Portable DPS Shallow Water Applications
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Lake Maracaibo is a large fresh water Lake in Northwestern
Venezuela. It measures about 100 miles from North to South
and about 80 miles from East to West at its widest point.
The lake is connected at its Northern end to the Gulf of Venezuela
through a narrow channel of about 5 miles wide. A number
of rivers flow into the lake, keeping the water fresh.
The lake is very shallow, with an average depth of 60 ft.
and a maximum depth of 120 ft. Pipelay operations often
occur in waters as shallow as 10 ft. or less.
The lake is covered by a forest of oil derricks, approximately
11,000, most owned by PDVSA, Venezuela's national oil company.
On the bottom of the lake is a spider web of oil and gas pipelines
and electrical cables, many of them old and undocumented.
Pipelines are not buried, but are suspended in a fine sludge
covering the bottom of the lake. Pipeline sizes range from
2 inch to 40-inch diameter. Roughly 1700 kilometers of
new pipelines are laid every year, 80% of it in diameters from
2 inches to 6 inches. New pipe runs are typically not more than
2 kilometers long, most of them shorter. There are about
1600 repair jobs per year, fixing leaks in old pipelines.
95% of these repairs concern pipes with diameters from 2 to 6
Pipelay and pipe repair is currently being performed by contractors
on the lake, using barges with typical lengths of 40 to 50 meters.
The barges use anchor-mooring systems to hold them in position
and to move forward while laying pipe. The anchors frequently
damage other existing pipelines resulting in oil and gas leaks
and requiring additional repairs. PDVSA wants to solve
these problems by banning` the use of anchors in these operations.
This requires dynamic positioning of the pipelay and pipe repair
Dynamic positioning is a relatively new technology.
It was primarily developed for deep water drilling applications,
where the use of anchor mooring systems was no longer practical.
The first DP vessel was the Cuss I, which was used
in 1961 for core drilling operations off California and Mexico
in deep water, up to 3,500 meters. The vessel used four
(4) deck mounted azimuthing thrusters mounted over the side at
the four corners of the vessel. The thrusters were direct
engine driven and were manually controlled. The vessel
could stay on station within a radius of about 180 meters.
Due to the depth of the water, the great length of the drilling
stem had enough flexibility to allow such a large operating envelope.
Simultaneous manual control of the four thrusters was a difficult
task, so the idea was developed to use a computer to control
the thrusters. This was done for the first time in 1961
on the coring drill vessel Eureka operated by the Shell
Many vessels have since been equipped with DP capability.
As computer technology and experience and knowledge about dynamic
positioning increased, the systems became more sophisticated
and reliable. Today, there are three major suppliers of
DP controllers and software. First there is Kongsberg-Simrad,
with the combined technology and experience of Robertson, Albatross,
Kongsberg and Simrad. Second, there is Nautronix, who acquired
the Honeywell technology and experience. Third, there is
Alstom who bought Cegelec. In addition to these three majors,
there are some small companies that have recently developed their
own DP systems. These new systems are relatively simple
and lack many of the built-in special software and safety features
that come with the time-proven systems of the three established
major suppliers. Although the new systems are offered at
much lower cost, there are not many of them in use. So
far, these newly developed systems have been used in non-critical
applications only and there is insufficient operating experience
to determine their reliability. They have never been used
in pipelay service.
Dynamic positioning is used in quite a few different applications.
Some of these are simple and not very critical, like the DP system
on a deep water supply boat that just has to stay in the vicinity
of an offshore platform while offloading supplies. In such
applications, accuracy of 5 meters is perfectly acceptable and
heading is unimportant. The vessel will normally turn its
bow into the wind. Once the cargo is offloaded, the vessel
goes off DP and uses its normal propulsion to sail back to port.
If, during offloading, the DP system were to fail, the Captain
can take over with manual controls and complete the operation.
