Portable Dynamic Positioning Systems
(PDPS) White Paper
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> Portable Dynamic Positioning Systems (PDPS) White Paper
Since their introduction in the early 1960's, dynamic positioning
systems continue to become more commonplace in the offshore industries.
The first dynamic positioning systems were used exclusively for
deep sea drilling applications. As these systems became more
reliable and more affordable, they found their way into many
other applications, including pipe laying, cable laying, offshore
construction, supply boats, crane vessels, diving boats, salvage
vessels, ROV support, offshore tanker loading, workover rigs
and seismic vessels, to name a few. This paper discusses Thrustmaster’s
portable dynamic positioning system, allowing vessels of opportunity
or any floating structure to be easily retrofitted or temporarily
equipped with dynamic positioning capability.
A dynamic positioning system is a computer controlled propulsion
system allowing a vessel to maintain its position in open waters
against wind, waves and current. The system consists of computer
controlled thrusters whereby the computer calculates and
controls the amount and direction of thrust necessary to counteract
wind, wave and current forces in order to prevent or correct
deviation from the desired position and heading of the vessel.
Position reference sensors, combined with wind sensors, current
sensors and gyro compasses, provide information to the computer
pertaining to the vessel's position and the magnitude and direction
of environmental forces affecting its position. The computer
program contains a mathematical model of the vessel that includes
information pertaining to the wind and current drag of the vessel
and the location of the thrusters. This knowledge, combined with
the sensor information, allows the computer to calculate the
required steering angle and thruster output for each thruster.
A combination of fixed main propellers with tunnel thrusters
is often used in applications where heading is unimportant to
the mission of the vessel. This allows the DP system to point
the bow towards the brunt of the environmental forces using the
main propulsion to counteract those forces while the sideways
holding force is controlled by the transverse tunnel thrusters.
Quite often, a fixed heading without fish-tailing is required.
The heading may be dictated by mission requirements, rather than
weather conditions. For those applications, a system utilizing
multiple azimuthing thrusters is much more appropriate. It allows
the computer to not only control the magnitude of the thrust
vectors, but also their angular direction. By doing so, the computer
can utilize each thruster most effectively, permitting the designer
to accurately define thruster capacity for a vessel operating
in a defined sea state.
The most common
and most efficient thrusters used for dynamic positioning applications
are azimuthing propeller thrusters using large diameter propellers
in Kort nozzles. These thrusters typically develop 25 to 30 pounds
of net bollard thrust per delivered horsepower. A comparison
to jet thrusters, which produce only 11 to 14 pounds of thrust
per horsepower, explains why jet thrusters are less practical
for use in DP applications. Jet thrusters require more than twice
the installed horsepower and use more than twice the fuel of
equivalent propeller thrusters.
Accurate control of thruster output is essential in DP applications.
When using a fixed pitch propeller, thrust is controlled by controlling
the propeller speed. The computer must be able to control propeller
speed from zero RPM to full speed. Direct diesel engine drive
through a fixed reduction is not suitable as the usable engine
speed range does not cover slow operating RPM's resulting in
unstable control of the vessel. Many of the existing DP systems
use DC electric motor driven propeller thrusters. The drive system
includes diesel generators, SCR controllers and variable speed
DC electric motors driving the propeller shaft through right
angle gear transmissions. Overall transmission efficiency from
diesel engine to propeller is typically in the range of 75 to
80 percent at full load and about 70 to 75 percent at half load.
Variable frequency AC drive systems have similar characteristics.
Azimuthing thrusters using controllable pitch propellers are
used on some high horsepower applications. Thrust output is controlled
by pitch adjustment, so the thrusters do not require a variable
speed drive system. While controllable pitch propellers provide
acceptable reliability in large ship propulsion systems, incorporating
the complex propeller pitch control system in azimuthing
thrusters presents a serious challenge, as pitch controls and
feedbacks have to go through complex slipring mechanisms to facilitate
the 360° steering of the propeller. The system contains
many hundreds of moving parts that have limited life and reliability
when used in a demanding DP system requiring continuous pitch
adjustments. While drive efficiency is high ( 90 to 95 percent),
the large propeller hub and poor pitch distribution of the blades
result in lower propeller efficiencies than those obtained with
fixed pitch propellers.
