Subsea-to-shore innovation
North Sea
GAS
Subsea-to-shore
innovation
The Laggan-Tormore project west of
the Shetland Islands is the first
subsea-to-shore development in UK
waters. Nic Newman reports.
D
Installation of import
pipeline at Orka Voe,
Shetland Islands
Source: Total
eveloped by Total E&P UK
(operator, 60%) in
partnership with SSE E&P
UK (20%) and DONG E&P UK
(20%*), the innovative LagganTormore project is located in blocks
206/1a and 206/5a, in water
depths of 600 metres. Total field
reserves are put at 1tn cf of gas and
condensates – a substantial
addition to the UK’s known
reserves.
The £3.5bn project comprises
the development of two deep
offshore gas fields – Laggan and
Tomore – the first to be developed
in the Atlantic Margin, a region
that until now was known
primarily for its oil output. First gas
32 Petroleum Review | August 2017
was produced on 8 February 2016,
via a semi-autonomous system
lying on the seabed connected by a
143-km subsea export pipeline,
known as the Laggan Tormore
export pipeline (LTEP). The LTEP
links the gas fields with a new
£800mn gas plant located in
Shetland, next to the Sullom Voe
oil and gas terminal. The facility is
able to process 500mn cf/d –
enough gas to supply two million
households or about 8% of the UK’s
energy needs. After processing, the
gas is piped through a second
243-km newly constructed
Shetland Islands Regional Gas
Export (SIRGE) pipeline which
connects with the Frigg-UK
pipeline and then into the UK’s gas
pipeline network in Scotland.
A combination of the extreme
weather experienced in the
Atlantic Margin, including
hurricane force winds in winter;
water depths of some 600 metres,
where temperatures are commonly
–1°C; typical current speeds as high
as 0.64 m/s; and the large volumes
of gas to be developed made it
unrealistic to employ a traditional
platform or a FLNG (floating LNG)
vessel to collect, process and store
gas ready to be offloaded onto LNG
tankers. Instead, a subsea-to-shore
development concept was chosen
– a UK first.
Essentially, the Laggan and
Tormore fields are each serviced by
a semi-autonomous subsea
production system consisting of a
set of six-slot manifold templates
lying beneath 600 metres of water
on the seabed. Each manifold
template measures approximately
30 metres wide, 40 metres long
and 21 metres tall, weighing in at
900 tonnes. At present, five subsea
wells are in operation out of a
12-well capacity.
These subsea installations
represent two major engineering
achievements for the UK. The first
is the installation of a subsea
production system with remote
controlled wells, and the second is
the longest tie-back between
offshore wells and an onshore
terminal.
Installation challenges
The depth of water at LagganTormore precluded the use of divers
so Heerema Marine Contractors’s
Thialf heavy-lift vessel was used to
transport and install the
production systems on the seabed.
Subsea work was sub-contracted to
Aberdeen-based Specialist Subsea
Services (S³), who provided and
operated remotely controlled
underwater vehicles (ROVs) for the
subsea installation work.
The project has a field life
expectancy of 20 years. As with
any production facility, wellhead
pressures naturally decline over
time and the pipeline export flow
will fall below a critical value. At
this juncture compression boosters
will most likely be installed at each
wellhead and one for each export
pipeline in order to boost
production and recovery from the
reservoir by reducing backpressure
on the wells by increasing the flow
rate in the export pipelines, thus
extending field production.
In the pipeline
Two sets of new pipelines were
constructed for the project, the first
being the LTEP network linking the
fields with the newly constructed
Shetland gas processing plant. A
second pipeline system exports the
processed gas from the gas plant to
the UK mainland. The entire
pipeline network required 200,000
tonnes of high-grade steel.
Internally the pipes are protected
by a three-layer polypropylene
anti-corrosion coating, internal
flow-efficiency coating and
concrete-weight coating. The
pipeline network is regularly
inspected and can be cleaned by a
remotely controlled pig.
The LTEP comprises three
pipelines and a communication or
control umbilical. On the seabed
this network consists of two
18-inch diameter flow-lines that
carry gas from the fields to the
Shetland gas plant for processing
and an 8-inch diameter pipeline
carrying MEG (mono ethylene
glycol) from the gas plant to the
Laggan-Tormore field’s production
sites. MEG is used to prevent the
condensate from freezing at the
start of its journey, much like
antifreeze is used in a car. When
the gas arrives at the processing
plant, the MEG is removed and
pumped back to the wellheads, to
start its journey again. Gravity
causes the contents in the gas
pipelines to separate into gas in
the top half of the pipe, whilst the
heavier gas liquids settle along the
North Sea
Gas processing plant,
Shetland Islands
Source: Total
bottom half. The critical element,
the control umbilical, monitors and
controls all the subsea
infrastructure. Both the MEG
pipeline and control-umbilical are
buried under newly laid stone to
protect them whilst the gas export
pipelines are exposed to the
elements.
Construction of the Shetland gas
processing plant began in 2010 and
was handed over to Total by lead
contractor Petrofac Offshore
Engineering & Operations in
December 2015 for testing the
complete range of command and
control systems. As noted earlier,
first gas arrived in February 2016
and in April the same year the
second gas compressor came
online. It took a huge effort to get
to this point – taking some 42mn
person-hours over two years,
employing 800 personnel on
temporary contracts for the
construction, and currently
employing 70. By May 2015
Laggan-Tormore was producing at
a peak 90,000 boe/d.
The gas processing plant
separates the well fluid into gas, oil
and water with the three fluids
being discharged separately. The
associated liquid hydrocarbons are
exported to the Sullom Voe oil and
gas terminal. The treated gas from
the plant is exported to the Frigg
UK pipeline in the North Sea via
the SIRGE pipeline system, which
then links with another pipeline to
deliver the processed gas to the St
Fergus gas terminal, near
Peterhead in the north-east of
Scotland for final processing.
The plant can process up to
500mn cf/d of gas, which is more
than Laggan-Tormore is likely to
produce. This is in anticipation of
future successful exploration in the
West of Shetland region, an area
estimated to contain as much as
17% of the UK’s remaining
yet-to-find gas reserves.
Development of the nearby
Edradour and Glenlivet fields by
Total and its partners, starting in
2017 and 2018, will necessitate
installation of new mercury
removal facilities at the Shetland
gas plant, and will ensure
production is kept at peak rates
until at least 2020.
Looking ahead, Laggan-Tormore
will have a key role to play,
allowing future gas finds in West
of Shetland waters to be easily
transmitted to the UK mainland
via the existing infrastructure. ●
*DONG’s entire oil and gas business,
including its interest in Laggan-Tormore, was
sold to Ineos in late May 2017 for $1.05bn,
plus a contingent payment of $150mn
relating to the Fredericia stabilisation plant
and a contingent payment of up to $100mn
subject to the development of the Rosebank
field.
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Petroleum Review | August 2017 33