1) There are currently no internationally agreed upon stability requirements specifically for anchor handling tug supply (AHTS) vessels. 2) After the 2007 accident of the AHTS Bourbon Dolphin, initiatives were taken to improve design, operations, and stability requirements for AHTS vessels, including guidelines from the Norwegian Maritime Directorate. 3) The guidelines from the Norwegian Maritime Directorate propose criteria for limiting the heeling moment on AHTS vessels during anchor handling operations based on the angle of heel equivalent to 50% of the maximum GZ, the angle of flooding of the work deck, or 15 degrees, whichever is smallest.

1. Formulating Stability Requirements for
Anchor Handling Tug and Supply
(AHTS) Vessels
Dephne Chea Wei Peng, Arun Kr Dev, Ivan Tam, (Newcastle University)
Kenneth Hanks, Kalyan Chatterjea (EMAS Training Academy & Simulation Centre)
1

2. BACKGROUND
 No specific internationally applicable stability requirements for
AHTS vessels.
 After AHTS “Bourbon Dolphin” tragedy in April 2007, initiatives
were taken for improving
 Design
 Operational safety & specifically
 Stability and the performance of anchor handling winches.
 Probably the most important initiative is from the
 Norwegian Maritime Directorate (NMD):
NMD Circular - Series V, RSV 04-2008, 14 July 2008.
2

3. IMO Guidelines…BV Presentations, 2010 @ KL
 Refer to Intact Stability Code, 2008, IMO Res. MSC.267(85).
 Code covers
 intact stability criteria &
 addresses offshore supply ships & special purpose ships,
 requirement for a minimum freeboard at the stern of at least 0.005L
to be maintained in all operating conditions.
 Code does not include any specific stability criteria for towing and
anchor handling operations
 Minimum required GM0 = 0.15 m should NOT be considered
as sufficient stability margin for towing and anchor handling.
.3

4. IMO Criteria for OSV
 Max GZ of an OSV is allowed to
occur at much smaller angle of
heel than normal, providing
that the positive stability up to
this angle of heel (area A) is
greater than for a cargo ship
 This requires a large upright
GM
 Minimum freeboard criteria for
OSV is inadequate for anchor-
handling operation
 Clark & Hancox (2012)
4

5. Recommendation from
Marine Safety Forum (MSF)
 Quoting from Anchor-handling Manual template by
Marine Safety Forum (MSF), incorporating
recommendations from NMD.
 Prior to sailing, a document must be displayed on the
bridge, where it is visible to be navigator on duty, to
show the acceptable vertical and horizontal transverse
force/tensions to which the vessel can be exposed.
 This should show a sketch of the GZ curve and a table
of the tension/forces which give the maximum
acceptable heeling moment. 5

6.  Further quoting from the Anchor-handling Manual
template by MSF incorporating recommendations from
NMD.
 Calculations must show the maximum acceptable tension in
wire/chain, including transverse force, that can be accepted
in order for the vessel’s maximum heeling to be limited by
one of the following angles, whichever occurs first:-
o Heeling angle equivalent to a GZ value equal to 50% of GZ max.
o The angle of flooding of the work deck – i.e. the angle which
results in water on working deck when the deck is flat.
o 15 degrees.
Recommendation from
Marine Safety Forum (MSF)
6

7. NMD Criteria
1. Heeling angle
equivalent to a GZ
value equal to 50% of
GZ max.
2. The angle of flooding of
the work deck – i.e. the
angle which results in
water on working deck
when the deck is flat.
3. 15 degrees.
 Vessel‟s Maximum
Heeling to be limited by
one of the following
angles, whichever
occurs first:
 Heeling moment must be
calculated as the total effect of
the horizontal & vertical
transverse components of the
force /tension in the wire/chain
 The torque arm of the
horizontal components shall
be calculated as the distance
from height of the work deck at
the guide pins to the centre of
the main propulsion propeller
or to the centre of stern side
thruster if it projects deeper
 The torque arm of the vertical
components shall be
calculated from the centre of
the outer edge of the stern
roller & with a vertical straining
point on the upper edge of the
stern roller
7

8. External Acting Forces & Healing Moments
…NMD Proposal to IMO
 α – angle between towline & ship’s centre line
 β – angle between towline & the waterplane
 y & v – refers to application point of the of the
line tension
 Ft – towline tension
Unified Stability Criteria & Operational Guidance for AH Vessels
Application Point & Direction
of a Towline in the Stern of a
multi-operational mode vessel
8

9. External Acting Forces & Healing Moments
…NMD Proposal to IMO
 Heeling Moment MH = (Ft .sin α.cos β x v)+(Ft .sin β x y)
Application Point & Direction
of a Towline in the Stern of a
multi-operational mode vessel
Transverse
Component
Vertical
Component
Unified Stability Criteria & Operational Guidance for AH Vessels 9

10. Tension Directions vs Healing Moments MH
…NMD Proposal to IMO
A typical example
of Heeling Moments
with some specific
‘y’ & ‘v’ levers
β
α
So, in general,
 As ‘α’ increases
MH is increasing
 As ‘β’ increases
MH is decreasing
Unified Stability Criteria & Operational Guidance for AH Vessels
10

11. Influence of ‘α’ & ‘β’ on MH
…NMD Proposal to IMO
α
 As ‘α’ is increased
MH could reach to
an unacceptable
level!
β
MH
Unified Stability Criteria & Operational Guidance for AH Vessels
11

