687

1 INTRODUCTION

Failure of a ship’s own propulsion is a serious danger

not only for the ship not under command but also for

other vessels. From the definition of a ship underway,

you can define the drift process and then go to the

definition of a free-drift ship.

Influence of the vector of disturbed motion VZ on

the vessel VS results in the resultant drift vector ΔV as

shown in Figure 1a.

Figure 1. Ship drifting on the water surface

VZ – external disturbances vector, Vd – vector of object

movement over ground [Vd = f(VZ, A)], ΔV – resultant

ship's drift vector, A – extended section of the ship

When the ship loses the ability to move ahead the

vectors VZ and Vd are observed (Fig. 1b). During

external disturbances, a vessel or other free-floating

object loses its intended direction of movement. A

ship without its own propulsion cannot counteract

external disturbances, i.e. drift, thus becoming an

object not under command.

The value of the motion of a disabled ship depends

on:

− disturbing parameters (VZ)

− ship's geometric parameters (A).

2 COMPONENTS OF EXTERNAL DISTURBANCES

General division of ships in terms of external

disturbances according to geometric parameters:

− A1 – vessels with large windage areas – more

sensitive to wind load influence

− A2 – vessels with low free board – more

susceptible to current influence.

Processes of a Freely Drifting Vessel

M. Jurdziński

Gdynia Maritim

e University, Gdynia, Poland

ABSTRACT: The article describes the rules for planning a ship's navigation in the event of loss of propulsion. A

disabled ship drifting freely at sea is a potential danger to the crew and the marine environment. Lack of

propulsion means that the ship cannot give way to other ships/keep out of the way of another vessel. One of the

main elements of danger for a drifting ship is the possibility of grounding in restricted areas. The aim of the

article is to draw the attention of navigators to the dangers to navigation resulting from ships drifting without

their own propulsion, disabled ships.

http://www.transnav.eu

the

International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 14

Number 3

September 2020

DOI:

10.12716/1001.14.03.22

688

Figure 2. A1 and A2 Vessels and various FP/FW ratios

An example of a container ship surface is given for

comparison. The pressure from the wind on its

different side surfaces depending on the number of

layers of containers is surprisingly high. For example:

a container ship with a length of 400 m and a side

height of 35 m gives a lateral windage area of 14,000

m2. Table 1 gives wind load values on the ship’s hull .

Table 1. Wind load on the side area of 14,000 m

_______________________________________________

Wind speed [m/s] Pressure [t]

_______________________________________________

16 199

20 311

26 526

30 700

_______________________________________________

Drifts which are influenced by two major

elements, i.e. currents and winds of two different

ships have been analyzed here. Ships with A1

geometric structure are much more strongly

influenced by wind (passenger ships, ro-ro vessels,

etc.), whereas ships with A2 geometric structure (bulk

carriers, tankers, etc.) are affected more by the

influence of sea currents (tidal streams).

Ratio:

FP1/FW1 > 5/1 – (A1)

FP2/FW2 > 1/5 – (A2)

2.1 Components of external disturbances

The components of external disturbances are the

following:

− wind vector – VW, KW [knots, degrees]

− wave vector – hi, TF, KF [m, sec, degrees]

− swell vector – hM, TM, λp, KM [m, sec, m,

degrees]

− ocean current vector – Vp, Kp [knots, degrees]

− tidal current vector – Vpp, Kpp [knots, degrees]

− wind current vector – Vpw, Kpw [knots, degrees].

Additionally large vessels are influenced by a

Coriolis force.

2.2 Hull inclination of a drifting disabled ship

Location of the FP and FW gravity centers determines

the hull in motion in the process of free drift. Shifting

the FP point aft relative to the position closer to the

bow will cause the hull to turn windward, assuming

that the wind factor will be stronger than the current.

The vessel is said to tend to be windward.

The situation will be the opposite when the FP

point is closer to the forward part of the vessel than

the FW point – the ship will be leeward. It will also

depend on the value of wind and current forces as

well as the Coriolis force.

The pattern of the wind direction acting on large

ships and the direction of drift also depend on the

ship's trim, usually small on large ships. Such

arrangements are shown in Figure 3a and 3b.

Figure 3. Position of hulls depending on wind when the

vessel is adrift

a) with trim 1/50 by the stern and wind speed 30 knots

b) under ballast with trim 1/50 by the stern at 50 knot wind

speed

The drift velocity of a tanker depends on the

loading condition of the vessel such as ballast or full

load.

