DP Conversions of Existing Vessels
with Thrustmaster's Modular DPS
Abstract
As older offshore production areas become more congested and
new exploration moves to deeper waters, the demand for
dynamically positioned vessels, barges and platforms increases
rapidly. Rather than scrapping older vessels and replacing
them with DP-capable newbuilts, many existing vessels and rigs
can be upgraded for DP at a fraction of the cost of replacement.
A good DP system uses multiple azimuthing thrusters with either
variable speed, fixed-pitch propellers or fixed-speed, controllable
pitch propellers. Either way, the system is complex and
comprises many components and subsystems. Proper integration
of all parts of the system requires a good understanding of the
interdependence of all critical system components. Acquiring
and installing the sub-systems in an existing hull is always
challenging and often requires major vessel modifications requiring
extensive design work and lengthy dry-docking.
This paper discusses a different approach to these challenges.
The packaged system approach introduced here uses standard, pre-engineered
modules provided by a single source as an integrated, factory-tested
system. This system is designed to allow installation on
any vessel, barge or platform up to 600 ft. without the need
for modifications or dry-docking.
Real-world experiences with the implementation of this approach
on a number of different vessels and barges are discussed.
A detailed description of the system is provided including DP
system, deck-mounted thrusters and deck-mounted diesel-hydraulic
power units. Thruster-to-vessel interface is discussed,
including the effects from propeller ventilation, vessel motions,
wave action and slamming impacts.
An overview is presented on how such a system satisfies Classification
Society Rules for DPS-1, DPS-2 and DPS-3, including Failure Modes
and Effect Analysis (FMEA) and Power Management System (PMS).
Introduction
Adding dynamic positioning capability to an existing vessel
generally involves adding thrusters and machinery to power and
control the thrusters. Ideally, the thrusters will allow
true thrust vectoring whereby the thrust force can be steered
to any horizontal direction and the amount of thrust force can
be set at any magnitude between zero and maximum thrust (Ref.
1). This can be accomplished by using azimuthing thrusters
with controllable-pitch propellers running at constant speed
or by using azimuthing thrusters with fixed-pitch propellers
running at speeds that can be freely controlled from zero to
maximum RPM. In recent years, several DP specialists have
addressed the relative merits of the two concepts and concluded
that fixed-pitch at variable speed is preferable over controllable-pitch
at fixed speed (Refs 2, 3, 4). The author of this paper
concurs with that conclusion.
Most of the DP conversions in recent years have involved installation
of retractable thru-hull azimuthing thrusters with diesel-electric
drive. Those conversions required the creation of additional
machinery space, acquisition and installation of diesel-generator
sets, SCRs or variable frequency controls, and electric motor-driven
retractable thrusters. The projects required substantial
planning, engineering and coordination. It required that
the vessel be taken out of service and put in dry dock for an
extended period of time. Let’s take a look at how
these conversions are currently done.
The Conventional Method of DP Vessel Conversion
A typical vessel conversion to DP capability starts out with
several studies. These include:
- Definition of Vessel Capability Requirements
- Determination of equipment and machinery to be added to meet
capability requirements
- Determination of where to install the new machinery (new
thrusters, generators, control cabinets) and how to create space
for it.
- Stability analysis based on removal/addition of weight and
changes in centers of gravity and vessel rotation.
- Preparation of project proposal with budget and time line.
The next step is procurement of long lead items and contracting
with a shipyard to modify the vessel and install the new machinery
and equipment.
These conversions are costly. In addition to equipment
purchases and installation costs, the support systems are a major
cost component. They involve lots of shipyard work, including
installation of ventilation blowers and ducting for the new machinery
spaces, ladders and platforms, fire walls and fire protection
equipment, bilge water pumps and piping, thruster wells, electric
starters for hydraulic steering pumps, diesel engine starting
systems, fuel storage and treatment and transfer systems, exhaust
piping and mufflers and stacks, thermal and noise insulation,
cooling water pumps and piping, seachests, alarm systems, machinery
controls, power management system, and lots of plumbing, cabling
and wiring. And then there is the cost of engineering and
project management and obtaining class approvals for all these
vessel modifications.
