Zeszyty Naukowe 29(101) 69

Scientific Journals Zeszyty Naukowe Maritime University of Szczecin Akademia Morska w Szczecinie

2012, 29(101) pp. 69–75 2012, 29(101) s. 69–75

Problems of towing of damaged tankers

Problemy holowania uszkodzonych zbiornikowców

Lech Kobyliński

Ship Handling Research and Training Centre Fundacja Bezpieczeństwa Żeglugi i Ochrony Środowiska 14-200 Iława, ul. Chodkiewicza 12, e-mail: lechk@portilawa.com

Key words: tanker casualties, system of preventing marine environment pollution

Abstract Tanker casualties that happen from time to time in the world cause serious damage to environment with

disastrous effects. In particular, casualties of tankers “Prestige”, “Erika” and “Nakhodka” where ships were

seriously damaged, broken in the middle or separated into two parts showed that salvage operations in many

cases were not effective. Such casualty in the Baltic Sea, that is closed area, may lead to the total destruction

of the environment. The paper describes a proposed system of preventing marine environment pollution from

damaged tankers. Problems related to towing of damaged tankers to the safe harbour or area, that consist an

important element of the system, are discussed in more detail with reference to model tests of towing of

damaged tanker performed at Iława Training and Research Centre for Ship Manoeuvrability.

Słowa kluczowe: katastrofy zbiornikowców, system zapobiegania zanieczyszczeniom morza

Abstrakt Katastrofy zbiornikowców, które zdarzają się co jakiś czas, powodują poważne szkody w środowisku mor-

skim. W szczególności katastrofy zbiornikowców „Prestige”, „Erika” czy „Nakhodka”, gdy statki zostały

poważnie uszkodzone, przełamane w połowie lub rozbite na dwie części, wykazały, że akcja ratownicza

w wielu przypadkach nie była efektywna. Gdyby taka katastrofa zdarzyła się na Bałtyku, które jest morzem

zamkniętym, mogłoby to spowodować całkowite zniszczenie życia w morzu. W artykule zaproponowano sys-

tem zapobiegania zanieczyszczaniu morza przez uszkodzone statki. Problemy odholowania takich statków do

bezpiecznego portu czy akwenu, stanowiące ważny element systemu, omówiono szczegółowo w powiązaniu

z badaniami holowania uszkodzonego zbiornikowca przeprowadzonymi w Badawczo-Szkoleniowym Ośrod-

ku Manewrowania Statkami w Iławie.

Introduction

From time to time the public opinion is shocked

by the information that somewhere in the world

there was serious shipping accident where damaged

large crude oil tanker caused disastrous pollution of

the sea environment. In consequence, hundreds

kilometers of the seaside were polluted and were of

no use for tourism, recreation and for fishery, sea

life was damaged for a long time and resulting cost

of recovery was counted in hundreds of millions of

US dollars. Those accidents happen in spite of en-

hanced international safety rules adopted regarding

construction of tankers during the last period,

where double hull, segregated ballasts, duplicated

rudder gear amongst other measures are required.

Avoiding disastrous consequences of such acci-

dents requires immediate localization of the casu-

alty, securing the damage ship as far possible, tow-

ing it to the safe place where oil spill could be

contained and removed. Rescue and cleaning action

has to be quick and effectively performed.

Important oil spills and their consequences

The most serious tanker casualties that happened

in years 1967–2008 are shown in the table 1 (from

[1]). In the last column amount of oil that went to

the sea is shown, and largest oil spills polluting the

coastline are marked by star. In older times when

Lech Kobyliński

70 Scientific Journals 29(101)

the size of tankers was generally rather small, casu-

alties, if they happen, did not cause large pollution

because of small amount of oil spilled. The rapid

increase of the amount of oil transport and of the

tankers size in the sixties of the last century has

changed the situation.

Table 1. Most serious tanker casualties and oil spills from

tankers in years 1967–2007

Tabela 1. Najpoważniejsze awarie zbiornikowców i wylewy

ropy naftowej w latach 1967–2007

Month /

Year Ship’s name Place

Oil spill /

sunk (t∙103)

