Floating Production Storage Research Articles

Analysis on nonlinear characteristic of coupling heave and pitch motions of sandglass-type floating body

For its special shape of floating body with oblique side and external expansion, the heave and pitch motions of the sandglass-type Floating Production Storage and Offloading unit (FPSO) have coupling influence and instability phenomenon under harmonic wave excitation force. In order to study the nonlinear characteristic of the sandglass-type floating body, this paper presents the improved incremental harmonic balance (IHB) method in conjunction with the incremental arc-length method, and adopts the Floquet theory to determine the stability of the periodic solutions. Therein, the discrete Fourier transformation is introduced in the traditional IHB method to solve the problem of no analytical expressions for nonlinear FK and hydrostatic forces of the sandglass-type floating body. Considering the dependence of the heave and pitch hydrodynamic coefficients on wave frequency, the coefficients of the Fourier series in the IHB method are modified to cope with the memory effect. Taking a classic Spar platform as the test case, the accuracy of the method in this paper is validated and can be used to analyze the nonlinear coupling stability of the large-scale floating body. Then, the nonlinear responses corresponding to single-DOF heave and single-DOF pitch respectively are studied, and the nonlinear characteristics of hardening-type stiffness are verified by considering the different effects of damping and wave amplitude. Next, the interactive effects between pitch and heave motions of the sandglass-type model are further discussed, which are mainly reflected in two frequency bands representing the hardening-type nonlinearity of pitch and heave motions. Finally, the influence of different upper inclination angle of the floating body on heave and pitch coupling motions is analyzed. The results show that the smaller the upper angle, the larger the nonlinear motion responses. Consequently, the nonlinear characteristics of the heave and pitch motions should be fully noticed and considered in the shape design of the sandglass-type FPSO.

Effects of the current-wave interaction on a cylinder platform

ABSTRACT This article investigates the motion responses of floating production storage and offloading systems (FPSO) under wave-current interactions. Experiments were conducted on a cylindrical FPSO in a wave-current combined wave basin. Two methods were employed to compare the effects of currents: applying an equivalent force to a specific location in the platform and generating real currents in the basin. The primary objective of this study is to explore the sensitivity characteristics of the platform's dynamic response in the presence of wave-current coupling. The findings reveal significant phase shifts in the low-frequency motions of the sway and pitch responses due to the interaction between waves and currents. Moreover, the six degree-of-freedom motions of the platform experience amplified overall energies in both low-frequency and wave-frequency motions. A notable observation is that wave-current interaction leads to a considerable increase in surge motion amplitude, even when the reduced velocities are smaller than the lock-in region.

Reversed tension characteristics in subsea mooring connectors for an FPSO SPM in shallow water

The reversed tension on subsea mooring connectors is studied to prove its appearance. The mooring line is chosen from the real single point mooring system of Floating Production Storage and Offloading Unit (FPSO) wording in the South China Sea. The forced motion excitations are applied on the top end of the mooring line with the data monitored by the GPS device on the FPSO. The simulations are carried out with the lumped mass model and the nonlinear finite element model to verify the accuracy of the whole process. In the results of the forced excitation case, the reversed tensions are found three times. The occurrence of the revered tensions is consistent with the minus peak of the vertical displacement in time history, and the vertical displacement is recognized as the key factor for the reason of reversed tension. The reversed tension will apply a compression force on the rigid subsea mooring connect after a generous vertical displacement change. It may lead to the failure of the connector, and then the mooring line. Therefore, the minimum tension design is suggested to be added to the rules of classification society to avoid the reversed tension and slack condition of the mooring line.

The structure design and offshore oilfield experiment study of industrial compact flotation unit

