63
1 INTRODUCTION
Norway has a very long coastline with many wide
and deep fjords. The increased focus on efficient, safe
and environmentally friendly traffic in coastal areas
has led to a debate on different ways for fjord
crossing. The plan for a ferry-free main road from
Kristiansand to Trondheim along the western part of
Norway is shown in figure 1. The plan has been
prepared by the Norwegian Road Authority. To fulfil
the objective of the plan, eight existing ferry routes
must be removed and replaced by other types of fjord
crossing, such as very long bridges (including floating
types) and subsea tunnels. Based on new cost
estimates for different fjord crossing scenarios, some
of the crossings will continue to be operated with
ferries. However, to fulfil the national goals for coastal
shipping [7] the ferry fleet must be renewed by
introducing low, preferably zero, emission vessels.
For some of the long and exposed routes in the
Manoeuvring Study Norwegian Double-Ended Ferry
T.E. Berg
1
, Ø. Selvik
1
, K. Steinsvik
2
& D. Leinebø
2
1
SINTEF Ocean, Trondheim, Norway
2
HAV Design, Fosnavaag, Norway
ABSTRACT: The Norwegian coastline has many long fjords where crossings are necessary for transportation of
goods and passengers. In the last decade, the focus on reduced travel time along the main roads in coastal areas
has increased the building of bridges and subsea tunnels. However, at present and in the future many fjord
crossings will depend on ferries. The Norwegian government [7] requires that ferries, like all coastal ships in
Norwegian waters, should be designed for zero or low greenhouse gas (GHG) emissions to meet the national
goal of 50% reduction of GHG from coastal shipping by 2030. As ferry services are regulated by national or local
governmental bodies, all new ferry operations should be performed using zero- or low-emission ferries. Thus,
ferry companies require new and innovative ferry designs with reduced resistance, resulting in reduced
installed propulsion power.
This paper describes work done by the ship designer HAV Design AS (former Havyard Design & Solutions AS
(HDS)) to meet the governmental request for ferries with a low environmental footprint. Work on a double-
ended ferry design is described. In the early design phase manoeuvring performance is not a priority item,
partly due to lack of a simple and reliable manoeuvring performance prediction tool for unconventional ship
designs. It is well known that optimization of resistance can be at the cost of manoeuvring performance. In this
paper, a specific double-ended ferry design will be used as a case. Outcomes of design simulation of
manoeuvring performance are compared to manoeuvring full-scale tests in deep, calm water. Full-scale test
results will later be used to tune a simulation model for a future training simulator for double-ended ferry,
where full-scale manoeuvring tests have been performed, will be used as a test case. This paper shows how the
designer has worked with these two topics in parallel in the final design stage where both experimental and
numerical tools have been used for design verification.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 15
Number 1
March 2021
DOI: 10.12716/1001.15.01.05
64
northern part of Norway, low-emission ferries are the
only solution. Previous tender documents prepared
by the national road authority and regional authorities
responsible for transport services ask for zero or low
emissions fiord crossing ferries. In the process of
evaluating service offers, the two main parameters
have been operational costs and emission footprint. In
these tenders, the cost is weighted 70% and
environmental aspects 30% when comparing offers
from ferry companies, meaning that the concept with
the lowest price will not necessarily win. Lately, the
new tenders from different governmental bodies
require zero emission for fiord crossing ferries. Thus,
the only criteria when comparing offers will be the
total costs (investments and operations) for a given
licence period.
In the competition for new transport contracts, all
ferry companies need new vessels or retrofitting old
vessels with zero/low emission engines. As a result,
national ferry companies have requested new vessel
designs from many Norwegian ship designers. This
paper describes design challenges related to double-
ended ferries, how design tools and model tests were
used to verify a specific ferry design (H-936)
developed by HAV Design for a 120 car capacity
ferry. Sea trials were used to validate manoeuvring
performance of these classes of ferries. Four H-936
ferries have been built and operating on different
routes in Norwegian fjords.
Figure 1. Future ferry free main road (E 39) from
Kristiansand to Trondheim (courtesy Norwegian Road
Authority).
