Towingrecomends

69
Day 1
Paper No. 7
DNV Towing Recommendations
Rolf Hilmar Hansen, Det Norske Veritas, Norway
SYNOPSIS: This is a review of some important towing related requirements and recommendations given in Det
Norske Veritas (DNV) Rules for Planning and Execution of Marine Operations, ref. /1/, hereafter referred to as
“VMO Rules” (VMO = Veritas Marine Operations). The VMO Rules was issued in 1996, based on the “DNV
Standard for Insurance Warranty Surveys in Marine Operations” of 1985. Latest update of the VMO Rules was
in 2000. Our experience with these Rules is discussed in the paper, and some possible future changes are
indicated as well. DNV is planning to issue new, completely revised VMO Rules in 2005.
1 INTRODUCTION
1.1 Application of VMO Rules
The VMO Rules is a free-standing document within the DNV
hierarchy of Rules, Standards and Certification Notes, and
cannot be directly used in connection with the classification of
vessels.
As the full title of the VMO Rules indicates, it covers
planning and performance of marine operations, including
towing operations.
The Rules contains the following parts and chapters:
– Part 1- Planning, stability, design loads, structural design.
– Part 2, Chapter 1 – Load Transfer Operations
– Part 2, Chapter 2 – Towing
– Part 2, Chapter 3 – Special Sea Transports
– Part 2, Chapter 4 – Offshore Installation
– Part 2, Chapter 5 – Lifting
– Part 2, Chapter 6 – Sub Sea Operations
– Part 2, Chapter 7 – Transit and Positioning of Mobile
Offshore Units
The overall objective of the Rules is to ensure that marine
operations are performed within defined and recognised
safety levels.
When DNV is acting as Marine Warranty Surveyor (MWS)
of a marine operation, review of documents and survey
during preparations and execution of the operation is
performed according to the VMO Rules. When the required
documentation of an operation has been received, reviewed
and approved, the prevailing conditions have been found
acceptable, and all surveys completed to the DNV Marine
Surveyor’s satisfaction, a Marine Operation Declaration may
be issued:
– For towing of a barge or a ship, DNV “Towing
Declaration”, will normally be used.
– For all other operations, DNV “Marine Operation
Declaration” will be used.
Also, the VMO Rules is often a contractual document for
development of offshore oil/gas fields, which contractors have
to deal with and warranty surveyors have to inspect in
accordance with, regardless of the name of the actual MWS-
company.
1.2 Salvage operations
Salvage operations are not particularly covered in the VMO
Rules. It is realised that during salvage of a vessel (in
particular with crew and passengers onboard) one often has
to use the equipment and resources available on site,
regardless of rules and regulations. However, the
recommendations to capacity and strength of tugs and
equipment given in the VMO Rules may still be useful during
salvage operations.
1.3 References
/1/ DNV Rules for Planning and Execution of Marine
Operations, 1996/2000.
/2/ DNV Rules for Classification of Ships.
/3/ IMO Circular MSC/Circ.884 – Guidelines for Safe Ocean
Towing, 1998.
/4/ ITS 2002 – Paper “Bollard Pull” by Hannu Jukola and
Anders Skogman, Steerprop Ltd.
2 OPERATIONS
2.1 General
Regardless of the technical condition of towing vessel,
equipment and towed object as addressed in the following
chapters, the success of the towing operation is highly
depending on the standard and experience of the crew, and
in particular the master. This issue is not discussed further in
this paper; it is assumed that the crew holds the relevant
certificates and that proper seamanship will be exercised.
2.2 Weather routing
Towing operations may be categorised as;
– unrestricted or
– weather routed.
Marine operations with an operation reference period,