689

Figure 4. Graph of drift velocity of VLCC tanker loaded

and in ballast .

Figure 4 presents the drift velocity of a tanker in

ballast and loaded to capacity conditions as a function

of wind speed. In ballast condition vessels have a

larger windage area (FP) in relation to the submerged

part (FW).

Plan of the ship's drift close to dangers to

navigation is shown in Figure 10.

Figure 5. Situational plan of a disabled ship’s drift

The method of letting go the towing line onto a

ship in a drift is shown in Figure 6.

Figure 6. Chosen method of sending the tow onto the

disabled vessel

Three phases of sending the towline from the port

side of the tug H onto the vessel adrift where a

heaving line was sent to the disabled vessel and then

a towing line onto the vessel 1. Phase 2 – making the

tug fast. The last phase is towing and agreeing on the

speed of the tow VS(H3).

Figure 7. In bad weather pouring wave quelling oil in lateral

drift

2.3 Calculation of the bollard pull needed to tow a

disabled vessel

The formula can be used to calculate the bollard pull

for a free drifting vessel:

( )

0.06

120 60

M Bh K





= + ×× ×







(1)

M – required bollard pull in tonnes

D – full displacement of towed vessel in tonnes

VH – tow speed in knots

B – breadth of towed vessel [m]

h – height of the exposed transverse section of the

towed vessel including deck cargo, measured above

the waterline [m]

K – a factor that reflects potential weather

K = 1.0–3.0 – for exposed coastal tows

K = 0.75–2.0 – for sheltered coastal tows

K = 0.5–1.5 – for protected water tows

Another formula can also be used to calculate the

bollard pull for towing a disabled ship

( )

0.6

7.4M DWT= ×

(2)

In bad weather, ships over 100,000 (DWT) require

3 to 4 tugs of 3000 HP each .

The result of determining the bollard pull will

allow deciding on the number of tugs to stop the

ship's drifting and the method of holding the ship in

position when assistance of more tugs is required for

towing the ship to the place of refuge. In order to

reduce seaway parameters when the towing lines are

sent to the drifting vessel a method of wave quelling

oil spread on the sea surface may be used. See Figure

690

3 OPERATION DIAGRAM FOR PREPARING A

TOW PLAN FOR A DISABLED SHIP

Figure 8. Operation diagram for preparing a tow plan for a

disabled ship

4 MATHEMATICAL BASIS FOR A MODEL OF

THE SHIP’S DRIFT

The ship's drifting movement is of a stochastic nature,

it results mainly from the nature of the elements

disrupting the free drift of ships. The random nature

of the ship's drift can be described as the Makarov

process. This movement can be determined by the

formula (3) :

( )

d V x t dt d

= +

(3)

x – motion of the drifting ship

x – position

t – time

ε - error of motion determination

If it is assumed that the drift results from the

impact of wind and current then formula (3) can be

represented as a formula (4) :

(

) (

)

( )

x t x V t dt L t U t dt

−= − +





∫∫

(4)

x(t) – route for a specific moment t

0 – initial drift path

L(t) – wind drift

U(t) – sea current drift

The drift velocity can be obtained using the

following formula:

V V adt

∆

= +

∫

(5)

0 – is the initial drift velocity

Δt – the time of drift

a – is drift acceleration

The method of obtaining short distance of the drift

path can be calculated using the formula:

y x x Vdt= = +

∫

(6)

V – drift velocity

Figure 9 shows the model of the ship's drift under

the influence of sea current and wind. External

disturbances may also be affected by waves. An

important element of drift control is the use of

simulation. In such cases, a wide range of prognostic

information about hydro-meteorological factors

should be used whereas the details of drift model

description can be found in .

Figure 9. Pseudo ship’s drift

691

5 FREELY DRIFTING VESSELS – CASES TO BE

SOLVED

5.1 Information on the weather condition in drift areas

Knowledge of weather factors is an important

element in safe towing of disabled ships. The

following is the minimum scope of meteorological

data in planning and implementing ship towing.

Standard information on facsimile maps

transmitted to ships in international navigation

includes such information as :

1 Analysis of sea surface weather.

2 Sea surface weather forecast.

3 Analysis of the windage area.

4 Analysis of sea waves.

5 Seaway forecast.

6 Analysis of surface water temperature.

7 Forecast of surface water temperature.

8 Information regarding ice cover and icebergs.

9 Significant weather condition.

10 Information on sea currents.

Information on the weather forecast onboard a

drifting ship is needed to reduce the risk of ship’s

damage when navigating in difficult areas. A high

risk of failure arises when a ship without shelter drifts

freely in the regions oil rigs and platforms. Similar

situation is observed when a drifting ship carrying

dangerous cargo approaches navigational obstacles.