2 DESIGN CONSIDERATIONS
Many of the double-ended car ferries in Norway
operate in sheltered waters, but these waters have
many obstacles, such as islets or underwater reefs,
combined with narrow passages and shallow water
effects close to the ferry quays. Transit routes are often
short, many times shorter than thirty minutes. This
means many docking operations every day, often
between twenty and forty.
The main operation can be divided in four phases:
acceleration transit retardation and manoeuvring /
berthing. Very often both retardation and final
manoeuvres overlap. Before entering the quay, waters
are often both narrow and shallow introducing
shallow water and bank effects changing the
manoeuvring performance. In addition to these
effects, the captains have experienced unusual
manoeuvring responses during retardation before
arrival at the ferry port. This has led to development
of special operational procedures on how to use the
fore and aft azipulls in the retardation phase. Adding
the influence of harsh weather conditions increase the
challenges to be coped with by the bridge crew.
When ferry companies compete for governmental
fjord crossing contracts, two factors are weighted in
the selection process operational costs and emission
footprint. In addition, operational regularity is a
challenging as it is required that operation should be
possible under all weather conditions, except extreme
cases where the weather gets a specific name. As
operational costs are strongly related to energy
consumption (electricity, different types of fuels or
hybrid solutions), the design will focus on resistance
reduction and propulsion efficiency in addition to
selection of engine power sources.
To be able to fulfil governmental requests for
reduced GHG emissions from new ferries, the ferry
companies have requested new vessel designs from
many Norwegian ship designers. Double-ended ferry
designs are often selected to reduce turn-around time
at the ferry quays. As most of the crossings are inside
the fjords, the critical factors for an energy efficient
hull design are:
Low calm water resistance
High propulsive efficiency
Controllability in strong winds near quays
Additional items to be considered are green
water/sea spray on the car deck and directional
stability (to reduce the additional resistance from
using control units).
Figure 2 is a generic illustration of a ship design
process. As can be seen from the “Performance” list,
manoeuvring characteristics is missing. This could
result in new ship designs giving ship masters that is
difficult to handle, especially in confined waters and
ports.
Figure 2 Design process used for a double-ended ferry.
65
To illustrate design tools used by HAV Design, the
work related to development of their H-936 (120 car)
double-ended ferry is used. The main particulars of
the vessel are listed in Table 1 and a picture of the
vessel in the transit phase is shown in Figure 3. A brief
overview of the tools (experimental and numeric)
used by HAV Design will be described. For
illustrative purposes, the H-936 is used as a case.
Tools applied are listed in Table 2.
Table 1. Main particulars and propulsion units of the HAV
Design H-936 double-ended ferry design.
_______________________________________________
Geometric parameters
_______________________________________________
Length overall Loa 111 m
Length between perpendiculars Lpp 84 m
Breadth B 17,5 m
Max draught Tmax 3,6 m
Test draught T 3,0 m
Block coefficient CB 0,36
Main propulsion 2 x 1200 kW
2 x Rolls-Royce AZP85-EL FF 12.5 TME
_______________________________________________
Figure 3. Double-ended ferry HAV Design, double-ended
design H-936.
Table 2. Hydrodynamic tools used for designing the H-936
double-ended ferry.
_______________________________________________
Experimental hydrodynamics (model tests)
_______________________________________________
Open water tests, stock and design propeller
Thruster fwd/aft load variations
Resistance tests, 2 waterlines
Self-propulsion tests, 2 waterlines
Trim optimization tests
Seakeeping tests, regular waves
Seakeeping tests, irregular waves - free running
Calm water stop and acceleration tests
_______________________________________________
Numerical hydrodynamics
_______________________________________________
VeSim (seakeeping, manoeuvring and dynamic positioning)
CFD (resistance, manoeuvring, hull optimization)
Stability
Internal weight analysis
Internal route, energy and fuel study
Station keeping analyses for thruster capability
Veres motion code:
- Added resistance indication
- Motion characteristics optimization
CAESES
- Automated CAD/CFD optimization of hull
- Ship speed and powering
- Calm water resistance
- Propeller diameter optimization
- Performance in real seas
Star CCM
Calm water flow analyses optimization
_______________________________________________
3 INITIAL DESIGN VERIFICATION TESTS
Using the given design, HAV Design contracted
SINTEF Ocean to conduct a series of different model
tests and numerical studies (see Table 2) to investigate
design characteristics and to verify the numerical
models used in the early design phase. Experimental
studies were performed in SINTEF Ocean’s Towing
Tank. At this stage studies of manoeuvring
performance were not included. Model scale used for
these tests was 1:12.637.
