Catenary Anchor Leg Mooring Buoy Research Articles

Prediction of Chinese-lantern submarine hose tension variation in extreme sea states based on data-driven methods

To predict the variation of Chinese-lantern type submarine hose tension in extreme sea states is of great significance for the monitoring, operation and lifetime assessment of catenary anchor leg mooring (CALM) systems. In this research, the machine learning algorithm in radial basis function (RBF) neural network and the deep learning algorithm in long short-term memory (LSTM) neural network model were used to predict the variation of submarine hose tension respectively, and the prediction performance of the two algorithms on the time-series variation of submarine hose tension was evaluated. During the neural network construction, the six-degree-of-freedom (6-DOF) motion of CALM buoy is taken as the training parameter, while the historical tension variation data is also integrated for the correction of the prediction results. The training data are collected by hydrodynamic coupling simulation of the established CALM system with submarine hoses. Different parameter values selection of the neural network model is investigated to determine the optimal structure of the RBF and LSTM neural network models, including spread, initial learning rate, number of nodes in the hidden layer and batch size. Since the direction of environmental loads has a large effect on the response of the submarine hose, four combinations of wind, wave and currents under 100-year self-existing sea state are considered in the research. Through the case study and sensitivity analysis, it is demonstrated that the non-linear relationship between 6-DOF motion and hose tension is varied for different environmental load directions and LSTM neural network is suitable for predicting the variation of Chinese-lantern type submarine hose tension in CALM systems under different combinations of extreme sea states.

Fatigue Analysis of the Oil Offloading Lines in FPSO System under Wave and Current Loads

In this paper, fatigue analysis of oil offloading lines (OOLs) in the floating production storage and offloading (FPSO) catenary anchor leg mooring (CALM) buoy offloading system under wave and current loads in the West Africa Sea area is carried out by the numerical simulation method. The hydrodynamic coupling response is calculated, and fatigue damage is analyzed. Firstly, the numerical model is verified by comparison with the experimental results. Then, according to the environmental statistics in West Africa, the influence of various parameters on the fatigue damage of OOLs is analyzed, including tension characteristics, wave parameters, and structural parameters. Additionally, the effect of current load is studied. Results show that accumulated fatigue damage mainly occurs near the CALM buoy and is mainly caused by the 0° wind wave. Appropriately reducing the cover length of buoyancy material and increasing the wall thickness can reduce fatigue damage. Moreover, the effect of the shuttle tanker can increase the fatigue damage of the OOL near the CALM buoy by about 1.5 times, and the effect of vortex-induced vibration can increase the fatigue damage of the OOL in the middle part by up to 5–10 times.

Coupled CFD-FEM simulation for the wave-induced motion of a CALM buoy with waves modeled by a level-set approach

The Finite-Analytic Navier-Stokes (FANS) computational fluid dynamics (CFD) code was coupled with an in-house nonlinear finite element (FEM) mooring analysis program, MOORING3D, to study the dynamic responses of a catenary anchor leg mooring (CALM) buoy system in waves. The hydrodynamic loading on the moored buoy was estimated by the CFD module, in which large eddy simulation (LES) was used as the turbulence model. The FANS code was solved in conjunction with the level-set (LS) formulation to model waves. A motion solver in six degrees of freedoms (DoFs) was coupled with the MOORING3D and the CFD to achieve the coupled analysis. Model-scale simulations of free-decay motions of the buoy were performed and validated by experimental comparison. Simulations of the wave-induced motion of the buoy were performed, and the response amplitude operators (RAOs) were compared to experiments. The agreement with experiments manifested the advantages of the coupled CFD-mooring method in addressing viscous and turbulent effects and nonlinear hydrodynamic effects due to free surface. The sensitivity study of the skirt demonstrates the skirt exerting limited influence on the buoy RAOs, which appeared to be dominated by the inertial and viscous effects of the buoy hull. The conclusions demonstrate that the coupled FANS-MOORING3D code provides accurate prediction of wave-induced motion of floating systems.

