Subsea LNG Transfer System
Authors: Mathews William Sean , Gentry Mark C. , Ehrhardt Mark 
Published: March 2015

Many offshore liquefied natural gas (LNG) import terminal designs have been proposed by industry in the last several years. Most base-load offshore LNG import terminals currently under development utilize either a bottom-founded structure, such as a gravity base or a jacket structure, or a floating structure such as a moored ship-shape hull. In either case, LNG transfer from the LNG carrier (LNGC) to the LNG import terminal is performed through side-by-side offloading. LNG transfer is accomplished with loading arms equipped with constant motion swivels and a wire-guided connection system. Side-by-side offloading has acceptable availability for mild environmental conditions. However, when applied in more severe environments, this type of LNG transfer system can limit the terminal throughput because of low berthing and offloading availability.

This limitation has been addressed for oil transfer systems by utilizing disconnectable turret-mooring buoys. High availability has been achieved with this type of transfer system in harsh environments, e.g., the crude oil export system for the Heidrun Tension Leg Platform in the North Sea. Leveraging off this proven system, ExxonMobil Upstream Research Company carried out conceptual engineering studies with component suppliers to develop an LNG transfer system utilizing a disconnectable cryogenic turret-mooring buoy for challenging environments. System design requirements included a minimum water depth of 45 meters, a minimum LNGC connection limit of 4 meters significant wave height (Hs), and a minimum departure limit of 8 meters Hs. Study results included conceptual designs for all system components including the disconnectable cryogenic turret-mooring buoy, dynamic flexible cryogenic risers, subsea cryogenic pipelines, cryogenic swivels, subsea cryogenic valves, and all corresponding shipboard modifications. These studies also produced a budgetary system cost estimate. This work demonstrated the technical feasibility and economic benefit of such an LNG transfer system.

Introduction

A key element in delivering an uninterruptible supply of natural gas from an LNG import terminal is obtaining the necessary availability to berth and offload the incoming LNG carriers (LNGCs). In the case of an onshore LNG import terminal, a protected harbor is used to provide the sheltered environment in which the LNGC can perform the necessary offloading operations. Generally, in the case of an onshore LNG terminal, a traditional berth equipped with a conventional mooring system is used to moor the LNGC, and hard arms are used to connect to the offloading manifold, which is located amidships. These side-by-side offloading operations are limited to benign environmental conditions, on the order of 1 meter waves, as measured by significant wave height (Hs). The sheltered environment of a protected harbor is generally required to meet this restriction and ensure that the necessary berthing and offloading availability can be achieved.

As LNG import terminals move offshore, the creation of a protected area for offloading operations becomes increasingly difficult, if not impossible. In these open-sea environments, the use of traditional berthing and offloading equipment, i.e., side-by-side offloading, may lead to reduced berthing and offloading availability. For an offshore LNG import terminal with similar storage and gas sendout rates to those of an onshore LNG import terminal, this may lead to challenges with maintaining an uninterruptible supply of natural gas to the consumer. In order to achieve uninterruptible natural gas sendout, either the LNG storage capacity must be increased, or the natural gas sendout rate must be reduced. Both of these solutions allow for longer periods between LNGC offloads without hitting LNG tank bottoms and interrupting natural gas sendout. LNG import terminal economics, however, are reduced for either of these options, through either an increase in capital expenditure or a reduction in terminal revenues. In an extremely harsh offshore environment, it is possible that this could lead to an uneconomic project.

A solution that allows the LNG import terminal to maintain natural gas sendout rates without increasing the LNG storage capacity is to extend the limits for offloading operations. Side-by-side offloading using hard arms with constant motion swivels and a wire-guided connection system or tandem LNG offloading systems utilizing flexible cryogenic hose or hard pipe may extend the operational limits for offloading into the 2 to 3 meter Hs range.