The outcomes of these tests were compared to
numerical predictions from the tools described in the
previous section. Figure 4 compares predicted power
(on the electric motor) and measured values for sea
trial with four sister vessels. The figure shows that
predictions based on the model test overestimates the
measured from the four sister vessels of the H-936
design for speeds above 11 knots. For the first three
delivered vessels (ships 1 - 3), trial trip results have
been corrected using the ISO standard [4]. This will
also be done for the fourth ship (yard trials not
analysed yet). The figure includes computational fluid
dynamics (CFD) calculated resistance. As can be seen,
these results compare well with sea trial data.
Figure 4. Comparison between sea-trial data on sister
vessels and model test results.
4 DESIGN VERIFICATION OF MANOEUVRING
PERFORMANCE BASED ON MODEL TESTS
For an MSc thesis [5] HAV Design and NTNU
(Norwegian University of Science and Technology)
collaborated to fund the building of a new hull model
and to run additional model tests in SINTEF Ocean's
Towing Tank. For Planar Motion Mechanism (PMM)
and oblique towing tests the model scale was 1:15.33
giving an approximately 8 m long mode (somewhat
smaller than the model used for resistance and
propulsion tests). Figure 5 shows the model in the
Hexapod system.
66
The focus of these tests was to investigate the
quality of numerical tools for simulation of
manoeuvring performance. In principle, two methods
were used to generate input data to SINTEF Ocean's
six degree for freedom (6 DOF) time-domain vessel
simulation tool VeSim [9]. The first one is purely
numerical using the Hullvisc program to generate the
hydrodynamic input file for VeSim [6]. It is based on
linear slender body theory and a cross-flow drag
formulation for non-linear damping forces. The
second one is based on experimental data from model
scale oblique towing and PMM tests. In the thesis,
Leinebø compared outcomes from these sets of input
data. A mathematical three degree of freedom (3 DOF)
solver for the non-linear coupled surge, sway and yaw
equations (SIMAN), was also used with only
numerical input data. The simulations were
performed with the aft propulsion unit working and
the fore azipull turned off.
Figure 5. HAV Design H-936 connected to seakeeping
carriage for PMM tests.
Challenges were experienced using an azipull
propulsion model, which were solved by utilising a
simplified propulsion and rudder model. They were
tuned to and compared against a working model in
SIMAN (previous time-domain manoeuvring
simulation tool used by SINTEF Ocean). This model
showed good agreement with different standard
manoeuvring tests [3] with rudder/azipull angles up
to 20-25 degrees (with the same numerical hull input).
Figure 6 shows the results of a complete spiral
simulation using both time-domain simulation tools.
In figure 7 there is good agreement for the full-scale
sea trial and predicted turning circle manoeuvre when
VeSim used a 20- and 25-degrees rudder angle and
SIMAN used a 35 degrees azipull angle.
Figure 6 shows some challenges when using the
numerical HullVisc tool (based on slenderbody and
strip theory) to create the input field for VeSim. This
is mainly due to the design of double-ended ferries
where the combination of main dimensions is outside
the parameter range for regression formulas used
when developing the present numerical tool. The
numerical method experienced some unstable
solutions because of this. A new numerical tool based
on CFD are under development at SINTEF Ocean, and
it is presently tested for a smaller HAV Design (50 car)
double-ended ferry. Until CFD results are validated
against model test results, the simpler and faster
HullVisc tool will be applied in the early design
phase.
Figure 6. Complete spiral manoeuvre simulations of H-936.
