Proceedings of the PRADS2013
20-25 October, 2013
CECO, Changwon City, Korea
Dynamic Loads on Mechanical Azimuthing Thrusters
Jie Dang1), Jos Koning1), Joris Brouwer1) and Johan de Jong1)
1)
Maritime Research Institute Netherlands (MARIN), The Netherlands
Abstract
Mechanical azimuthing thrusters have been applied in
various ship operations in the past decades, such as low
speed manoeuvring; dynamic positioning (DP); bollard
pull (BP); high speed transit trips and at continuous full
power as the main propulsion system of ships. The ship
types on which these thrusters are used include tug
boats, offshore supply vessels, pipe layers, drill ships,
ferries, fast transport vessels, etc. Despite this widely
spread use of such thrusters, however, damages on gears
and bearings have been reported in some cases. According to the statistics of survey records of classification
societies, gear and bearing failures are at the 2nd and the
3rd place on the failure list of parts of mechanical azimuthing thrusters, right behind the propellers as the No.
1 vulnerable parts, which are exposed directly to sometimes quite harsh environmental conditions.
To help the industries to get insight into the failures,
MARIN has initiated a Joint Industry Project (JIP) on
the hydrodynamic loads and shaft responses of mechanical azimuthing thrusters, called SHARES JIP.
Thruster, gear and bearing manufacturers, shipyards,
ship operators and classifications have been teamed up
in the JIP. Studies have covered operational investigations, extensive dedicated model tests for dynamic loads
and full-scale trials and monitoring on a ship – the largest pipe layer of the world: Allseas’ SOLITAIRE with
10 sets of 5MW azimuthing thrusters. Results of the
study are summarized and presented in this paper.
Keywords
Azimuthing thruster; Dynamic load; Interaction; Ventilation; Gear damage; Bearing damage
Introduction
In the past decade during the global shipbuilding industries boom, a considerable amount of mechanical azimuthing thrusters (typically of the pushing type with
ducted propellers or the pulling type with open propellers) have been manufactured and installed on various
types of vessels, covering wide ranges of operational
profiles. Good manoeuvrability, superior position keeping capability, large pulling forces at bollard and low
speed operations, and high efficiency for free sailing
and transit conditions, have distinguished them from
ships with traditional propulsion systems. However,
failures of gears and bearings have been reported after
some of those thrusters were used in service operations
for some time, irrespective of the thruster manufacturer
or ship operator. In extreme cases, the failures occurred
within one year of operation. Obviously the operation of
those mechanical azimuthing thrusters has exceeded the
design constraints and limits, which are based on the
present understanding of the hydrodynamic loads on the
thrusters and their shafting systems, including gears and
bearings.
At least two possible causes have been identified which
can be attributable for these mechanical failures. One of
them is extreme manoeuvring with azimuthing thrusters,
including also thruster-thruster interactions, typically for
offshore ships and offshore structures where the thrusters are mainly used for transit and DP-mode. Another
cause is thruster ventilation, which can occur both during DP and also during free sailing operations when the
thrusters are fitted to the vessel at a location which is
too close to the free water surface. In both cases, large
unforeseen variations of the hydrodynamic loads on the
propeller blades and dynamic responses of the shafting
system occur, which propagate through the propeller
hub and shaft to the underwater gears, to the pinion
shaft and to its bearings.
In addition, most mechanical thrusters have a very short
and torsionally stiff propeller shaft connected directly to
the bevel gear. On the other side of the gears is often an
electric motor with a large mass moment of inertia.
Since the elasticity of the system is rather low, the propeller blade impacts are transmitted directly into the
gear without damping.
Typical damages found in a thruster are one broken
tooth of the bevel gear and worn or burnt bearings of the
pinion shaft, although many safety factors have been
already applied in the design stage, which often include
appropriate safety factors for surface pitting damage,
sub-surface fatigue, tooth root damage, loss of lubrication film thickness and tooth interior fatigue fracture
(TIFF). It is understood nowadays that the TIFF, is one
of the major damage mode that causes the fatal failure
of some mechanical azimuthing thrusters.