Here are a few examples of such conversions
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- Figure 1 - The Global Explorer
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- Figure 2 - Retractable Tunnel
Thruster
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- Figure 3: Elkhorn River
Thrusters
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- Figure 4: Elkhorn River
Thrusters
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- Figure 5: Retractable Azimuthing
Thruster with Electric Drive
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- Figure 6: Retractable Azimuthing
Thruster with Hydraulic Drive
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- Figure 7: Cuss I
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- Figure 8: Packaged DP System
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- Figure 9: Arctic Discoverer
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- Figure 10: Arctic Discoverer
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- Figure 11: Arctic Discoverer
thruster
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- Example 12: Thai cable-lay
barge
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- Example 13: Seabeach
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- Figure 14: NAWC 38
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- Figure 15: Propeller ventilation
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- Figure 16: Long Stems
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- Figure 17: Pirelli Cable
Lay Barge
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- Figure 18: Pirelli Barge
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- Figure 19: Ulises
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- Figure 20: Ulises
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- Figure 21: Nippon Salvage
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- Figure 22: Titan 2
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- Figure 23: Titan 2
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The Global Explorer is a 279 ft. cargo vessel recently
converted to a multi-purpose offshore construction support vessel
(Fig. 1). The conversion included addition of a DPS-2 System.
This involved acquisition and installation of an Alstom DP System,
three 715 kW and one 1440 kW Caterpillar generator sets, SCR
systems, and DC electric motor-driven thrusters provided by Thrustmaster.
The two stern thrusters are 1100 HP azimuthing L-drives, one
of the bow thrusters is a 66-inch, 1000 HP tunnel thruster, and
the other bow unit is a 1000 HP retractable combination thruster
which is freely azimuthing in the lowered position, while functioning
as a tunnel thruster in the retracted position (Fig. 2).
Each thruster is driven by a GE 752 variable-speed DC traction
motor.
Installing the DP computers was easy. Finding a place
for the generators, SCRs and thrusters and then installing that
equipment with all its interconnecting buses, cables, controls
and ancillary equipment was a complicated, time-consuming and
expensive project. Some of the work was done dockside,
but installation of the thrusters could only be done in dry dock.
Another recent conversion was the Elkhorn River,
a 218 ft. PSV that started out as a 178 ft. twin-screw OSV (Fig.
3). Besides a 40 ft. mid-body extension, the conversion
included adding a Kongsberg-Simrad DP System and two Thrustmaster
500 HP retractable combination thrusters, one in the bow and
one in the stern skeg between the two shaft lines (Fig.4).
Each thruster uses a Cummins KTA 19M diesel engine driving a
hydrostatic transmission that powers the podded hydraulic propeller
drive.
The vessel has a shallow hull depth of only 15 ft. and is
frequently used on projects in very shallow water. On those
projects, the thrusters are retracted and work in tunnel mode
on DP in combination with the CP main screws and rudders.
When working in deeper water, the thrusters are dropped down,
and the 50" props in nozzles are freely azimuthing.
For this vessel, the diesel-hydraulic thruster drive was selected
because it was less expensive, and the diesel hydraulic power
packs are a lot smaller in size and easier to install than diesel
generators with SCRs or VFDs (Fig. 5). This is generally
true for projects where total thruster power is less than 4000
HP and individual thruster sizes are 1000 HP or less, unless
the vessel already has a large electrical plant.
The mid-body extension required lengthy dry-docking, and thrusters
were installed during the same period. The total cost for
conversion and upgrade approached the cost for a new-built vessel.
While expensive, these conversions were economically feasible.
Changing market conditions had rendered the original vessels
obsolete, and they were laid up. The DP conversions turned
those non-performing assets into very marketable vessels with
excellent utilization at high day rates.
