Turret Moored Floating Production Storage And Offloading Research Articles

Numerical investigation on the multi-body hydrodynamic interactions under Caspian Sea environmental conditions

Floating production storage and offloading (FPSO) platforms are among the most important platforms in the offshore oil and gas industry. These platforms are generally anchored to the seabed through mooring lines. Usually, the FPSO and the oil tanker are nearby before starting the offloading operation. In this research, the hydrodynamic interactions between a turret-moored FPSO and shuttle tanker were examined using the boundary element method for coupled time-domain dynamic analyses. To apply the sea conditions, the environmental conditions of the Caspian Sea were considered. In this regard, considering the values of hydrodynamic forces on the shuttle tanker, the main purpose of this study was to evaluate the most appropriate vertical position of the shuttle tanker in the vicinity of FPSO. Therefore, 19 vertical center-to-center distances with steps of 20 m (from 80 m to 440 m) were considered between the two vessels. A comprehensive assessment of the dynamic responses of the two floating vessels was then conducted at different angular positions of the shuttle tanker relative to the FPSO. The numerical results indicated that the minimum hydrodynamic force was applied to the shuttle tanker for the distance of 400 m between the centers of the vessels. Also, the 60-degree position of the shuttle tanker relative to the FPSO within a 400-m radius was the most critical position in terms of hydrodynamic forces on the shuttle tanker.

Coupled dynamics between a turret-moored FPSO and a semi-submersible accommodation platform

In multi-body offshore systems, a telescopic gangway is often adopted for personnel transfer, the feasibility and safety of which are crucial. The coupled dynamics and gangway responses of a turret-moored floating production storage and offloading (FPSO) unit and a semi-submersible accommodation platform (AP) were investigated numerically in this study. Both side-by-side arrangement and the configuration that AP is tandem at the stern of FPSO were considered. The hydrodynamics considering multi-body wave interaction were calculated in frequency domain based on potential theory. The non-linearity of second-order wave drifts, moorings, and turret was fulfilled in time domain. We focused mainly on the effects of multi-body interactions on hydrodynamics and gangway responses. The hydrodynamic interactions have a negligible effect on the global motions of FPSO, whereas it more strongly affects AP. The interference is more intense in side-by-side scenario than in tandem configuration. The gangway responses are governed by relative translational motions between two vessels and are also related to its initial arrangement. Since weathervaning of FPSO can cause a large yaw offset, non-collinear sea states are considered, in addition to cases with different significant wave heights. Extreme gangway responses are predicted according to Weibull distribution, by which the environment criteria are determined.

Real-time estimation of riser's deformed shape using inclinometers and Extended Kalman Filter

The real-time monitoring of underwater risers, cables, and mooring lines by multiple sensors is in great demand but still very challenging. In this study, a new real-time riser monitoring method based on an Extended Kalman Filter (EKF) is proposed. It estimates the overall shape of riser in real-time utilizing the measured signals from multiple bi-axial (inclination and heading) inclinometers along the riser. The novel EKF algorithm is shown to be robust against sensor noises and successfully reproduces the actual riser profiles at each time step, which has been verified by multiple tests through numerical simulations. For verification, a turret-moored FPSO (Floating Production Storage and Offloading) with a SCR (Steel Catenary Riser) is employed in four different random waves and currents. Subsequent algorithms are also developed so that the corresponding bending and axial stresses along the riser can also be estimated in real time from the obtained riser shape, which can further be used for the real-time estimation of fatigue-damage accumulation.

Effects of water depth, mooring line diameter and hydrodynamic coefficients on the behaviour of deepwater FPSOs

Abstract Floating Production Storage and Offloading (FPSO) platforms are the most used floating structures in the offshore oil and gas industry. They are generally kept in place with the aid of mooring lines anchored to the seabed. In deep waters, the responses of mooring lines become more significant and difficult to be accurately captured through experiments or numerical quasi-static models. Thus, this paper assesses the influence of water depth, mooring line diameter as well as added inertia and drag coefficients on the behaviour of a deepwater turret-moored FPSO platform. The commercial software, AQWA, is used to conduct fully coupled time-domain dynamic analysis for the FPSO subjected to a unidirectional random wave. The results highlighted the influence of water depths and mooring line diameter in reducing surge motions and increasing mooring line tensions, at the same time furnishes valid information on the fluctuation trend of these responses. Drag and inertia coefficients were found to have little impact on the dynamic motions of FPSO.