TIFF starts as a small crack below the surface of the
active flank, most often within the transition zone be-
tween case and core material. During operation the
crack grows gradually without notice inside the tooth
towards the root area of the non-active flank. A single
high transient load, impurities in the material or inconsistent material treatment (usually carburized case hardening) may be the origin of the initial crack.
Currently most thrusters are designed for a single
(static) design condition. In reality however, thrusters
do not operate exactly at the design point. Due to lack of
knowledge, the extreme static and dynamic loads are
often not, and in most case not able to be, assessed in
the design stage. Off-design conditions may overload
the thrusters and result in damages that impose risks for
the operators.
Studies on static loads on azimuthing thrusters, at all
possible steering angles in inclined inflows, can be
traced back to the work of Oosterveld and van Oortmerssen (1972), after their successful study of the
Wageningen B-series propellers and the Ka-series propellers for the entire operation regime in four quadrants
(MARIN 1984, Kuiper 1992). In their work, the propeller thrust and torque, the unit steering moment and the
unit side force have been measured at selected steering
angles between ±90o and in the first and fourth quadrants. However, this has only been carried out for an
open propeller from the B-series – B4-70 at pitch setting
P0.7R/D=0.6 and for a ducted propeller from the Kaseries – Ka4-70 at pitch setting P0.7R/D=1.0. Both are
considered to be the most frequently used propellers for
low speed and DP operations of azimuthing thrusters.
In the last decade, a large number of new studies have
been carried out at MARIN for podded propulsors and
thrusters under all possible steering angles, by complete
6-component force and moment measurements on both
the propeller shaft as well as on the total unit. Dedicated
shaft transducers with high accuracy have been used,
both in open water conditions, in behind conditions and
also with ice impacts (Hagesteijn, et al. 2012). Recently,
systematic controllable pitch propeller series – the
Wageningen propeller C-series and D-series have been
tested at all possible pitch settings for their entire 2quadrant operations (Dang, et al. 2012, 2013), together
with measurements of the blade spindle torque.
Measuring dynamic loads is extremely difficult. Care
has to be paid to the response of the measuring system
with the transducers and the object upon which the
hydrodynamic loads are measured, such as a single
blade of a propeller. The natural frequency of the system is preferred to be as high as possible, while sufficient elasticity of the transducers should be allowed for
the strain measurements. This requires the transducers
to be stiff enough with high sensitive strain gauges and
to be made of light material to reduce the inertia of the
whole system. Hagesteijn, et al. (2012) have explained
the problems and the solutions and Brouwer, et al.
(2013) has further elaborated how this test set-up can be
used to accurately measure impact loads of ice on one
propeller blade.
Dynamic loads on a propeller blade during ventilation
events have been investigated and studied by means of
model tests (van Beek and van Terwisga, 2006). The
influence of the cavitation or the ventilation, and the
combination of cavitation and ventilation have been
studied on one propeller blade for a fast container ship
in behind condition. The blade loads, mainly the blade
spindle torque, have been found to be heavily affected
by the amount of ventilation.
A series of model tests on thruster ventilation has been
carried out recently by Koushan, et al. (2011). The ventilation inception mechanism and the scaling laws have
been reviewed, together with categorization of the various ventilation events (Kozlowska, et al. 2009). The
sudden drop of the mean propeller thrust and torque
when ventilation starts and the increase of the dynamic
loads on the blade have been observed and measured.
Ventilation phenomena have been illustrated by using
high speed cameras.
Similar studies have also been carried out by Amini and
Steen (2011) both experimentally and also theoretically.
