FLNG Design Issues - Building on known technology
Authors: Fagan Conn , Sele Hans Olav , Oestvold Tom Klungseth 
Published: July 2010

Floating LNG production units are being proposed for a number of applications worldwide. The majority of those being proposed are based on use of a ship-shaped hull and intend to use some technology similar to that used in the LNG shipping industry. This paper will address how far the experience and technology used in shipping applications can be utilised directly in an offshore LNG project and how the gaps are being tackled today.

DNV has been involved in many of the ongoing projects looking at technical feasibility and more detailed design of a number of FLNG concepts and has also carried out studies to address some of the key technological gaps identified between existing experience and proposed applications.

These issues include limitations on use of traditional LNG containment systems, particularly of the membrane type, considerations of impact of FLNG operational modes on some design details traditional to the shipping industry, some consequences on layout and arrangement to take account of topside hazards, and some consequences of selection of the various liquefaction technologies available. This paper will address :Gas Ship versus FLNG applicationsCurrent thinking on Sloshing in membrane tanksDesign for continuous operation offshoreSome key issues and hazards influencing layout and arrangement

Introduction

The LNG chain requires at one end, facilities for liquefaction of the gas (an export terminal) and at the other end, facilities for regasifying it (a receiving terminal). Traditionally these terminals are located on land. The gas in the form of LNG is transported by ships designed specifically for that purpose (gas carriers). This in itself is well known technology, and a technology which has an extremely good safety record. However the current trend is to locate some of these terminals offshore, a trend which is driven, in some cases, by such factors as unavailability of suitable land sites, problems and delays in permitting, public opposition, and security considerations. In other cases the trend is driven by cost and schedule considerations.

Solutions chosen for such offshore terminals may include concepts based on bottom-fixed structures or floating technology. Of the floating solutions the majority of those currently proposed are based on a ship-shaped hull (rather than a barge) and may involve either a purpose-built design or a conversion from an existing ship, typically a gas carrier.

While each project would generally like to be considered as unique and adapted specifically to the specific application and location for which it is intended., it is not desirable to purpose-build any installation from first principles without taking account of technology used and experience gained in similar projects. For novel applications there may not be extensive equivalent experience on which to draw, however there will typically be elements of existing projects in related applications which may be utilized.

This will be the case for Floating LNG units which will combine technology used in the land-based LNG industry (liquefaction and regasification plant), the offshore oil and gas industry (gas pre-treatment and FPSO design and mooring) and the LNG marine transport industry (ship design and LNG containment system design).

A significant challenge for floating offshore LNG terminal design is to determine the relevance and applicability of experience and technology used in related industries. In some cases application of such experience may lead to solutions which are overly conservative, in other cases, to solutions which are not sufficiently conservative. In other cases again, it may be that there are issues which have not been addressed previously at all and which are unique to the offshore LNG application.