Equilibrium Position Analysis for Offloading Operations With Turret-Moored FPSO

With the expansion of oil exploration in deep waters, assessing the risks associated with offloading operations becomes essential in preventing accidents that may cause huge environmental disasters. In this paper, the system that composed of a turret-moored floating production storage and offloading (FPSO) connected to a conventional shuttle tanker, which is assisted by a tug boat to maintain its position during an offloading operation, will be studied. Using environmental data collected over a period of 6 years, from 2004 to 2009, from the Campos Basin in Brazil, the equilibrium positions of the system were calculated, considering its constraints (operational criteria defined by Petrobras) and verifying the stability of those equilibrium points. The hydrodynamic and aerodynamic static forces were calculated using models validated in the literature. Dynamic effects and oscillations are taken into account by adding safety margins to the operational sectors. With this analysis, we calculated the FPSO heading probabilities during an offloading operation and the expected downtime of operation in Campos Basin. We concluded that the downtime of the offloading operation with a conventional shuttle tanker is close to that with a dynamic positioned (DP) shuttle tanker (10% downtime). Furthermore, the results from the stability analysis were used to generate a simplified set of rules to classify the environmental conditions into four classes of operational risk by applying an unbiased decision tree. This method obtains practical rules based on measurements of wind, wave, and current, allowing the operator to quickly evaluate the risk level before starting the operation.

Extreme Response Prediction of Steel Risers Using a Four Parameter Distribution

Short-term extreme response estimates are required in many areas of ocean and offshore engineering, such as steel risers design. As in many cases, the response in non-Gaussian, a theoretical solution, is usually not readily available for this purpose. Hermite transformation and Weibull-based models, among others, are some alternatives that have been used in connection with sampled response time series. In this work, a new approach is investigated. Recently, a four-parameter distribution known as the shifted generalized lognormal distribution (SGLD) has been presented in the literature. One of its main advantages is that it covers regions of skewness–kurtosis not covered by other distributions of common use in engineering. In this paper, the performance of this distribution is evaluated in the extreme values' estimation of the utilization ratios of steel riser sections. Three alternatives for using SGLD are investigated in two case studies of different dynamic behavior. The first one is a steel-lazy wave riser (SLWR) connected to a turret-moored FPSO (floating, production, storage and offloading unit) in 914 m water depth, and the second is a SLWR connected to a spread-mooring FPSO in a water depth of 1400 m. The results obtained by the SGLD-based analysis, which considered several simulation lengths, are compared to those obtained by means of an extreme value distribution fitted to episodical extremes obtained from many distinct realizations. The results of a traditional Weibull-fitting approach to the response peaks and those obtained with a Hermite transformation-based model are also presented for comparison.

Dynamic analysis of turret-moored FPSO system in freak wave

Freak wave is the common wave which has significant wave height and irregular wave shape, and it is easy to damage offshore structure extremely. The FPSOs (Floating Production Storage and Offloading) suffer from the environment loads, including the freak wave. The freak waves were generated based on the improved phase modulation model, and the coupling model of FPSO-SPM (Single Point Mooring) was established by considering internal-turret FPSO and its mooring system. The dynamic response characteristics of both FPSO and SPM affected by the freak wave were analyzed in the time domain. According to the results, the freak waves generated by original phase modulation model mainly affect the 2nd-order wave loads. However, the freak waves which are generated by random frequencies phase modulation model affect both 1st-order and 2nd-order wave loads on FPSO. What is more, compared with the irregular waves, the dynamic responses of mooring system are larger in the freak waves, but its amplitude lags behind the peak of the freak wave.