Most recently, propeller ventilation phenomena have
been studied also in behind conditions with podded
propulsors fitted to a cruise ship, in combination with
blade cavitation in the renovated Depressurised Wave
Basin (DWB) of MARIN (Brouwer & Hagesteijn 2013
and Hagesteijn & Brouwer 2013). Dedicated transducers with high accuracy have been used to obtain reliable
dynamic loads up to 500 Hz on one blade on model
scale, giving enough resolution over one propeller revolution. The measured loads have been synchronized
with the high speed videos so that the dynamic loads
can be re-played and visualized. This helps to understand the ventilation events and the associated peaks of
the transient dynamic loads.
However, systematic studies for dimensioning thrusters
gear parts are still lacking. The thruster manufacturers
are eager to obtain such information both from model
tests and also from full-scale measurements. In addition,
the operation instructions for avoiding over-loading
thrusters during operations are also very limited. No
correlation between model test results and full scale
trials have ever been established.
In order to help the thruster manufacturers, the gear
makers, the operators and the classification societies,
MARIN initiated a Joint Industry Project (JIP) on the
hydrodynamic loads and the shaft responses of mechanical azimuthing thrusters, called SHARES JIP.
Within this JIP, operational studies have been carried
out by investigating thruster damages on the type of
parts used, and the way the thrusters are actually operated. Systematic model tests have been carried out with
Wageningen Series C4-70 and D4-70 propellers at various pitch settings for both the pulling type thrusters with
open propellers and the pushing type thrusters with
ducted propellers. Three different 6-component transducers have been used: one on the key blade of the
propeller, one on the duct and one on the total unit.
Tests have been carried out both in extreme manoeuvring and interaction conditions and also in ventilating
conditions. Full-scale dedicated trials and long term
thruster monitoring are being carried out at this moment
on the world largest pipe layer – SOLITAIRE of Allseas.
Operational studies and investigations
Operational studies have been carried out and used to
map the operational experience and perceptions of risk
on damaging the mechanical azimuthing thrusters
among officers, operators and owners who operate vessels with thrusters, and to identify the limits related to
extreme manoeuvring and thruster ventilation, aiming at
describing and closing the gaps between how the thrusters are used in practice and what the assumptions and
limits are in the designs, which have been set by the
manufacturers and the classification societies.
The study identified which dynamic loads are avoidable
and which are not.
For the avoidable loads, together with the results of the
studies from the model tests and the full-scale trials, the
findings will be translated into guidelines on designing
and operating azimuthing thrusters for extreme manoeuvring and ventilation conditions in order to avoid
unnecessary high dynamic loads for the operators, the
owners, the designers and the manufacturers.
For the unavoidable dynamic loads, the results of the
study will be used to detail the model testing and investigation programs in order to define the magnitude of
those unavoidable dynamic loads and the related load
characteristics for dimensioning the mechanical thrusters and their shafting systems.
The service database of the classification societies involved in the present project have been studied in order
to understand the problems and the size of the damages.
Table 1 shows the amount of damage in percentage
found on thrusters during their regular services. Those
damages are not necessarily related to each other (for
instance, propeller blade damages are often local and
may not result in gear or bearing damages). Although
not all damages are fatal, the size is rather large.
Table 1:
Model testing
The models
Two generic thruster housings, struts and fins have been
designed which represent the contemporary thruster
designs used for novel ships. The pulling type thruster
with open propellers has a streamlined slender body, as
shown in Fig. 1. The pushing type thruster with ducted
propellers has however a blunt and short housing used
mainly for low speed operations, see Fig. 2. The propeller blades, the housings, struts and fins, and the propeller hubs are all made of aluminium in order to limit the
influence of the mass and the mass moment of inertia of
the models. The duct has been partly made from a
PMMA (polymethyl methacrylate) block which is
transparent and partly 3D-printed of semi-transparent
ABS plastics so that it is hollow and the transducer can
be fitted inside the duct to measure the forces and moments.