70
exceeding 72 hours are normally defined as unrestricted
operations. Environmental criteria for these operations shall
be based on extreme value statistics. However, the design
criteria for determination of minimum towing force for open
sea towing (i.e. unrestricted towing) is fixed (see sect. 3.2)
and not based on actual statistics for the towing route and
season.
A towing operation may be weather routed (i.e. weather
restricted) due to limitations on the towed object. Weather
routed tows shall seek shelter if weather situations exceeding
the operation criteria are forecasted or experienced. Ports of
refuge and/or area of shelter shall be defined in the towing
procedures. Entrance, geography and size of shelter shall be
considered. Weather routing criteria will normally be given as
recommendation on the towing certificate/declaration issued
by the MWS.
Regardless of category (unrestricted/weather routed) of
the towing operation, a tow out criteria of Beaufort Force 5 or
better for the coming 24 hours is normally applicable. The
intention with the tow out criteria is to allow time for
familiarisation with the tow, and to ensure adequate distance
to shore in case of adverse weather conditions.
DNV experience: There have been weather routed towing
operations where the forecast for the route seems not
being conscientiously assessed onboard the tug, and bad
weather encountered resulting in towline breakage and/or
damage to the towed object. In principle, such action of the
tug master (or lack of action!) may be classified as “breach
of warranty” and may render the insurance of the towed
object invalid.
2.3 Voyage reporting
Daily voyage reporting to owners of tug and towed object is
normally required, and should at least contain position, sea
condition, speed, ETA and status of tug and towed object.
The reports should be copied to the MWS.
DNV experience: Even if voyage reporting is given as a
recommendation on the DNV Towing Declaration, the
report is often not copied to DNV. This is unfortunate in
case of incidents occurring during the tow, and may cause
embarrassment towards the insurance.
2.4 Towing manual
A towing manual shall be prepared and distributed to key
personnel.
The manual shall normally contain information regarding;
– tow out criteria,
– criteria for seeking shelter,
– towing route,
– ports/areas of shelter,
– estimated towing time (ETD, ETA),
– environmental limitations w.r.t. structural capacity of
object, seafastening, etc.,
– contingency actions,
– description of the ballast condition,
– reporting routines for progress of the tow, ETA, status,
etc.,
– contact persons and telephone numbers,
– expected environmental conditions for the intended towing
route for the relevant season, and
– procedures for departure and arrival as well as calls at
intermediate ports.
The tow master (usually the tug master) shall familiarise
himself with the towing manual.
3 TUG
3.1 Type, size and classification of tug
The towing vessel can be a tug, an anchor handler, or a type
that combines these functions with several others, and have
the Class Notation “Tug”. The main requirement is that the
vessel must be equipped with a remote operated towing
winch. The background for the towing winch requirement is
at all times to have the flexibility to adjust the length of the
towing line according to environmental conditions, water
depth, other traffic, and width of the navigated area.
Formally there is no minimum size/length of a tug, but
vessels for open sea towage shall not have any service
restrictions. Note that the VMO Rules penalise tugs of less
than 45m length with respect to towing efficiency, see 3.2
below.
DNV is considering redundancy requirements on critical
systems (propulsion, steering, winch, etc.) for tugs
performing towing of “high value” objects in the next revision
of the VMO Rules.
The tug must, of course, hold valid statutory certificates
and trading permit and will be inspected by the MWS prior to
commencement of the towing operation. Class is formally not
required; a certificate from the national maritime authority is
also acceptable.
Tugs with a towing hook only (i.e. no towing winch) are not
accepted by DNV for ocean towing, but may be permitted for
towing in harbours or towing jobs of short duration/length in
sheltered waters.
DNV experience: There have been a few cases where tugs
have been turned down at the time of inspection by the
DNV marine surveyor due to gross overdue validity of
certificates or a general unsatisfactory condition of the tug
(e.g. fuel oil spill/leaks in the engine room).
3.2 Minimum bollard pull of tug
According to the VMO Rules towing force for open sea
towing shall be sufficient to maintain zero speed under the
following conditions:
– sustained wind velocity VW = 20 (m/s),
– head current velocity VC = 1 (m/s), and
– significant wave height HS = 5 (m).
Towing force for coastal towing and towing in narrow or
shallow waters representing a danger for grounding, shall be
sufficient to maintain a speed over ground, in safe direction,
of minimum 2 knots under defined environmental design
conditions. Note that for an area with strong current this may
lead to higher towing force than required for ocean tow. Also,
passage through narrows may require additional tug(s)
connected to the stern of the towed object.
Required tug bollard pull (BP) shall be estimated based on
calculated required towing force and tug efficiency in waves.
DNV experience: The DNV BP criteria seems to be

