Turret-moored FPSO Research Articles

Real-time prediction of 6-DOF motions of a turret-moored FPSO in harsh sea state

Predicting the motions of a floating offshore structure could be useful for a motion compensation system and could provide useful early warning information for motion-sensitive operations. In this study, a deep learning (DL) model was developed to provide deterministic predictions of 6-DOF motions of a turret-moored floating production storage and offloading unit (FPSO) in a harsh sea state. These predictions rely purely on past time series of the motion itself. Training and test databases were obtained using a scaled model test in a wave basin. The 6-DOF motions were fed into the DL model simultaneously, and the predictive motion series was obtained from corresponding 6 standalone subnets. The prediction extended by approximately 15 s (one wave cycle) into the future with good accuracy for all 6-DOF motions. By concentrating the predictions in order, a 3-hour time series of the motions was reconstructed. Hyperparameter importance was also evaluated for dropout probability, number of RNN layers, number of fully connected layers, and number of neurons in each layer in terms of accuracy and training duration. Without the requirement for wave measurements, the proposed model is closer to real applications in offshore engineering.

Optimization of mooring line design parameters using Mooring Optimization Tool for FPSO (MooOpT4FPSO) with the consideration of integrated design methodology

Optimization of mooring line design parameters including line azimuth angles, line diameter, line length and mooring radius is presented for a turret-moored FPSO. The optimization procedure is implemented using a Mooring Optimization Tool for FPSO (MooOpT4FPSO), which is an in-house optimization tool purposely developed for this purpose. The tool is a synchronisation of the Regrouping Particle Swarm Optimization (RegPSO) algorithm with commercial software, OrcaFlex. Case studies using a validated numerical FPSO model moored with multicomponent mooring lines acted upon by non-collinear wave, wind and current were analysed using the developed tool. To take into consideration the interaction of the riser system in the optimization procedure, the integrated design methodology was adopted where the riser safe operation (SAFOP) zone diagram combined with the offset diagram is used for the verification/assessment of the design criteria of the risers and mooring lines. The optimized FPSO model offsets in eight directions are found to be within the riser safe operation zone. Based on the results, the tool was able to simultaneously optimise the mooring line diameter, line length, mooring radius, and azimuth angles of the turret FPSO to achieve a specific offset. Application of the tool can help the industry save material (by reduction of line diameter and length) and consequently the overall project cost, in addition to the reduction of structural payload exerted on the platform. Furthermore, the tool has an automatic search capability, which is an improvement to the conventional mooring design approach that is based on a trial-and-error approach.

Effect of Various Mooring Materials on Hydrodynamic Responses of Turret-Moored FPSO with Emphasis on Intact and Damaged Conditions

The behavior of different mooring line materials has a significant influence on the behavior of the mooring system and, consequently, the dynamic responses of the floating platform. Although there have been previous studies on FPSOs and their mooring systems, the influence of mooring line failure scenarios associated with different mooring materials has received less attention, particularly for turret-moored FPSOs with taut moorings. Thus, this paper investigates the behavior of different mooring line materials in intact, single-line, and double-line damaged conditions on the hydrodynamic responses of the FPSO, restoring behavior, mooring, and riser tensions considering wave conditions in the Gulf of Mexico. Mooring lines including Aramid, HMPE, polyester, and steel wire were considered in the middle segment, which was the segment of interest in this study. The restoring forces of the mooring system were found to increase with increasing mooring stiffness, and a higher stiffness resulted in a higher loss of restoring force in the case of single-line failure. In all cases, the submerged weight and material stiffness had a significant influence on dynamic responses, mooring tension, transient responses, riser tension, and especially on the ability of the mooring system to resist the case of single-line failure. Each material was observed to behave differently in each degree of freedom (DOF), showing the necessity to pay close attention to the selection of mooring material for specific objectives.