6-C transducer
for the unit
Percentage of mechanical thrusters experienced
damage (averaged classification database)
Type of thrusters
Percentage of damages
Main propulsion thrusters
23%
DP thrusters
7%
Auxiliary thrusters
7%
When looking into the type of damage, the propeller is
on the top of the list, which is well understandable since
the propeller is directly exposed to the harsh environment (Table 2). Not surprisingly, the gears and bearings
follow the propeller at the second and third positions on
the damage list. However, there is no reported damage
found on the steering gears.
Table 2:
trigger gear and bearing damages too.
Within the gear damages, the TIFF damage and the
scuffing damage of the gear surface are at the top of the
list and next to each other. Both damages indicate the
possible high single load peak occurred during operation of the thrusters.
The damages on the housing and strut of the thrusters
and the clutches on the shafts are very limited, being
lower than 3%.
6-C shaft transducer for one key blade
Fig. 1:
The generic pulling thruster with Wageningen
Propeller Series C4-70 at various pitch settings
6-C transducer
for the duct
6-C transducer
for the unit
Type of thruster damage (averaged)
Components
Percentage of damages
Propellers
24%
Gears
12%
Bearings
11%
Ducts
3%
Steering gears
0%
Besides the damages listed above, there is also the regular replacement of seals which accounts for a large part
of the maintenance for thrusters. Failure of seals may
6-C shaft transducer
for one key blade
Fig. 2:
The generic pushing thruster with Wageningen
Propeller Series D4-70 at various pitch settings
in 19A duct, propeller model diameter - 21.9cm
In order to get cross-references with standard series
propellers, the Wageningen Propeller C-series and Dseries have been selected for the studies. For the pulling
type thrusters, the C4-70 propellers have been used with
4 blade designs at design pitch settings of P 0.7R/D = 0.8;
1.0; 1.2 and 1.4. For the pushing type thrusters, the D470 propellers have been used in combination with a 19A
duct, also at design pitch settings of P 0.7R/D = 0.8; 1.0;
1.2 and 1.4 (Dang, et al. 2012 and 2013).
interaction terms. Good linearality has been found. The
calibration uncertainties are about 0.3%.
The driving system and the transducers
For the pulling type thruster with relatively large
underwater housing, an electric motor has been fitted in
the model. Good quality of signals have been obtained
without any interference from its driving system.
However, with their short and blunt form of the housing
for the pushing thruster with ducted propellers, a rightgear driving system has to be used. Care has been paid
in the design of the shafts and bearings and the selection
of the gears and the number of teeth, in order to prevent
noise pollution to the signals, especially at the gear
meshing frequency. In order to obtain reliable
measurements on the blade dynamic loads up to 500Hz
in model scale, the gears have been chosen with such a
ratio that they result in a gear meshing frequency of
over 700Hz at 900RPM shaft rotational rate. Fig. 3
illustrates the driving system with the transducer for one
of the propeller blades (the others by-pass the transducer)
and the transducer for the duct.
Fig. 3:
The driving system with transducers for the
pushing type thruster with ducted propellers
To measure the pure hydrodynamic forces and moments
on the key propeller blade without damping of any
impact due to the elasticity of the model shafting system,
the mass of the key blade has been reduced as much as
possible (aluminium blade) on one side of the
transducer. The mass and the mass moment of inertia
have been increased as much as possible on the other
side of the transducer. This has been made possible by
using an electric motor for the pulling thruster and by
adding a flywheel to the pushing thruster, see Fig. 3.
The transducer for the shaft and the duct are both 6component and specially designed for the present
measurements. The duct transducer is made of two stiff
stainless steel rings, with 6 individual 1-dimensional
force transducers in between (Fig. 4). Both 6component transducers have been calibrated with all
Fig. 4:
The 6-component transducer frame fitted into
the transparent hollow duct model with 3-point
connection to the thruster housing and struts
The test programme
In the first test campaigns, the following tests have been
carried out:
- Open water tests for all propellers at their design pitch settings of the C4-70 series and the
D4-70 series in a 19A duct for all steering angles between -180o and +180o at advance ratio’s covering J=0 to KT=0, with step - J=0.2.