03/1967 Torrey Canyon* Scilly Isles 119 000

03/1971 Wafra South Africa 65 000

08/1974 Metula Magellan Strait 50 000

01/1975 Jacob Maersk Oporto, Portugal 88 000

05/1976 Uruiola Galicia, Spain 101 000

01/1977 Nakhodka* Japan Sea 6 000

02/1977 Havaian Patriot Hawaii 95 000

03/1978 Amoco Cadiz* Brittany, France 223 000

11/1979 Independent* Bosphorus 95 000

07/1979 Atlantic Empress* Tobago, West

Indies 287 000

02/1980 Irenes Serenade Navarino Bay 100 000

01/1983 Assimi Oman 53 000

08/1983 Castillo de

Bellveder* South Africa 252 000

11/1988 Odyssey Nova Scotia, 132 000

03/1989 Exxcon Valdez* Alaska 37 000

12/1989 Khark 5 Morocco 80 000

04/1991 Haven Genoa, Italy 144 000

05/1991 ABT Summer Angola 260 000

04/1992 Katina Mozambique 72 000

12/1992 Aegean Sea Galicia, Spain 74 000

01/1993 Braer Shetlands 85 000

02/1996 Sea Empress* Milford Haven 72 000

12/1999 Erika* Brittany, France 20 000

11/2002 Prestige* Galicia, Spain 76 000

08/2003 Tasman Spirit Karachi 28 000

12/2007 Hebei Spirit South Korea 10 500

The first serious tanker casualty that happened

in European waters was in 1967 when the super-

tanker “Torrey Canyon”, because of the master

fault, hit the Seven Stone rocks near the Scilly

Island. The hull of the tanker was ruptured and

spilled oil caused the most serious ecological disas-

ter. Several other serious tanker casualties hap-

pened in years 1967–1977, many of them close to

the coast of America that stirred public opinion and

maritime authorities and as a result extraordinary

conference was organized by the International

Maritime Organization (IMO) in 1978 and Protocol

SOLAS and MARPOL Conventions was adopted

that included important amendments to those

conventions related to tanker safety [2]. This confe-

rence almost coincided with the most serious disas-

ter of the tanker “Amoco Cadiz” in 1978 close to

shores of Normandy that was caused by rudder gear

failure. Disabled ship drifted towards the rocks

where the hull was eventually split into two parts

and more than 200 000 tons of oil transported

spilled into the sea.

More recently, in November 2002 in Greek

owned tanker “Prestige” one of the tanks started

leaking due to hull damage in Atlantic Ocean close

to Portugal. Because all ports refuse to allow the

leaking tanker to enter, the master was forced to

sail farther from the shore. Attempts to tow, the

disabled ship failed and during the stormy weather

the ship was broken into two parts as shown in

figure 1, that ultimately separated and finally foun-

dered. The spilled oil drifting towards the Spanish

coast caused the worst disaster in the history of

Spain.The consequences of such large oil spills are

disastrous for the sea life (birds, animals and fish),

for the economy (fishery, tourism, recreation) and

cleaning operations costs usually many millions

euro.

Fig. 1. Tanker “Prestige” broken down drifting in Atlantic

Ocean

Rys 1. Zbiornikowiec „Prestige” przełamany, dryfujący na

Atlantyku

Casualties of the Russian tanker “Nakhodka” in

1997 that was broken into two parts in Japan sea, of

the tanker “Erika” in1999 that was also broken into

two parts near Brest, and in particular of the tanker

“Prestige” where attempts to tow the disabled ship

failed drew attention to the problems of towing

disabled ships.

System of preventing marine environment pollution from damaged tankers

Understanding that such disasters, in spite of

whatever prohibitive measures are taken, are un-

avoidable, people realized that it would be neces-

sary to develop a system to prevent or mitigate

consequences caused by spilled oil, if such casualty

Problems of towing of damaged tankers

Zeszyty Naukowe 29(101) 71

happens. This was done for the first time after the

“Nakhodka” disaster, where in Japan important

project of this type was installed under the leader-

ship of professor Hara. This was reported in several

places, (e.g. [3, 4]). Attempts to develop certain

tools designed to help the salvage organization to

deal with the consequences of similar disasters

were undertaken also after the “Erica” and “Pres-

tige” disasters [5].

Baltic Sea is a closed sea particularly vulnerable

to pollution. If disaster of a large tanker, such as

described above, happens in the Baltic, the results

would be catastrophic to the countries surrounding

Baltic Sea. In the extreme case, the whole life in the

Baltic might be destroyed and the beaches made

unusable for the long time. Fortunately, such disas-

ter did not happened yet, but the risk that it may

happen in future is rather high. Some indication

what may happen was shown in December 2009,

when the bunker ship “Romanka” lost manoeuvra-

bility, drifted towards the Latvian coast and

grounded. The ship was without cargo and on the

sandy beach hull was not ruptured. Fortunately,

nothing serious did happen [6].