71
established and accepted by the industry, even though the
current criteria is 1.0 m/s, compared with 0.5 m/s given in
the IMO Guidelines ref. /3/. Usually the current resistance
is the least part of the calculated total towing resistance
(contributions from wind, waves and current).
3.3 Tug efficiency
Unless more accurate calculations or model tests of towing
efficiency of the tug in waves are made, the continuous
static bollard pull stated in the bollard pull certificate shall be
multiplied with an efficiency factor:
γTE = 0.75(1 – γL)
where
γTE : tug efficiency factor
γL : tug length factor, γL = (1 – L/45)2
L : tug length (m), not to be taken more than 45 m
For tugs performing weather routed towing or towing in
protected areas/harbours, a tug efficiency factor below may
be used instead:
γTE = (0.875 – γW/8)(1 – γL * γW)
where
L : length of tug (m), not to be taken more than 45 m
γL : tug length factor, γL = (1 – L/45)2
γW : wave factor, γW = HS/5
HS : limiting significant wave height (m) for the weather
routed towing operation, or the probable significant
wave height in the protected area/harbour. HS is not
to be taken less than 1 meter and not more than 5
meter in this equation.
The resulting tug efficiencies from the formulas above are
given in Table 1. It is clear from the formulas and the table
that for a tug of more than 45 m length its efficiency for
ocean towing (i.e. unrestricted towing) is found to be, γTE =
0.75 and for weather routed towing maximum γTE = 0.85.
3.4 Bollard pull certificate
All towing vessels should have a bollard pull (BP) certificate.
Such certificate is also required in the VMO Rules. Usually
the BP certificate is issued by the classification society of
the vessel, after a BP test. Several of the classification
societies (including DNV) have procedure for such tests
which is more or less in line with the bollard pull test
procedure given in Appendix A of the IMO Guidelines ref. /3/.
The adequacy or precision of the various BP test
procedures may of course be discussed, see e.g. ITS 2002
paper from Steerprop Ltd., ref. /4/. The DNV BP test
procedure is therein considered to be one of the better ones.
We do however agree with Steerprop Ltd. that a more
detailed commonly accepted test procedure would be
advantageous for the towing industry and their clients.
Note that according to the VMO Rules the bollard pull
certificate (and the test!) shall not be older than 10 years.
Also, if the vessel has undergone significant structural or
machinery changes a new bollard pull test may be required
and could also be advantageous with respect to the BP
figure.
DNV experience: There are tugs with dubious BP
certificates, either far too old or issued by unknown
bodies/consultants. We have also experienced certificates
based on theoretical calculations, which is of course
unacceptable as a BP certificate.
When acting as MWS, DNV will in such cases either reject
the tug or determine the BP to be used as follows:
– Tug with propeller nozzle: BP = BHP/90
– Tug without nozzle: BP = BHP/110
(BHP = brake horse power of the propulsion machinery)
3.5 Equipment of tugs
– For ocean towing the towing vessel shall be equipped
with a certified towing winch of capacity matching the
bollard pull, remote operated from the wheelhouse and
with towline load monitoring.
– One spare towline of minimum length and strength as
required for the main towing line shall be onboard the
towing vessel, preferably on a separate drum on the
towing winch or on a storage drum.
– At least one suitable workboat with propulsion should be
Table 1 Tug efficiency, γγTE
Weather routed towing/harbour
Tug length (m) Unrestricted towing Sign. wave height HS (m)
1.0 2.0 3.0 4.0 5.0
10 0.30 0.75 0.63 0.51 0.40 0.30
20 0.52 0.80 0.72 0.65 0.58 0.52
30 0.67 0.83 0.79 0.75 0.71 0.67
40 0.74 0.85 0.82 0.79 0.77 0.74
50 0.75 0.85 0.83 0.80 0.78 0.75
60 0.75 0.85 0.83 0.80 0.78 0.75
70 0.75 0.85 0.83 0.80 0.78 0.75
80 0.75 0.85 0.83 0.80 0.78 0.75
90 0.75 0.85 0.83 0.80 0.78 0.75
100 0.75 0.85 0.83 0.80 0.78 0.75