- Thruster-thruster interaction tests at downstream thruster setting angles between -90o and
+90o, with distance between the two thrusters
at 4 different distances between 5D to 20D for
a few selected advance ratio’s, while the fore
thruster is sweeping ±45o, for all propellers and
thrusters mentioned above.
- Thruster ventilation tests both in waves and in
vertical heaving mode at selected sets of advance ratio’s and various immersions, with and
without cavitation on the blades.
More tests have been planned after studying the results
of the first set of tests, zooming into more details, especially during the ventilation events.
Data acquisition and reduction
The signals from the transducers for the unit and duct
loads were sampled at 1kHz frequency. Due to the relative elasticity of these, it is not expected that any meaningful signal is to be measured above this frequency. To
study the dynamic load behaviour on the blade, a 5kHz
sampling frequency has been used for the shaft transducer. The position of the blade is accurately constructed at 5kHz by means of measuring the raw position encoder signals at even higher frequencies.
The sampled data were further filtered and reduced to
maximise the signal to noise ratio, which were low-pass
filtered to remove the natural blade frequency amplification and the gear teeth noise, the harmonic components caused by the magnets in the electric drive, and
the centrifugal forces. A correction for gravity or buoyancy forces of the blade’s own mass is applied.
To determine the cut-off frequency, FTT analyses of the
sampled raw data from all sensors have been carried out
and studied. Three typical examples for the test set-up
of the generic pushing thruster model with ducted propeller (see Fig. 3) are plotted for indication in Fig. 5,
Fig. 6 and Fig. 7 for the blade thrust, the duct thrust and
the unit thrust, respectively.
Fig. 5:
MDx, MDy and MDz, the moments on the key blade MBx,
MBt and MBr, and the torques on the propeller Qx, Qy and
Qz, as shown in Fig. 8.
Raw and filtered signal of the blade thrust
Fig. 8:
The coordinate systems and the forces and
moments
In order to make easy use of the data for the future in
supporting the thruster designs, all the nondimensionalized coefficients K have been fitted with
Fourier series with respect to the steering angle .
N
Fig. 6:
K    Ak sin(k )  Bk cos(k )
Raw and filtered signal of the duct thrust
(3)
k 0
for  180o    180o
which has been truncated at the Nth harmonic.
Test results of the loads
Fig. 7:
Raw and filtered signal of the unit thrust
A very high natural frequency of the key blade of its
first mode has been found - about 750Hz, as shown in
Fig. 5, which is as expected and as designed for. The
natural frequencies of the measuring systems of the duct
and the unit loads have been also measured at about
43Hz and 12Hz, respectively.
To obtain purely hydrodynamic loads and to remove
any possible resonance of model test set-up from the
signal, a cut-off frequency at 500Hz has been applied to
the blade loads and a cut-off frequency at 10 Hz has
been applied to both the duct loads and the unit loads.
The filtered forces and moments are further nondimensionalized into the following coefficients.
F
 n2 D 4
M
KM 
 n 2 D5
KF 
Most tests have been performed under quasi-steady
conditions (Dang, et al. 2012) with most parameters
kept constant except for one that varies slowly over
time. In the present case,  is varied. This is done twice
with both an increasing and a decreasing rate to cancel
the inertia, lag or memory effects. The results of the
increasing and decreasing path are fitted and averaged.
The propeller thrusts, in 3 directions, are plotted in Fig.
9 as a typical case for the generic pulling type thruster.
