Challenges on the Safety of Floating Liquefied Gas Terminals - Hull, Topside and Interface
Authors: Wang Xiaozhi , Sun Haihong , Patel Harish N. , Hwang Jeong Heon , Liao Ping 
Published: April 2012

The increasing demand for natural gas and interest in offshore gas fielddevelopment has led to significant activity in studies of floating liquefiedgas terminals for production, storage and transfer of liquefied gas to vessels. Based on experience from Floating Production, Storage and Offloading (FPSOs)systems, LNG carriers and onshore facilities, the challenges for new FloatingLiquefied Gas (FLNG) terminal designs remain primarily in the safety of theunit. This includes hull design, topside and hull-topside interfaces.

FLNG terminals are still an emerging technology which is of great interest tothe offshore and energy industry. Current FLNG terminal designs are not only oflarger size than conventional FPSOs, but also incorporate new design features, in particular the integration of the containment system as well as topsideprocess facilities. The purpose of this paper is to present experience gainedthrough various FLNG terminal projects and to describe safety requirements forFLNG terminals, including prescriptive and risk assessment criteria. This paperpresents an easily adopted methodology to evaluate FLNG terminal designs with afocus on the overall safety of the structure and facility, and also introducesa tool developed for an integrated structural analysis methodology for FLNGterminals.

Introduction

Various FLNG terminal concepts proposed to date have several common elementsfrom technology and application perspectives. All concepts have proposed to usevarious existing technologies and combined them for use on FLNG terminals. These existing technologies have extensive experience in either land-based ormarine applications. Gas processing, liquefaction and the fractionationtechnologies selected are based on land-based experience. Cargo storagetechnology (liquefied natural gas (LNG), liquefied petroleum gas (LPG), andcondensate) is based on transportation of liquefied petroleum gases in bulk ina marine environment such as LNG and LPG carriers. Technologies for hull, mooring, accommodation, subsea production, process facility integration, etc.are from offshore FPSO experience. New technologies are considered to includecryogenic cargo transfer in an offshore environment.

The challenge for FLNG terminals is to determine the applicability and safetyof the selected technology for offshore application. A significant issue is theintegration of the entire facilities and operations - i.e., production, processing, liquefaction, support systems, marine systems, storage and cargohandling, while minimizing the environmental " foot print".

Technical challenges in FLNG terminal design are being driven by a combinationof design and operational issues. These include the increase in the size ofterminal hulls and LNG containment systems, shallow water load effects, frequent partial filling, offloading operations and critical interfaces betweenthe hull and topside structure and between the hull and position mooringsystem. For example, the Prelude FLNG terminal, which is currently underconstruction, is to be 488m long and 74m wide, and when fully loaded it willweigh approximately 600,000 tonnes - roughly six times as much as the largestaircraft carrier.

Design of an FLNG terminal can be based on the application of prescriptiverequirements, sea-keeping studies, structural strength and fatigue analysis, station keeping system evaluation, containment system assessment, and caninclude sloshing analyses. For the topsides, the gas processing andliquefaction plants or the re-gasification modules use advanced risk analysisto verify that accepted safety standards are attained.