In the Baltic Sea 55 000 ships pass the Danish

Strait every year, and about 6000 of them are large

or medium size tankers carrying dangerous goods;

more than 50 ferry routes cross the Baltic Sea.

Collision and grounding may easily happen. There-

fore, it is extremely important to assess risk of such

a situation and appropriate salvage action has to be

prepared.

The attempt to mitigate consequences of such

disaster materialized in the proposal to develop

a system for preventing disastrous consequences in

case of tanker casualty in the Baltic Sea [7]. Such

system developed in Japan was the result of 5 years

project installed after “Nakhodka” disaster and was

named Optimum Towing Support System (OTSS)

[4]. Unfortunately, only short publications that in-

clude description of the system and some results of

the model test of damaged tankers are available,

and access to the system that is owned by Japanese

authorities is not possible (see references above).

Moreover, the system is geared to the local condi-

tions in Japan and model tests comprise damage

cases that do not include cases that happened in

“Prestge” or “Erika” casualties.

The system proposed (EPM System) should be

for use in the Baltic, therefore should be tailored to

local conditions. The system should made use of

the Japanese experience, but will not necessarily be

modeled in the same way. The block diagram of the

proposed system is shown in figure 2. In order to

Containment and cleaning

oil spilled

Information about ship’s type,

cargo, dimensions extent

of damage and other relevant

information

Information on hydro-

-meteorological conditions

and weather forecast

Approx. estimation ship

parameters required for

further calculations

Prediction of the path

of drifting damaged ship

Planning of optimum towing

operations to safe place

Information on availability

of tugs, on availability of safe

harbor etc.

Towing operation,

supervised and corrected

Information on the true

position of damaged ship

under way and weather

conditions

Information on the amount

and location of spilled oil etc.

Information on availability

of equipment

CASUALTY

Fig. 2. Block diagram of the EPS system

Rys. 2. Schemat blokowy systemu EPS

Lech Kobyliński

72 Scientific Journals 29(101)

accomplish aims of the EPM system, the following

tasks have to be undertaken:

Task 1: Identification of the condition and impor-

tant parameters of the disabled ship.

Task 2: Identification and prediction of the hydro-

-meteorological condition in the area of

casualty.

Task 3: Prediction of the path of drifting damaged

vessel.

Task 4: Developing of the optimum towing system

of the damaged vessel to the safe place.

Task 5: Developing optimum system of contain-

ment of the pollution and removal spilled

oil.

Task 6: Developing user friendly fast computer

code for the use of salvage organization.

Research project on prevention of oil pollution from damaged tankers

Bearing in mind the possibility of a large tanker

being damaged because of grounding or collision

in the Baltic Sea, the programme of research was

installed in the Foundation for Safety of Navigation

and Environment Protection with the aim to de-

velop the system mentioned above aimed at preven-

tion of oil pollution from damaged tankers. Part of

this programme constituting important element of

the system was already completed. The aim of this

part was prediction of drift of a damaged tanker

under the influence of wind, waves and current

allowing fast intervention of the salvage ships. The

results of this part were already reported [8].

The other part of the project, now under consid-

eration, is aimed at estimation of forces required to

tow the damaged tanker to the safe place where the

oil spill may be contained and ultimately remove-

dand to recommend optimum towing arrangement.

It is necessary to estimate forces required for tow-

ing which in turn affect the choice of tugs (neces-

sary to do this work) and provide recommendations

regarding the technique of towing. This part is

almost completed and is based on model tests of

towing of the model of disabled tanker in different

conditions of damage. Large model of damaged

ship was used for this purpose.

Towing tests of the disabled tanker

Within the scope of the project model tests using

large model of a tanker in different damage condi-

tions were arranged on the lake in the Research and

Training Centre for Ship Manoeuvrability in Iława

in order to study the behaviour of damaged tanker

when towed and to develop computer code for pre-

dicting forces required for towing. Those tests are

unique, because no such tests were performed any-

where yet. As a sample ship for model tests tanker

of the common size met in the Baltic Sea was

chosen. This was the same tanker as used for tests

within the first part of the programme of research

[8]. The dimensions of the chosen ship are shown

in the table 2, together with dimensions of the large

model used in experiments on the lake. Figure 3

shows conditions of the damage tanker that were

used in model tests.