(1)
(2)
where F represents all forces, which consist of the
forces on the unit FX, FY and FZ, the forces on the duct
TDx, TDy and TDz, the forces on the key blade TBx, TBt and
TBr, and the forces on the propeller Tx, Ty and Tz; and M
represents all moments, which consist of the moments
on the unit MX, MY and MZ, the moments on the duct
Fig. 9:
Propeller thrusts of the generic pulling type
thruster, P0.7R/D=1.0, J=0.8
The dotted lines are the processed quasi-steady time
series and the solid lines are the Fourier fits of this data
representing the mean loads. Similarly as for the other
pitch settings and the advance ratio’s, the thrusts show
strong periodic variation when the steering angle is
larger than ±90o. Tx reaches the maximum around ±90o
or over. Compared to the mean values, the amplitude of
the fluctuations is not negligible. The propeller torques
show also the same characteristics. This is also true for
the pushing type thruster with ducted propeller.
Fig. 10 and Fig. 11 show the unit hydrodynamic bending moments MX, MY and the steering moment MZ for
the pulling and pushing type thrusters, respectively.
Thruster-thruster interactions
The objective of the thruster-thruster interaction tests
was not the thrust degradation which has been intensively studied before (Cozijn, et al. 2013), but on the
dynamic loads on the propeller and its shafting system
when interaction occurs. Fig. 12 shows a screen shot of
the underwater video’s taken during the interaction
tests, where an additional stock thruster has been used.
A stock thruster
used to generate
dynamic flow
Fig. 10:
Unit moments of the generic pulling type
thruster with open propeller, P0.7R/D=1.0, J=0.8
Fig. 11:
Unit moments of the generic pushing type
thruster with ducted prop., P0.7R/D=1.0, J=0.8
The hydrodynamic bending moment at the thruster
foundation consists of mainly the side bending moment
MX and the forward bending moment MY. Both reach
their peak value around ±90o or over. This is true for
both pulling and pushing type thrusters. Although the
absolute values depends strongly on the geometry the
thruster housing and struts, the results shown in the
above figures are very representative since the two generic thrusters as designed and used for this project,
represent the most contemporary thruster designs.
The steering moment MZ on the pushing type thruster
(Fig. 11) shows a typical self-restoring ability where the
hydrodynamic steering moment is negative for a positive steering angle around 0o, while the pulling type
thruster lacks this ability (Fig. 10).
It should be noted that although the Fourier fittings
represent the mean values, the filtered data with the
spikes on Fig. 9 through Fig. 11 does provide the dynamic loads information for both the propeller blade, as
well as for the duct and the total unit.
Instrumented generic
thruster to measure
blade/shaft dynamic loads
Fig. 12:
Thruster-thruster interaction tests, P0.7R/D=0.8,
J=0.6, =30o, stock thruster sweeping ±45o
Fig. 13:
Shaft thrust dynamics, P0.7R/D=0.8, J=0.6,
=0o, stock thruster sweeping ±45o, x/D=5
Fig. 14:
Shaft torques dynamics, P0.7R/D=0.8, J=0.6,
=90o, stock thruster sweeping ±45o, x/D=5
Fig. 13 shows both the mean propeller thrust and its
dynamic fluctuations at 0o setting angle in the
downstream of another thruster when it sweeps over
±45o. The dynamic thrust doesn’t show strong
fluctuations. This is also true for the propeller torque
(Fig. 14). However, when the downstream thruster is at
a setting angle of 90o, which may occur during extreme
manoeuvring and harbour operations, significant load
fluctuations are measured. In addition to the shaft torque
Qx, the shaft bending moment Qy and Qz are at the same
order of magnitude as Qx, both for their mean values as
well as for the fluctuations.
Test results of dynamic loads on thruster blades
In Fig. 17 and Fig. 18, a test result is shown during the
passage of a wave under atmospheric conditions with
fully-ventilated propeller. Fig. 17 is a snapshot when the
wave crest is passing over the propeller. At this moment
the propeller is not sucking in new air, but still discharging the air it caught before during the trough passage.