The repair function is a probability function for the

duration on an engine failure.

( )

EF s

Ptt

(7)

( )

EF s

Ptt>

– probability of an engine failure of ts hour

or longer

In case of such event on board a vessel there must

be another function to be used, i.e. repair function .

( )

ER s

Ptt>

(8)

( )

ER s

Ptt>

– is the probability of the main engine

repair in t

s hour or longer

This assessment will allow you to make a decision

to call for external assistance to tow the ship away

from navigational dangers & navigational obstacles.

An example of a ship's drift near oil rigs is shown

in Figure 10.

Figure 10. Risk of drifting towards an oil field

5.2 Ship’s data recording

With the change of weather parameters the

parameters of the ship's drifts also change, therefore it

is necessary to monitor the ship's drift all the time.

A practical method of assessing the control of the

ship's position and the parameters of the ship's drift,

speed and direction over ground must be recorded in

the Ship Logbook.

Data Recording of ship’s event in the operating

process in accordance with SOLAS Convention 174

Reg. V/28 is the duty of the captain of a merchant

ship in international shipping. Data on the ship itself

and its operation can be divided into groups in

individual phases of navigation in various operating

conditions in a continuous system.

Entries include:

− general information during normal ship operation

− information on the description of special or

unusual events .

The daily record in the tow log should include

details such as:

1 Ship’s geographical position.

2 Weather conditions (sea state).

3 Tug(s) status.

4 Speed of the tow.

5 ETA at the port of refuge.

6 Technical condition of the towed ship.

7 Towing route.

These records can be made as follows:

− manual entry in the Ship Logbook

− constant automatic voyage data recording (VDR)

− electronic recording in the central ship computer

(Log Towing ).

During free drift of a ship, records must reflect the

exact chronological sequence of events of the entire

ship failure process.

Figure 11 shows how to register a ship's drift path.

692

Figure 11. Assessment of the ship's motion in the drift

Fi – position of the ship

i – moment of observation

A – position of the bridge (antenna)

Observation of ship’s position at frequent intervals

allows to control the motion of the ship. On this basis,

further parameters of the ship's drift can be predicted

based on external disturbance conditions (prediction).

5.3 Predicting the track of a disabled vessel drifting in

tidal areas

In tidal regions, values of current change within an

hour, so a graphic picture of current changes over

time should be made. Additional drift resulting from

a wind of constant direction is added to get the plan

of the track of ship's drift. This example is shown in

Figure 12.

Figure 12. Way of predicting the ship's drift path in tidal

areas

5.4 Types of options of vessels’ failure in the drift

1 Damage to the ship's propulsion away from

navigational obstacles and away from assistance

(tugs), in deep waters without the possibility of

using anchors to control the drift.

2 Ship’s drift far from assistance in shallower waters

with the possibility of lowering the anchors to

control the drift.

3 1st and 2nd case of the ship’s drift with the

possibility of the propulsion system to become

operational within specified time.

4 Ship’s drift in shallow water in the vicinity of

navigational obstacles without the possibility of

repairing the propulsion system failure in a short

time which may result in risk of collision with the

navigational obstacle.

5 Ship’s drift near obstacles without tugs assistance

and without the possibility of repairing propulsion

systems in the vicinity of dangers to navigation.

6 Drifting ship in tow with a tug not powerful

enough; there is a drifting system near dangers to

navigation.

7 A drifting system that is at anchors and the tug

while waiting for assistance of greater towing

power.

8 Drifting tow with tugs unable to remain on the

planned route to the port of refuge in conditions

when wind and currents are increasing.

6 SUMMARY

− Ship's drift may occur in bad weather as a result of

a limited propulsion system or a reduction in its

power.

− Ship's drift may result from the ship's breaking off

the anchor in bad weather (after damage to the

anchor system).

− Ship's drift is caused by damage to the steering

and/or propulsion system.

− Ship’s voyage planning should also include

contingency plans showing how to act in case of

lost propulsion or steering system.

− In any case there is a phase of rescue or towage to

ports of refuge when it is not possible to repair

damage to the ship's technical systems.

− Rescue of a freely drifting ship is carried out with

the help of professional rescue teams or with the

participation of merchant ships (see Annex 1) .

Annex contains the ANNEX to the IMO MSC/Circ.

