Floating Production Storage Offloading Research Articles

Predicting heave and pitch motions of an FPSO using meta-learning

Real-time motion prediction is helpful in guaranteeing the operation stability of a Floating Production Storage Offloading (FPSO) unit. Recurrent neural networks (RNNs) are becoming feasible alternatives to numerical simulations for motion prediction as artificial intelligence develops rapidly. In this study, model-agnostic meta-learning (MAML) is combined with RNNs to deterministically predict the heave and pitch motions of a ship-shaped FPSO. This approach is motivated by the fact that MAML improves training efficiency without losing accuracy. The data came from a scaled model test conducted at Shanghai Jiao Tong University’s deepwater wave basin. Before introducing MAML, we verified that long short-term memory (LSTM) and gated recurrent unit (GRU) could accurately predict the heave and pitch of about 7.68 s into the future. With fewer learnable parameters than LSTM, GRU demonstrates slightly better accuracy. Therefore, this study focuses particularly on the combination of GRU and MAML. The parameters of MAML, including order of derivative, step size, number of adaption gradient updates, and batch size of the tasks, are evaluated systemically in terms of accuracy and training efficiency. With the assistance of MAML, GRU’s training efficiency for heave and pitch has significantly improved, increasing by approximately 65% and 55%, respectively. Meanwhile, the prediction error for both has decreased by about 10%. Notably, MAML’s performance is minimally affected by variations in incoming wave direction and sea state, as well as the randomness and temporal variability of the motion. MAML is a powerful tool that enables RNNs to achieve real-time prediction of FPSO motion.

On the hydrodynamic response and slamming impact of a cylindrical FPSO in combined wave-current flows

Owing to the lower construction and transportation costs, cylindrical floating production storage offloading (CFPSO) has received extensive attention in the industry. Currently, attention has primarily been focused on the damping performance of heave plate while few studies have touched upon hydrodynamic characteristics under complex combined wave-current condition. In this paper, we numerically investigate the hydrodynamic response and slamming impact of a CFPSO under combined wave-current flows. The active wave generating-absorbing boundary condition (GABC) along with the buoyancy-modified k-omega SST turbulence model are utilized to generate high-precision waves and current based on the open source computational fluid dynamics (CFD) framework OpenFOAM. A self-developed six degree of freedom (6DOF) motion module is used for solving rigid body motion and updating mesh motion. The numerical results of motion response and impact pressure are compared with the experimental data to verify the accuracy of present simulations. The correlation between impact pressure and relative wave elevation and wave velocity is analyzed and three types of slamming events have been identified. The flow fields such as vorticity, pressure and streamlines in the vicinity of the CFPSO are also presented and discussed, which provides a reference for structural design for the CFPSO under complex sea conditions.

Dual-Algorithm Hybrid Method for Riser Structural Health Monitoring Using the Fewest Sensors

This study suggests a novel riser structural health monitoring methodology based on a dual algorithm (DA). In this method, the displacement tracing algorithm first traces the node displacement and tension up to the last sensor position called the target point. Then, the movement and tension at the target point are used for boundary conditions of the finite element (FE) simulator to obtain displacements and stresses below the target point. The developed method is validated through numerical simulations by comparing riser behaviors/stresses from the fully coupled model with those from the proposed method with numerical sensors. For that, a moored FPSO (floating production storage offloading) system with SCR (steel catenary riser) or SLWR (steel lazy-wave riser) is employed. Only three angle sensors are used at the top portion to monitor the entire length of riser. Much simpler forced top oscillation method is also investigated, which only uses riser top movement for running FE simulator, which cannot accurately reproduce the dynamics of the upper portion of riser since real-time wave action is ignored. The developed DA riser monitoring methodology can reproduce the movements and stresses along the entire length within around 5% error regardless of riser shapes and materials.