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

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

Espirito Santo: Design and Operational Expereince of the Use of Steel Risers on a Turret-Moored FPSO

Summary The floating production, storage, and offloading (FPSO) facility Espirito Santo, located offshore Brazil in the Parque das Conchas (BC-10) field, is the world's first turret-moored FPSO facility to use steel risers for fluid transfer. The FPSO facility is moored in a water depth of 1780 m, and the internal turret incorporates a total of 21 riser and umbilical slots. The steel risers, which are in a lazy-wave configuration, were pulled into the turret through inclined I-tubes. Clamps at the top of the I-tubes retain the risers, thus transferring axial loads from the risers to the turret. A clamp casting welded at the bottom of the I-tubes houses a stopper arrangement designed to transfer shear forces and moments from the risers to the turret. The FPSO facility began oil production in July 2009, and now has more than 5 years of operational experience. During this period, inspection of the riser system and the associated flex joint has confirmed the integrity of the design, giving further confidence in the use of steel risers in turret-moored systems. This paper summarizes the basis for selecting steel risers for the development, how the risers and umbilicals interface with the turret, and the impact of the riser choice on the turret design. The in-service inspection of the riser system is described, and the results are reported.

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

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

Experimental investigation of an external turret-moored FPSO system by VALM arrangement

This article deals mainly with an experimental study on a typical external turret-moored floating, production, storage and offloading (FPSO) system by a vertical anchor leg mooring (VALM) arrangement in order to investigate the effect of parameters influencing the mooring and hawser line forces and motion response for different loading conditions with different hawser lengths. A 1:100 scale model of 140,000 DWT turret-moored FPSO system was tested for three loading conditions (i.e. 40% DWT, 70% DWT and 100% DWT) with three hawser lengths (i.e. 15%, 20% and 25% of the length of the FPSO model). The tests were conducted in a 2-m wide wave flume at the Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai. The model tests were carried out under regular waves for the wave frequencies ranging from 0.55 to 1.25 Hz in steps of 0.04 Hz at a water depth of 1 m for head sea condition. The motion response was measured using rotary-type potentiometers, and mooring forces were measured using specially fabricated ring-type load cells. The test results of the mooring forces and motion response are analysed and presented with discussion.

Experimental and Numerical Study of the Motions of a Turret Moored FPSO in Waves

This paper presents the results of an experimental program carried out with a model of a FPSO (Floating Production, Storage and Offloading) unit moored and subjected to incoming waves. In regular waves, a wide range of wavelengths were tested and the effect of the wave amplitude was also investigated. In irregular waves the model was subjected to different sea states, including very severe significant wave heights. The measured responses include the six degrees of freedom absolute motions, relative motions, and the mooring forces. The experimental data of surge, heave, and pitch is compared with calculated results from a Green’s function panel method and a strip theory program. In general, the agreement between experimental and numerical data is very good.

Hull/mooring/riser coupled dynamic analysis and sensitivity study of a tanker-based FPSO

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum). The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.

TotalFinaElf awards Angolan work

A computer program is developed for hull/mooring/riser coupled dynamic analysis of a tanker-based turret-moored FPSO (Floating Production Storage and Offloading) in waves, winds, and currents. In this computer program, the floating body is modeled as a rigid body with six degrees of freedom. The first- and second-order wave forces, added mass, and radiation damping at various yaw angles are calculated from the second-order diffraction/radiation panel program WAMIT. The wind and current forces for various yaw angles of FPSO are modeled following the empirical method suggested by OCIMF (Oil Company International Marine Forum).The mooring/riser dynamics are modeled using a rod theory and finite element method (FEM), with the governing equations described in a generalized coordinate system. The dynamics of hull, mooring lines, and risers are solved simultaneously at each time step in a combined matrix for the specified connection condition. For illustration, semi-taut chain-steel wire-chain mooring lines and steel catenary risers are employed and their effects on global FPSO hull motions are investigated. To better understand the physics related to the motion characteristics of a turret-moored FPSO, the role of various hydrodynamic contributions is analyzed and assessed including the effects of hull and mooring/riser viscous damping, second-order difference-frequency wave-force quadratic transfer functions, and yaw-angle dependent wave forces and hydrodynamic coefficients. To see the effects of hull and mooring/riser coupling and mooring/riser damping more clearly, the case with no drag forces on those slender members is also investigated. The numerical results are compared with MARIN's wave basin experiments.