Other applications are much more critical. An example
is oil drilling in deep water. That requires the vessel
to be on DP 24 hours a day for months at a time. In this
application, reliability is extremely critical. If the
vessel were to drive off location, the drill stem breaks and
a blow out may result. The associated costs of such a drive-off
are extremely high. Consequently, those systems require
a high degree of reliability and redundancy. Accuracy is
not that critical, as excursions of up to 10 meters generally
do not cause any problem due to the great length of the drill
stem in deep water.
Pipelay is one of the most critical applications for dynamic
positioning. Not only are environmental forces acting upon
the vessel, but also the suspended pipe will create additional
forces acting upon the barge. The vessel must maintain
position and accurate heading control. If the vessel loses
it’s heading, the pipe comes under great bending load,
which may result in pipe breakage or failure of pipe handling
equipment on deck. This may cause damage and injuries.
The need for accuracy becomes increasingly important in shallow
water depths where the length of the pipe suspended between the
barge and the bottom is short. This short length does not
allow much flexing to accommodate for vessel drive-offs or heading
changes. Very good slow speed tracking control capability
and very accurate control of position and heading is essential
in this application.
Today's dynamic positioning systems use software that includes
a mathematical model of the vessel. This is developed for
each individual vessel and it contains information about vessel
wind drag, vessel current drag, center of gravity, center of
rotation, location of thrusters, etc. With the help of
this mathematical model, the DP controller issues the proper
commands to the thrusters on a continuous basis to maintain desired
position, heading and speed of the vessel.
The controller gets frequent updates from position reference
sensors (such as DGPS), heading references (gyro compass) and
the speed and direction of the wind (anemometer). There
are two different ways that prompt the DP controller to take
corrective action. The first is position feedback. If the
position reference signal indicates that the vessel is off location,
the controller takes corrective action by making the thrusters
get the vessel back on its required position. The second
is wind feedforward. As wind direction and speed change,
the controller makes the thrusters correct for this, before the
vessel is moved off position.
Thrusters Used for Dynamic Positioning
Most DP applications are for large vessels operating in deep
water. Modern drill ships use thrusters in the range of
4,000 to 8,000 HP. They are electric motor driven with
variable speed control.
Platform supply boats normally use tunnel thrusters or retractable
thrusters for station keeping. They are mounted inside
the hull and they may be direct, electric or hydraulic drive.
Deep-water pipelay barges typically use retractable thrusters
mounted inside the hull. The thrusters are retracted during
long tows and when going in and out of port. The barges
normally have one or two large engine rooms with diesel generators
and electrical control equipment. The thrusters are driven
by electric motors with speed control.
On small barges, the hull depth is usually insufficient to
use retractable thrusters inside the hull. Consider that
a 300 HP retractable thruster already requires a hull depth of
not less than 12 ft. For conversion of existing barges,
creation of large machinery spaces and installation of diesel
generator sets, electrical control equipment and retractable
thrusters is not practical. For these applications, deck
mounted thrusters are much more suitable and economical.
The typical deck mounted steerable propulsion unit, used on
self-propelled barges, uses a diesel engine with clutch driving
a cardan shaft connected to a Z-drive style azimuthing thruster.
This type of thruster uses a horizontal input shaft with an upper
right angle gear assembly driving a vertical drive shaft which,
through a lower right angle gear assembly drives the propeller
shaft (like the letter Z, hence the name Z-drive). They
steer through 360 degrees without stops. They are commonly
referred to as rudder propellers. The concept is old and
well proven. Such units were used for positioning of the
very first DP vessel, the Cuss I. However, these
units have certain limitations that make them unsuitable for
use in shallow water pipelay operations. An explanation
as to why conventional rudder propellers with direct engine drive
are not suitable for this application follows below.
Sizing of thrusters is critical. Nobody ever got in
trouble because of too much thruster power, but many people got
in trouble because of insufficient power.
Thrusters must be sized for the worst environmental conditions
that the vessel may operate in. In considering this, note
that the weather may change while in the middle of a pipelay
job. The barge must be able to complete the job under those conditions.
