ABSTRACT A one-mill ion-pound break strength rope of high strength aramid fiber was designed, produced, and field evaluated as a mooring line on the drilling rig Zapata Lexington. The rope was used in an insertion mode between a length of conventional steel wire rope and chain. The aramid rope was designed to be torsionally compatible with the steel wire rope to avoid torsional fatigue of the aramid fibers. In addition to describing that aspect of rope design, this paper discusses the mooring line performance, and the results of rope testing prior and subsequent to use. INTRODUCTION Despite depressed crude oil and natural gas prices, the ability to cost effectively drill and produce in deeper water and harsher environments remains a key industry need for the 1990' s. One potential route to meet that need is to utilize high specific strength mooring lines, such as those produced from para-bonded aromatic polyamide fibers such as Kevlar* 29 and the higher modulus Kevlar* 49, both manufactured by Du Pont. The reduced weight (Table I), vs. steel wire rope and chain, of an equivalent strength para-aramid catenary mooring line can provide significant benefits, improving the station keeping of deep water semisubmersible exploration rigs and floating production platforms. Tethers of Kevlar 49, because of their high specific strength and specific modulus, are well suited for mooring lightweight tension leg platforms in intermediate water depths. Of the existing 285 semisubmersible and floating rigs, 3/4 are limited to water depths of less than 1500 feet and only 40 have capability of drilling in depths beyond 2000 feet. As the need to increase the capability to explore and produce in deeper water grows, it becomes incumbent to evaluate the alternative routes available. One can either construct new rigs and platforms or modify existing ones to increase their depth capabilities. One potential depth increasing modification is to replace steel wire rope or chain mooring lines with lightweight para-aramid ropes in the suspended part of the catenary. This paper describes a Conoco evaluation of a para-aramid mooring line in the Gulf of Mexico. BACKGROUND All-chain mooring systems can be used satisfactorily in water depths up to about 1500 feet. In shallow water, the high weight (low specific strength) of chain is desirable, because it produces sufficient catenary sag to provide an acceptable level of restoring force to counter vessel excursion. In deep water, on the other hand, an all-chain system is inadequate. The high weight of the chain produces excessive catenary sag (and poor station keeping) which can only be overcome by undesirably high pretension (or through the use of flotation devices, such as mid-line buoys). Additionally, the weight of the chain can significantly reduce the rig's variable deck load in transit. The suspended weight of chain required to adequately 8-point moor a "typical rig" in 2000 feet of water is 1.8 million lbs. Combination chain/wire rope mooring lines alleviate the problems inherent with all chain and become the system of choice in water depths beyond 1500 feet until they too become victims of the same problems associated with all-chain moorings: excessive catenary sag, poor station keeping.