Aerodynamic and Hydrodynamic Model Tests of the ENSERCH Garden Banks Floating Production Facility

Abstract

ABSTRACT This paper presents the results of aerodynamic and hydrodynamic model tests of the ENSERCH Garden Banks, a semisubmersible Floating Production Facility (FPF) moored in 2, 190-ft waters. During the wind tunnel tests, the steady component of wind and current forces/moments at various skew and heel axes were measured. The results were compared and calibrated against analytical calculations using techniques recommended by ABS and API. During the wave basin test the mooring line tensions and vessel motions including the effects of dynamic wind and current were measured. An analytical calculation of the airgap, vessel motions, and mooring line loads were compared with wave basin model test results. This paper discusses the test objectives, test setups and agendas for wind and wave basin testing of a deepwater permanently moored floating production system. The experience from these tests and the comparison of measured tests results with analytical calculations will be of value to designers and operators contemplating the use of a semisubmersible based floating production system. The analysis procedures are aimed at estimating 1)vessel motions, 2)airgap, and 3)mooring line tensions with reasonable accuracy. Finally, this paper demonstrates how the model test results were interpolated and adapted in the design loop. INTRODUCTION ENSERCH Production Operating Limited Partnership (EPO) has selected the Glomar Biscay I, an Ocean Victory-class semisubmersible MODU for conversion to the FPF and renamed it the ENSERCH Garden Banks. The FPF will be located in Garden Banks Block 388 in the Gulf of Mexico in 2,190 feet of water. The FPF will be permanently moored using a 12 point combination chain and wire rope system. The mooring system is designed for the 10o-year hurricane condition at the site. In this paper results of 1) wind tunnel testing, 2) global performance analysis, 3) mooring system design and analysis, 4)wave basin model testing, are presented. DESIGN PREMISE The mooring system has been designed according to practices and procedures recommended by ABS [1] and API [2]. Mooring design criteria are summarized in Table 1. The worst one-line damage condition was determined by analyzing cases with either the most loaded line or the second most loaded line broken. Mooring Line Scope The mooring line scope was designed to ensure no uplift of the mooring line at the seafloor in the 10o-year storm condition with all lines intact. Also, no mooring line was allowed to contact an environmentally sensitive seafloor area within the mooring pattern. Platform Offset Requirements The vessel offset requirements used in the mooring system design for the ENSERCH Garden Banks are shown in Table 2. Metocean Criteria The following three conditions are used to design the permanent mooring system:1ao-year hurricane survival condition,1a-year winter operating condition and5-year winter storm combined with loop current operating condition. Metocean criteria for a similar region (27º N and 86º W) of the Gulf of Mexico, studied by API and presented in [3], suggested that a set of reduction factors may be applied to the omni-directional wave height to account for the directionality of the severest hurricanes in this region.