Thruster sizing is normally done by a computer program and
is represented in polar diagrams called ACapability Plots@.
They are unique for each vessel and preparation of these plots
requires input of physical details of the vessel, such as dimensions,
draft, displacement, outboard profile and thruster size and locations.
In simplified form, we can manually estimate the forces acting
upon the vessel by wind and current. We take the worst
case whereby wind and current are both directed towards the same
side of the vessel. The following formula is used:
For wind load, F is the force on the barge from the wind.
Cd is the drag coefficient of the upper vessel structure.
v is the wind velocity. g is gravity acceleration.
A is the projected area of the vessel above the waterline, exposed
to the wind. ρ is the density of air.
For current, F is the force on the barge from the current.
Cd is the drag coefficient of the submerged portion of the hull.
v is the velocity of the current. A is the projected area
of the vessel below the waterline exposed to the current and
ρ is the density of water.
Suppose that we design for a wind velocity of 45 knots and
a current of 1.5 knots. Then, let us assume that the wind
force calculation indicates that we need 600 HP of equivalent
thrust and the current force calculation indicates that we need
200 HP of equivalent thrust. So the total HP required to
counteract wind and current would be 800 HP. Since redundancy
(DPS-2) is required, the vessel must still be able to hold position
under these conditions upon failure of one thruster. Accordingly,
let us assume that we calculate that we need four (4) each 300
HP thrusters to do this (1200 HP total).
Now, let's assume that we start operating with
this barge and the wind is blowing at 14 to 25 knots and there
is no current. Since wind velocity is squared in the formula,
the total force from the wind would be equivalent to 60 to 180
Since we have four thrusters, each thruster needs to develop
one fourth of this, i.e., 15 to 40 HP. However, we have
four thrusters of 300 HP. So, when wind speed is
14 to 25 knots, the thrusters are running at only 5 to
15 percent of their capability. This is very typical
of DP applications. Most of the time, DP thrusters are
running at only a small percentage of their rated output.
Direct Engine Driven Thrusters
With diesel-electric or diesel-hydraulic
thrusters, all thrusters work
together as a
team, each thruster
contributing in countering
With direct engine driven thrusters,
of the thrusters must counteract
of the other half, in
addition to countering
wind and current loads.
Ideally, all four thrusters are working together to counteract
the wind force. But this is a problem when using direct
engine driven thrusters. At full engine speed, say 1800
RPM, the thruster puts out maximum thrust. But even at
low idle speed, say 600 RPM, the thruster still produces 12 to
13 percent of its maximum thrust. At 14 knots wind, that
is far too much, since we only need 5 percent of maximum thrust.
Accordingly, we cannot operate with the thrusters shown in the
arrangement of Figure 1. We will have to steer two thrusters
against the wind and two thrusters in the opposite direction,
as shown in Figure 2.
Thrusters 1 and 2 are running at low idle speed (each at 13
percent output) while thrusters 3 and 4 are not only counteracting
the wind force, but also counteract the thrust produced by thrusters
1 and 2. As the wind speed changes, thrusters 3 and 4 speed
up and down to hold the barge in position, while thrusters 1
and 2 continue to run at idle speed in the opposite direction.
Diesel engines are designed to operate within a certain speed
range. For instance, an engine rated at 1800 RPM runs well
in the range from 1200 RPM to 1800 RPM. At slower speeds,
the engine runs at poor fuel efficiency and it becomes difficult
to control the speed of the engine. Also, at slow speed,
the engine produces very little torque, usually only slightly
more than the torque absorbed by the propeller. There is
not much excess torque available for acceleration. This
results in slow response when increasing the speed setting.
Moreover, the engine has no capability of forcing the propeller
speed down. Reducing the speed is done by reducing the
fuel rack setting, which makes the engine coast down. Response
to DP commands for changing speed is therefore slow and sluggish
at these low engine speeds. This results in lack of station
keeping accuracy of the barge during changes in wind and current
speed and direction.
It really gets bad when one of the thrusters were to fail.
