number of design cycles. To take aim at these challenges, this paper studies the application of an optimization algorithm to the Floating Production Storage and Offloading (FPSO) mooring system design with an internal turret system at deep-water locations. The goal is to minimize mooring system costs by satisfying constraints, and an objective function is defined as the minimum weight of the mooring system. Anchor loads, a floating body offset and mooring line tensions are defined as constraints. In the process of optimization, the mooring system is analyzed in terms of the frequency domain and time domain, and global and local optimization algorithms are also deployed towards reaching the optimum solution. Three cases are studied with the same initial conditions. The global and local optimization algorithms successfully find a feasible mooring system by reducing the mooring system cost by up to 52%.

Effect of keel plate on the performance of FPSO suitable for arctic ice environment

ABSTRACT The purpose of the paper is to carry out experimental and numerical investigations on a 1:45 scaled model of a non-ship-shaped FPSO vessel that could work in clear water open-sea and in ice-covered arctic environments with least ice-management requirement. The FPSO vessel is designed with a turret mooring system which is disconnectable during emergency and inclement weather conditions. As a first phase of the study experiments were conducted on a 1:45 scale model with three different mooring arrangements viz. 100% turret mooring, 50% vessel and 50% turret mooring, and 100% vessel mooring. Experiments are performed on the model with different sizes of damping plates attached at the keel to study its influence on the responses of the vessel. A numerical study has been carried out in WAMIT on the scaled model and the results are compared with those obtained from model tests in the wave basin.

원통형 문풀 구조물의 탑재 시 변형과 수정

Turret mooring type Floating Production Storage and Offloading (FPSO) is designed to rotate the hull around a turret system. The system is mounted inside a cylindrical moon-pool structure of the ship hull structure. The upper part of the moon-pool structure called Bogie Support Structure (BSS) is supported on ring type rail structure (bogie), so high roundness is required at the top of the structure. In this study, the deformation measured during BSS installation was compared with the predicted values through the thermal elasto-plastic analysis, and the causes of deformation were analyzed. Deformation behavior of cylindrical structure with a very large diameter compared to the thickness was investigated. In addition, a proper welding sequence and correction method for the deformed structure were proposed. This study can be an example of the solution to the tolerance problem of large cylindrical structures.

Modelling of the passive turning of a turret moored vessel in conditions of compact ice

The concept of the offshore oil and gas field development using floating ship-shaped platforms, frequently named floating production units (FPU further), with turret mooring is widely used in the world, including regions with harsh environment. The direct transfer of this concept to the Arctic seas is not possible, mostly due to the difficulties of maintaining a weathervaning, or passive turning regime in heavy ice conditions. The main danger relates to the fact that the expected FPU rotation under the action of high ice loads can be accompanied by a translational displacement of the FPU away from the mooring point. As a result, the mooring system may reach an overloaded state until the FPU turns to a favourable position relative to the ice drift direction. In the paper, we are focused on the investigation of a mathematical model of the passive FPU turning on a spot under the assumption that the ice cover is described by a rigid-plastic continuum. The study is performed both analytically and by numerical simulations. A number of specific FPU motion patterns are analytically derived from the model in quasi-static approximation in the form of successive limit states of the system FPU – ice continuum. Some results of the corresponding numerical simulation are presented that confirm the existence of similar solutions in the full dynamic setting of the problem. A partial parametric analysis of the problem is also performed.

Fatigue Life of Mooring Lines on External Turret Floating LNG for Different Pretension and Water Depth

This paper studies fatigue life of mooring lines applied on turret-moored Floating LNG (Liquefied Natural Gas). Several case studies were designed to investigate the influence of mooring pretension and water depth on the fatigue life of mooring lines. Floating LNG with permanent external turret mooring system consists of 12 catenary lines arranged in 3 group with 4 lines each. Time domain simulation applied to calculated coupled dynamic response and mooring tension. Fatigue life is assessed using rainflow counting method, T-N curves and Palmgren-Miner rule. Analysis results for mooring pretension case show that case study 2 with lowest pretension (14%MBL) yields shortest mooring fatigue life of 1814.20 years compared to case study 1 (18%MBL) and 3 (22%MBL) with 2034.61 years and 2983.33 years respectively. Despite has the lowest dynamic line tension, case study 2 has larger tension range that results in the increased of fatigue damage. The increase in water depth will increase the mooring line length and its weight, so it results in the increase of fatigue damage. This is reflected in case study 5 (903m water depth), which has shortest mooring fatigue life of 1842.65 years compare to case study 1 (602m water depth) and 4 (301m water depth) with 2983.33 years and 3363.62 years respectively.

The Investigation of Internal Turret Single Point Mooring Slip Ring Structure Design

Research the structure of turret mooring system slip ring abroad, for the first time in China presents a slip ring structure design, contact fatigue, finite element analysis of the key components of the brush, and through the research on the feasibility of assembly precision test equipment, main circuit resistance and withstand voltage, power frequency withstand voltage and dynamic continuous high current test of static electric slip ring in the working process and rationality.