Data assimilation for deterministic prediction of vessel motion in real-time

This paper describes the development of a data assimilation framework for deterministic vessel motion prediction in real-time, through the coupling of artificial neural network (ANN) and Ensemble Kalman Filter (EnKF). The ANN uses measurements of vessel motion and upstream wave elevation as inputs to predict future vessel motions. However, due to the uncertainties in the measurements and ANN, large errors may be built up over time. To improve the accuracy of the predictions, an EnKF has been used to combine measured data with the ANN predictions, to generate an updated prediction, which is subsequently used as input for the next prediction. Using a turret-moored Floating Production Storage Offloading (FPSO) vessel, the developed framework was first tested using synthesized measurements generated from numerical simulations and subsequently applied on experimental measurements of vessel motion. Sensitivity study was carried out to determine the data assimilation parameters, including the ensemble size and data assimilation interval. The data assimilation framework using ANN-EnKF is promising, in that it is able to improve the motion prediction accuracy for heave and pitch as compared to ANN, which do not incorporate measurements. The improvement is more obvious for the experimental measurements that have higher uncertainties.

PRESSURE-IMPULSE DIAGRAM OF THE FLNG TANKS UNDER SLOSHING LOADS

Sloshing impact loads can cause severe structural damage to cargo tanks in liquefied natural gas floating production storage offloading units (LNG-FPSOs or FLNGs). Studies of sloshing can be classified into two types, namely, hydrodynamics-related and structural mechanics-related studies. This study is a sequel to the authors’ previous studies (Paik et al. 2015; Lee et al. 2015), but is more related to issues of structural mechanics. In this study, a new method for probabilistic sloshing assessment, which has been previously developed by the authors, is briefly explained. The nonlinear impact structural response characteristics under sloshing impact loads are examined by a nonlinear finite element ANSYS/LS-DYNA method. An iso-damage curve, representing a pressure-impulse diagram, is derived for the self-supporting prismatic-shape IMO B type LNG cargo containment system of a hypothetical FLNG. The insights developed from this work can be useful for the damage-tolerant design of cargo tanks in FLNGs.

Probability of detection of discontinuities by ultrasonic phased array inspection of 9% Ni steel joints welded with alloy 625 as the filler metal

For the first time, the 9% Ni steel is being used in CO2 injection units in floating production storage offloading vessels. Pipe joints welded with nickel-based superalloy 625 as the filler metal present difficulties in ultrasonic inspection owing to the sonic attenuation and deviation caused by the anisotropic and coarse-grained weld metal. This study aims to conduct a probability of detection (POD) to evaluate the discontinuity detection reliability in 9% Ni steel arc welded joints. The inspections were performed using ultrasonic phased array transducers with transversal and longitudinal waves, by using both a conventional wedge formed of Rexolite® and an adapted wedge filled with water to simulate the immersion inspection. POD curves were developed using the hit/miss and â vs a models. The inspection performed with transversal waves detected much fewer discontinuities than that performed with longitudinal waves. Furthermore, inspection performed with longitudinal waves after weld metal cap pass removal exhibited much more reliability in 9% Ni steel welded joints. The obtained POD curves were shown to be a suitable tool to evaluate the inspection of 9% Ni steel welded joints, as they confirmed that the best inspection procedure involves a combination of three different phased array ultrasonic testing configurations.