72
carried onboard for transferring personnel and
equipment from the tug to the towed object.
DNV experience: In the next revision of the VMO Rules it
will probably be required that ocean-going tugs shall have
the spare towline on a separate drum on the winch.
4 TOWING LINE
4.1 Minimum length and strength of towing line
For unrestricted towing VMO Rules require that the main
towing line shall have a length of not less than 2000BP/MBL.
The certified minimum breaking load (MBL), in tonnes, of the
main and spare towing lines (steel wire ropes) shall be taken
as:
– 3.0 BP BP ≤ 40
– (3.64 – 0.8 BP/50) BP 40 < BP < 90
– 2.2 BP BP ≥ 90
where
BP : continuous static bollard pull of the vessel in
tonnes.
These recommendations to towline strength are equal to
the requirements for the additional Class Notation “Tug” in
DNV Rules for Classification of Ships, ref. /2/.
The resulting towline length and MBL is shown in Table 2.
DNV experience: The towline MBL requirement is now
found to be reasonable after being reduced for smaller
tugs in the 2000-revision of the VMO Rules. Most tugs
seem to comply with the minimum requirements, even if
the 2.2 factor for tugs of more than 90 t BP is higher than
the 2.0 factor recommended in the IMO Guidelines ref. /3/.
4.2 Fibre rope pennant
Synthetic fibre rope pennant (“stretcher”) should normally
not be used where there is adequate depth and sea room to
allow for sufficient shock absorbing in the tow line catenary.
If fibre rope pennants are used the pennants shall be in as
new condition.
Minimum breaking load of any fibre rope pennants shall
not be less than;
– 2.3 times the tow line MBL for tugs with bollard pull less
than 50 tonnes,
– 1.5 times the tow line MBL for tugs with bollard pull
greater than 100 tonnes, and
– linearly interpolated between 1.5 and 2.3 times the tow
line MBL for tugs with bollard pull between 50 and 100
tonnes.
The resulting fibre rope MBL is included in Table 2.
DNV experience: DNV is in principle reluctant to the use of
fibre rope pennant/spring during ocean towing. Our
experience is that these pennants are often “used up”
before new ones are purchased. However, we are at
present looking into the 1.5 – 2.3 additional safety factor
on fibre rope pennants required by the VMO Rules and
may reduce this factor in the next revision. It will then be
even more important that such pennants are in “as new”
condition, as recommended in the VMO Rules.
4.3 Condition of towing lines
Before departure an inspection of the towing wire ropes
shall be performed by the DNV marine surveyor. At least the
first 50 metres of the towing wire should be streamed for
inspection.
According to the VMO Rules the towing line shall not be
used if the reduction of towline strength due to wear,
corrosion and broken wires exceeds 10 per cent and there is
severe kinking, crushing, or other damages resulting in
distortion of the rope structure. Also, the tow line should be
subject for special evaluations if number of broken wires
over a length of 7 times the tow line diameter exceeds 6 per
Table 2 Minimum towline length and MBL of steel wire rope and fibre rope pennant
Tug bollard pull Towline length (m) Steel wire towline MBL Fibre rope pennant MBL
(tonnes) (tonnes) (tonnes)
20 667 60 138
30 667 90 207
40 667 120 276
50 704 142 327
60 746 161 344
70 794 176 349
80 847 189 344
90 909 198 329
100 909 220 330
120 909 264 396
140 909 308 462
160 909 352 528
180 909 396 594
200 909 440 660
250 909 550 825
300 909 660 990