Reliability-based Event Driven Backstepping Positioning Control for a Turret-moored FPSO Vessel with Unknown Slow Time-varying Disturbances

Reliability-based Event Driven Backstepping Positioning Control for a Turret-moored FPSO Vessel with Unknown Slow Time-varying Disturbances

Application of a convolutional neural network for mooring failure identification

A novel application of a convolutional neural network (CNN) for the identification of mooring line failure of a turret-moored FPSO is demonstrated. The CNN was trained on images of the turret horizontal displacement history, simulated for both an intact mooring and a system with one line that had failed. When tested on operational and extreme environments representative of the North West Shelf of Australia, the CNN successfully distinguished between the turret responses associated with the intact and broken mooring. Classification accuracy was found to be lower for relatively benign conditions when the turret offset response was minimal. This was significantly improved through the use of additional hidden layers and retraining. As the CNN does not explicitly utilise metocean data as input, apart from training, it is envisaged that it offers an effective and lower-cost alternative to existing mooring failure detection approaches for the offshore industry.

Effect of the Turret’s Rotational Damping on the Heading Stability of a Turret-Moored FPSO

Effect of the Turret’s Rotational Damping on the Heading Stability of a Turret-Moored FPSO

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.

Motion behavior in a turret-moored FPSO caused by piston mode effects in moonpool

The water trapped in a moonpool area by a turret-moored Floating Production, Storage and Offloading unit (FPSO) in some cases can give rise to resonant periods due to piston or sloshing modes that cause unstable behaviors in heave and pitch. In this work, experimental tests have been conducted in order to analyze the dynamic effects of water trapped in moonpools and to determine how they affect the global performance of a turret-moored FPSO. Two series of experimental test have been conducted in two different configurations: one using a turret that prevents water inflow, and the other using a turret that allows water inflow between the moonpool and the internal cylinder. The obtained results show that this so-called piston mode appears in the turret model with moonpool area, while at the same time instabilities in dynamic response are observed in FPSO. The experimental results seem to indicate that the piston effect has some contributions in this behavior related to the yaw motion instabilities at regular wave period of 10.0s.

북해설치 FPSO의 플레어 슬래밍 하중에 대응한 선수 설계

This paper is about the bow structure design of the ship-typed and turret moored FPSO which is subjected to the bow-flare slamming load in harsh North Sea environments. Quad 204 FPSO project involves the redevelopment of the existing Schiehallion FPSO which is damaged by impact wave loads. Normally all offshore systems including FPSO are designed to withstand the 100 year storm I.e. the storm that happens once every hundred years at the location where the system is installed. Several incidents have revealed that impact loading is important issue for moored floating production systems. In this paper, the design impact loads are estimated considering the ship owner’s specification, measured data from model tests, requirements of the classification society rules and results of numerical simulation analyses. The impact pressure by numerical analysis is 1.8 times greater than required value by CSR adopted by IACS. Based on the selected design load, plastic design formulae allowing the local material yielding are applied for the initial scantling of the bow structure. To verify the structural integrity, FE analyses are carried out considering the local area subjected to the impact wave loads. Their results such as structural arrangement, design loads and scantlings are shown and discussed. It is found that plastic design formulae in adopting Initial design phase give sufficiently conservative results in terms of structural strength.

Dynamic behaviors of conventional SCR and lazy-wave SCR for FPSOs in deepwater

The general and detailed structural/dynamic performances of steel catenary risers (SCRs) and lazy-wave steel catenary risers (LWSCRs) are directly compared under the same storm and floater (deepwater turret-moored FPSO) conditions. Their global performances are simulated by using the hull/mooring/riser fully-coupled dynamic analysis program developed by authors. This study proves that conventional SCRs connected to the FPSO hull exhibit significant dynamic responses and negative tensions, which may induce large structural stress amplification, local dynamic buckling, and serious fatigue damage at the members nearby TD (touch-down) region. The generation, propagation, and decay of the elastic waves generated by the temporal dynamic buckling are investigated in detail. On the other hand, LWSCRs can eliminate the local dynamic buckling for the same floater/storm condition and significantly reduce the stress level and fatigue damage near TDZ (touch-down zone) due to the motion isolation effect. It is because the modified shape with intermediate sag and arch portions can effectively isolate and absorb the downward riser motion. Therefore, LWSCRs can effectively be used for FPSO under very harsh environmental conditions. Despite the big advantage in dynamic characteristics, LWSCR is generally more expensive, more difficult to design, and may have slugging problem for internal flows.