Test results of ventilated thrusters
Fig. 15 and Fig. 16 are snapshots of high speed video
recordings showing thruster ventilation for the generic
pulling thruster in bollard pull condition without waves.
Fig. 17:
Fig. 15:
Thruster ventilation, P0.7R/D=0.8, J=0,=0o,
shaft immersion hs/D=0.5, wave height hw/D=0.
Thruster ventilation, P0.7R/D=0.8, J=0,=30o,
shaft immersion hs/D=0.5, wave height
hw/D=0.5, during a wave crest
Fig. 18 shows the time series of the thrust of the key
blade. The snapshot of Fig. 17 was taken during a moment placed at the far right of the graph.
Snapshot

Trough
Extreme event
=0o
=30o
=60o
Extreme event
Crests
Fig. 18:
=90o
=120o
=150o
=180o
=210o
=240o
Fig. 16: Thruster ventilation, P0.7R/D=0.8, J=0,=0o,
shaft immersion hs/D=0.5, wave height hw/D=0.
For this partial ventilation, ventilation starts from the
top region when the propeller tip interacts with water
free surface. It is clearly seen that once ventilation occurs, it remains attached to the propeller blade for one
complete or more revolutions.
Crests

Time trace of TBx during approximately one
wave period
Several distinct regions can be identified. During the
crest regions, the oscillating motion of the force has the
largest amplitude. This oscillation has the frequency of
the propeller revolution and is therefore identified as
being caused by the local change of inflow to the blade
during each revolution.
In the trough region this propeller frequency oscillation
has a much lower amplitude while the mean force actually increases. This is due to the wave orbital velocities
being different from the crest region. Both the average
inflow velocity, the angle of attack and its amplitude to
the blade have changed.
Apart from these oscillations, more random motions
with higher frequencies are observed during the entire
time series. These are caused by a ventilation event and
the discharge of the air later on. The bandwidth of this
random component is most severe suddenly during the
wave trough. This is when the blades start to pierce
through the surface and are severely ventilating. Once
the surface piercing condition is over some ventilation
can exist for a short moment while the water surface is
still near the propeller blades. As the discharge of air
follows, the random motions start to decrease since the
turbulence created by air decreases.
Two extreme events are also identified. It is not clear
why these are caused. Up to now they seem to appear a
bit random. A possible cause for these events is a sort of
collapsing or slamming situation of a large air pocket.
propeller, electric motor, gear meshing and bearings.
Data can be captured and stored 24 hours per day or at
preset intervals for a given number of minutes per each
hour (Fig. 22).
Full-scale trials on SOLITAIRE
A full scale campaign aimed in capturing the dynamics
in a real life situation is currently being conducted on
SOLITAIRE of Allseas (Fig. 19). The full scale measurements were focused on capturing what kind, and
what level of dynamic response occurs under varying
operational and off-design conditions.
Allseas’ SOLITAIRE
The operating conditions of SOLITAIRE provide a
specific opportunity to investigate thruster dynamics.
The thrusters are designed for a compromise between
transit conditions, sailing around 14 knots, and DP operation where the thrusters have to provide station keeping as well as pipe pretension at zero speed. In more
shallow water the pipe pretension can go up to various
hundreds of tons putting the thruster in a high power
bollard pull.
The number 10 port side aft thruster was extracted for
the project specifically, see Fig. 20. With joint effort
from Allseas, Wärtsilä and MARIN, the thruster was
disassembled and outfitted internally with strain gauges
and accelerometers, Fig. 21.
The sensors and locations were designed to capture
overall thrust, thruster dynamics and shaft drive torque.
The combination to allow derivation of dynamics in the
bevel gear, propeller disk, and the bearings.
