FPSO Systems Research Articles

Design optimization of compact gas turbine and steam combined cycles for combined heat and power production in a FPSO system–A case study

This case study aims to cover a wide range of relevant aspects related to combined cycle design, mechanical integrity and operational reliability for cogeneration of heat and power in FPSO systems. The methods consist of combined optimization of combined cycle thermodynamic design and geometry of steam generator; vibration analysis for flow induced vibrations; and thermal stress estimation of casings during cold start-stop scenarios. Challenges and opportunities for reliable water treatment systems are explored. The results show that small tubes, a compact tube bundle and a low condensation temperature reduces the once-trough steam generator (OTSG) weight. The vibrations numerical simulations in this work support the standard recommendations of using 35 times tube OD as upper limit for the unsupported tube length, which could be used as a reasonable design criterion. Thermal stresses analysis indicates that the design of beam arrangement, location, and stiffness of beams has a major impact on thermal stresses, and can be optimized to different plate thicknesses in order to avoid fatigue damage. Focus should be on reducing leaks of deaerator, steam turbine and condenser. It is recommended to add Na sensors after condenser and investigating the use of Electrodeionization (EDI) technology for make-up water production from seawater.

Quantitative Risk Analysis of Oil and Gas Fires and Explosions for FPSO Systems in China

The LH11-1 FPSO is an 80,000 t cylindrical structure that is responsible for the processing, storage, and offloading of process oil from existing and newly developed oilfields. In this paper, a full probabilistic analysis was developed based on very detailed CFD simulation results to evaluate ventilation, gas dispersion, explosion, and fire scenarios. A detailed fire and explosion risk analysis of LH11-1 FPSO was performed based on NORSOK Z-013 and FABIG Technical Note 11. The risk-based calculations were performed applying FLACS, KFX, and DNV GLEXPRESS Fire. Finally, the oil and gas dispersion, fire, and explosion consequence risks were calculated under the credible combination of leak frequency and leak location. By this probabilistic risk analysis, it was found that the west wind could generate optimal ventilation conditions for the topside process area of FPSO compared to other wind directions, while the hull region was poorly ventilated for all wind directions. The explosion risk analysis showed that the FPSO system would take no action in terms of explosion risk according to the acceptance criteria of 10−4 occ/year. Meanwhile, the fire risk analysis demonstrated that the PR1 first and second deck, PR2 first and second deck, process deck, offloading on starboard, and the base of the flare tower would have different impacts from the fire, and the PR1 first deck and PR2 first deck at the topside deck would be severely impacted regions.

EFFECT OF MOORING LINE AZIMUTH ANGLE ON THE HYDRODYNAMIC PERFORMANCE OF A MOORED FPSO IN ULTRA-SHALLOW WATER

As the development of floating structure is rapidly growing in demand, the attention has turned towards finding alternative to move in ultra-shallow water locations. One of the most important factors affecting the performance of floating structure is the station keeping system. The station keeping system includes configuration of azimuth angle and pretension forces due to mooring line arrangement. In this study, the hydrodynamic responses of spread moored FPSO systems have been conducted by using numerical simulations. Time domain analysis of the FPSO mooring system was carried out to simulate a fully coupled integrated model using AQWA software to obtain the system responses subjected with different orientation for the azimuth angle of mooring line. The investigations were carried out to determine the impact of angle and pretension forces of each mooring line condition on the dynamic performance of FPSO, as well as to identify the critical mooring lines.

Failure Statistics Analysis Based on Bayesian Theory: A Study of FPSO Internal Turret Leakage

The load and corrosion caused by the harsh marine environment lead to the severe degradation of offshore equipment and to their compromised security and reliability. In the quantitative risk analysis, the failure models are difficult to establish through traditional statistical methods. Hence, the calculation of the occurrence probability of small sample events is often met with great uncertainty. In this study, the Bayesian statistical method is implemented to analyze the oil and gas leakages of FPSO internal turret, which is a typical small sample risk but could lead to severe losses. According to the corresponding failure mechanism, two Bayesian statistical models using the Weibull distribution and logarithmic normal distribution as the population distribution are established, and the posterior distribution of the corresponding parameters is calculated. The optimal Bayesian statistical model is determined according to the Bayesian information criterion and Akaike criterion. On the basis of the determined optimal model, the corresponding reliability index is solved to provide basic data for the subsequent risk assessments of FPSO systems.

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.

