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When developing and producing Flowline status analysis
deepwater oil fields, flow
assurance aspects always Investigation was performed with two different
assume critical importance methods. First one applied was Eni patented
to grant uninterrupted and Primeflo, pressure signals transient inspection:
economical operations working principle is based on the generation of a
fast and sharp pressure input signal, on a media
of subsea production systems. Gas injection preferably full of liquid, and subsequent record
of associated system response. High resolution
system herein discussed, characterized by pressure behavior is then compared to flowline
simulated signal, in order to qualitatively reproduce
light hydrocarbon high pressure flow and low observed characteristics of the real trace and
interpret them. Primeflo highlighted a combination
temperature environment, may be affected by of partial restriction plus gas pocket at dynamic
riser touch down point (600 m downstream of
hydrate formation in case proper preventive actions FPSO, with vertical section full of methanol) and
a complete blockage in proximity of subsea XT
are not taken. (figure 2).
Injection system, composed by a flexible flowline Gas injection system herein discussed,
characterized by light hydrocarbon high
and two layers intelligent completion well, showed
pressure flow and low temperature
since life inception poor performances, probably environment, may be affected by
hydrate formation in case proper
due to sand control issues not allowing to reach preventive actions are not taken.
designed rates, with detrimental effects on Further diagnostic assessment was performed
with support of Remotely Operated Vehicle (ROV)
reservoir pressure support and production. In order deployed gamma ray scanning tool. Method
principle relies on irradiating flexible line with a
to reduce the skin, remedial acid treatments in bull- gamma ray source and recording intensity of output
radiation, through a receiver frame diametrically
heading were carried out from Floating Production placed on opposite side, to infer nature of content
from calculated average density. Gamma ray
Storage Offloading unit (FPSO) along subsea scanning completed Primeflo interpretation of
line status in proximity of riser touch down point,
flowline. After last pumping operation, flowline displaying gas presence beside a zone which
content density was consistent with hydrate.
was found to be completely obstructed, with lost Additional information was also obtained when
possible hydrate lenses were observed along the
communication between FPSO and subsea Xmas flowline, not only nearby XT, but also scattered in
the horizontal section (figure 3).
Tree (XT). Having well section of injection system
not delivering planned rate and flowline section with
compromised functionality, two parallel activities
were established:
• mobilize a Mobile Offshore Drilling Unit (MODU)
to re-enter well;
• assess completion status, while flowline
diagnostics was ongoing to identify location
and nature of blockage (figure 1).
Removal strategy
Considering flowline status, with no hydraulic
communication and possible presence of
several hydrate plugs, depressurization below
hydrate formation curve (15 bars at 5 °C sea bed
temperature) was the only way to recover system
functionality. Taking advantage of MODU presence,
a three level action program was planned to
intervene on injection line.
Fig. 1 – Injection System
Layout
Impiantistica Italiana - Settembre-Ottobre 2015 51