The Compact Flotation Unit is widely used in the offshore oilfield produced water treatment based on the characteristics of relatively small footprint and high oil removal efficiency, but the structure details of the Compact Flotation Unit have been rarely described up to now. Few literature is talking about the Compact Flotation Unit design process and systematically analyzing the influence of operating parameters on the oil removal efficiency, especially on the large-capacity industrial-level Compact Flotation Unit products. In this paper, the working principle and structure design process of the Compact Flotation Unit from Beijing Institute of Petrochemical Technology cooperated with China National Offshore Oil Corporation Research Institute were systematically introduced. Furthermore, the single-stage Compact Flotation Unit from Beijing Institute of Petrochemical Technology of industrial scale with a treatment capacity of 120 m³/h was manufactured and tested on floating production storage and offloading unit located in the South China Sea. When the inlet average oil content was 280.3 mg/L, the outlet average oil content was 39.9 mg/L, and the corresponding average oil removal efficiency was 85.8%. The flow split ratio, back pressure, and inlet flow rate have a significant impact on the oil removal efficiency, while the effect of the gas-liquid ratio and outlet water recycle ratio are not sensitive. The main reason can be attributed to the dissolved gas components in the produced water. A large quantity of microbubbles is generated after depressurization which directly participates in the flotation process. Besides that, the single-stage Compact Flotation Unit prototype can tolerate the fluctuation of inlet flow rate and oil content.

Maintenance Technology and Practice of Mooring Anchor Leg in Deepwater Floating Production Facility

“Nanhai Shengli” is a floating production storage and offloading unit (FPSO) in the Liuhua 11-1 oil field in the South China Sea. During its service, the FPSO suffered severe broken anchor legs due to the hostile impact of several strong typhoons. In order to ensure the safety of its mooring system, emergency maintenance operations were carried out on 10 mooring anchor legs under the condition of non-release and non-production of FPSO, which was the first time in deep water in China. Its construction is difficult, high risk, weather and FPSO draft time window requirements are high, the use of reasonable technology and construction methods is the key to efficient completion of the operation, such as simulation analysis, reasonable selection of machinery and resources, efficient control of site construction, accurate environmental monitoring, etc. Taking the release and tieback of the mooring system of "Nanhai Shengli" FPSO in Liuhua 11-1 oilfield as an example, various technical difficulties and key points in the process of release and tieback construction are introduced, and the rules are summarized and analyzed. The safe and efficient completion of this construction operation is not only the embodiment of management level, but also the result of daring to innovate and face challenges. It has a typical guiding significance for the maintenance of mooring anchor leg of similar deep-water floating production facilities in China.

FPSO offloading operational safety study by a multi-dimensional reliability method

In contrast to the well-known bivariate statistical approach, which is known to properly anticipate extreme response levels for two-dimensional systems, the research validates a unique structural reliability method that is particularly appropriate for multi-dimensional structural responses. Traditional dependability techniques that deal with time series don't have an edge when it comes to handling high-dimensional systems and cross-correlation between several dimensions. A Floating Production Storage and Offloading Unit (FPSO) is a tanker that can produce, store and transport hydrocarbon products such as oil. The hawser during the offloading process is an important part used during operations, and a thorough prediction of its extreme tension is essential for operational safety. Failure of the offloading process may lead to great environmental harm and economic loss. This study assessed hydrodynamic forces acting on FPSO vessels in real-world environmental conditions based on the potential flow theory. A brief discussion of the experimental verification of numerical results is also included. The outlined method may be effectively employed during the vessel design phase for choosing the best vessel specifications to reduce potential FPSO hawser tension. The ability to foresee high hawser strains during the FPSO offloading operation is essential for reliability and safety.

Nonstationary response of floating structures to random waves

Even if ocean waves are treated as a stationary random process, dynamic responses of floating structures to random waves at the transient state are always nonstationary. When nonstationary response statistics is desired, a common technique is to apply Monte Carlo simulations; however, its implementation is costly in computational time. Analytically, this article develops an efficient method for computing nonstationary response statistics, including evolutionary power spectrum and time-varying mean-square values. Assuming a hydrodynamic software has been employed to get various frequency response functions, a prerequisite of the proposed method is to get the elevation-to-motion transfer function formulated in its pole-residue form. The proposed method is applicable to arbitrary wave spectrum and has been based on pole-residue operations implemented in the Laplace domain to obtain closed-form solutions for the response evolutionary power spectrum. Numerical examples choose a single-degree-of-freedom Spar model and a six-degree-of-freedom Floating Production Storage and Offloading model to a Pierson–Moskowitz wave spectrum, and the correctness of the computed mean-square values is verified by Monte Carlo simulations.