So why don’t we see more of these conversions?
- Many vessels are dismissed as viable candidates for conversion
due to lack of space for new machinery or insufficient hull depth
for installation of adequate drop-down thrusters (Fig.6).
- The projects are complicated. A lot of study has to
be done before you can even start working on a rough cost estimate.
A substantial amount of money must be spent, just to find
out if the project is technically and economically feasible.
- The projects are risky. There are many opportunities
for surprises: Design modifications may be based on old drawings
that turn out to be inaccurate; one of the equipment suppliers
may be late causing delays and extra dry-docking time; during
dry-docking for conversion, vessel structure or hull plating
may be found to require replacement; equipment may not fit or
be incompatible with other equipment; major obstructions may
be found when routing new exhaust, ventilation ducts, fuel lines,
cooling water, electrical cables, etc.; the conversion may affect
gross tonnage, changing USCG requirements for manning the vessel;
etc.
In summary, vessel DP conversions are expensive and have great
potential for cost overruns and schedule delays. Is there
a better way?
The Packaged System Approach
As far back as the 1960's, Murray and Tregurtha built a number
of packaged, deck-mounted propulsion units for dynamic positioning
applications (Refs 5, 6). An example is the Cuss I,
a 260-ft. drilling ship for core sampling in up to 12,000 ft.
of water (Project Mohole) (Fig 7). The vessel was fitted
with four diesel engine-driven right angle drives over the side,
one on each quarter.
Following the same basic concept, Thrustmaster developed a
packaged system for DP conversion using deck-mounted over-the-side
thrusters with deck-mounted self-contained power units, DP system,
sensor
suite and all interconnecting cables
and interface for ready installation on any vessel or barge from
100 to 600 ft. long (Fig 8).
The first such system was installed in the late 1980's on
the Arctic Discoverer, a 300 ft. trawler converted to
a dynamically positioned treasure hunter that found the wreck
of the Central America in deep waters off the Carolinas
(Fig. 9, 10). With an ROV, the vessel recovered some $300
million in gold over a period of about 5 years. The system
was comprised of two 500 HP cradle-mounted azimuthing thrusters
stuck to the side shell, two Caterpillar powered hydraulic power
units sitting on the aft deck, and a Robertson DP system with
DGPS.
To effectively produce omni-directional thrust, the thruster
propellers had to extend below the bottom of the vessel.
This required a stem length of over 26 feet (Fig. 11).
In a dynamic environment with continuous vessel motions, varying
thrust forces and wave action, a long stem containing right-angle
gears, drive shafts and bearings has limited prospects of longevity.
This is due to continuous flexing or deflection of the stem as
well as lateral and torsional vibrations of the lengthy drive
shafts at varying RPM and torque. It’s a mechanic’s
nightmare.
To avoid this, we used podded hydraulic propeller drives on
these thrusters. The benefits of podded propulsion have
recently been rediscovered and described in several papers (Refs
7,8,9). While podded electric drives are still relatively
new and unproven (Refs 10,11), Thrustmaster has been building
podded hydraulic drives for many years. A lot of these
units, like the 600 HP retractable azimuthing thrusters on the
DP pipe lay barge Chickasaw, have been operating for
more than ten years without a single failure.
With the podded hydraulic drives, the long stems contain hydraulic
hoses only. There are no moving parts in the stem other
than the hydraulic fluid flowing through these hoses. Flexing
of the lengthy stems has no effect on the drivelines whatsoever.
The DP conversion of the Arctic Discoverer took only
a couple of weeks and was done dockside.
During the 1990's, the packaged system approach was used on
a number of temporary DP conversion projects whereby a vessel
of opportunity was temporarily equipped with thrusters and a
DP system for the performance of a specific contract. Here
you see a deck barge being outfitted in Singapore for a cable-lay
project in Thailand, using 500 HP deck-mounted thrusters and
diesel-hydraulic power units (Fig. 12). Each thruster uses
its own dedicated hydraulic power unit. Hydraulic hoses
run between thruster and power unit using quick disconnects at
the ends. The diesel-hydraulic power packs are very compact,
suitable for outdoor installation and completely self-contained.