Real-time trace of riser profile and stress with numerical inclinometers

In this paper, the methodology for the real-time tracing of riser profile and bending moment by multiple inclinometers along the riser is presented with the assumption that its top (by GPS; global positioning system) and bottom (anchoring) points are known. The riser is modelled as a series of slender-beam segments and at each time step, the riser nodal points are progressively traced. For riser x-y-z displacement and bending-moment tracing, quadratic and cubic interpolation functions for each line element between two neighboring bi-axial inclinometers are employed with respect to the global and generalized coordinate systems. Analytical solutions are derived for the instantaneous riser profile and bending moment at each line's mid-point. To validate the developed riser-monitoring algorithms, a FPSO (floating production storage offloading) system with SCR (steel catenary riser) or SLWR (steel lazy wave riser) was simulated for 1-yr and 50-yr collinear wind-wave-current storm conditions with 45-deg heading and the corresponding numerical sensor signals were inputted to the algorithms for reproducing the real-time riser profiles and bending stresses. Based on the comparisons between the algorithm-predicted and actual values, the developed method is validated. The methodology can be applied to any kinds and shapes of lines and cables too. • The real-time tracing of riser profile and bending moment by multiple numerical inclinometers along the riser is presented. • Analytical solution using numerical inclinometer is derived for the instantaneous riser node displacement. • The analytic curved line length formula in 3D space is derived. • Simplified analytic formula for the bending moment at the mid-point of each riser segment is suggested.

Fault current limiters: a case study of protection and operational continuity for FPSOs

The Floating Production Storage Offloading offshore (FPSO) has been employed in the oil and gas exploration at the Brazilian pre-salt fields. Due to the increasing production capacity of process plants and consequently its power demands, FPSO power systems are experiencing higher fault current levels, which can present peak values of almost 200 kA in the medium voltage circuit. One solution to keep fault currents within safe values for the new electrical installations is to split the main switchgear busbar in two parts interconnecting them with a pyrotechnic device (PD). However, this typical fault current limiter (FCL) imposes operational restrictions to the FPSO; when the PD is triggered, the production of barrels is severely reduced. This work proposes two alternative FCLs to mitigate this problem: a Pyrotechnic Device (PD) associated with a parallel Air-Core Reactor (ACR) and a resistive Superconducting Fault Current Limiter (r-SFCL). As described in this manuscript, both alternatives allow full operation of the FPSO even after its actuation. The simulation model used to reproduce the transient behavior of two FCLs and the power system of typical FPSO was developed in the software Alternative Transients Program. The results show that the alternatives can limit the fault current below the main switchgear supportability limit, improving the reliability of FPSO’s production.

The assessment of impact damage caused by dropped objects on floating offshore structures

Offshore facilities are exposed to the risk of accidental damages caused by fire, explosion, dropped objects, extreme environmental events and vessel collision poses a big threat to the safety and operation. During any lifting work, either from a supply vessel to the installation or from within the installation or plant itself, there is a potential danger when objects are being lifted for a dropped object accident to take place. This paper introduces extensive dropped object analyses applied in the design of Floating Production Storage Offloading (FPSO) offshore units, where non-linear finite element analyses are performed using an explicit LS-DYNA tool. In the structural evaluation, the hull parts such as infill deck, galley, and main lay-down located on poop deck, and cargo deck are taken into account. The dropped objects study is to find out what happens to the FPSO hull structure if the heavy items are dropped. The drop object impact analyses show that even though some structural members were permanently deformed by drop loads, however, there was no failure taken place in accordance with the plastic strain criteria as per NORSOK Code. It is discovered that the pillars supporting the main lay-down area have not been suffered from buckling failure through examined reaction forces. Further, the impact resistance of a deck depends on the plate thickness and the size and spacing of the supporting stringers.

Numerical Assessment of FPSO Platform Behaviour in Ship Collision

Offshore platforms may potentially collide with vessels of various types, including visiting ships such as supply ships and passing ships. The most critical and relevant conditions, including the analysis and design approaches are introduced. Different ship types having different displacements and structural designs exert different vessel impact loads on impacted structures. This paper presents the findings of collision impact analyses of the side shell panel, bow and stern structures of Floating Production Storage Offloading (FPSO) platforms in case of impact, e.g. by a supply vessel or methanol tanker. As collision impact simulations continue to be conducted conservatively, the colliding positions of the striking vessel are presumed to be bow and stern only, with side force. In order to assess hull strength in collision events, non-linear FE simulations were performed by means of the MSC / DYTRAN tool, as these collision events result in more complex reactions. The degree of hull damage suffered by an FPSO vessel in different collision scenarios and at varying impact energy levels was determined in accordance with the NORSOK N-004 standard guidelines. Post-collision analyses were conducted to establish the structural integrity of the damaged hull after being exposed to environmental conditions for one year. The reduction of hull girder strength associated with the worst damage was evaluated and accounted for in the present study, providing no further damage occurs. Furthermore, the acceptance criteria for evaluation and corresponding consequences are calculated and discussed in detail. Finally, the findings from the present paper will help clarify the impact response of offshore structures and evaluation approaches and give valuable guidance for the design and operation of FPSO platforms.

