Page 22 - Handbook Of Multiphase Flow Assurance

16 1. Introduction

line was considered among the four likely root causes for the condensate leak and fire which
sank the platform (Cullen, 1989), with 167 fatalities.
In other situations, the disaster caused by solid blockages leading to a pipe rupture and a
release of hydrocarbons may be narrowly averted. In one FPSO in West African waters, an ice
blockage formed in a flare relief line when a cold gas stream and a warm gas stream carrying
water moisture combined, which led to a rupture and formation of an explosive gas cloud,
but the wind on that day was blowing away from the furnaces.
In another example from US deepwater, nature may intervene to help remove a blockage,
such as when a hurricane cleared a hydrate blockage. A deepwater chemical injection sys-
tem methanol line connected to a scraping crossover valve got plugged as produced fluids
hydrocarbon and water back-flowed past a checkvalve into the methanol line and formed
a hydrate. The methanol line remained plugged until a hurricane led to an evacuation of
the platform. This, in turn, triggered an automated opening of the scraping crossover valve.
Warm water, accumulated behind the crossover valve in a dead leg of an actively heated
flowline, flowed through the crossover valve and past the methanol line plugged with hy-
drate. The warm water flow initiated in the automated response to the hurricane heated
and cleared the methanol line of hydrate, which was understood upon the restart of the
field. This example teaches us to seek sources of energy available and accessible to over-
come a blockage.
In a yet another example, an onshore oil field in Siberia experienced multiple hydrate
blockages in early summer within months of being put on production because the initial wa-
ter cut was low (under 5%), and methanol was not being injected. Additional resources were
provided to supply methanol to treat the produced fluids. However, this was costly as meth-
anol had to be airlifted by a helicopter during the summer months because the roads were
impassable. A storage facility was then constructed to provide methanol supply through the
year, but a longer-term ingenious solution was implemented, which came from a local tech-
nologist, who suggested to convert one of the several producer wells to a water-producer
by re-perforating the well in the aquifer zone. Heat carried by water from one well was suf-
ficient to keep produced fluids from all the wells warm and outside of the hydrate stability
region. This was possible because all wells were equipped with ESPs for artificial lift. This
example not only induces us to seek the nearby energy sources which can be used for flow
assurance but also shows that operators of the field have a better understanding of the field's
capability and should be consulted with during concept evaluation.
Blockages in onshore wells and flowlines are more routine and are much less costly to
deal with. In one onshore field in North America, partial hydrate blockages occurred in
wells nearly daily during the cold season, and were cleared promptly by methanol injection
from a pump truck.
Engineers and chemists perform various analyses of properties for reservoir fluids,
including hydrocarbons, water and gas. In some cases, the fluids are not sampled ade-
quately, and some properties, such as the presence of H 2 S or mercury, may be not noticed
until after the startup. Retrofitting a facility to take care of such problems, if they are not
known at first and discovered later, is both costly and time consuming. Proper sampling
is the foundation on which the good flow assurance design and production chemistry
selection are based.