Figure 7. Turning circle manoeuvre. Simulations vs full-
scale of H-936 (identical SB and PS simulations)
Two different ways of using experimental results
were applied. Both are based on mapping force
measurements from the experiments to common time
series for surge and sway forces and yaw moment. A
modified least square method was then used to
estimate hull force coefficients in a 3 degree of
freedom model developed by Ross [8] integrated into
the six degree of freedom model used in VeSim. In the
first one, only PMM test data was used and in the
second the combined oblique towing and PMM tests
were used. Including the oblique towing force
measurements gave a large difference in some of the
hydrodynamic coefficients (also for added mass
coefficients). This showed how different set of
coefficients in the motion equations could be
developed from model tests. Even if the values of the
individual coefficient in the simulation varied
significantly, the outcomes from VeSim simulations
were nearly the same for IMO's turning circle test [3].
67
But this will give challenges for manoeuvres outside
the measured areas.
Table 3 compares numerical and experimental
input data used in VeSim with the average overshoot
angles from full-scale trials. The table presents the
differences between the full-scale average overshoot
angles and the predicted overshoot angles using
different VeSim input. Negative values represent
lower overshoot angles in the simulations compared
with full-scale results, and positive values vice versa.
The experimental input shows the improvement of the
overestimation of the vessel’s course-keeping and
course-changing ability in the numerical input. The
RMS values from the sea trials varied between 1-2
degrees, resulting in a high percentage deviation at
low overshoot angles. The 5°/5° zig-zag manoeuvre
was requested by the master during sea trials in May
2019 (see section 5).
Guidance notes from NAUT(AW) class notation
states [1]: The characteristic parameters should be within
15% of the parameters obtained from the full-scale trials. If
deviation exceeds this figure, the whole full-scale trial
program should be completed.
Comparing overshoot angle predictions from
VeSim with the measured sea trial data, it is seen that
some results deviate more than 15%.
Table 3. Zig-Zag manoeuvre test, numerical (HullVisc) and
experimental input (PMM) data in VeSim compared with
full-scale results of H-936.
_______________________________________________
Zig-Zag Manouvre Overshoot angles differences
Simulations 1st (Deg) 2nd (Deg) 3rd (Deg)
_______________________________________________
Numerical input 5/5 + 0.7 - 1.5 - 0.6
10/10 - 1.9 - 5.0 - 4.7
20/20 - 4.9 - 2.8 - 5.6
Experimental 5/5 + 1.3 + 0.3 + 1.3
Input 10/10 - 0.1 - 1.8 - 1.7
20/20 - 1.2 + 0.6 + 2.3
_______________________________________________
When applying the present simplified numerical
tool HullVisc to generate input data to VeSim, the
result is large discrepancies between simulated
turning circle characteristics to the ones coming from
experimental input data. This is an expected result as
the HullVisc tool used for a double-ended ferry is
outside of the validity range of correlation factors
used in HullVisc as they are adapted to conventional
merchant ships. The hydrodynamic coefficients in the
manoeuvring equations coming from experimental
PMM results give better prediction of IMO standard
manoeuvres. In the early design phase, there is a need
for a numerical tool for generating input, for
unconventional ship designs such as the double-
ended ferry, to time-domain manoeuvring simulation
tools. As mentioned earlier, there is an ongoing
activity to develop a specific CFD code for better
estimation of hydrodynamic coefficients for the
manoeuvring equations for unconventional ship
designs.
Both numerical and experimental input in the
turning circle simulations showed a much higher peak
in rate of turn in simulations, with this a quicker
established drift angle. The simulations also gave too
low a period between overshoot angles, because of
higher peak values in rate of turn. Further
investigation showed that the simulation did not
capture the full effect of the fore azipull.
5 VALIDATION OF MANOEUVRING
PERFORMANCE IMO STANDARD
MANOEUVRES
For documentation of the manoeuvring performance
of the H-936 design, the ferry company Fjord1 made
the double-ended ferry MF Suløy available for sea
trials in May 2019. The tests were performed in
Vartdalsfjorden where the water depth is 300 m,
figure 8. Weather conditions were excellent, with no
appreciable waves and a very low wind speed (2.8
m/s, direction 166°). Tidal current during the test
period was not measured. Two sources were used for
data collection. The main data source was the vessel's
Integrated Automation System (IAS), where among
position, heading, thruster settings, rpm and thruster
angle were measured. In addition, SINTEF Ocean a
dual Global Positioning System (GPS) for measuring
position and heading. All signals were recorded
synchronously as time series with a sampling rate of 1
Hz.