A fuel day tank is incorporated in the unit. It also includes
a local control panel for start-up and emergency operation.
Controls use battery power from the engine start system and the
engine has an alternator for battery charging. No vessel utilities
are required: no cooling water, no electrical power, no
air, and no control power. All it needs is daily diesel
fuel refills.
Another temporary DP conversion was the Seabeach,
a 270-ft. vessel chartered for cable lay in the Bay of Campeche
(Fig. 13). Four 500 HP thrusters and HPUs and a Nautronix DP
system were leased as a “Portable Dynamic Positioning System.”
The U.S. Navy leased a pair of 500 HP deck-mounted thrusters
and HPUs with a Simrad SDP-01 for temporary conversion of the
NAWC 38 (Fig. 14). This vessel acquired acoustical
profiles of NATO submarines in the Caribbean.
Western Geophysical used a pair of 500 HP units mounted on
platforms cantilevered off the side shell of
one
of their ocean bottom seismic data processing vessels, the Western
Orient.
A cable repair vessel owned by Delba Maritima in Brazil was
equipped the same way, using two platform-mounted thrusters and
two deck-mounted diesel-hydraulic power units connected to a
Kongsberg-Simrad DP System.
Dynamic Loads on Thrusters and Platforms
When deploying thrusters over the side of a vessel, the propeller
typically extends below the baseline, putting it deep in the
water. Nevertheless, propeller ventilation may occur if
the vessel has significant roll. This is because the thruster
is quite a distance outboard from vessel centerline and thus
has a large radius of rotation from roll. The thruster
travels a substantial amount in vertical direction and the propeller
and nozzle may break the water surface frequently during bad
weather, exposing the drive to heavy shock loading. Fortunately,
a podded hydraulic propeller drive can cope with this without
any harmful effects. The rotating inertia is quite small
and the hydraulic drive dampens the shocks from these load variations
while pressure compensators and reliefs provide the perfect torque
limiter.
In addition to instant and frequent thrust load variations,
the thruster is exposed to the following dynamic loads (Fig.
16):
- Acceleration loads (g-forces) from vessel motion.
- Drag loads from the submerged portion of the thruster thrashing
through the water due to vessel motion.
- Wave slam impact loads.
All of these loads are transferred through the stem to the
support platform or the main deck. This results in appreciable
flexing of the stem during rough weather operations. Since
the stems to not contain any rotating shafts, gears or bearings,
this flexing does not have any adverse effects on the reliability
of the thrusters. Structural fatigue design of the thrusters
is such that the stem and top structure are capable of easily
dealing with all dynamic loads on a continuous basis.
Classification
The Classification Societies have established rules for DP
class. Even though there are differences between the rules
of each Society, the basic concept is pretty much the same.
There are various levels of system redundancy, generally referred
to as DPS-0, DPS-1, DPS-2 and DPS-3.
The Packaged System Approach lends itself very well for DPS-2
or DPS-3 (Ref 12). DPS-2 requires that the vessel can hold
station upon any single point of failure. (Failure of any
system component or sub-system). This requires a second
DP system and redundant thruster capacity. In the Packaged
System Approach, each thruster has its own dedicated power unit.
There is no sharing of power. There are no common cooling
water systems or fuel supplies. Control power of each thruster
is from the battery of its own HPU. There are no common
points of failure on thrusters or HPUs. This makes the
Failure Modes and Effect Analysis (FMEA) very straightforward.
It also eliminates the need for a Power Management System (PMS).
For DPS-3, the loss of a vessel compartment must be considered
a single point failure also. As thrusters and HPUs are
deck-mounted and not installed in a compartment, the only requirement
for upgrade to DPS-3 involves adding another DP computer with
operator control separated by a firewall from the primary DP
computers on the bridge.