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 ...

Technology Focus: Offshore Facilities (February 2015)

Technology Focus Last year’s Offshore Facilities Technology Focus (February 2014) highlighted the slowdown of new-project approvals that the inflated cost of doing business in the oil and gas sector had already brought about. That slowdown has continued throughout the past year, although the high cost of goods and services has been replaced as the driver by the much lower revenues now attainable from the oil and gas produced. In many cases, the currently required capital investment cannot be justified. Of course, the cost of goods and services will gradually moderate as the demand from operators temporarily wanes, but the project activity will really pick up again only when the value of the hydrocarbons recovers to a point where confidence in longer-term economic viability is restored. Meanwhile, we risk witnessing yet another cycle of layoffs and redundancies, even while the wailing over resource shortages for the tasks ahead still echoes around the upstream corridors of power. One of the phenomena examined in this year’s feature is the destabilizing escalation in weight, and hence cost, of offshore facilities that has been an increasingly common feature of our industry after project sanction at a given commitment level. This is clearly something the industry has to get a proper grip on before confidence can be restored fully. We also continue to highlight a theme that has appeared in each of the last four editions of the Offshore Facilities feature, namely the critical importance and benefits of effective collaboration at all levels and between sectors of our industry in honing the efficiency with which development objectives are achieved. Availability and application of new ways of tackling tough development challenges remain key components of this collaborative quest for ever-better operational efficiency. We delve into several recent examples of how this has been achieved, using new technologies, methods, and materials. It is hoped that these might serve as a timely reminder to avoid temporary layoffs by encouraging instead a redeployment of resources temporarily liberated by the current lull in field-development activity. Many of these could, and perhaps should, be tasked with qualifying the next tranche of new approaches that will benefit the eventual resumption of unfettered field- development activity. Innovation is the life-blood of any high-tech industry, and the operators are the primary beneficiaries of its application. If we do not take advantage of opportunities such as those currently presented, to focus liberated resources onto developing and qualifying the innovations needed for enabling the next set of challenges to be overcome, then we will have only our collective selves to blame. JPT Recommended additional reading at OnePetro: www.onepetro.org. OTC 25354 Espirito Santo: Operational Feedback on the Use of Steel Risers on a Turret-Moored FPSO by Andrew Newport, SBM Offshore, et al. OTC 24834 Concept Evaluation of Concrete Floating Liquefied Natural Gas by A. Nezamian, WorleyParsons, et al. SPE 170696 Upstream-Offshore- Facility Weight-Growth Study by Ray Rui, Independent Project Analysis, et al.

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.

Full-scale measurement investigation of the hydrodynamics of a turret-moored FPSO in a typhoon and deduction of its mooring loads

As a type of offshore structure, an FPSO unit is typically permanently moored to a designated position. These structures experience very complex sea states during which the motion response of the FPSO is enhanced, resulting in extreme loads on the mooring system. Full-scale measurement of the motion characteristics of an FPSO unit was carried out by the Shanghai Jiaotong University and the China National Offshore Oil Corporation from 2007 to 2009, lasting 25 months (Hu Z. 2010. Report for the full-scale measurement on the motion response of FENJIN FPSO in South China Sea and investigations on the deduction of the mooring line loads. Shanghai: State Key Lab of Ocean Engineering, Shanghai Jiao Tong University). We analysed the FPSO unit's motion responses during the passage of the typhoon ‘Koppu’ and its hydrodynamic performance. The loads acting on the mooring system were deduced through a decoupled analysis with the measurement data as input. The ability of the mooring system to hold station and the performance of the vessel in actual sea states are also presented to contribute important input to the future designs of FPSO systems.