Coupled FPSO roll motion response and tank sloshing in a pair of two row cargo tanks

The advantages offered by Floating Production Storage and Offloading Units (FPSOs) continue to make them high in demand and widely used. Continuous loading and offloading of FPSOs means the presence of slack cargo tanks is inevitable and can impact the seakeeping performance of FPSOs. Published experimental data on this impact is limited. This paper experimentally studies liquid cargo and its effect on roll motion in a pair of two-row cargo tanks of a FPSO model in regular beam waves. The results show that the liquid cargo motion effect on floating vessels cannot be studied using solid cargo. The results also show that varying the liquid cargo loading affects floating vessels’ Response Amplitude Operators (RAOs).

Guest Editorial: Special Section on Sustainable and Innovative Use of Emerging Ocean Structural Systems

Near-shore and open-ocean areas offer abundant energy resources. There is an ever-increasing demand to harness these resources in a safe, cost-effective, and sustainable manner. Innovative ocean structural systems can play an important role to meet this demand. Along with advances in engineering design methods and computational power, the development of such structural systems appears to be more efficient than ever before. Still, the design process is nontrivial and often requires physical insights combined with practical considerations. On the one hand, the structural system can be highly complex and may consist of multiple components, some of which require active control. On the other hand, the system can be subjected to extreme environmental loads from wind, waves, and ocean currents, and the structural integrity of the system then becomes a great concern. This Special Section aims to highlight recent efforts in addressing the challenges of next-generation ocean structural systems.There is a wide range of papers included in this Special Section. One study by Li and co-authors proposes a modular floating structure (MFS) that consists of inner hexagonal tension leg platform modules and outer floating artificial reef modules. Relative motions between adjacent modules of the MFS are utilized by wave energy converters (WECs) in the form of ball connectors. Through dynamic response analysis of this novel modular system, the authors demonstrate its technical feasibility in operational and extreme sea states.The subject of WECs is further pursued by Husain and co-authors. In their work, important design parameters are studied of a variable-geometry oscillating WEC. This system resembles a paddle pitching about a fixed hinge, and its geometry variation is achieved by opening controllable flaps on the paddle. By numerical simulations in the time domain, the authors gain insights into structural loads and dynamics, power generation, and the design of such systems.Another study by Ma and co-authors deals with the station-keeping control of an interesting subsea shuttle tanker concept. This tanker was originally proposed for efficient underwater transport of liquid carbon dioxide; ocean currents pose maneuvering challenges during the offloading process. The authors propose and develop a comprehensive maneuvering model and a linear quadratic controller for hovering the tanker in stochastic currents.Additionally in this Special Section, the concept of using a spar floating wind turbine to power a floating production storage and offloading unit is addressed by Schnepf and co-authors. Through numerical simulations and by comparing the dynamics of the structural systems, insights are provided into the design of suspended power cables. In another study, an improved actuator line method was developed by Fan and co-authors and then used to study the wake characteristics and power performance of two wind turbines in tandem.

Sea state estimation based on the motion data of a moored FPSO using neural networks: An evaluation with multiple draft conditions

Useful information for offshore systems can be obtained by monitoring sea state parameters, but the accurate estimation of the on-site wave condition is challenging. This paper deals with motion-based wave estimation using neural networks. The main novelty of this contribution is the evaluation of the performance of the data-driven inference system with motion data from different draft conditions. The study is based on the simulated motions of a Floating Production Storage and Offloading (FPSO) unit with multiple loadings, subjected to typical sea conditions observed offshore the Brazilian coast. The inference models were trained to estimate two independent sets of wave related parameters, corresponding to modal wave statistics, which were post processed to identify single and double-peaked sea states. Results showed that estimation errors were quite similar along the evaluated drafts, with an overall good agreement between estimations and reference values. However, a limitation of the method regarding less frequent sea conditions was observed, even when significant wave response is induced. Also, an analysis of the robustness of the estimation models with respect to a draft variation of ± 0.5 m was carried out considering the global wave statistics, presenting small average deviations compared to the expected values.