Installation Examples
Here you see a packaged system on a cable-lay barge operated
by Pirelli-Jacobson in Seattle, WA (Fig. 17, 18). It is
comprised of four 500 HP platform mounted thrusters with deck-mounted
HPUs and a redundant Kongsberg-Simrad DP system. It started
out as a leased system but was soon purchased.
Another interesting DP conversion was the Ulises,
an oceanographic research vessel (Fig. 19, 20). Two 500
HP thrusters were installed, one on a platform off the bow and
the other on centerline at the stern on the launching ramp.
The vessel reportedly found an ancient city on the ocean floor
off Cuba. Some people speculate that they have found the
lost city of Atlantis.
On this reel pipe-lay barge from Nippon Salvage, we used a
Kongsberg-Simrad DP system with electronic chart display and
special pipe-lay software (Fig.21). It was provided with
four 500 HP deck-mounted thrusters and HPUs.
The Titan 2 is a 440 x 177 x 43 ft. twin hull offshore
construction vessel (Fig. 22, 23). It is under long-term
charter with Global Industries, who put a DNV AUTR DP system
on it with all the bells and whistles. They added eight Thrustmaster
1000 HP thrusters, installed on platforms off the hulls.
Thrustmaster also provided the eight 1000 HP diesel-hydraulic
power units. The conversion was completed ahead of schedule
and within one percent of budget. The ship is operating
in the Bay of Campeche and is holding station within half a meter.
All of these DP conversions have a few things in common: they
were done dockside with only minor vessel modifications, completed
within a few weeks, and the owner knew exactly how much it was
going to cost.
Conclusion
Quick and relatively inexpensive conversions of existing vessels
and barges to DP capability are quite feasible using the Packaged
System Approach. In many cases, the total cost is only
half of what a conventional conversion would cost. Almost
any vessel or platform can be converted regardless of hull depth
or machinery congestion. Since there is just one single
equipment supplier involved, project budget and schedule are
easily established with a high degree of accuracy. While deck-mounted
power units and outboard thrusters may not be very elegant, they
allow leaving the vessel in its present state, avoiding major
surgery. And while outboard thrusters are somewhat exposed
to collision damage, repair can be done quickly, without dry-docking.
For DP conversions of existing vessels, the Packaged System Approach
is the better way.
References
Deter, Dietmar, 1997: Principle Aspects of Thruster Selection,
DP Conference, Houston, TX.
Adnanes, Alf Kare, 1996: Variable Speed FP vs. Fixed Speed
CP, ABB Industri AS.
Adnanes, Alf Kare, et al, 1997: Essential Characteristics
of electrical Propulsion and Thruster Drives in DP Vessels, DP
Conference, Houston, TX.
Kallah, Amrik, 1997: A Comparison of Thruster Propellers and
Variable Speed Drives for DP Vessels, DP Conference, Houston,
TX.
Dewhurst, Peter K., 1969: Steerable Right Angle Drive Units
for the Main Propulsion of Ships, SNAME Great Lakes and River
Section.
Dewhurst, Peter K., 1969: Experience in the Control of Ships
by Right Angle Drive Thrusters, Second Ship Control Systems Symposium
Annapolis, MD.
Raynor, S.J., 1998: The Benefits of Podded Propulsion in the
Offshore Market, DP Conference, Houston, TX.
Strand, Jann Peter, et al, 2001: Compact Azipod Propulsion
on DP Supply Vessels, DP Conference, Houston, TX.
Adnanes, Alf Kare, et al, 2001: New Thruster Concept for Station
Keeping and Electric Propulsion, DP Conference, Houston, TX.
Woodyard, Doug, 2001: Popular Pods still striving for Perfection,
Marine Propulsion International.
Woodyard, Doug, 2002: Electric Propulsion exerts strong Attractions,
Offshore Support Journal.
ABS, Rules for Building and Classing Steel Vessels 2001, Part
4.
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