The sloshing effects on distribution characteristics of gas–liquid mixture in spiral-wound heat exchanger

Spiral-wound heat exchanger (SWHE) is important equipment for industry, and it is suitable for floating production storage off-loading (FPSO). However, two-phase mixture maldistribution always restricts the heat transfer capacity of SWHE in FPSO. Few studies have focused on the maldistribution of SWHE. According to this, an experimental device was fabricated to evaluate the maldistribution of a novel distributor on the sloshing platform. The effects of vapor qualities and mass flow rates were analyzed at the range of 0.106–0.702 and 608.5–1491.6 kg h−1, respectively. The following conclusions are drawn: With the augmentation of vapor quality, the gas–liquid uniformity deteriorates after optimizing under steady and sloshing conditions. Heave condition improves gas and liquid flow uniformity by 2.9–11.0% and 1.7–3.7% separately, and pitch condition improves the gas flow uniformity by 0.5–3.7%, and roll condition lowers the liquid flow uniformity by 2.0–7.2% in the variation of vapor quality. It is also proved that the flow uniformity achieves better performance with the increasing mass flow rate under steady, heave and pitch conditions. Heave condition improves the distribution characteristic by 4.4–8.7% for the gas phase and 2.0–9.5% for the liquid phase, and pitch condition elevates the gas flow uniformity by 3.1–7.7%. Under the roll condition, the distribution performance of the experimental model changes worse after better and obtains the best performance at the mass flow rate of 1183.8 kg/h. At large mass flow rates, gas entering into liquid space under roll condition can bring the structural problem. Increasing the tube diameter and making the upper baffle tilt from shell to center barrel are beneficial to keep the distributor structure safe.

Ultimate strength analysis of stiffened plate variation on double bottom of Floating Production Storage Offloading (FPSO)

Double bottom is part of the ship that must be evaluated from the loads for instance wave load acts on side hull and pressure from the outer bottom. The loads must be evaluated and investigated the effect on the double bottom structure. The objective of the present study is to analyze the ultimate strength of stiffened plate variation on the double bottom of the floating production storage offloading. The small section of stiffened plate with attached plating at the double bottom is considered to the analyzed. The numerical analysis is adopted to analyze the model. The stiffened plate is variated with the distance in terms of model breadth. The boundary condition is applied to the model including axial compression load. The behavior of the model is represented by stress and deformation. For the simple analysis, the welding residual stress, crack, and initial imperfection are not taken. The result obtains by the numerical analysis is presented together with the behavior in term of stress and deformation.

Feed-forward control for dynamic positioning in random waves based on quadratic impulse function and real-time wave measurement

The real-time estimation of second-order difference-frequency wave forces using real-time random-wave measurement is developed for the FF (feed-forward) control based dynamic positioning of floating offshore vessels and platforms. The efficacy of the developed FF control scheme is validated by using the in-house hull-mooring-riser-thruster fully coupled time-domain computer simulation program through comparisons with the results by the conventional feedback-control-only case. The feedback (FB) control intends to reduce the accumulated position-excursion error, meanwhile the proposed feed-forward control compensates the controllable slowly-varying wave loads by activating thrusters in advance based on the real-time estimation of the second-order difference-frequency wave loadings using the real-time signal of random incident wave. The real-time estimation of the second-order difference-frequency wave loads is done by using the double-convolution integral with pre-calculated QIF (quadratic impulse function). The numerical DP system is successfully implemented with the FF control algorithm in the vessel-thruster fully coupled time-domain simulation program. The developed schemes are applied to a turret-moored FPSO (floating production storage offloading) with six dynamic-positioning (DP) azimuth thrusters in two non-collinear storm conditions. It is clearly demonstrated that the developed FF control scheme performs much better than the conventional feedback-control-only case. The corresponding reductions in horizontal offsets, motions, mooring tensions, and fuel consumptions by using the developed FF control scheme are underscored.