Mechanical modifications were installed to guide cabling from the pressurized gear oil interior into the
thruster room. Data logging equipment was installed on
the azimuthing part of the thruster. All channels are
logged synchronously at a sampling rate of 1kHz to
recognize principal characteristic frequencies from
Fig. 20:
No. 10 thruster – underwater de-mountable
Fig. 21:
Locations of load sensors on No. 10 thruster
Fig. 22:
Data logger fitted on steering hub
Fig. 19:
The detailed dynamic data set from the thruster is combined with the operational data logged by the DP control system including the rpms, azimuth angles and
power demand of all thrusters and the ships speed, position and heading.
The full scale tests are aimed at two aspects of thruster
operation. The first is on their performance under off
design conditions. These are manoeuvring at high loads,
crash stop, bollard pull, oblique inflow and operation
with thruster-thruster interaction. The SOLITAIRE crew
will deliberately bring the vessel in the required conditions to capture the behaviour.
Measurement program
The second is on the performance under regular operational conditions including transit and DP pipelaying at
various speeds and pipe pretensions. In particular the
operation at high pretensions under DP conditions are
expected to raise strong dynamics. This is due to the
combination of high propeller loads in combination with
expected thruster–thruster interactions.
Status
The instrumentation was installed during a docking in
October 2012 and will remain in operation until end of
2013. After the installation the vessel sailed from Italy
to the Gulf of Mexico where she operated in deep water
for four months. She then came back to Italy for a drydocking after which she continued on a transit to Australia where she will be operated until mid 2014. First
data from the transit to US and operation in the Gulf of
Mexico were received in March 2013. The transit condition to and from US was found to be fairly mild. The
DP operation in the Gulf of Mexico in deep water only
required moderate DP power due to the minor pretension of the pipe in deep water.
DP GoM – Vs=0
Free sailing
in transit
Fig. 23:
Power absorption of No. 10 thruster
The performance of the sensors in the thruster was
found to be excellent. The strain gauges produce consistent and strong signals. Fig. 23 shows the relation
between the propeller shaft rotational rate and the shaft
torque registered during the operation. Fig. 24 is a typical strain gauge response to the shaft rotational rate.
The acceleration sensors both inside the thruster and on
the steering hub are fully operational. Evaluation of
time series, frequency spectra and trending can be performed on the measured data. This allows trending of
response signature of vibrations over time, but also
selection of short term highlighted events, breakdown in
rms and peak levels per frequency band etc. Contributions of gear meshing, blade harmonics, etc. show up
nicely as illustrated in Fig. 25.
Fig. 24:
Strain gauge output vs RPM at zero speed
Fig. 25:
Trending of response spectra
Continuous data logging is in progress now during transit to Australia and will continue. Interesting regular
operating conditions are expected directly at the start of
the oncoming project when a maximum pulling force
has to be applied to test the pipe mooring.
The off-design tests will be scheduled over the coming
months.
Conclusions
Mechanical thruster failures have been investigated in
order to bridge the gaps between the assumptions used
in the designs and the operations in practice. Systematic
model tests with propeller series fitted to a generic pulling type thruster with open propeller and a generic
pushing type thruster with ducted propeller have been
carried out at open water, interaction and ventilation
conditions. 6-component forces and moments have been
measured on the propeller shaft, on the duct and on the
total unit at all steering angles. Both the static and dynamic loads on contemporary azimuthing thrusters have
thus been studied thoroughly and systematically for the
first time. Full-scale sea trials and thruster monitoring
provide valuable correlations with the model test results.
The results of the studies, with valuable database, form
the foundation for dimensioning mechanical azimuthing
thrusters and for defining operational guidelines for
them.
Acknowledgement
The authors of this paper are grateful for all participants
of the SHARES JIP, being ABS, Allseas, ATA-Gears,
Brunvoll, DNV, IHC, Kawasaki, Klingelnberg, LRS,
MARIN, Niigata, SKF, Voith, Wärtsilä and ZF Marine.
Special thanks are extended to R. Bosman for designing
and building the transducers for model tests and to J.H.
Allema for reviewing and correcting the manuscript of
this paper.
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