1

2

3

4

Fig. 3. Conditions of the damaged tanker tested

Rys. 3. Badane stany uszkodzonego zbiornikowca

Problems of towing of damaged tankers

Zeszyty Naukowe 29(101) 73

Table 2. Dimensions of the sample ship and its models

Tabela 2. Wymiary statku przykładowego i jego modelu

SHIP [m] MODEL 1 [m]

Scale 1 1:41.06

LOA 324.0 7.89

B 54.9 1.34

H 32.25 0.785

T1 15.50 0.377

T2 21.50 0.523

As the towing ship model of the large tanker

“Blue Lady”, normally used for training sea pilot

sand ship masters was used. Large displacement

of this model caused that effect of the towing ship

(or tug) on the behavior of the towed ship was neg-

ligible. During tests towing force was measured, as

well as velocities and paths of both ships. Also

wind velocity was measured, although all tests were

performed in calm weather with almost no wind in

order to exclude the wind and waves effects on

towing force. Those effects will be added to calm

weather towing force by calculation.

Fig. 4. Towing of the model of damaged tanker (condition 3)

Rys. 4. Holowanie modelu uszkodzonego zbiornikowca

(stan 3)

Fig. 5. Towing of the model of damaged tanker (condition 4)

Rys. 5. Holowanie modelu uszkodzonego zbiornikowca

(stan 4)

During tests three lengths of towing line were

used, corresponding to 500, 750 and 860 m. Tests

were performed at different towing speeds

Programme of tests of towing disabled tanker

in four different conditions of damage has been

already completed. Figures 4 and 5 show examples

of towing arrangement.

Ocean rescuetowing

In rescue towing as rule damaged ship is not

controlled and there usually are no crew members.

This is usually posing serious problems in bringing

the towing hawser that is rather heavy (2–3 tons)

onboard and fixing it. These problems are, how-

ever, not discussed here.

When the non controlled ship is towed, usually

yawing motion around the straight course occurs.

Yawing motion is connected with dynamic instabil-

ity of the towed ship and is difficult to avoid.

An example of the trajectory of the towed vessel is

shown in figure 6.

Fig. 6. Recorded trajectories of towed ship and tug [9]

Rys. 6. Zapisane trajektorie statku oraz holownika [9]

Yawing causes reduction of speed of towing,

increases the danger of breaking of towing line

because of overloading and possible lost of tow.

Yawing phenomenon was studied by several

authors (e.g [10, 11]) and in general, it was con-

cluded that this phenomenon is very difficult to

avoid when towing conventional ships or towing

objects unless special additional arrangement in

form of fitting fins or skegs or drag anchors are

fitted, which cannot be done with rescue towing.

Yawing, in general, depends on the inherent dy-

namic stability of the tow and on the length of the

towing line. Generally, the longer towing line, the

smaller amplitude of yawing. This relation is shown

in figure 7 derived from calculations (after [11]).

This, however, was calculated taking as an example

of towed ship a naval ship that was inherently

stable on straight course when towed. Most ships

when towed, reveal larger or smaller amplitude

of yawing. Tests with towed gas carrier model

confirmed this effect [12].

Wind 195 deg.

8.8 m/s

Wave 150 deg.

1.5 m

Current

1 knot

Tow ship

Towed ship

[m]

[m]

Lech Kobyliński

74 Scientific Journals 29(101)

Fig. 7. Relation between course stability and length of the

towing line [11]

Rys. 7. Zależność pomiędzy statecznością kursową a długością

holu [11]

The negative result of yawing motion of the tow

is jerking of the tow line. When the tug is pulling

even at constant speed, yawing of the tow is caus-

ing that the pulling force is not constant, but is

changing in time. When the yaw angle of the tow is

at its extreme, then the tow accelerates the towing

line became slack and the jerking occurs when

the tow rope is again taut. Therefore, on the steady

or quasi-static pulling force jerking force is over-

lapping as shown in figure 8 (after [13]). In this

diagram, additional time varying components of

tension caused by waves are also shown.

The extreme time varying tensions are compen-

sated by the construction of the towing hawser that,

when wire rope is used, always include highly elas-

tic part, as well as by the towing winch that auto-

matically releases rope when the tension increases

Fig. 8 Time relation of extreme tensions in towing hawser [13]

Rys. 8. Maksymalne naprężenia holu w zależności od czasu [13]

Extreme Tension (Te)

Wave-Induced (Twave)

Yawing and

Surging (Tyaw)

Steady or

Static (T)

Time (Minutes)

Ten

sio

n,

Kip

s

0

5 000

10 000

15 000

20 000

25 000

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5

Rs [kN]

Vs [knots] l = 500 m l = 750 m l = 860 m

Fig. 9.Comparison of towing forces for different hawser lengths. Condition 3

Rys. 9. Porównanie siły holowania dla różnych długości holu. Stan 3

Problems of towing of damaged tankers

Zeszyty Naukowe 29(101) 75

over certain value, but even so, the towing hawser

diameter should be calculated taking into account

additional jerking tensions. The behaviour of the

towing hawser is very complex and several authors

attempted to investigate it and to develop computer

codes able to calculate extreme tensions (e.g. [14,

15]), but this subject it out of the scope of this

paper. On rough recommendation as used by the

U.S. Navy is to use safety factor 1.5 [13].

