SUBSEA
taken to the vehicle’s storage facility.
Samples are carried to a second location with temperature and pressure
intact, ensuring safe delivery for laboratory analysis.
Subsea fuid sampling provides insight into process conditions at the
time of a sample collection. The objective is to collect a representative sample which can be defned as having its
physical or chemical characteristics
identical to the fuid properties being
sampled. This enables the adjustment
of SMPFM calibration, bringing many
benefts to operators.
Accurate data and detailed knowledge of the reservoir enables effcient
production on brownfeld and greenfeld developments. For felds that have been
in production for more than 10 years with life
extension capacity, the need for accurate data
is vital. This fact, connected to the uncertainty
of accuracy in the data currently available or
simply the lack of detailed knowledge, refects
the need to acquire reliable data to support
EOR programs.
Subsea sampling module
The subsea sampling module (SSM) is ROV
operated, providing connections to the subsea
sampling interface (SSI). It operates the SSI
pressure barriers and captures the required
fuid samples by pumping the fuids from the
production fow loop or sample point into the
sample bottles. It is designed to capture representative samples of the three phases (oil, water,
and gas) in isobaric and isothermal conditions.
It can meet different requirements in quantity in
one subsea deployment, and for use in a wider
range of applications.
The SSM is compatible with ROV interface for fexibility of differing sampling applications such as fow rates, water cuts, gas
volume fraction (GVF), and viscosities.
Subsea sampling interface
The subsea sampling interface (SSI) is installed in the subsea tree or manifold to capture sample fuids upstream of the SMPFM. It
can be installed as a hardware structure or be
included in a retrievable fow control module
(FCM) – SMPFM and choke module – on the
SPS. In this case, the sampling interfaces, barriers, etc., are packaged in a dedicated fuid sampling FCM with provision for interchangeability
to standard production FCM. It is designed to
capture a representative sample of each phase
(oil, water, and gas) in a wide range of fow
conditions, taking into consideration fowrates,
phase distribution, and physical properties.
The SSI includes sampling lines that tap
into the production fow path, and the re-
motely activated valves, 2 or 4 on each sam-
pling line, depending on requirements, all
of which are fail-safe in the close position.
The ROV provides the connections between
the SSI and the sampling skid. The SSI can
be confgured for vertical access sampling,
as is normally required for manifolds, or for
horizontal access as the preferred option for
integration into an SPS tree.
Sampling operation is done in a steady condition to secure representative samples over a certain time period in the fow-loop. This ensures
sample accuracy in case of unstable fow and
provides the volume needed to perform laboratory analysis topside. The SSM can house 9 to
12 sample bottles, based on project specifc requirements. To maximize value on the sampling
operations is to incorporate the subsea fuid sampling program into the periodic subsea intervention operations to reduce the high cost of vessel
entry and ROV deployments.
While there are benefts, there are still
limitations to their deployment, recovery
methods, payload ratings, dexterity, and
available power sources. Equipment for retrieval and deployment can be mounted on
the ROV, allowing for greater payload and
complexity, especially in the case of the subsea sampling operations. This could restrict
access due to the physical size of the combined ROV and sampling skid during operations. However, research on ROV manipulation and deployment is ongoing for subsea
intervention operations. By exploring cost
effective solutions, such as the virtual fuid
sampling model, the subsea industry can
manage the challenges faced by reservoir
production uncertainty.
Integrated virtual model
The development of an integrated fuid
sampling model was based on virtual compositional fuid tracking results. This captures
the essential elements of the simulation
model to compare both the input and output
of simulated results to check and verify the
performance of the SMPFM.
This development is a cost-effective sub-
sea reservoir fuid sampling approach
to reduce the frequency of operations
in retrieving a subsea sample with the
potential to reduce the costs of interven-
tion and risk of exposure to the subsea
environment. To achieve operational
success with the integrated virtual fuid
sampling model, an optimized novel
sampling strategy was developed for
deepwater feld development.
This sampling strategy is required
on each individual production well, depending on the pressure profle of the
well during early-, mid-, and late-life of
subsea feld operations. The integration
and benefts of applying this strategy
add value to the virtual compositional
fuid tacking model application. The vir-tualfuid sampling model would be a useful
predictive toolfor operators and regulatory
authorities to manage the challenges on subsea fuid sampling operations for accurate
understanding of the reservoirs and impact on
production facilities.
A deepwater feld off West Africa validated
the results in a case study. It showed a simulated pressure profle with compositional
fuid tracking, which was compared with the
experimental base pressure data from a test
loop facility.
The validation provides accurate PVT and
compositions of reservoir fuid properties of
the subsea tree. A transient multi-phase fow
simulation environment was selected to develop
the virtual fuid sampling model, capturing the
essential building blocks of the SPS and simulations to test the model.
Pressure results of the test show that the
convergence of pressure drops from 726.6psi
( 50 bar) down to 725 psi ( 49. 9 bar) over 20,000
seconds. Comparing the simulated numerical
results with experimental results, the pressure
at 730 psi ( 50. 3 bar) shows that at 300 m (984
ft) of pipeline length, the simulated results
have a 3% drop in pressure from the experimental result. This is from the relatively low
pressure due to slip mode effect, liquid hold
up, slugging, or turbulence, in the multi-phase
fow caused by pressure fuctuations. Also, as
the pressure drops to 715 psi ( 49. 2 bar) along
the pipeline length of 2,650 m ( 8,694 ft), the
simulated pressure was in phase with the
experimental pressure. This is due to steady
conditions of the multi-phase fow. So at a 50%
drop of total pressure, the pressure is representative of the experimental results.
A convergence test analysis shows a pressure
drop of 700 psi ( 48. 2 bar) at the 4,700 m ( 15,420
ft) along the pipeline. The simulated pressure
exhibited the same result with less than 2% slip
mode effect of the fuid compositions on multi-phase fow. This demonstrates that both results
are represented with the pressure trend. Therefore, acquired results of the validation of the
Convergence tests on the simulated numerical pressure
results at 5s, 10s and 20s time interval over 6,000 m ( 19,685
ft). (All graphs courtesy Eni)