역 일계이차 모멘트법을 이용한 설계 해상상태의 결정에 관한 연구

Response-based approach is getting more preferred in determining the design sea-state for offshore structures because traditional environment-based approach is known to yield a much conservative design condition. This paper introduces the inverse first-order second-moment (IFOSM) method as a response-based approach, which is expected to give a more feasible design condition at the cost of reasonable number of motion analyses. The IFOSM method is based on the theory of probability and adopts an optimization scheme to determine the design point. Both the design maximum response and design sea state can be obtained straightforwardly from the optimum. The IFOSM method has been applied to a turret-moored FPSO's design problem and showed its effectiveness in practical use.

Vessel/mooring/riser coupled dynamic analysis of a turret-moored FPSO compared with OTRC experiment

A vessel/mooring/riser coupled dynamic analysis program in time domain is developed for the global motion simulation of a turret-moored, tanker based FPSO designed for 6000-ft water depth. The vessel and line dynamics are solved simultaneously in a combined matrix for the given environmental and boundary conditions. The vessel global motions and mooring tension are tested at the OTRC wave basin for the non-parallel wind–wave–current 100-year hurricane condition in the Gulf of Mexico. The same case is also numerically simulated using the developed coupled dynamic analysis program. The numerical results are compared with the OTRC 1:60 model-testing results with truncated mooring system. The system's stiffness and line tension as well as natural periods and damping obtained from the OTRC measurement reasonably match with numerically simulated static-offset and free-decay tests. The numerically predicted global vessel motions are also in good agreement with the measurements. It is underscored that the dynamic mooring tension can be underestimated when truncated mooring system is used.

Assisted Dynamic Positioning System for a FPSO Based on Minimization of a Cost Function

This paper presents a two-layer controller for a turret-moored FPSO. The first layer evaluates ship optimal heading, based on the minimization of a cost function relating important operational parameters, namely first-order roll oscillation, dynamic traction on risers, mooring lines offset and fuel consumption. Since this layer requires an estimate of the environmental conditions, it is proposed an indirect evaluation of the wave spectrum by the measurement of ship first-order oscillations. The second level receives the set point from the superior level and commands the thruster system. This layer is composed by a robust heading sliding-mode controller and the surge and sway motions are damped by a simple feedback linearization scheme. The controller was tested through numerical simulations of a moored VLCC under common environmental conditions found in Campos Basin, Brazil.

H∞ Controller Design for Dynamic Positioning of a Turret Moored FPSO

This paper presents the controller design of a dynamic positioning system for a FPSO-turret moored in deep water, using a H∞ methodology. The controller must counteract slow-drift oscillations due to waves and static offsets induced by current, wind and waves. The controller must not respond to zero mean high frequency motions caused by first order wave forces. To the controller synthesis, a linear model is used, but tests are carried out with a complete non-linear FPSO model. Simulations considering several environmental conditions and two distinct turret positions confirm the robustness of the control system.

An experimental study of turret-moored floating production systems

The paper reports on an experimental study of two turret-moored FPSO systems —an internal turret system and a bow-mounted external turret system. The set-up for the model tests is described in detail, including the characteristics of the systems tested and the environmental test conditions. Model test results are presented for motions and mooring line tensions for a range of long-crested and short-crested seas. The motion results for the two systems are used to identify general motion characteristics of turret-moored systems. Mooring line tension measurements are separated into high and low frequency components to enable line tension to be related to turret resonse. The important role played by turret position and wave spreading is highlighted. Consideration is given to how the maxima of the high and low frequency components of line tension combine to produce the maximum dynamic line tension.