Offshore LNG Transfer - A New Flexible Cryogenic Hose for Dynamic Service

Abstract

Abstract This paper reviews flexible cryogenic hose constructions aimed at the ship to ship transfer of LNG in an exposed marine environment. This application requires a large bore hose to meet minimum flow rate requirements and it must be sufficiently compliant in flexure so that it may be deployed with relative ease between the two moving vessels. A new flexible cryogenic hose is presented that meets the pressure and inner diameter requirements while offering a relatively small operational bend radius in dynamic service. Introduction A number of marine applications require a compliant link for the transfer of cryogenic fluids between two units that experience relative movement such as the offloading of LNG from a FSRU to a tanker, ship to ship transfer of LNG from an FPSO to a shuttle tanker in an exposed location and ship to shore LNG transfer. This is a challenging requirement as the low temperatures involved (e.g. 77 K associated with LN2 testing) severely restrict the range of materials available that can be used for this service in bulk form. Austenitic stainless steel 316L and fluoropolymer PTFE are respective examples of suitable metals and polymers for cryogenic service. There are two main methods for providing a conduit for conveying cryogenic fluid between two moving bodies. These are (a) linkage mechanisms incorporating "hard" stainless steel pipe work with articulated joints (swivels) and (b) flexible hoses. The use of linkage mechanisms is a geometric solution where relative motion between two linking elements is restricted to planar rotation by the swivel. The length and orientation of the links are designed to accommodate the required motion envelope. The complexity of the linkage mechanism reflects the number of individual fluid paths and the extent of the relative motions. The key component of such a system is the swivel. A number of swivel designs have been proposed and in one of its simplest manifestations consists of a stainless steel spring energised U-shaped ring made from PTFE resin between the metallic male and female cylinders undergoing relative rotation about their common longitudinal axis where the male cylinder passes through a set of bearings housed within the female receptacle. As with any cryogenic equipment including flexible hoses due account needs to be taken of the common and differential contraction of the components as a result of the change in temperature from ambient to cryogenic conditions. Large bore (16" ID) cryogenic loading arms are examples of linkage mechanisms that have been in service for a number of years. The alternative technical solution is to use a flexible hose for the cryogenic fluid transfer and this is the subject of this paper. The main types of flexible hose for cryogenic service are presented with a discussion of their performance characteristics.