884 publication from 21 December 1998 under the

title Guidelines for Safe Ocean Towing. The annex

presents the method of towing a ship by a

commercial ship.

− Towing plan based on the tow contract must be

prepared before towing.

− The main points of the contract are the efficiency

of towing operation.

− The towing plan covers a safe towing route and

speed to the port of refuge.

− The weather is constantly monitored on the towing

route and the place of towing, as well as the ETA.

693

− An important element of towing planning is

establishing a method of communication and

cooperation regarding management of the tow.

− An equally important element of the towing plan

is risk assessment of towing process.

− The level of towing risk throughout the entire

towing operation depends on the following

elements :

1 Quality planning of the route of the tow.

2 Quality and reliability of communication in the

tow and outside the system.

3 Good supervision of the operation (operation

management).

4 Experience of tug crews.

5 Technical condition of tugs (towlines, winches,

engine power, etc.)

6 Lack of commercial pressure on the tugs crews

from outside.

7 Unexpected changes in the marine

environment, such as sudden changes in

weather (high seaways).

8 No unexpected increase in ship traffic along the

towing route.

9 Area free from pirate attacks.

ANNEX

Basic safety elements of the towed ship

Prior to towing operation, the towed vessel must

meet certain technical parameters in order to be

accepted on the tow in order to move from position A

to position B.

1 Training the crew in the safe operation of technical

systems related to the tow.

2 Before towing, the vessel must prepare lights and

signs prescribed for towing.

3 The vessel must have a specified draft (trim) for

the entire towing period.

4 The towed vessel must be watertight (when

making heavy weather).

5 The towed vessel must have a well secured cargo

(ballast water with no free space) etc.

6 The towed vessel must have safe stability in all

wave conditions and external disturbances.

7 Steering system, propeller (the rudder to be held in

the forward and aft position- midships). If

possible, the propeller should be secured against

turning.

8 Details on the technical and operational condition

of the ship should be collected on an ongoing basis

and ready to be reported.

9 Weather forecasts and sea state are to be

controlled. The captain must establish constant

contact with the mainland (shipowner, SAR, etc.).

10 Special Towing Booklet throughout the entire

towing passage must be kept.

BIBLIOGRAPHY

Annex Draft Guidelines For Safe Ocean Towing, IMO

MSC/Circ. 884, 21 December 1998.

DNV Towing Recommendations,

www.tngmasters.org.2014/07 Towingrecomends.pdf.

Dongdong Au. et allia, Research on Drift Path Prediction

Technology Ships Advances in Intelligent Systems

Resewal (AICR), vol. 151, (CMSA) 2018, p. 236.

Galt J.A., Hanson R., Aleutian Disabled Ship Drift Study,

Genwest Technical Publication 16-001, Dec, 2015.

Guide to Marine Meteorological Services (2018 edition),

WMO, No. 471.

Haisheng Xiong, Xiaolin Wang, Effect of Leeway and Dryft

of Ship Navigation and Determination the Hod, MATEC

Web oif Conferences 267, 01005 (2019),

https//dvi.org/10,1051/matecconf/201926701005.

IMO Resolution A.916(22), Guidelines for the Recording of

Ivents Related to Navigation, London 22 January 2002.

Jurdziński M., Nawigacja morska, Wydawnictwo AMG,

Gdynia 2014.

Jurdziński M., Planowanie nawigacji w żegludze

przybrzeżnej, WSM w Gdyni, Gdynia 2002.

LOSS PREVENTION Tugs and Tow – A practical Safety and

Operational Guide, SHIPOWNERS Security for small &

pecialist vessels,

www.shipownersclub.com/media/2015/08/PUBS-Loss-

Prevention-Tag-an…

LOSS PREVENTION Tugs and Towns – A Practical Safety

and Operational Guide, Shipowners security for small &

Specialist Vessels.

Marine 6 Contact Drift Model, Raport No.

18591.620/tech_doc/2, www.iala_aism.org.

MARIN Report No.18504.620/TECH_DOC/2.

Christian Michelsen Research AS, cmr Computing,

www.emr.no.

Preparation Emergency Towing of Ship-10 Important Poins,

https://www.marinerisight.com

Sjöberg H. et al., Soft hand lines of Large Container Ships in

Strong Winds, Final Report-2016-12-01, Gothenburg

Pilot.

Smeaton G.P., Mathematical Model of the Disabled Large

Tankers, Symposium on Behaviour of Disable Large

Tankers, London 1981. (Paper 5).

Takuzo Okada, Ship Maneuvering Technical Reference,