Figure 8. Test area for manoeuvring test with car ferry "MF
Suløy".
An overview of the main test program is given in
Table 4. The first part of the test program included
IMO standard tests. Outcomes of these tests were
used in the preparation of the vessel's Manoeuvring
Booklet [2]. The second part was manoeuvres
specified by the master on the ferry. Finally, some
IMO standard tests were repeated with a small
forward trim (obtained by positioning heavy trucks at
the bow).
Table 4. Manoeuvring tests from MF "Suløy" test campaign
May 2019. Test speed 13 knots.
_______________________________________________
Test type Number Test parameters Comment
of tests
_______________________________________________
Turning circles 7 Azipull angle 35° One 72
Zig-Zag 11 5°/5°, 10°/10°,
20°/20°
Direct spiral 4
Reversed spiral 9 5°/min, 10°/min, Investigating
15°/min width of
hysteresis loop
Stopping 21 Varying engine
control modes,
manual control
_______________________________________________
68
Figure 9 is a picture of the vessel performing an
IMO zig-zag test. As can be seen, the sea in the test
area is calm. Some initial validation studies using the
full-scale tests have been described in section 4. Figure
7 (in section 4) compares turning circle paths and
ways to handle the deviations between measurements
and predictions by either changing the control angel
of the azipull unit or replace the unit by a
conventional rudder/propeller system. It was found
that the applied generic azipull model gave too high
control forces for high angles. Simulations were thus
also run using different azipull angles the best
turning circle results were obtained using a 25°
control angle to port and 20° to starboard. Also, for
the overshoot angles, there are some differences
between measured and predicted values. Improved
outcomes from VeSim are obtained by tuning the
hydrodynamic coefficients and especially control
system parameters using the deviations between the
initial VeSim predictions and full-scale
measurements.
There are two main reasons for the difference
between sea trial manoeuvres and VeSim predictions.
The first one is due to limitations introduced by the
Hexapod system used by SINTEF Ocean for PMM
tests. For highly manoeuvrable vessels, such as
double-ended ferries with azipull systems, it is not
possible to obtain the high drift angles and yaw rates
that are measured during turning circles and other
tests applying large control unit angles. The second
one comes from the existing model of the hull and
azipull unit interaction in VeSim. CFD is presently
used to study these interactions and new models for
interaction effects will be developed for double-ended
ferry designs.
Figure 9. Zig-zag test seen stern-wise from the bridge of MF
Suløy.
The results of the manoeuvring sea trials have been
used by SINTEF Ocean and HAV Design to tune hull
and propulsion force models for the VeSim simulation
tool. The goal of this work has been to reduce the
differences between sea trial measurements and
VeSim predictions. The reliability of VeSim with
numerical input for new hull forms needs to be
improved so it can be used as one of the design tools
by HAV Design in their work to develop new zero-
/low-emission double-ended ferries with high
manoeuvring performance. Based on tuning
experience, it is concluded that approvements are
needed with respect to:
Four quadrant asipull model
Interaction effects between hull and asipull
Two alternative ways of improving VeSim input
for early design phase studies are presently
investigated. One is to extend the present HullVisc
tool to unconventional ship designs by using 3D
potential flow theory. The other one is to generate
input data from a CFD based PMM.
6 SOME COMMENTS ON OPERATIONAL
EXPERIENCE
The IMO standard tests give information for the ship
designer more than for ship captains. Despite double
ended ferries showing a low degree of directional
instability, based on direct and reverse spiral tests,
and qualifying well within IMO’s criteria on zig-zag
tests, there is no guarantee that the ferry is steerable in
critical and typical manoeuvring situations, especially
during the retardation phase. One specific effect that
is purely related to the hull and propulsion units is
the behaviour during retardation. Some of the ferry
captains initially reported unusual effects related to
control of the vessel during retardation manoeuvres.
Analysing the reports, it was concluded that lack of
experience with the actual control system (azipulls
fore and aft) caused some of these effects. The higher
the initial speed when initiating retardation, the more
unstable the vessel will be in the retardation phase.
