Advanced Mooring Method for Installation of Enserch Mooring Legs Garden Banks 388 FPF

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ABSTRACT In the fall of 1994 HeereMac v.o.C installed the 12 mooring legs for Enserch Exploration's Floating Production Facility in Garden Banks Block 388 in the Gulf of Mexico with the SSCV Balder. The installation of the catenary mooring system, each leg comprising several raying sections of spiral strand wire and chain, required sufficient handling and manocuvrarability power of the vessel, while enough holding capacity and stiffness of the system had to be provided. The most important aspects of the actual installation of the mooring legs are explained, for example, the w of a purpose built tipping winch. The method selected by HeereMac for station-keeping the Balder was to use a minimum number of anchor lines in combination with a tug, in order to maintain position and at the same time have an easy and controlled method of manoeuvring to a new position. Themethod of station-keeping the SSCV in this way is part of a development towards full position control with a spread of tugs. In this paper the station-keeping system is described and the offshore experiences with the system are discussed. Some future developments with respect to tug-assisted station-keeping systems are highlighted. INTRODUCTION In November 1994 HeereMac v.o.f installed the mooring system for the Ensach Garden Banks Floating Production Facility (FPF). The installation took place in Garden Banks Block 388 in the Gulf of Mexico and was performed by the Semi-Submersible Crane Vessel (SSCV) Balder. The water depth at the installation site varied from 2100 ft. to 2550 tl.. The FPF mooring system consisted of 12 mooring lines in a regular pattern (at intervals of 30 degrees) with leg lengths varying between 7650 ft. and 9000 ft. (see Figure 1). Allmooring leg components were delivered by Enserch to the HeereMac base port in Fonrchon (Louisiana), where all components werechecked and marked, and trial tit-ups were performed. The mooring leg components were transported from the base pat to the installation site on three barges, a 250 tl. × 72 tl. barge for the transport of the chains of the fust six mooring lines, a 260 ft. × 72 tt. barge for the chains of the remaining six mooring lines, and the MWW03, a 400 tl. × 105 ft. barge, for the transport of all the othermooring leg components. Only twice was it necessary to transfer a full batch of all components of six mooring legs from the cargo barge to the &ck of the SSCV (see SSCV deck lay-out, Figure 2). Before the job started all systems had been extensively tested in order to avoid surprises during the installation. Each anchor had to be set within an envelope measuring 100 ft.perpendicular to the leg and 50 ft. along the leg direction around the anchor setting co-ordinates.. The anchor lines had to be installed within a corridor of 50 R. on either side of the target-setting line. This paper gives a description of the project and the installation aspects. Special attention is paid to the station-keeping of the SSCV.