12. Concept of Constant Heeling Moment
…NMD Proposal to IMO
 The Norwegian proposal is based on a concept of α constant
heeling moment MH , providing limits
 to the line tension only dependent on sideways angle ‘α’
Vertical
Component
Transverse
Component
Unified Stability Criteria & Operational Guidance for AH Vessels
12

13. Fixed Moment vs Tension
…NMD Proposal to IMO
 Tension
distribution with a
constant Heeling
Moment
Unified Stability Criteria & Operational Guidance for AH Vessels
13

14. Operating Tension vs Angle Alpha
…NMD Proposal to IMO
 Finally, a
simplified Angle
Alpha vs Tension
could be created
 For use by the
operators
15 30 60 90
Unified Stability Criteria & Operational Guidance for AH Vessels
14

15. Graphical Presentation
…NMD Proposal to IMO
 A simplified graphical
presentation on bridge
 T1 – 0 to 150
 T2 – 15 to 300
 T3 - 30 to 600
 T4 - 60 to 900
Unified Stability Criteria & Operational Guidance for AH Vessels
DF – 1.4 to 1.6 times
the static load is common
Sector-1
Sector-2
Sector-3
Sector-4
15

16. Stability Limiting Curves
…NMD Proposal to IMO
Unified Stability Criteria & Operational Guidance for AH Vessels
 For AH-Operations,
the stability limiting
curves should be
developed as a
function of
draught/displacement
against initial KG or
GM & applied tension
 Covering lightest
anticipated draught to
Summer Load Line &
trim range
Heeling Angle Flooding Angle
16

17. Stability Limiting Curves
…NMD Proposal to IMO
Unified Stability Criteria & Operational Guidance for AH Vessels
 The minimal residual
area between the
righting lever curve
and the heeling lever
curve ≥ 0.055 m-rad
 [θe to θf or θc
whichever is less]
17

18. IMO Amendment - Part B 2008 IS Code
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19. Ultra Deep Water Multifunctional AHTS Vessel
LOA – 93.4m
LBP – 82.0m
B – 22.0m
D – 9.5m (main deck)
Design Draught – 6.5m
Bollard Pull – 300 tonnes
AH/Towing Winch – 500 tonnes pull [600 tonnes brake]
Propulsion - 23,467 BHP
DP II
Deadweight Approx. 4,700 T
GRT Approx. 6,000 T
Lewek Fulmar
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20. Lewek Fulmar uses AUTOLOAD 6
AUTOLOAD 6 ASUMPTIONS:
 Tension from chain is placed at the full breath of aft roller, & the chain
angle (tension) is calculated from 0o(vertical) to 90o(horizontal)
 The vertical moment arm from main thrusters (or aft side thrusters if
existing) and up to aft roller is kept constant, independent of vessels
heel angle.
 The horizontal moment arm from end of aft roller to centerline is kept
constant, independent of vessels heel angle.
 Both the effect of the horizontal forces and the tensions offset from CL
is converted to an external moment acting on the vessel.
 To keep correct displacement during all calculations, the vertical
tension component (the weight) is placed at centerline on top of aft
roller. 20

21. Lewek Fulmar uses AUTOLOAD 6
Tank Sensors in AUTOLOAD 6:
 Autoload installations can be fitted with a Tank Sensor
program that interfaces directly with Autoload
 Thus providing immediate tank loadings for an accurate and
up-to-the-minute analysis of the vessel’s stability.
21

22. NMD stability criteria for anchor handling is
incorporated in AUTOLOAD 6
22

23. NMD stability criteria for anchor handling is
incorporated in AUTOLOAD 6
23

24. General Interface in AUTOLOAD 6
Hydrostatics Anchor-Handling Margins
24

25. IMO 469 Intact Stability in AUTOLOAD 6
Righting Arms vs. Heel - IMO 469, INTACT STABILITY
Heel angle (Degrees)
A
r
m
s
i
n
m
0.0 10.0p 20.0p 30.0p 40.0p 50.0p 60.0p
0.0
0.5
1.0
Righting Arm
R. Area
Flood Pt
25

26. NMD Criteria in AUTOLOAD 6
Righting Arms vs. Heel - NMD ANCHORHANDLING CRITERIA
Heel angle (Degrees)
A
r
m
s
i
n
m
0.0 10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s
-0.5
0.0
0.5
1.0
Righting Arm
Heeling Arm
Equilibrium
Crit. Pt
26

27. NMD Criteria in AUTOLOAD 6
Righting Arms vs. Heel - NMD ANCHORHANDLING CRITERIA
Heel angle (Degrees)
A
r
m
s
i
n
m
10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s
-1.0
-0.5
0.0
0.5
Righting Arm
Heeling Arm
Equilibrium
Crit. Pt
27

28. NMD Criteria in AUTOLOAD 6
Righting Arms vs. Heel - NMD ANCHORHANDLING CRITERIA
Heel angle (Degrees)
A
r
m
s
i
n
m
10.0s 20.0s 30.0s 40.0s 50.0s 60.0s 70.0s
-1.5
-1.0
-0.5
0.0
0.5
Righting Arm
Heeling Arm
Equilibrium
Crit. Pt
28

29. Conclusions
 Recommendation for Stability Requirements during Towing &
Anchor-handling Operations are likely to be covered by the Par „B‟
of the Intact Stability Code, 2008, IMO Res. MSC.267(85) in
2014
 We should be prepared with a suitable course to support the
professionals at the operational level
 We hope there will be continuous support from the academia
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30. Thank You!!
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