Risk assessment for structural design criteria of FPSO systems. Part II: Consequence models and applications to determination of target reliabilities

The paper presents applications of the framework for generic risk assessment for FPSO systems described in a companion paper [1]. Consequence models for structural risk assessment are developed for failure of hull girder sections, hull components and mooring lines using Bayesian Probabilistic Networks (BPN). The estimation of conditional probabilities required in the BPN models, such as the probability of a combined failure of several components and system failure conditional on components failure, are discussed. The risk assessment framework is applied for decision making on target reliabilities for ULS design of mooring lines. The target reliabilities are found by optimizing expected life-cycle costs and verifying compliance with risk acceptance criteria developed according to the Life Quality Index (LQI) principle. It is shown that the optimal reliability index, which minimizes total expected costs, may not always lead to an acceptable solution from the point-of-view of the LQI criterion.

Risk assessment for structural design criteria of FPSO systems. Part I: Generic models and acceptance criteria

A generic framework for consequence assessment and risk analysis of FPSO systems for the purpose of establishing structural design criteria is introduced, taking basis in recent work by the Joint Committee on Structural Safety (JCSS) addressing the issue of system representation through exposure events, direct and indirect failure consequences. The scenarios considered for risk-based calibration of a design code safety format for FPSO systems are outlined. It is shown how these scenarios may be represented in a generic risk assessment model using Bayesian Probabilistic Networks (BPNs). Risk acceptance criteria related to consequences to humans are determined based on the Life Quality Index (LQI), which is outlined and discussed in this paper. The generic risk framework and acceptance criteria are then applied in a companion paper [1] to build consequence models and to determine target reliability indices for structural design of FPSO components.

Nested reliability analysis of mooring lines for floating systems

A formulation for the nested reliability analysis of mooring lines is presented, which handles the uncertainty in the maximum dynamic line tensions that is assessed conditional on the uncertain environmental variables describing extreme sea states. The formulation is applied to calibration of partial safety factors for the ultimate limit state. Response surfaces are used to describe the mean line tension and the probability distribution parameters of the maximum dynamic tension as functions of the environmental variables. The appropriateness of using simplified formulations of the maximum dynamic tension in the limit state function is examined, as well as the effect of reducing the number of environmental variables that are treated as uncertain. Numerical results are given for mooring lines designed for FPSO systems at 500 m, 1000 m, and 1500 m water depths in the Bay of Campeche (Gulf of Mexico) using environmental models based on hurricane and storm hindcast data.

A Method for Fatigue Analysis of Piping Systems on Topsides of FPSO Structures

Pipelines located on the decks of FPSO systems are exposed to damage due to sea wave induced random loading. In this context, a methodology for estimating the fatigue life of fluid-conveying pipelines is presented. The pipeline is subjected to a random support motion that simulates the effect of the FPSO heaving. The equation of motion of the pipeline is derived by assuming small amplitude displacements, modeling the empty pipeline as a Bernoulli-Euler beam, and adopting the so-called “plug-flow” approximation for the fluid (Fluid-Structure Interactions Slender Structures and Axial Flow, Academic Press, San Diego, Vol. 1). Random vibration analysis is carried out by the Galerkin method selecting as basis functions the natural modes of a beam with the same boundary conditions as the pipeline. The discretized equations of motion are used in conjunction with linear random vibration theory to compute the stress spectrum for a generic section of the pipeline. For this purpose, the power spectrum of the acceleration at the deck level is determined by using the Response Amplitude Operator of the FPSO hull. Finally, the computed stress spectrum is used to estimate the pipeline fatigue life employing an appropriate S-N fatigue curve of the material. An illustrative example concerning a pipeline simply supported at both ends is included in the paper.

A Procedure to Analyze the Maneuvering Stability of FPSO Systems in the Design Stage

The use of large floating units to operate in deep water appears as a solution adopted by the industry to produce and storage the oil and finally to transfer the oil to shuttle ships. In the particular case of Petrobras the conversion of VLCC to FPSO has been adopted to work in the exploitation of oil in deep water. This has motivated many studies about the dynamics of this kind of structures exposed to the environmental conditions: wind, currents and waves. The present paper deals with the dynamics of ships in the horizontal plane under the action of a current. A maneuvering model is used to express the hydrodynamic forces. Analytical expressions for the eigenvalues of the linear stability analysis are obtained. These expressions are used to analyze the influence of the main dimensions in the design of new structures and the use of some stabilizers.

FPSO/FSO: State of the art

Floating productions systems have been utilized in remote offshore areas without a pipeline infrastructure for many years. However, they have become even more important with the push by the offshore industry into ever deeper waters. Floating production, storage, and offloading/floating storage and offloading (FPSO/FSO) systems have now become one of most commercially viable concepts for remote or deep-water oilfield developments. In this article, the advantages of FPSO systems are explained, and their present status of maturity and utilization around the world is reviewed. Recent trends in mooring systems, hull construction, safety, and operational issues are summarized from a technical viewpoint. Finally, the technical challenges and future prospects of two significant growth areas, i.e., gas-field development and deep-water development, are discussed.

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.