46
LNG
INDUSTRY
SEPTEMBER
2016
resources or associated gas that could not be tied-back to a
major LNG plant onshore or otherwise valorised. It has also
been seen as an opportunity to reduce time for gas to
market in many places, while maintaining a competitive
advantage for small to medium scale liquefaction plants.
However, it took some time for the concepts and
technologies to mature before the installation of the first
permanent offshore regasification terminal near Rovigo,
Italy (a gravity-based structure), in 2009. In the same year,
the first floating storage regasification units (FSRUs) berthed
at quayside in sheltered areas in Brazil. At the end of 2013,
OLT Offshore LNG Toscana’s floating regasification terminal,
FSRU Toscana
, was permanently moored off the Italian
coast, between Livorno and Pisa. Now, the first floating
liquefaction units are under construction and will probably
commence LNG exports in 2017. Despite a challenging oil
and gas market with low prices offshore, LNG projects and
technologies are continuing to attract a great deal of
attention from both major and smaller operators. This article
outlines the various types of offshore LNG unit.
FSRUs
The number of operating FSRUs has been continuously
increasing over the past few years, with a dozen now
operating throughout the world. A relatively simple process
ensured that these regasification facilities were the first
LNG plants to be installed on a floating facility. A major
reason for the success of FSRUs is the flexibility offered
by a floating facility in a commercial leasing scheme,
e.g. regasification vessels can be used on a seasonal basis.
However, even on a permanent basis, the FSRU concept
remains an attractive alternative to traditional land-based
onshore regasification terminals. Indeed, few modifications
are needed to convert a standard LNG carrier (be it for
a conversion or on a purpose-built hull) to an FSRU. For
instance, the regasification module can be compact, and
the transfer systems can be moved onto a jetty if the FSRU
is berthed at quayside, etc. Moss Maritime, a Saipem
subsidiary and developer of the spherical tank containment
systems for LNG carriers, performs these conversion studies
using in-house technologies. Several of the currently
operating FSRUs are a result of Moss design.
Even a complete, truly offshore FSRU, with mooring and
LNG offloading functions included, can be competitive
against onshore terminals and provide an answer to the
challenge of installing regasification facilities in
industrialised coastal areas where available space is often
limited, authorisations can be difficult to obtain, and local
populations can be hostile to the installation of such plants
in their communities. An example of a truly offshore FSRU is
the
FSRU Toscana
(OLT) built by Saipem. The project
consisted of converting an LNG carrier into an FSRU through
the addition of a module for regasification, marine loading
arms for offshore LNG transfer, a turret for the permanent
mooring of the vessel, and thrusters to assist offloading
operations.
Offshore liquefaction
The development of offshore FLNG facilities has been
slower than the development of FSRUs due to the more
challenging technological hurdles to be overcome and a
higher perceived risk due to this additional complexity.
However, the first facilities are now in the final stage of
construction and floating liquefaction will soon be an
operating reality.
The incentives to move a liquefaction export terminal
offshore are different from those of a regasification terminal.
Indeed, the purpose here is usually to produce remote or
isolated offshore gas fields that are either too small to
justify the investment in an onshore liquefaction plant, or are
too far from existing plants to make a tie-back to such a
plant viable. However, an offshore floating liquefaction
plant can also prove to be more cost-effective and faster to
build than an onshore plant, especially when no
infrastructure exists onshore, since a floating LNG (FLNG)
unit can be fully built in shipyards, under cost-effective
working conditions. However, the large, but still limited,
area available on the deck of an FLNG unit (designs usually
consider hull lengths up to 500 m) limits the production
capacity that can be achieved on a single floater, typically
approximately 6 – 7 million tpy of LNG for the most
favourable cases, i.e. for lean gases that do not require
significant NGL and LPG extraction and production in
particular.
Saipem has been developing and improving
technologies that help make FLNG a competitive and
enabling solution for producing offshore gas fields. Some of
these developments relate to the core of the process design
of the plant, i.e. the liquefaction cycle. For example,
optimisations have been made to open-art nitrogen
liquefaction cycles, which are robust and easy to operate at
Figure 1.
Side-by-side offloading on a converted floating
storage regasification unit (FSRU).
Figure 2.
A typical 3 million tpy floating LNG (FLNG) unit.