Suppose that in Figure 2, thruster #3 suddenly failed.
Now, thruster #2 is helping the wind pushing the bow of the barge
around. Of course, the DP system will immediately try to steer
thruster #2 in the opposite direction, but this takes time (at
least 10 to 15 seconds). By that time, the barge completely lost
Station keeping with direct engine driven thrusters is certainly
possible, and we have done it in the past. However, the
accuracy of station keeping is not very good and recovery from
mishaps is very slow. While that may be acceptable in some
applications, it is not acceptable for pipelay operations, especially
when working in shallow waters.
Diesel-Electric and Diesel-Hydraulic Driven Thrusters
With diesel hydraulic and diesel electric drives, the engines
run continuously at their rated speed, like a generator set.
In a diesel electric system, the thruster is driven by an electric
motor with a variable speed drive. In a diesel hydraulic
system, the thruster is driven by a hydraulic motor operating
in a closed loop hydrostatic system. The propeller speed
is controlled by the swash plate controller of the pump, which
gets its electrical control signal from the DP controller.
The propeller speed is infinitely variable from full forward
to full reverse. Full torque is available at any propeller
speed, so changes in speed setting are very fast and accurate,
even at very low propeller speeds. This makes for a very
responsive system whereby all four thrusters are positively contributing
to counteracting the forces from wind and current, as shown in
Figure 1. Reactions to changes in wind or current speed
and direction are very fast, maintaining very steady position
and heading of the barge. And if one of the thrusters were
to fail, the other thrusters instantly take over, since they
are already in the proper steering orientation.
For the above reasons, applications requiring accurate dynamic
positioning use diesel electric or diesel hydraulic thruster
drives. While diesel electric and diesel hydraulic drives
are much more expensive than direct engine drives, the performance
and the reliability is much better. More importantly, it
provides the level of accuracy necessary for shallow water pipelay
Brown Water Operation
Deep water operation (also referred to as blue water), poses
little risk of physical damage to the thrusters. However,
in shallow water (also referred to as brown water) there are
real dangers of the thrusters hitting large objects on the bottom
or other mishaps, such as ropes or tree trunks getting stuck
in the propeller. Therefore, thrusters used in brown water
must be stronger and better capable of coping with grounding
and sudden propeller blockage. Thrustmaster's podded hydraulic
drives are specifically designed for brown water applications.
If the propeller is suddenly blocked on a direct engine driven
Z-drive, the engine flywheel inertia will shear a shaft or break
gears in the thruster. Repair is complicated and time consuming.
Replacement gears and bearings are very expensive and gears have
long delivery times.
Propeller blockage on a Thrustmaster podded hydraulic drive
is easily absorbed by the pump pressure compensators and hydraulic
reliefs. Since the rotating inertia is very small (only
the propeller, propeller shaft and rotor of the hydraulic motor)
there is usually no damage at all. The hydraulic tilt of
the thruster allows easy removal of the object blocking the propeller,
and the thruster can be put back into use immediately.
A direct engine driven Z-drive uses right angle gear transmissions
and drive shafts in the stem. The whole outdrive is completely
filled with lube oil. If the nozzle were to hit a large
object on the bottom of the lake, the stem breaks and the lower
end of the thruster falls down on the bottom of the lake.
All the lube oil is spilled into the water.
Thrustmaster's hydraulic podded drives use a very strong stem
that contains hydraulic hoses only. The drive does not
use any right angle gear transmissions or drive shafts.
If the nozzle were to hit a large object and the stem were to
break, the hydraulic hoses will prevent the lower unit from falling
to the bottom. Repair is easy and there is no loss of oil
into the water.
Using dynamic positioning for pipelay and pipe repair operations
on Lake Maracaibo is certainly feasible and will result in more
efficient operations. A high degree of positioning accuracy
is required. This requires a top class DP controller system
with accurate sensors and very responsive thrusters with full
speed range capability in forward and reverse. Thrustmaster
of Texas has the experience, the know-how and the right products
for this application.