73
cent of total number of wires in the rope, if significant wear
of outer layer of wires are found or if the tow line is found
significantly corroded.
Wire rope end sockets or other end connections should
not be older than two years, depending on the extent of use
(wear and tear). Soft eye is not accepted, and neither is the
use of wire rope clips to make an end eye on the towing line.
A log for the towing lines shall be kept onboard, giving the
following information on each rope;
– date taken in use,
– records of inspection,
– date of renewal of end sockets or other end connections
and
– report on damage to the rope.
DNV experience: There have been several cases where the
towing wire rope has failed inside the spelter socket
probably due to improper casting with resin compound. It
requires special training to perform proper “socketing” of a
new spelter socket to the tow wire. Bad “socketing” may
not be visual afterwards.
5 TOWED OBJECT
5.1 Towing bridle and brackets
A bridle should preferably be used for connection of the tow
line to the towed object. Chains should be used in the way of
chafing areas such as fairleads.
Each single leg, components and connections (shackles,
rings etc.) in the bridle shall have a MBL not less than the
MBL of the main tow line. Reductions of equipment MBL due
to bending in way of fairleads, end connections etc. shall be
considered. Fairleads shall have a shape preventing
excessive bending stress in the chain links/wire.
Shackles, rings etc. are normally acceptable if stated safe
working load (SWL) is minimum 1/3 of the main towline
MBL.
A towing bridle should normally be attached to towing
brackets. Towline brackets/attachments (including
emergency towline attachment) shall be designed to resist
towline pull from any likely direction (usually ± 90o
), with the
use of fairleads if necessary. The ultimate capacity of any
towline attachment (bracket, bollard and their foundations)
shall not be less than 1.3 times the MBL of the towline.
DNV experience: There have been cases where the tug has
an oversized towline (MBL more than 4 times the BP of
the tug) leading to an unusual high requirement to towing
bracket capacity. In the next revision of the VMO Rules it
will probably be introduced alternative recommendations
(technical or operational) covering such cases.
The towing bracket capacity may also be insufficient if
the actual tug is significantly more powerful than needed or
planned. This problem is usually overcome by giving the
master of the tug a recommendation on maximum towline
pull to be exercised during the tow.
5.2 Emergency towing line
An emergency towing wire rope of minimum length equal to
the length of the towed object shall be connected to a bridle
or single leg connection, and lashed to the barge side for
easy release. A recovery trailing line with a pick-up buoy
shall be fitted to the emergency towing wire rope. The trailing
line shall be of floating material and shall have a minimum
breaking load not less than 30 tonnes. The distance from the
aft extremity of the towed object to the buoy shall not be less
than 50 metres. In addition to the trailing line, a messenger
line of length 100 metres may be considered necessary
between the buoy and the trailing line.
DNV experience: It is crucial that the emergency towing line
arrangement has sufficient length and strength to facilitate
pick-up and connection to the towing vessel and
continuation of the towing operation .
5.3 Behaviour of towed object
The steering quality and the rolling of the towed object may
depend on several factors; shape of underwater body, trim,
draught, stability, rudder/skeg, bridle or single leg, towing
speed, etc. The VMO Rules recommend minimum 0.5% (of
object length) trim by stern to help the steering
characteristics.
DNV experience: There have been cases where the towed
object yawed violently even with towing in a bridle, and call
at the nearest port for corrective actions became
necessary. Rudder or skeg will always be advantageous,
and change in ballast condition may also help.
5.4 Other requirements
– The towed object (barge, ship or any other self-floating
structure) shall have sufficient stability and water- and
weathertight integrity for the towing operation. The
stability shall be documented to be in compliance with
relevant criteria.
– If bilge pumps are not fitted on the towed object, bilge
suction may be arranged by portable pumps placed on
board, powered from the tug.
– There shall normally be an anchor with cable onboard for
emergency use, rigged for easy release.
– For unmanned towed objects, means for entering from
the tug shall be arranged on both sides.
– Navigation lights and signals to be installed on the towed
object.
– Towing of ships; securing of propeller shaft axial and
against rotation if shaft is disconnected from
engine/gearbox, securing of rudder if disconnected from
steering gear.
DNV experience: Watertightness of the towed object is an
obvious important element. There have been cases where
a partly completed hull has been towed to a new shipyard
for completion. Propeller and shaft have been installed
before departure, but not gearbox/engine. Due to
insufficient securing of the propeller shaft it came loose
(slipped backwards) and caused leakage through tailshaft
sealing which flooded the engine room and nearly sunk
the vessel.