Offloading operation bivariate extreme response statistics for FPSO vessel

The Floating Production Storage and Offloading unit (FPSO) is an offshore unit producing and storing crude oil prior to tanker transport. An important design concern is an accurate prediction of risky dynamic hawser tensions during FPSO offloading operations. Bivariate extreme hawser tension contours are important for selecting proper design values. This paper employed the AQWA hydrodynamic software to analyze vessel hydrodynamic wave loads dynamic response, acting on FPSO vessels under realistic sea state conditions. This paper presents an efficient method for estimating FPSO bivariate response statistics based on numerical simulations validated by various experiments. The bivariate Average Conditional Exceedance Rate (ACER2D) method offers an accurate bivariate extreme value probability distribution and return period contours estimation, utilizing available data efficiently. The two-dimensional probability contours, corresponding to low probability return periods, are easily obtained by the ACER2D method. The performance of the presented method has shown that the ACER2D method provides an efficient and accurate prediction of extreme return period contours. The suggested approach may be used for FPSO vessel design, minimizing potential FPSO hawser damage. Bivariate contours yield bivariate design points, as opposed to a pair of uncoupled univariate design points with the same return period as currently adopted in the industry.

Numerical Investigations on Suspended Power Cable Configurations for Floating Offshore Wind Turbines in Deep Water Powering an FPSO

Abstract Floating offshore wind turbines (FOWTs) are an opportunity for floating production storage and offloading units (FPSOs) to reduce emissions. To avoid long connecting power cables with long transmission distances between a FOWT and an FPSO, the novel concept of a suspended power cable in a large water depth of 1000 m is investigated in this study. The power cable is kept floating between the sea surface and the seabed without touching either of them. A catenary configuration and two configurations with subsea buoys attached at different locations along the cable are investigated. The OC3-Hywind 5 MW reference FOWT is set up with a deepwater mooring system, and a spread-moored FPSO is modeled with characteristics similar to existing FPSOs. Steady-state and dynamic simulations are carried out in the numerical software OrcaFlex. The different configurations are first evaluated in steady-state analyses. The largest tensions are observed for the catenary configuration, whereas it shows the lowest horizontal cable excursions. Buoys attached along the center section of the cable lift it into regions with strong currents. This results in a large horizontal excursion of the cable and large tensions. The suspended configuration with buoys attached evenly over the cable results in significantly lower tensions than the other two configurations. It is studied further with dynamic analyses. The tensions at the floater hang-offs increase by a maximum of 24% compared to steady-state results indicating that dynamic analysis is crucial for the design of suspended cable configurations.

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.

Profiles of Two JOMAE Associate Editors (The Fifth in a Continuing Series)

Profiles of Two JOMAE Associate Editors (The Fifth in a Continuing Series)

Deepwater Artificial Seabed (DAS) Production System: An Innovative Approach to Cost-Effective Deepwater and Ultra-Deepwater Field Developments

The prevailing offshore field development solutions, i.e., dry tree and wet tree systems, are confronted with serious technical and economic challenges in deep and ultra-deep waters resulting from the large depth of water, far offshore distance, and harsh ocean environmental conditions, as well as high cost. In response to these challenges, an innovative Deepwater Artificial Seabed (DAS) production system is proposed in this article. The DAS production system concentrates on well access and riser design, which enables shallow-water-rated subsea production systems to develop Deepwater (DW) and Ultra-Deepwater (UDW) fields. First, DW & UDW field development drivers are discussed and presented. This is followed by a detailed discussion of the merits and demerits of the prevailing dry tree and wet tree field development solutions. On this basis, the design philosophy and main characteristics of the DAS production system are presented and discussed in detail. Dynamic survival analysis for the fully coupled Floating Production Storage and Offloading (FPSO)-DAS production system is carried out. The artificial seabed stability is systematically investigated for both intact and damaged conditions. The global analysis results indicate that the DAS production system as developed experiences quasi-static responses even under extreme storm conditions, due to the location of the artificial seabed and the decoupling effects of the flexible jumpers. The new DAS production system is considered to be a competitive and cost-effective field development solution in depths of up to 3000 m.

The multi-visit-multi-voyage scheduling of the heterogeneous shuttle tanker fleet via inventory-oriented joint planning

The deep-sea oil exploitation leads to the growing utilization of the Floating Production Storage and Offloading (FPSO), where the crude oil is processed and stored until it can be transferred to a shuttle tanker. The conventional scheduling method of one shuttle tanker serving a fixed FPSO is challenged by the increasing numbers and capacities of the deep-sea FPSOs. This paper proposes a collaborative scheduling method by pooling the transportation capabilities of the shuttle tankers and conducting inventory-dependent route planning. The inventory headroom of the FPSO is taken as a key variable for the route planning of the shuttle tankers. A multi-visit-multi-voyage model is constructed to schedule the optimal routes for the shuttle tanker fleet in the multi-voyage time period. Case studies demonstrated that by implementing the collaborative scheduling, the proposed model could achieve 11.14% transportation cost saving in the planning period when compared with the conventional shuttle tanker scheduling model. By proper combining the shuttle tanker types, further cost savings can be obtained while using less shuttle tankers to execute oil transportation.

