Computer simulations of oil & gas field operations are extremely helpful in planning field developments in deep water. They may be used to identify optimum or robust strategies, and to assess trade-offs among alternative scenarios. Through series of simulations, different development scenarios can be compared, and the sensitivity to diverse parameters investigated and quantified. The SLOOP simulator was developed in a Joint Industry Project to model the complete operation of offshore oil and gas facilities, including weather and other sources of downtime, well maintenance & subsea interventions, processing, storage and off-loading. SLOOP (Simulation of Long-Term Offshore Oil & Gas Production) has been applied to a range of deep water production scenarios to assess different concepts, and to quantify sensitivities to a wide range of influences. These include well performance and equipment reliability, weather severity and forecast accuracy, intervention resource availability, and thresholds for various operations. Example results are presented to compare simulated production performance for a number of scenarios, and illustrate the influence of reservoir performance and other uncertainties and options, on the production and operability. Introduction Designs of oil and gas production facilities are developed based on economic evaluations, which are in principle optimised on such measures as net present value and rate of return on investment. Three critical inputs to such evaluations are capex (installed cost of the facility), opex (cost to operate the facility through its life) and production efficiency (output of the facility compared to its target capacity). The three are obviously interrelated. Capex investment, such as in more reliable equipment or in higher storage capacities, lowers opex and increases efficiency. Opex investment, such as having dedicated repair and maintenance resources, increases efficiency, and may compensate for, or be necessitated by, lower capex. Even for simple facilities, quantifying the trade-offs among capex, opex and production efficiency is not easy. For deep water developments with extensive subsea systems, comparative assessments must at present be based on very limited experience and track record. For first time applications of system concepts or components, the exercise verges on the intuitive. The number and range of variables are daunting, and include times between failure for diverse hardware, consequences of failure and times to repair, mobilisation time and operating limits of repair resources, capacities of storage and transport, reservoir performance and types, frequencies and durations of well interventions. Virtually all offshore operations have limiting environmental conditions, and are thus subject to weather fluctuations, sometimes accurately forecast and sometimes not. The performance of such complicated systems can be realistically estimated via time-domain simulations, which model all the influences summarised above in as much detail as the data describing them allow. The SLOOP simulator (Figure 1) has been developed specifically for deep water oil and gas facilities in severe environments. In such applications, weather conditions significantly influence production efficiency, and therefore strategies affecting capex and opex investment. SLOOP's origins date from 1995 [1], when Conoco and BMT Fluid Mechanics developed quantitative predictions of the performance of different production systems on the Vøring Plateau, offshore Norway.
Tranche-6 in the Faeroe-Shetland Channel is a deepwater site characterised by complex currents of potentially high magnitude. Preparation for drilling at Tranche-6 included development of extreme and operational environmental criteria. Wind and wave conditions could be estimated with adequate accuracy from existing hindcast studies. Information on ocean currents is in general lacking, and hindcast modelling for currents is not nearly as accurate as for waves. Measurement programs collected data near the site in June-November 1992 and April-September 1995. Recorded data confirm strong currents varying in direction with depth. The data are the basis for preliminary extreme criteria and generic current profiles used in operability studies. Realtime current measurements were also made during Tranche-6 drilling, which began in May 1998, to support station keeping and riser management decisions. A weakness in advisory capabilities exists due to the lack of reliable current forecasts.
An analytical approach via frequency domain techniques is applied to determine the power spectra of total base shear and bending moment acting on an offshore structure. The particular structure considered was the Ocean Test Structure (OTS) located in the Gulf of Mexico. Measurements of these power spectra for a number of different wave records were established by analysis of the measured force data in the OTS project. The present paper provides a theoretical analysis applied to this data in order to determine the power spectra from first principles based upon knowledge of the geometry of the structure and the power spectrum of the incident wave system. The basic force model used is the Morison equation representation applied to a structure made of a number of vertical legs, horizontal bracing elements, and inclined members. The methods and approximations used in the analysis are described, with the final results compared with the measured spectra. Good agreement between theory and experiment is exhibited. There by providing a useful tool for applications involving studies of fatigue other dynamic response aspects of offshore platforms. Introduction Among the various forces acting on an offshore structure, which should be properly predicted for design use, are the total base shear and the bending moment. In the present study, an analytical approach via frequency domain techniques is applied to determine the power spectra of total base shear and bending moment acting on an offshore structure. The particular structure considered is the Ocean Test Structure (OTS) which was located in the Gulf of Mexico and used for an extensive series of tests and analyses by a consortium of organizations engaged in off- shore engineering. Computer analysis of measured test data in order to obtain the various spectra and related quantities for the structure was carried out by Richman and Bendat [1]. The results giving these spectra are available on a magnetic tape associated with that study. There has not been any theoretical analysis which was directly applied to the measured OTS data in order to determine these power spectra, based upon knowledge of the geometry of the structure and the power spectrum of the incident wave system. Directional wave analysis procedures were utilized by Borgman and Yfantis [2] in the analysis of force relationships, where the procedure used in that analysis increased the effective vertical leg volume to account for all of the other structural members. The effects of phase due to different spatial locations and orientations of such members cannot be accurately represented by such a projected area approximation. In establishing the analysis of the power spectra for the total shear and bending moment on the structure, the basic local force model used is the Morrison equation representation. The method of statistical linearization is applied to the quadratic form of the drag force term in order to allow ease of analysis using such a model. The mean square were velocities are assumed to be independent of the orientation of the member and equal to the horizontal mean square velocity.
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