74
you have all the papers, the certificates and so on, for the
personnel, it’s very rare that the marine warranty surveyor
has any additional requirements in this regard.
Joop Timmermans
The surveyor puts these additional recommendations to the
tug’s crew. I think you could do more in that respect at DNV.
Rolf Hilmar Hansen
OK. What we should like to see is to get something written on
this, because we could then be able to include that in our
next revision of the rules, and I guess your comments now
will be included.
Johan de Jong, MARIN
I would like to ask you a question about a table you presented
on the tug efficiency; this is also on page 3, and it shows the
relation between tug lengths and sea state and the resulting
tug efficiency. Could you explain to me a little bit what the
underlying data are for that formula? Is it based upon
experience from DNV with executed or performed tows, or is
it based on any research data. So, I’m referring to Table 1,
Page 3, which I think you also showed on one of the slides.
Rolf Hilmar Hansen
The table there is based on some stats on some papers we
had on tug efficiency, but then we just developed some
curves, which approximately go through those points. It also
takes into account that this 45m length of a tug, which some
30 years ago, defined in DNV internal guidelines for towing,
the tug should preferably be more at 45m in length. And then
we just developed some curves off that, and we ended up
with these factors here. For ocean-going tows, you will rarely
have a tug with less than 30m in length, I guess. So the part
of the table for the really small tugs is not so relevant for
ocean-going tugs.
But back to your question, it’s not based on any special
towing test or anything from this. It’s just a couple of figures
that we had from some old papers.
Robert Underhill, Jayco Mooring & Rigging
A rope salesman’s question – has any consideration been
given to looking at the use of HMP ropes, Spectra Dyneema-
type ropes in place of wire?
Rolf Hilmar Hansen
Yes, we are in fact looking into it. So it may happen in the
next revision of the rules. But I guess this is very interesting
for the tug owners; I don’t know whether any tug owners had
taken Dyneema or similar ropes in use as the main towline
yet. I don’t think so, but it is very interesting, so we may in fact
try to include something as I said, in the next revision if it’s
not too early. We’ll look into it.
Joop Timmermans, International Transport Contractors
Management BV
I have two questions. The IMO guidelines for safe ocean
towage feature quite prominently in your paper. My
understanding is that they have never been adopted. Is that
correct? And why do they then feature so prominently?
Rolf Hilmar Hansen
You mean that the IMO guideline is not adopted?
Joop Timmermans
No
Rolf Hilmar Hansen
OK
Joop Timmermans
I am asking that question of you.
Rolf Hilmar Hansen
I don’t know, but you can find it in the IMO Official…..
Joop Timmermans
As far as I know, they have never been adopted. Secondly,
has DNV ever rejected a towing vessel based on the
experience of the tug’s crew? Of the lack of experience of
the tug’s crew?
Rolf Hilmar Hansen
Yes, once.
Joop Timmermans
Only once. I feel there is quite a debate about bollard pull
certification, but there is never a debate about the experience
of a tug’s crew. Surveyors never put criteria on what the
experience is of a master of a tug – have you done so many
ocean crossings; have you done this or that? That is never
put on paper.
Rolf Hilmar Hansen
I’m not quite sure, in fact.
Joop Timmermans
We towed some of the ‘ghost ship’ fleet from the United
States to the UK, and one of the recommendations of the
surveyor was that the master had to have experience with at
least three double-tows across the Atlantic. The chief mate
had to have experience with at least one double-tow across
the Atlantic, and the bosun had to have at least three double-
tows across the Atlantic, as well. I thought that is something
which not many companies can do. Fortunately, we could
do it, but I think those are the recommendations we are
looking at.
Rolf Hilmar Hansen
You mean this was a requirement from DNV? In our view, if
DISCUSSION – DAY ONE, PAPER No. 7

Towingrecomends

More Related Content

What's hot (20)

Similar to Towingrecomends (20)

Recently uploaded (20)

Towingrecomends