Impact of Paris Agreement on financing strategy: Evidence from global FPSO industry

This study explores the impact of the Paris Agreement on the determinants of firm-level capital structure decisions of listed contractor-owned Floating Production Storage and Offloading (FPSO) companies in the oil and gas (O&G) industry from 2000 to 2019. The study identified various financing structures between contractor-owned FPSO companies due to their individual and institutional characteristics. Tangibility, profitability, market-to-book (growth), size and effective tax rates are critical determinants of capital structure. Overall, the results support applying the pecking-order theory (PoT) from a firm-level and macro-economic context. The 2015 Paris Agreement ratification significantly impacted the capital structure determinants; the dynamic association has changed in the post-Paris period. Besides, the impact of the global financial crisis on leverage ratios was potentially mitigated by the upward trend in Brent crude oil price between 2007 and 2013.

Fatigue assessment for FPSO hawsers

Floating Production Storage and Offloading Unit (FPSO) is designed to produce, store and transport hydrocarbon products. FPSO's hawsers may be exposed to both extreme and fatigue loads during operations. Hence prediction of their fatigue life is important for operational safety. During some unloading operations, consistent hawser tensions could develop as a result of internal friction in nylon ropes, casing wear and accumulated fatigue damage. Methodology, suggested in this study, may be effectively employed at the vessel design phase, when optimizing vessel parameters, reducing potential FPSO hawser tension fatigue damage. This study aims to contribute to development of novel fatigue assessment approaches, in order to use limited available datasets more effectively. Stresses occurring within FPSO hawsers have been modelled, using actual in situ environmental conditions. Simulated continuous stress time series were used as input for the rainflow counting analysis; the cumulative fatigue damage was then evaluated. Note on experimental validation has been provided.

Study on a Circular Cylindrical Floating Device and Its Motion Response Characteristics

In this paper, a circular cylindrical floating device design is proposed by combining the structural features of the cylindrical Floating Production Storage and Offloading (FPSO) and the deep-sea Spar platform. It is suitable for deep-water oil and gas field development environment in the South China Sea. This floating device adds six floats with a Spar platform hard tank role based on the cylindrical FPSO to improve the buoyant center. This paper analyzes the motion response characteristics of the circular cylindrical floating device in waves by the Eulerian multi-phase flow model of STAR-CCM+. The results show that the pitch motion characteristics of the circular cylindrical floating device are better than those of the cylindrical FPSO and ship-type FPSO in the reference [8]. In addition, the paper also investigates the motion response of the circular cylindrical floating device with different directions of the incoming flow. The analysis shows that this floating device has the lowest motion response at 30° direction incoming flow. And the Response Amplitude Operator (RAO) of pitch motion shows a negative correlation with the incoming flow direction. The research results of this paper can provide a reference for the future development of marine resources, and have a certain practical significance.

Designation of the Boundary Conditions for Estimating the Thrust Loss due to Thruster-Hull Interactions

The azimuth thruster is mainly installed on a vessel that requires a dynamic positioning (DP) function for special purposes. When the azimuth thruster on a vessel operates for DP, the thrust loss is induced by the thruster-hull interaction. This study examined the influence of boundary conditions in numerical simulations for predicting thrust loss. Wind turbine installation vessels (WTIV) and floating production storage and offloading (FPSO) were chosen as a target vessels. In this study, two types of boundaries were defined. The first consideration is that the boundary condition was assigned with consideration of the azimuth angle of the thruster, whereas it is fixed regardless azimuth angle of the thruster. The predicted thrust loss according to these boundary conditions showed a difference. This observation originated from the current load of the vessel. Therefore, the boundary conditions for which the current load is not induced need to be designated to obtain a realistic thrust loss in a numerical simulation.