Hydrodynamics of a conceptual FLNG system in side-by-side offloading operation

ABSTRACTThis paper investigates the hydrodynamics of an floating liquefied natural gas (FLNG)–liquefied natural gas (LNG) offloading system in a side-by-side configuration using potential flow solver AQWA. The system is moored by an internal turret mooring system allowing itself to weathervane under external disturbances and is designed to operate in a sea environment close to that on the coast of Western Australia. Initially, validations are presented for the computed wave loads, representative motion responses and mooring loads. The results are compared against experimental data and those gathered from literatures. Time domain analyses are carried out for the FLNG–LNG system coupled with hawser, fender and mooring systems under the combination of wind, current and waves. The relative motions between the FLNG and LNG in the horizontal plane as well as the mechanical loads on the hawsers, fenders and moorings are computed. Furthermore, the effects of varying hawser pretension and stiffness on the hydrodynamic performance are investigated.

Yaw motion analysis of a FPSO turret mooring system under wave drift forces

In order to evaluate the dynamic behavior of a FPSO-turret system, experimental tests in engineering wave basin have been performed. During the realization of experimental test large slow yaw rotations were detected in the dynamic behavior of the vessel during regular and irregular wave conditions tests. In order to study this phenomenon, an analytical method was developed in order to predict the maximum yaw angle reached under the slow yaw varying oscillations of a single point moored FPSO-turret system dependent of the regular mean wave drift forces and the turret position. The results for the proposed analytical method were verified by comparing it with experimental and simulated results. Then, the proposed method allows to identify the critical regular wave periods than can provoke large yaw rotations as an alternative for the stability tests by bifurcation analysis. On the other hand, the performed yaw motion simulations take into account the following aspects: the turret position, and the force and momentum associated with the mooring system variations (in function of the FPSO center of gravity positions and regular mean wave drift forces). Analyzing the time domain simulations it is possible to identify that large yaw motion is reached faster when the turret is near the CG and so, as the turret moves away, the yaw motions become slower.

On critical parameters of squall associated with the mooring design of a turret-moored FPSO

ABSTRACTSqualls bring a unique challenge for the design and analysis of Floating Production Storage and Offloading Unit's (FPSO's) turret mooring systems in offshore West African and South Asia. This paper focuses on the characterisation of the squalls. The synthetic squall equations are introduced to simplify the actual squall time series. In addition, Monte Carlo method is used to calculate the response-based categorised estimates of maximum offsets and mooring line tensions. The hydrodynamic coefficients are calculated from ANSYS-AQWA and exported to ARIANE software in which the time-domain squall events can be considered as inputs. The system's responses, including turret offsets and mooring line tensions, are initially considered as the consequence of various critical parameters and instantaneous environmental parameters. Furthermore, a sensitivity check of critical parameters affecting the results has been carried out and discussed. Finally, the safety evaluation procedure required in the turret mooring system has also been presented.

Effect of Water Depths on the Hydrodynamic Responses of an FPSO Platform

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.

Dynamic Asynchronous Coupled Analysis and Experimental Study for a Turret Moored FPSO in Random Seas

The dynamic coupling analysis between floating platform and mooring/risers is one of the challenging topics in development of oil and gas exploration in deepwater area. In this paper, the dynamic coupled analysis model for a turret-moored floating production storage and offloading (FPSO) is developed. In order to improve the computational efficiency, the proposed asynchronous coupling model is extended to application for FPSO coupled dynamic analysis. In order to validate the developed dynamic coupling analysis model, the vessel global motion and mooring tensions for the turret moored tanker KVLCC2 were tested in China Ship Scientific Research Center (CSSRC) wave basin. In this paper, the numerically predicted coupled response is directly compared with those from model test in time domain. The research focus is in the following: (1) Through comparison of free decay surge motion in calm water/regular wave, the damping effect from mooring on drift motion is validated and confirmed; (2) The physical phenomena of wave drift damping effect on slowly varying surge motion are theoretically explained making use of the related hydrodynamic model and physical model test; (3) In random seas, the simulated vessel motions and mooring tensions generally show fairly good match in time records with model test. But there is phase difference between simulation and measurements after tens of minutes in model test scale, the exact reason is not known yet; and (4) Another interesting phenomena in model test are the existence of unstable horizontal sway and yaw motion, which are not well predicted by current numerical model, the further research is needed.

Ensuring Mooring Line Safety for Deepwater Turret-Moored FPSO in Extreme Seas

ABSTRACT Wang, Y.; Dou, X.; Ding, F.; Tuo, Y., and Liu, Y., 2015. Ensuring mooring line safety for deepwater turret-moored FPSO in extreme seas. This study addresses dynamic positioning technology of deepwater FPSO moored to the seabed via a turret based spread mooring system, referred to as Position Mooring (PM). In normal weather conditions, the mooring system constrains the FPSO to the predefined region. In extreme seas, however, the mooring system behaves in a limited fashion to constrain the position such that the thruster force is also needed in order to avoid damage to the mooring lines. A controller based on structural reliability is presented for turret-moored FPSO. First, a mathematical model of FPSO in terms of the kinematics and dynamics and the environmental disturbance is established. Second, a mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, the mathematical model for the whole turret mooring system is established. Third, a PID and a state ...