Spectral Wave Explicit Navier-Stokes Equations for wave-structure interactions using two-phase Computational Fluid Dynamics solvers

This paper proposes an efficient potential and viscous flow decomposition method for wave-structure interaction simulation with single-phase wave models and two-phase Computational Fluid Dynamics (CFD) solvers. The potential part - represents the incident waves - is solved with spectral wave models; the viscous part - represents the complementary perturbation on the incident waves - is solved with the CFD solver. The decomposition strategy is called Spectral Wave Explicit Navier-Stokes Equations (SWENSE), originally proposed for single-phase CFD solvers ( Ferrant et al., 2003). Firstly, this paper presents a new two-phase SWENS Equations with interface capturing technique. To achieve this single-phase and two-phase decomposition, the incident fields are extended in the air with a density-weighted pressure. Secondly, an accurate and efficient interpolation method is proposed to transfer High Order Spectral (HOS) wave model's result on CFD mesh, which reduces drastically the divergence error of the interpolated velocity. Implemented within OpenFOAM, these methods are tested by three verification, validation, and application cases, considering incident wave propagation, high-order loads on a vertical cylinder in regular waves, and a Catenary Anchor Leg Mooring buoy in both regular and irregular waves. Speed-ups between 1.7 and 4.2 are achieved. The wave models and the interpolation method are released open-source to the public.

Linearization of Quadratic Drag to Estimate CALM Buoy Pitch Motion in Frequency-Domain and Experimental Validation

The dynamics of an oil offloading catenary anchor leg mooring (CALM) buoy coupled with mooring and flow lines are directly related to the fatigue life of a mooring system, necessitating an accurate estimate of the buoy hydrodynamic response. Linear wave theory is used for modeling the surface boundary value problem, and the boundary element method is used to solve the fluid-structure interaction between the buoy hull and the incident waves in the frequency-domain. The radiation problem is solved to estimate the added mass and radiation damping coefficients, and the diffraction problem is solved to determine the linear wave exciting loading. The buoy pitch motion is investigated, and linearizations of the quadratic drag/damping term are performed in the frequency-domain. The pitch motion response is calculated by considering an equivalent linearized drag/damping. Quadratic, cubic, and stochastic linearizations of the nonlinear drag term are employed to derive the equivalent drag/damping. Comparisons between the linear and nonlinear damping effects are presented. Time-domain simulations of the buoy motions are performed in conjunction with Morison’s equation to validate the floating buoy response. The time- and frequency-domain results are finally compared with the experimental model test results for validations. The linearization methods applied result in good estimates for the peak pitch response. However, only the stochastic linearization method shows a good agreement for the period range of the incident wave where typical pitch response estimate has not been correctly estimated.

Erha Floating Production, Storage, and Offloading Vessel

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18659, "Erha and Erha North Development: Erha Floating Production, Storage, and Offloading Vessel," by S.M. Day and G.J. Parker, ExxonMobil, and P. Barclay and H. Boulais, Saipem, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. The new-build Erha floating production, storage, and offloading vessel (FPSO) is one of the largest in the world, at 285 m long by 63 m wide, with a 2.2-million-bbl-capacity hull and capacity for 210,000 B/D oil processing, 340 MMscf/D gas injection, and 150,000 B/D water injection. The full-length paper discusses the development of the FPSO, including design and operational features incorporated to meet field-specific requirements. The paper also addresses the project-management challenges associated with each phase of the job, with emphasis on the safe and timely execution of a sequential drill center startup while managing simultaneous production operations and in-field development drilling. Introduction The Erha and Erha North development, in 1200 m of water approximately 97 km offshore Nigeria, consists of three subsea drill centers tied back to the FPSO by steel catenary risers (SCRs). Injection fluids are transferred from the FPSO by the SCRs and flowlines for subsea injection at the drill centers. The FPSO is a new-build, spread-moored vessel consisting of two trains of three-stage oil separation. Processed crude is stored in the hull and offloaded regularly to tankers through offloading lines and a catenary anchor leg mooring (CALM) buoy. Associated gas is compressed and dehydrated to provide fuel gas, with the remainder reinjected into the producing reservoir. Water injection facilities inject treated seawater into the reservoir for pressure maintenance to enhance oil production. The design, fabrication, integration, and startup of the Erha FPSO was a global undertaking, with engineering performed primarily in France, India, and Nigeria; hull fabrication in Korea; and topside fabrication in Singapore, Malaysia, and Nigeria. The hull and topside were integrated in Singapore, and the FPSO was towed to offshore Nigeria for installation. This work was carried out under a single prime contractor, Saipem S.A., and its in-country affiliate, Saipem Contracting Nigeria Ltd. (SCNL) and included key design and execution interfaces with the Erha tiebacks and subsea-system prime contractors. Facilities Description The Erha FPSO (Fig. 1) is a barge-shaped, new-build monohull with all of the processing equipment on its top deck. It has a 2.2-million-bbl oil-storage capacity. Processed oil is offloaded to export tankers by the CALM buoy offloading system. The FPSO also has the capability to offload crude to tandem moored export tankers in a "bow-to-bow" configuration in the event that the offshore loading system is not available.