Penerapan Metode Multiple Scales untuk Masalah Galloping pada Mooring Cable FPSO

In essence, all objects in the universe have a natural frequency and when an object get an external force same with their natural frequency a resonance phenomenon will occur. In this study will be examined how the dynamic behavior of the mooring cable type Catenary Anchor Leg Mooring (CALM) on the Floating Production Storage Offloading (FPSO) when it gets external force with a very small frequency but near to the natural frequency itself. In the case of transmission cable, the phenomenon that describe this event is called the Galloping Phenomenon. The mathematics model of CALM system with the very small frequency will be solved using Multiple Scales method. The equilibrium point obtained is in the form of a center (stable). For negative, the amplitude value will be positive when the phase is zero (in-phase), at this condition if the initial conditions are chosen close enough to the equilibrium point then the cable will periodically oscillate with a limited phase value. Then if the initial conditions are selected a little further from the point of equilibrium, the cable will oscillate with amplitude the initial is the maximum amplitude and the value added of the dependent phase against time.

Risk and efficiency analysis of dual mixed refrigerant liquefaction process configurations for floating liquefied natural gas at conceptual design stage

Liquefied natural gas (LNG) floating production storage offloading, or floating liquefied natural gas (FLNG), is an offshore unit used to produce LNG from offshore gas reservoirs. The liquefaction is critical process for liquefying natural gas (NG) into LNG. Among NG liquefaction technologies used in the industry, single mixed refrigerant, dual mixed refrigerant (DMR), and the nitrogen expansion liquefaction process have been considered for FLNG on account of its space limitations and higher safety standards. In particular, the DMR liquefaction process is preferred for a large FLNG because of its high efficiency. Many studies have been suggested about an efficiency of DMR, but a few studies have been conducted on their process safety although different configurations in process concepts can cause meaningful differences in operating conditions and safety. In this study, two DMR process configurations were optimized to maximize the efficiency and conceptual explosion risk was analyzed to compare their risk at the conceptual design stage. The results showed a difference between the explosion risks by the differences in the optimal mixed refrigerant compositions and number of devices, with similar efficiencies. These results can provide insight for a risk management strategy at the conceptual design stage, to minimize the unexpected cost generation. © 2018 American Institute of Chemical Engineers Process Process Saf Prog 38: 87–98, 2019

Effects of geometric shape of LWSCR (lazy-wave steel catenary riser) on its global performance and structural behavior

This study aims to investigate the behavioral characteristics of the LWSCR (lazy-wave steel catenary riser) for a turret-moored FPSO (Floating Production Storage Offloading) by using fully-coupled hull-mooring-riser dynamic simulation program in time domain. In particular, the effects of initial geometric profile on the global performance and structural behavior are investigated in depth to have an insight for optimal design. In this regard, a systematic parametric study with varying the initial curvature of sag and arch bend and initial position of touch down point (TDP) is conducted for 100-yr wind-wave-current (WWC) hurricane condition. The FPSO motions, riser dynamics, constituent structural stress results, accumulated fatigue damage of the LWSCR are presented and analyzed to draw a general trend of the relationship between the LWSCR geometric parameters and the resulting dynamic/structural performance. According to this study, the initial curvature of the sag and arch bend plays an important role in absorbing transferred platform motions, while the position of TDP mainly affects the change of static-stress level.