Such an operational characteristic increases the stress
level for the captains during the final phase of a
voyage. To improve this behaviour, a set of new of
test manoeuvres have been suggested by the captains
to identify the best operational procedure for
operating both of the azipulls during retardation and
docking. The outcome of such tests, and a specific
operational guideline for these part of the operation
under varying environmental conditions, should be
documented in the vessel's Manoeuvring Booklet.
Based on discussions with double-ended ferry
masters, they are asking for better documentation of
low-speed manoeuvring performance, especially for
harsh weather conditions. This should also be part of
the vessel specific Manoeuvring Booklet. In addition
to these tests, which will reveal behaviour during sea
trials, it is even more important to detect
manoeuvring challenges during the design process
and hence modify design to overcome this. Hence, a
methodology for developing a reliable simulation
model that capture the behaviour during retardation
should be specified. The essence is to conduct
systematic CFD studies/ model tests to generate
hydrodynamic coefficient input to the time domain
simulation model so it can be used as an early design
tool. This tool (for instance a simplified version of
VeSim) could then be used by designers to investigate
manoeuvring performance, both IMO standard
manoeuvres and ship specific low speed manoeuvres
requested by captains.
69
7 VESSEL SPECIFIC MANOEUVRING
HANDBOOK
As described prior, the results from service speed
turning circles, zig-zags and stopping tests were used
to produce the Wheelhouse Poster and the Pilot Card.
Additional IMO manoeuvres such as spiral tests and
manoeuvres specified by the master (especially low-
speed tests) were used by HAV Design in the
development of the ship specific Manoeuvring
Booklet, see figure 10. As mentioned in the previous
section, vessel characteristics during the retardation
phase should be found here.
Figure 10. Front page of Manoeuvring Booklet for double-
ended ferry MF Suløy.
8 CONCLUSIONS
From the MSc study, it has been shown that present
numerical models to predict coefficients in
manoeuvring equations need to be improved. The
position of the separation point, used in the
calculation of hydrodynamic derivatives, deviates
significantly from values used for traditional
displacement vessels. The use of regression type
coefficients is not recommended for double-ended
ferries, partly due to the fore and aft symmetry of the
hull. Based on the comparison of simulated standard
manoeuvres and full-scale measurements, it is
concluded that more work is needed to understand
the influence of the forward azipull in all phases of
the vessel operation. Modifications of VeSim to
include this influence are presently investigated.
Application of least square methods to identify
linear and non-linear force coefficients from captive
model test must be used carefully. Motion parameters
in low-speed manoeuvres may be outside the speed
and acceleration domains used in the tests. More sea
trial data should be obtained for further validation
studies of the manoeuvring models for double-ended
ferries.
An early design phase tool for investigating
manoeuvring performance of double-ended ferries is
under development. Using sea trial data from a set of
double-ended ferries (50, 80 and 120 cars) developed
by HAV Design, SINTEF Ocean works to improve
methods (3D slender-body theory with empirical
corrections and CFD PMM) for creating the
hydrodynamic input files to the time domain VeSim
simulation tool.
IMO standard manoeuvring tests are of little value
for ship captains on double-ended ferries. They are
requesting more information on low-speed
manoeuvring performance which should be
documented in the vessel's Manoeuvring Booklet.
Addition to these tests, which will reveal
behaviour during sea trials, it is even more important
to detect manoeuvring challenges during the design
process and hence modify design to overcome this.
Hence, a methodology for developing a reliable
simulation model that capture the behaviour during
retardation should be specified. The essence is to
conduct systematic CFD studies/ model tests to build
a time domain simulation model to be used as an
early design tool. Such a tool would be used to predict
outcomes of IMO standard manoeuvres, low speed
manoeuvres requested by ferry captains, and special
off design performance characteristics of a specific
ship.
ACKNOWLEDGEMENT
The authors acknowledge the support from Fjord1 which
made it possible to use MF Suløy for an extensive full-scale
manoeuvring test program in May 2019. We also thank the
bridge crew on the ferry for their enthusiasm during the
tests and their proposals for low-speed tests whose results
are included in the Manoeuvring Booklet. Finally, we
appreciate the support from Norwegian Electric Systems
(NES) personnel in their assistance to link the vessel's data
logging system to SINTEF's sea trial instrumentation
package.
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