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The aforementioned wet gas compressor is
located at the M and L templates which are tied-back to the Gullfaks C platform.
The process design capacity of the compression system is 10 MSm3/d and it comprises:
2 multiphase compressors (WCG4000) that can be operate either in-parallel or
in-series, 2 coolers, 1 flow mixer and 3 bypass headers. Each compressor
generates 5MW (due to its double motor design) (Birkeland et al., 2016). The purpose
of the flow mixer, which is located at the inlet of the station, is to split
homogeneously the multiphase flow before it enters to the  compressor(s). With the aim of limiting the
discharge temperature and increasing compression efficiency, 2 coolers are
installed upstream the compression stage. Valves for shifting from in-parallel
to in-series operating conditions, and vice versa, are also ROV operable (Birkeland
et al., 2016).

Regarding multiphase flowlines, flow
assurance of the fluid recovered from Gullfaks South due to its high wax
content and hydrate control were the main factors for their design. The concept
chosen was the heating of these flowlines bundles by means of a hot-water
system (Knudsen, 1999). This process incorporated heat available from the gas
turbines on Gullfaks A (“Gullfaks oil,” n.d.).

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The HOST subsea and manifold system basically
consists of the following components: manifold module, centre section and mud
mat, protection structure, wing elements and wellheads. The manifold module is
the most customisable component in the HOST system due to its design is based
on field-specific requirements. This component allows for wellbay tie-in points
to be connected to the main headers by mean of valves and flow distribution
elements (Knudsen, 1999). Protection from dropped objects and fishing gear is
provided by the protection structure for wells, the manifold, flowlines and
umbilicals. The centre section is run alongside the mud mat and provides
positional reference and structural support for all HOST system components. A
hole can be drilled in the seabed through the centre section once this is
landed and levelled on the seabed. A hinge-over guidance structure (belonging
to the wing elements) is therefore required to land the drilling guide base for
each well to be drilled. The subsea wellheads are  standard 18¾” FMC UWD-15 and include
stacked casing hangers (Knudsen, 1999). In addition, a 3KSI system for drill
cutting injection was installed on each subsea template.

Back in 1994, a low-cost subsea concept was
identified as the preferred for developing the former three fields in terms of
flexibility. This concept involved the use of HOST technology (hinge-over
subsea template) which can be defined as a multi-well, guideline-based,
template system (Knudsen, 1999). Broadly, this system consists of lightweight
modules and allows for accommodation of up to five well slots. In the case of a
four-slot arrangement, it is possible to connect flowlines and control
umbilicals on two opposing sides where additional templates can still be tied
in (Knudsen, 1999).

Figure 1. General layout
of the Gullfaks field

The Gullfaks A, B and C are 4-shaft  GBS (gravity-based-structure) platforms,
however platform B has no oil storage facilities. Platforms A and C control the
fields Gullfaks South, Rimfaks, Skinfaks and Gullveig. These platforms process
and store oil coming from platform B which eventually is transferred to shuttle
tankers. Regarding gas, this is sent to Kårstø north to be treated and then
exported (“Gullfaks oil,” n.d.). Gullfaks A increased its capacity
for gas treatment and reinjection by installing modules for gas treatment and
pipeline pigging (“Gullfaks oil,” n.d.). This platform also has
production flowlines linked from the Rimfaks satellite which comprises of 3
subsea templates (“30 years,” 2016). Figure 1 shows
the general layout of the Gullfaks fields.

Subsea concept

Production of the Gullfaks underwent a
significant decrease back in 2010. However, during 2011 and 2012, wells were
repaired and water injection was implemented with satisfactory results. The
latter has actually been the prime reason for the high recovery factor (“30
years,” 2016). Later in 2015, the world’s first subsea wet gas compressor
was acquired and the overall production of the field has been raised by at
least 16MMbo since then (Vella, 2017). To date, the goal resides in increasing
such factor from 59% to 62% as well as summing efforts aiming for stretching the
Gullfaks’ production horizon towards 2040 (Vella, 2017).

The Gullfaks South, Rimfaks and Gullveig
satellites are part of the main Gullfaks field (located at south and
south-west) and have contributed to the recovering of reserves in two phases;
Phase I and Phase II. The former, approved in 1996 by the Norwegian authorities
and also called the oil phase, is
dedicated to the production of oil and condensate, whereas the latter mainly covers
gas reserves from Gullfaks South and Rimfaks (Knudsen, 1999). The three
production platforms; Gullfaks A (1986), Gullfaks B (1988) and Gullfaks C
(1989) are installed in these fields and have substantially contributed to the
recovering process of reserves (“Gullfaks oil,” n.d.). Eventually,
some other satellites have been linked to the Gullfaks. For instance, a
production system belonging to the Skinfaks satellite, which is located in the
Tampen area and whose production started in 2007, is linked to the templates in
Gullfaks South (“30 years,” 2016). It is also the case of the
satellites Gimle and Gullveig whose their production began in 2006 and 1998,
respectively (Vella, 2017).

The Gullfaks fields are located in block
34/10 on the Norwegian continental shelf, 175 kilometres north-west of Bergen in
water depths between 135 and 220 meters (Knudsen, 1999). Initially, recoverable
reserves of the Gullfaks fields were estimated in 200×106
Sm3 of oil and 23×109 Sm3 of gas, whose
their main reservoir is the Brent sandstone followed by the Cook and Statfjord
sandstones (Grinrod et al., 1988). Gullfaks production started in December
1986.

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