Externalタレットの係留設計要件に関する研究

In recent years, the use of natural gas as an oil alternative energy has been increasingly expected. To ensure the stable supply of natural gas, the construction of floating LNG (FLNG) is now under investigation around the world to develop small or middle scale gas fields, so-called stranded gas field. In most cases, the FLNG under investigation is equipped with a turret mooring system to keep its own position at each installed gas field. However, detailed methods to design the mooring system are not indicated in guidelines published by the typical classification society, or designers have to decide by themselves which of the guidelines they should adopt at the time of beginning design because of different design requirements for every guideline. In this paper, the author investigated combinations of severe direction of environmental forces, such as wind, current, wave and swell, selected from items indicated in the different design requirements. This paper also presents the results of investigation on the method of statistics analysis to evaluate the extreme value of line tension.

Optimal design of equivalent water depth truncated mooring system based on baton pattern simulated annealing algorithm

The highest similarity degree of static characteristics including both horizontal and vertical restoring force-displacement characteristics of total mooring system, as well as the tension-displacement characteristics of the representative single mooring line between the truncated and full depth system are obtained by annealing simulation algorithm for hybrid discrete variables (ASFHDV, in short). A “baton” optimization approach is proposed by utilizing ASFHDV. After each baton of optimization, if a few dimensional variables reach the upper or lower limit, the boundary of certain dimensional variables shall be expanded. In consideration of the experimental requirements, the length of the upper mooring line should not be smaller than 8 m, and the diameter of the anchor chain on the bottom should be larger than 0.03 m. A 100000 t turret mooring FPSO in the water depth of 304 m, with the truncated water depth being 76 m, is taken as an example of equivalent water depth truncated mooring system optimal design and calculation, and is performed to obtain the conformation parameters of the truncated mooring system. The numerical results indicate that the present truncated mooring system design is successful and effective.

Structural Reliability Based Dynamic Positioning of Turret-Moored FPSOs in Extreme Seas

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.

Static and dynamic mooring analysis – Stability of floating production storage and offloading (FPSO) risers for extreme environmental conditions

Floating production storage and offloading (FPSO) facilities are used at most of the offshore oil fields worldwide. FPSO usage is expected to grow as oil fields move to deeper water, thus requiring the reliability and stability of mooring wires and risers in extreme environmental conditions. Except for the case of predictable attack angles of external loadings, FPSO facilities with turret single point mooring (SPM) systems are in general use. There are two types of turret systems: permanent systems and disconnectable turret mooring systems. Extreme environment criteria for permanent moorings are usually based on a 100-year return period event. It is common to use two or three environments including the 100-year wave with associated wind and current, and the 100-year wind with associated waves and current. When fitted with a disconnectable turret mooring system, FPSOs can be used in areas where it is desirable to remove the production unit from the field temporarily to prevent exposure to extreme events such as cyclones or large icebergs. Static and dynamic mooring analyses were performed to evaluate the stability of a spider buoy after disconnection from a turret during cyclone environmental conditions.

On the Extreme Value Analysis of the Response of a Turret Moored FPSO

One of the standardized procedures used in the design of floating systems and their mooring and production lines is the so-called short-term design approach where the system is analyzed for some specific extreme environmental conditions. Along with this procedure, a nonlinear time-domain coupled dynamic analysis, considering the floater and its risers and mooring lines, can nowadays be incorporated as a feasible part of the design practice. One very important and challenging aspect of this process is concerned with the estimation of the characteristic short-term extreme values of the system response parameters based on the sampled time-series. In this paper a common procedure used to establish these extreme values for floater system response parameters, which is based upon a Weibull distribution model for the peaks of the time-series, is reviewed in the light of a recently proposed approach based on a general parametric model for the average conditional exceedance rate of peaks. It is shown that the former model corresponds to a particular case of the latter one. Numerical results are presented for the response parameters of a turret-moored Floating, Production, Storage and Offloading (FPSO) unit considering a short-term coupled analysis of the whole system under an extreme environmental condition of wind, wave, and current. Specifically, the extreme response of surge motion, top tension of the most loaded mooring line, and Det norske Veritas (DnV) codes utilization factor for the most critical section of an 0.20 m outer diameter SLWR (steel lazy wave riser) are investigated.

Hydrodynamic response of a non-ship-shaped FPSO vessel with damping plates

Experimental and numerical hydrodynamic investigations were carried out on a 1:45 scale model of a novel non-ship-shaped floating, production, storage, and offloading (FPSO) vessel that has been designed for use in the icy waters of the Arctic as well as the severe sea states of the North Sea. This FPSO vessel has a turret mooring system that is disconnectable during emergencies and inclement weather conditions. Experiments were conducted on a 1:45 scale model of the FPSO with damping plates consisting of skirt plates and different sizes of keel plates in order to study their influence on the response of the vessel. A numerical study of the scale model was carried out using the Wave Analysis at Massachusetts Institute of Technology code, and the results provided by it were compared with those obtained from model tests in the wave basin.