Developing And Using A Deepwater Predictive Soil Model To Optimize The Parque das Conchas (BC-10) FPSO Mooring Layout

Abstract

Abstract This paper provides a methodology for optimizing a deepwater mooring layout with respect to ground conditions and gives limitations of the method. Specifically, this paper:describes development of a predictive soil model based on integration of AUV geophysical survey and geotechnical data in an area characterized by complex and discontinuous sand strata interbedded with clays;describes using the predictive model to help optimize a FPSO mooring layout to meet design criteria;presents as-installed results for the suction anchors and demonstrates reliability of the predictive soil model;describes learnings from this project; anddiscusses limitations of the methodology. Suction-anchor installation was completed successfully and no problems were encountered with respect to soil conditions. The as-installed results thus confirmed the general reliability of the predictive soil model. Sand strata as thin as about 30 cm thick were predicted by the model and their presence was confirmed by increased pump underpressures recorded during anchor installation. The fact that the 5-m-diameter suction anchors " felt?? such thin sands was instructive. We demonstrate that, by using appropriate geophysical survey data calibrated with geotechnical information, a model of soil conditions can be developed and used as a tool to predict soil conditions away from geotechnical control points. Confidence in the predictions depends on the amount, type, spatial distribution, and quality of the data available and on the geology of the site. This case history is useful as a model approach for optimizing mooring layout with respect to ground conditions or for other applications where shallow soil conditions are sufficiently complex and variable to affect layout of foundation elements or design. Integrated study of the geophysical/geotechnical data was concurrent with mooring system design in a collaborative effort that directly influenced design iterations. Use of the integrated and iterative approaches described here will help future project teams develop more reliable mooring or other foundation solutions faster and thereby reduce costs. Introduction Background. The Parque das Conchas development is located off the coast of Brazil in the Campos Basin in Block BC-10 (Figure 1). Shell is the operator of BC-10 with 50% equity share with co-venturers Petrobras (35%) and ONGC (15%). The site is on the continental slope and is characterized by hummocky topography (Figures 2 and 3a) caused by buried mass-transport deposits (MTDs). Water depths across the mooring spread range from about 1740 m to 1800 m (Figure 3a). Production facilities feature the FPSO Espirito Santo with a clustered 3x3 suction-anchor mooring system; the mooring layout is shown in Figure 3a. Mooring radius varies slightly from cluster-to-cluster and ranges between about 2500 to 2700 meters. More details and other aspects of the FPSO Espirito Santo are discussed by Howell and others (2010).