Some results ot the towing experiments

Figure 9 (from [16]) shows partial results of

the experiments of towing damaged ship. Towing

forces measured are shown for three different

lengths of towing line for the model in condition 3

(model broken in the middle). It may be noticed,

that the forces re-calculated for full scale ship are

extremely large, therefore, towing speed should be

rather small, perhaps no more than 3–3.5 knots.

Large scatter of measured points is due to jerking

forces caused by yawing motion.

Conclusions

The tests with towing of damaged tanker in

different conditions of damage did show that this

operation may be very difficult and the forces re-

quired to tow such a ship might in certain situations

be extremely large. This will require employment

of powerful tugs having very large bollard pull.

Usually there will be also difficulties with bringing

heavy towing hawser onboard of the damaged ves-

sel. In order to avoid disastrous pollution of the sea,

a system of preventing marine environment from

spilling oil from damaged tankers should be devel-

oped as proposed. This is especially important for

the Baltic Sea.

Acknowledgement

The project described was supported by the

Polish National Committee for Scientific Research.

References

1. MOYANO H.: The Prestige case: four years after. TSDT 07

Conference, Newcastle on Tyne, 2007.

2. IMO: Protocol 1978 relating to the SOLAS Convention

and to the MARPOL Convention. International Conference

on tanker safety and pollution prevention London 1978.

3. HARA S.: Towing support tool and objects drift at sea.

From Nakhodka to Erika. Exchange of experience in at sea

response to offshore oil spills by passing ships. Proceed-

ings of the seminar between Cedre in France and SRI.

Brest 2000, France. Presentation.

4. HARA S., YAMAKAWA K., HOSHINO K., YUKAWA K.,

HASEGAWA J., TANIZAWA K., UENO M.: Development of

towing support tool named Optimum Towing Support Sys-

tem (OTSS). Proceedings of the Fourteenth International

Offshore and Polar Engineering Conference. Toulon 2004,

France.

5. CRANEGUY PH.: A simple operational model for the analy-

sis and forecast of pollutant and object drift. Sea-Tech

Week – Brest, 2000, 18–20.

6. Anonym: Lekcja bunkierki. Polityka, Nr 49, 05.12.2009.

7. KOBYLINSKI L., NOWICKI J., BURCIU Z.: System for prevent-

ing Marine environment pollution from damaged tankers.

Konferencja Transport XXI wieku. Białowieża 2010.

8. KOBYLIŃSKI L. and al: Drift path prediction of damaged

tankers. Proc. 18th HYDRONAV International Conference,

Gdańsk 2010.

9. KURODA T., HARA S.: Application of optimum towing sup-

port system in emergency towing operation. International

Conference on Towing and Salvage of Disabled Tankers,

Glasgow 2007.

10. JIANG T.: On the dynamic instabilities in towing process

and the possibilities of stabilization. JSTG, Vol. 90, 1996.

11. STRANDHAGEN A.G., SCHOENHERR K.E., KOBAYASHI F.M.:

The dynamic stability on course of towed ships. Transac-

tions SNAME, 1950.

12. KOBYLIŃSKI L.: Holowanie morskie statków uszkodzonych.

Azipilot Project Foundation for Safety of Navigaton and

Environment Protection. RAPORT No: 12G/2011.

13. Naval ships’ technical manual. Chapter 582. Mooring and

Towing, 2001.

14. MILGRAM J.H., TRIANTAFYLLOU M.S., FRIMM F.C., ANA-

GNOSTOU G.: Seakeeping and extreme tensions in offshore

towing. Transactions SNAME, 1988.

15. PELTZER T.J.: Computer Aided Decision Making for Ocean

Towing. Massachusetts Institute of Technology. 1979.

16. OLEKSY J.: Analiza wyników badań modelowych holo-

wania. Fundacja BŻiOŚ. Raport 7G/2011/2012.

Other

17. U.S. Coast Guard Research and Development Center.

Modelling of leeward drift. Final Report, September,

Report No. CG-D-06-99, 1998.