Most thrusters used on DP systems are mounted through-the-hull
in a well mounting configuration. Propeller and nozzle extend
below the baseline of the vessel to ensure clean water to the
propellers at any steering angle. Many installations use retractable
thrusters allowing the vessel into shallow water ports without
danger of running the thrusters aground.
Vessels of Opportunity
Some of the older offshore oil fields have become quite congested.
The Gulf of Mexico along the Louisiana coastline is a good example.
There are numerous platforms and sub-sea structures. A network
of pipelines and cables cover the bottom like a spider web. There
is always new construction, removal of old structures, repair
or modification of existing structures or pipelines involving
diving and ROV work. Mooring systems using anchors and winches
become less practical as the area becomes more congested. Anchors
tear up pipelines and cables. Moreover, the activity moves to
deeper water where anchor mooring systems are no longer cost
effective or practical. Dynamic positioning is the answer. However,
DP vessels are still few and far between and have high day rates.
Additionally, the available vessels with DP capability may not
be suitable or optimum for the mission at hand. Vessels or barges
that are just right for these missions could be retrofitted with
a dynamic positioning system. But these upgrade conversions are
very expensive and time consuming. Thruster installations require
extensive hull modifications in dry dock. Adding generator sets
and electric motor speed controls requires additional machinery
space which may not be available.
A Portable Dynamic Positioning System
Thrustmaster of Texas has developed a portable dynamic positioning
system consisting of modular, deck mounted, azimuthing thrusters
with separate hydraulic power units and a DP control console.
The whole system can be installed dockside, takes a minimum of
deck space and does not require any permanent vessel modifications.
Installation can be completed within days.
The system normally consists of four or more thrusters, four
or more hydraulic power units, one (1) central DP console and
the interconnecting hydraulic hoses between thrusters and power
units, and electrical control cables between power units and
console. Standard thruster sizes are 250 HP, 500 HP, 1000 HP,
1500 HP and 2000 HP. Using multiple units, systems ranging from
500 HP (2 x 250 horsepower) up to 24,000 horsepower (12 x 2000
horsepower) can be configured.
The portable thrusters are mounted on deck using a minimum
amount of deck space. They use direct hydraulic drive to the
propeller. The variable speed hydrostatic drive motor is in the
lower foot of the thruster directly in line with the propeller
shaft. This direct hydraulic drive eliminates the need for right
angle gear transmissions and drive shafts used on other thrusters.
Hydraulic hoses run from the deck mounted upper thruster assembly
down to the propulsion motor in the lower foot of the thruster.
The thruster stem contains these hydraulic hoses. There are no
moving parts in the thruster stem, other than the hydraulic fluid
running through these hoses. This makes for an extremely simple
and reliable thruster design. It allows mounting on deck without
intermediate stem support. The upper structure is designed to
handle the omni-directional cantilever moment from the thruster
in much the same way as a deck crane handles the cantilever moment
of its load. The stem length can easily be adapted to accommodate
different vessel depths.
The closed loop hydraulic drive of the propeller is highly
efficient. Transmission efficiency from diesel engine to propeller
is typically 80 to 82 percent at full load. It even increases
at partial load. Half-load transmission efficiency is typically
82 to 85 percent. Since DP systems run at partial load most of
the time, the partial load efficiency is of particular importance.
The hydraulic system acts as a vibration dampener. Propeller
induced vibrations and engine induced vibrations are dampened
by the hydraulic system and isolated from one another. There
are no torsional or lateral critical speeds within the operating
range of the equipment. The drive is extremely smooth.
