Marine Oil System (FPSO)
Marine Oil System (FPSO)
Introduction:
The diesel oil system has as its purpose the receiving, storage,
purification and distribution of diesel oil to the various users on the FPSO.
It is also capable of back loading diesel oil from the FPSO to Supply Vessel.
It comprises of the diesel oil storage system, the purifying and the distribution
system.
The Ships marine diesel engines run on Heavy Fuel Oil (HFO),
nowadays low sulfur content, or Marine Diesel Oil (MDO) or Marine Fuel Oil
(MFO).
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DIESEL OIL (BUNKERING HEATING SYSTEM, COURTESY GESAB FPSO) |
The diesel oil system has as its purposes are Receiving, Storage and Purification and distribution of diesel oil
to the various users on the FPSO.
The main functions of the diesel oil
system is to provide purified diesel oil to topside main turbo generator
packages (A/B/C); port and starboard (packages D,E,F); Fire water pump day
tank systems, Inert gas generators, Hull control Emergency/ Essential diesel generator tanks,
life boat refueling, crew boat refueling and fast rescue craft refueling
etc.,
The fuel oil system for a marine diesel engine can be considered in two parts—the fuel supply and the fuel injection systems.
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FLOW DIAGRAM |
Fuel supply
deals with the provision of fuel oil suitable for use by the injection system.
Marine Fuel oil system includes various piping systems provided for bunkering,
storage, transfer, offloading and treatment of fuel oils.
Fuel oil transfer system
This system receives and stores fuel and
delivers it to settling tanks. Fuel oils are loaded through deck fill
connections that have sample connections provided to permit the fuel to be
sampled as it is taken aboard. HFO is loaded in storage tanks fitted with
heating coils.
In preparation for use, HFO is transferred to the fuel oil settling tanks via
FO transfer pumps which are equipped with a suction strainer. Piping is so
arranged that the pumps can transfer fuel between storage tanks and then to the
deck connections for offloading. Settling tanks are used to permit gross water
and solids to settle on the bottom.
Fuel tank overflow system:
All tanks overflow to an overflow tank via a
line with an observation glass. This line also incorporates a flow alarm.
Fitted in the overflow tank is a level alarm which will be activated when the
tank is a quarter full.
All tank vents are fitted so that oil cannot overflow onto deck or into
machinery spaces which may lead to fires. The vent from the overflow tank is
led onto deck and fitted with wire gauze diaphragms.
Fuel oil supply for a two-stroke diesel engine:
A slow-speed two-stroke diesel
is usually arranged to operate continuously on heavy fuel and have available a
diesel oil supply for manoeuvring conditions.
In the system shown in Figure, the oil is stored in tanks in the double bottom
from which it is pumped to a settling tank and heated. After passing through
centrifuges the cleaned, heated oil is pumped to a daily service tank. From the
daily service tank the oil flows through a three-way valve to a mixing tank. A
flow meter is fitted into the system to indicate fuel consumption. Booster
pumps are used to pump the oil through heaters and a viscosity regulator to the
engine-driven fuel pumps. The fuel pumps will discharge high-pressure fuel to
their respective injectors.
The viscosity regulator controls the fuel oil temperature in order to provide
the correct viscosity for combustion. A pressure regulating valve ensures a
constant-pressure supply to the engine-driven pumps, and a pre-warming bypass
is used to heat up the fuel before starting the engine.
A diesel oil daily service tank may be installed and is connected to the system
via a three-way valve. The engine can be started up and manoeuvred on diesel
oil or even a blend of diesel and heavy fuel oil. The mixing tank is used to
collect recirculated oil and also acts as a buffer or reserve tank as it will
supply fuel when the daily service tank is empty.
The system includes various safety devices such as low-level alarms and
remotely operated tank outlet valves which can be closed in the event of a
fire.
Operation on Heavy Fuel Oil
Main engines designed to manoeuvre on heavy fuel oil are to be operated
according to the manufacturer’s instructions. All other types of main engines
are to be manoeuvred on diesel oil according to the manufacturers’
instructions.