Fuel-Optimal Thrust-Allocation Algorithm Using Penalty Optimization Programing for Dynamic-Positioning-Controlled Offshore Platforms

This research, a new thrust-allocation algorithm based on penalty programming is developed to minimize the fuel consumption of offshore vessels/platforms with dynamic positioning system. The role of thrust allocation is to produce thruster commands satisfying required forces and moments for position-keeping, while fulfilling mechanical constraints of the control system. The developed thrust-allocation algorithm is mathematically formulated as an optimization problem for the given objects and constraints of a dynamic positioning system. Penalty programming can solve the optimization problems that have nonlinear object functions and constraints. The developed penalty-programming thrust-allocation method is implemented in the fully-coupled vessel–riser–mooring time-domain simulation code with dynamic positioning control. Its position-keeping and fuel-saving performance is evaluated by comparing with other conventional methods, such as pseudo-inverse, quadratic-programming, and genetic-algorithm methods. In this regard, the fully-coupled time-domain simulation method is applied to a turret-moored dynamic positioning assisted FPSO (floating production storage offloading). The optimal performance of the penalty programming in minimizing fuel consumption in both 100-year and 1-year storm conditions is demonstrated compared to pseudo-inverse and quadratic-programming methods.

Use of built-up H sections as secondary members in topside modules

There have been many attempts to widely utilize built-up H sections to secure flexibility in structural design. Much research into the structural strengths and limit states of built-up H sections, therefore, has been carried out. However, a practical redesign methodology taking advantage of built-up H beams has yet to be introduced into the offshore industry. In this study, a comprehensive investigation into built-up H sections is carried out, based on which, a new redesign procedure for weight reduction herein is suggested. First of all, on the basis of the ANSI/AISC 360-10, the differences between the rolled H and built-up H sections are investigated in terms of their various strengths. Then, a secondary-member redesign procedure is established as a means of reducing structural weight by replacing rolled H sections with built-up H sections. In that procedure, the built-up H section cross-section is modified according to the failure modes of reference rolled H sections. The redesign procedure is verified by a nonlinear finite element analysis and four-point bending test. Through the nonlinear finite element analysis and experiment on the reference rolled H section and built-up H section obtained by the redesign procedure, it is observed that the weight of the built-up H section is reduced by about 15% while a flexural strength similar to that of the reference rolled H section is maintained. The suggested redesign procedure is then applied to three floating production storage offloading (FPSO) topside modules for demonstration purposes. In the results, the total structural weights of the reference topside modules are reduced by approximately 3%–5% by employing built-up H sections in secondary members in lieu of rolled H sections. The results indicate that, in many cases, built-up H sections can be used as secondary members to reduce the structural weight of topside modules.

Pressure Re-Impulse Diagram of the FLNG Tanks Under Sloshing Loads

Sloshing impact loads can cause severe structural damage to cargo tanks in liquefied natural gas floating production storage offloading units (LNG-FPSOs or FLNGs). Studies of sloshing can be classified into two types, namely, hydrodynamics-related and structural mechanics-related studies. This study is a sequel to the authors’ previous studies (Paik et al. 2015; Lee et al. 2015), but is more related to issues of structural mechanics. In this study, a new method for probabilistic sloshing assessment, which has been previously developed by the authors, is briefly explained. The nonlinear impact structural response characteristics under sloshing impact loads are examined by a nonlinear finite element ANSYS/LS-DYNA method. An iso-damage curve, representing a pressure-impulse diagram, is derived for the self-supporting prismatic-shape IMO B type LNG cargo containment system of a hypothetical FLNG. The insights developed from this work can be useful for the damage-tolerant design of cargo tanks in FLNGs.

Optimization of FPSO Glen Lyon Mooring lines from vessel positioning to detailed stress analysis

Optimization of FPSO Glen Lyon Mooring lines from vessel positioning to detailed stress analysis