Polyester Mooring Lines on Platforms and MODUs in Deep Water

Abstract

Abstract This paper is a review of the experiences and issues with the use of polyester fiber ropes as mooring lines in deepwater applications. Early studies showed that polyester rope taut-leg mooring systems could provide better offset-restoring properties in deep water than the traditional wire rope catenary mooring systems. But there was reluctance to use such moorings without further knowledge of fiber rope properties. Polyester and other fiber ropes were studied for deepwater moorings in several Joint Industry Projects (JIP) in the early 1990s. These studies provided vital information and answered many critical questions. They showed that polyester rope has desirable stretch characteristics and very good durability for use as mooring lines. The use of polyester mooring systems was pioneered by Petrobras in the late 1990s. Mobile Offshore Drilling Units (MODU) began using polyester mooring lines in the Gulf of Mexico (GOM) in the early 2000s. The first permanent applications of deepwater polyester mooring systems in the Gulf of Mexico were the Mad Dog and Red Hawk platforms, installed in early 2004. The use of polyester and other fiber rope mooring systems into even deeper water depths will present new challenges. Stiffer ropes may be necessary to achieve desirable mooring system characteristics. Longer, larger volumes of polyester rope will be difficult to handle. More knowledge of the properties of the alternative, high-modulus, high-strength fiber ropes may be needed. INTRODUCTION Fifteen years ago there was much reluctance to use polyester and other fiber ropes in deepwater mooring systems for oil exploration and production platforms. The tendency was to continue to use and adapt wire rope mooring systems into deeper water. That skepticism has now been overcome. Polyester mooring lines are now used on platforms in Brazil, the GOM and elsewhere. The experiences have been favorable. Several polyester platform moorings survived the recent hurricanes without incident. Now there is interest in installing polyester and other fiber rope mooring systems in even deeper water. But as water depth increases, other - stiffer and stronger - fiber ropes might be preferred. This paper discusses the history, present status, and possible future of polyester and other fiber rope moorings in deep water. ADVANTAGES OF FIBER ROPES Fiber ropes have a number of advantages over steel wire rope in deepwater mooring systems. Lighter Weight. Lower Loads on Platform The principal advantage is weight. Fiber ropes are nearly or essentially neutrally buoyant in sea water. They can be used in taut-leg mooring arrangements. The advantages of the taut-leg mooring might not be obvious. Figure 1 illustrates three possible ways of mooring a floating platform in deep water. Wire rope in the form of a catenary is the traditional way of mooring platforms. However, as water depth increases, the weight of suspended wire rope increases and the downward angle of the catenary at the platform becomes steeper. This results in a large downward pull on the platform which decreases payload or increases required buoyancy. (As discussed later, the steep catenary also initially produces very little horizontal restoring force)