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.
Estimate of the pitch motion of an oil offloading Catenary Anchor Leg Mooring (CALM) buoy is presented. Linearization of the quadratic drag/damping term is investigated by the frequency-domain analysis. The radiation problem is solved to estimate the added mass and radiation damping coefficients, and the diffraction problem is solved for the linear wave exciting loading. The equation of motion is solved by considering the linearized nonlinear drag/damping. The pitch motion response is evaluated at each wave frequency by iterative and various linearization methods of the nonlinear drag term. Comparisons between the linear and nonlinear damping effects are presented. Time-domain simulations of the buoy pitch motion were also compared with results from the frequency-domain analysis. Various linearization methods resulted in good estimate of 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.
This paper describes the evolution of the design leading to the selection, development and delivery of Stones disconnectable Buoyant Turret Mooring (BTM) buoy. The optimization process leading to the selection and configuration of the BTM buoy is discussed, with particular focus given to the mooring- riser system payloads supported by the BTM when in the disconnected state. A hybrid design, featuring syntactic foam buoyancy modules integrated into a steel structural skeleton, and incorporating a minimal number of steel ballast tanks, was selected over a conventional all steel buoyant hull structure. The selected design provides for a protective steel cage for the buoyancy modules, and a structural path for mooring-riser system loads into the Floating Production, Storage and Offloading (FPSO), while a large number of syntactic foam blocks are secured within the steel frame to provide adequate buoyancy to meet the in-service performance requirements. A qualitative discussion about the advantages of the selected geometry and hybrid steel-foam design is presented, including the performance characteristics of disconnection, submergence equilibrium and ballasting. Beyond the design evolution considerations including concept selection and optimization, attention is given to manufacturing, constructability, offshore installation and operability under the latest US Gulf of Mexico (GOM) regulatory regime. The buoy's transit voyage from Singapore to the GOM for the initial offshore integration with the FPSO necessitated Transport and Installation (T&I) requirements which were embedded into the design and integration of all BTM buoy components. Buoy ballasting and de-ballasting strategy after mooring hookup and riser connection played a pivotal role in sizing critical components and equipment. In this context, the paper will explore the interrelations between design and operations in order to optimize the loading requirements. The design decisions leading to the selection of geometries, materials and configurations of the largest and deepest offshore mooring installation are discussed. A novel solution is developed to accommodate a complex deepwater installation with a large payload mooring-riser system. Feedback on onshore fabrication and offshore integration and operations are presented.
A number of new developments based on floating production and storage facilities have suffered set-backs or been shelved due to escalating Capital Expenditure (CAPEX), increased top-side weight and complexity, and increasingly demanding regulatory requirements. Subsea tie-backs, and brownfield upgrades of floating facilities present operators with investment alternatives in their deepwater acreages while new technologies and improved business models are developed to lower cost and improve efficiencies. In this context, constructability offers an effective tool to influence project cost and schedule. As such, efforts to implement constructability in offshore oil and gas projects will set the stage for well-defined Exploration and Production (E&P) strategies while restoring confidence in the profitability and ultimately the safety of these developments. This paper presents a case study of implementing constructability program in the deep-water Gulf of Mexico (GOM) for a brownfield subsea tie-back development. Project constraints such as regulatory requirements, metering, field proximity, production capacity, weight limitation, bed space, shutdown, commissioning and start-up etc, are identified and investigated in details. Measures to minimize offshore work, and hot work are presented. Tools utilized in the design phase such as 3D laser scanning technologies in conjunction with CAD systems and field verifications are discussed. Contracting strategies and fabrication yard selection process are explored. Constructability parameters for the offshore campaign for hull piping and SCR/umbilical pull-in are outlined with goals to minimize offshore heavy lift and transport, diving operations, winch capacities and deck strengthening requirements. Additionally, riser installation methods and hang-off hardware are explored. Simultaneous Operation (SIMOPS) matrix development philosophy for the offshore campaign is proposed with focus on safety and uninterrupted production. The case study selected herein is for a Semi-submersible currently in production. Emphasis is given to effective work packages and job card development for effective offshore operations. Management of Change (MOC) processes are discussed. The findings and conclusions of this work are extended to other offshore production facilities. A road map extrapolating the outcome of this study is presented with the goals of implementing constructability as an effective tool for brownfield production floater development subject to risks of cost and schedule overruns.
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