Propeller speed is infinitely controlled through control of
pump displacement. The DP computer accurately controls thruster
output by comparing its electrical output signal to the pressure
feedback signal from the hydraulic drive. Hydraulic pressure
is directly proportional to propeller torque, so the feedback
accurately represents delivered thrust, unlike speed feedbacks
used on most older systems.
Hydraulic systems are extremely reliable, provided they are
properly designed and the hydraulic fluid stays clean and cool.
An appropriately sized heat exchanger on the power unit keeps
the fluid cool while charge filters, return filters, and flushing
filters keep the fluid at a high level of cleanliness. Pressurized
breathers virtually eliminate air ingestion to keep the moisture
out of the system.
Portable Hydraulic Power Units
Each thruster is powered by its own hydraulic power unit.
These power units are enclosed marine type hydrostatic
transmission units using a radiator cooled Caterpillar diesel
engine as prime mover. Some of these units use a standard 20
foot or 40 foot ISO container as enclosure. They may be installed
at any location based on deck space availability or optimum weight
distribution. The units are complete with fuel day tank, independent
battery powered electric start and control system with automatic
alarms and shutdown and are provided with critical grade muffler
and noise attenuating equipment. These power packs produce the
hydraulic power for the propeller drive system as well as hydraulics
for steering and auxiliary functions. This hydraulic power is
provided to the thruster through hydraulic hoses that run on
deck from the power unit to the thruster.
Portable Control Console
The systems use a Kongsberg or equivalent DP computer system
designed specifically for use with Thrustmaster's hydraulically
driven azimuthing thrusters. It is installed in a portable control
console for indoor installation on the bridge. Optionally,
Thrustmaster can provide a complete integrated control van with
DP System, manual controls, gyros, wind sensors, DGPS antenna,
etc. all installed and pre-wired. The display provides
a centralized report to the operator of all aspects of vessel
control. Thruster activity is reported in animated form as are
heading and position control activity and all incoming sensor
data. Selector keys are provided for DP or travel mode, screen
select and alarm acknowledgment. Joystick controls on the console
allow independent manual control of vessel heading and position.
The control panel houses the control computers and signal
processor units and the interface electronics that connect to
the thrusters and vessel sensors.
The DP computer program features sensor interface software
and a vessel mathematical model which is custom configured for
each installation by user definable parameters. The parameters
define vessel characteristics such as dimensions, displacement,
thruster locations, etc., and the type of position reference
systems and vessel sensors that are connected to the system,
such as DGPS, hydro-acoustic, laser, radio ranging, wind and
gyro. The sensor suite can be supplied by Thrustmaster or the
customer's own sensors can be used.
Installation and Startup
A Thrustmaster representative is required during mobilization
and demobilization of the system. This representative uses a
laptop computer for program fine tuning, troubleshooting and
performance analysis. At initial startup, main line hydraulic
filters are temporarily installed in conjunction with a portable
particle counter for on-line sampling of the hydraulic fluid.
The particle counter prints a fluid analysis every five minutes
complete with ISO and NAS cleanliness levels. The temporary flushing
filters are not removed until a cleanliness level of ISO
15/12 (NAS Class 6) is obtained. This is better than aircraft
quality. A number of equipment manufacturers, including SKF,
have found through endurance testing that life of rotating machinery
with small clearances, like ball bearings and piston pumps, is
dramatically increased when lubrication filtration levels are
increased. Cleanliness of hydraulic fluid is by far the most
important factor affecting life and reliability of a hydraulic
system. There are many documented cases where hydrostatic transmissions
exceeded 100,000 operating hours without interruption.
Depending on application requirements, additional thruster
units and a duplicate DP computer may be required to provide
the necessary redundancy. It is the user's responsibility to
clearly define redundancy requirements of the DP system.
Classification Society certification by ABS or DNV is available
up to DPS-3.