Extending the Innes Field Life by Cost-Effective Subsea Technology

Summary This paper describes an approach taken to extend the economic life of the Innes field, one of the North Sea's smallest producing oil fields. Emphasis is placed on the importance of minimizing production operating costs for developed fields. In the Innes field case, this was achieved by applying "low-cost technology" to change the Innes field production mode from a dedicated floating production facility to a subsea satellite manifold producing to a remote floating production facility (FPF). The feasibility study, the Innes field flow test, and the eventual project work leading to the demobilization of the Innes field FPF and the routing of Innes field fluids to the nearby FPF Deepsea Pioneer are described in this paper, which emphasizes the cost benefits obtained by using low-cost proven technology wherever possible and limiting the use of high-cost equipment. Introduction The Innes field, discovered in 1983, is located in the central North Sea in U.K. Block 30/24. The field, with ultimate reserves of about 5.0 million bbl [7.95 × 10(5) m3] of oil, is very small by North Sea standards. The reservoir is part of the Permian Rotliegendes sandstone of the Auk formation and was discovered during drilling of Well 30/24-24 in Jan. 1983. Production began in Jan. 1985 through the FPF Transworld 58 (TW58), located over Well 24. Stabilized crude oil was exported through a 6-m [15.2-cm] flowline to a catenary anchor leg mooring (CALM) buoy and shuttle tanker located in the Argyll field. Production from the Innes, Argyll, and Duncan fields was commingled at the Argyll base manifold (Fig. 1) and routed by the CALM buoy to the tanker. During Aug. 1985, a second Innes field development well. Well 30/ 24-32, was drilled 3,280 ft [1000 m] north of Well 30/24-24.ell 32 encountered oil-bearing Rotliegendes reservoir sand at a depth of 12,300 ft [3750 m]. Production was routed by a 4-in. [10.2-cm] subsea flowline to the TW58 in Nov. 1985. A third potential development well was drilled as a dry hole, thus confirming that the reservoir was indeed small. Initial combined production rates of about 10,000 BOPD [1590 m3/d oil] were achieved from the two development wells; however, production declined steadily to 5,000 BOPD [795 m3/d oil] by Dec, 1986 (Fig. 2). The Innes field reservoir production mechanism is depletion drive; hence, it was anticipated that production would continue to decline until the economic production limit of about 2,500 BOPD [397.5 m3/d oil] was reached in June 1987. Innes Field Feasibility Study The economic limit was rather high owing to the operating costs of the dedicated production facility, and declining production rates were anticipated. Thus, consideration was given to removing the TW58 and producing the Innes field directly to the Argyll/Duncan FPF. Several obstacles had to be overcome before this objective could be accomplished; hence, a study was initiated to propose a feasible scheme for the cost-effective tieback of Innes field production to the Argyll/Duncan FPF, the Deepsea Pioneer. The study highlighted a number of potential problems in directing Innes field fluids to the Deepsea Pioneer and proposed ways in which these could be overcome. Flow Instability. It was anticipated that the two-phase flow of Innes field well fluids, along an 8.1-mile [13-km] flowline connecting the field to the Deepsea Pioneer, may cause flow instability and process upsets on the Deepsea Pioneer. It was considered that a subsea slug catcher may be required upstream of the Deepsea Pioneer separators to solve this problem.