In the event of problems during manoeuvring on engines using heavy oil there
must be no hesitation in changing over to diesel oil irrespective of whether
the engines are being operated using bridge control, or using engine room
control.
It is the Chief Engineer's responsibility to inform the Master of the
particular engine type’s maximum period that it can safely remain in the
stopped position. He is also to inform the Master of the procedures which will
have to be carried out if the particular engine type’s maximum period at
standstill during manoeuvring is exceeded.
Fuel injection
The function of the fuel injection system is to provide the right amount of
fuel at the right moment and in a suitable condition for the combustion
process. There must therefore be some form of measured fuel supply, a means of
timing the delivery and the atomisation of the fuel. The injection of the fuel
is achieved by the location of cams on a camshaft. This camshaft rotates at
engine speed for a two-stroke engine and at half engine speed for a
four-stroke. There are two basic systems in use, each of which employs a
combination of mechanical and hydraulic operations. The most common system is
the jerk pump; the other is the common rail.
Various safety devices in a fuel system for a diesel engine are:
- Quick closing valves
on settling/service tanks
- Relief valves on 2
pumps/heaters
- Quick closing valve
on mixing/vent tank
- Pipes
lagged/save-alls under pumps and heaters
- Low fuel oil pressure
alarm
- High fuel oil
pressure alarm
- Low fuel oil
temperature alarm
- High fuel oil
temperature alarm
- Emergency remote
stops for pumps
- High pressure pipes
between fuel injection pump and injector are double skinned.
Pipes carrying fuel oil and flammable liquids:
There are two principal types of pipes that carry fuel and they are categorized by the pressure the pipe is designed to withstand. Low-pressure pipes are used
to move fuel from a storage tank to a service tank to an injection pump;
high-pressure pipes are used to deliver fuel from an injection pump to an
engine combustion chamber. Ships’ fuel is usually stored in double-bottom
tanks, deep tanks, side bunker tanks, settling tanks or service tanks. Piping
between a service tank and a fuel transfer or booster pump is rated as low
pressure. However, between each pumping stage, pressure increases.
It is a mistake to assume that even if a pipe’s pressure is relatively low,
fuel will not spray from a crack or small hole. Pipes from fuel tanks can pass
through ballast tanks and pipes serving ballast tanks can pass through fuel
tanks. Because of pollution risks, classification societies have stringent rules
restricting the length of any oil pipe passing through a ballast tank (and vice
versa); it must be short, have increased wall thickness and stronger flanges.
SOLAS Regulation:
The Safety of Life at Sea Convention (SOLAS) includes requirements for fire
safety in engine rooms. In particular, special double-skinned pipes must be
used to deliver fuel to engine combustion chambers. These are made of low
carbon steel alloys and operate at high pressure, between 600 and 900 bar.
Double skins are necessary because pipe fracture will cause fuel to spray in a
fine aerosol.
Fuel will ignite on contact with a hot surface, such as a turbocharger casing
or exhaust pipe. The second skin is to guard against direct spraying. The pipe
is designed so that fuel will be contained in the space between the outer skin
and the main pipe, and will drain into a collecting tank fitted with a
high-level alarm.
Low-pressure lubricating and fuel oil pipes passing close to a hot surface have
to be secured against the possibility of oil spraying from a flange. To prevent
this, the flange is usually taped. In addition, and whenever possible, the
pipes are routed clear of hot surfaces. Similarly, to prevent leaking oil
falling onto a hot surface, pipes should never be allowed to run above a hot
surface. Regular thermographic surveys of hot surfaces will identify those risk
areas that are sufficiently hot to ignite spraying or leaking fuel. Preventive
measures to be taken include additional lagging, spray or drip shields.
Two reasons why fuel pipes should be clipped and supported are to prevent
stress and fractures from vibration.
Fuel oil transfer pipes are usually mild steel and may corrode. The calculation
for minimum wall thickness includes a small allowance for corrosion. As a pipe
ages and corrodes, leakage can occur. Inspection programmes should concentrate
on identifying worn or corroded pipes.
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