Jarosaw Artyszuk
Maritime University of SzczecinFaculty of Navigation
TYPES AND POWER OF HARBOUR TUGS
- THE LATEST TRENDS
The manuscript delivered: April 2013
Summary: The aim of the present paper is to determine the availability and main features of existing
harbour tugs from the viewpoint of type and maximum bollard pull achievable nowadays. Particular interest is also focused on a tug's length as one of main dimensions representing the hull size. This length affects the manoeuvring area occupied by an assisting tug and expresses its basic turning and stopping performance. One can simply state that the involved manoeuvring area is proportional to the length. However, the same hull of a tug within the current shipbuilding practice, unlike in the past (when the higher propulsion power implied a larger hull) can accommodate propulsion units of quite different power. Thus, the higher propulsion power is installed the higher towing forces are achieved, and in addition the turning or stopping ability of a free-sailing tug is essentially improved. The conducted research is based on analysing the world fleet statistical data. The obtained results provide support for making operational and investment decisions by harbour authorities in the aspect of harbour modernisation or its traffic expansion.
Keywords: tug, bollard pull, propulsion, manoeuvring, towing
1. INTRODUCTION
The harbour authorities are obligated among others to take care of safety and effectiveness of the harbour traffic. This essentially includes the towing assistance to large ships while performing their transit, turning, berthing and/or unberthing operations. The authorities sometimes acquire for this purpose the most powerful tugs available on the market despite the overall future costs of maintenance and fuel consumption. The economical consequences, however, are not being dealt with hereafter. The present work is concerned with technical details of harbour tugs in the top range of installed power.
The main objective of this contribution is to determine the availability and main features of existing harbour tugs from the viewpoint of maximum bollard pull (BP) achievable nowadays. Particular interest is also focused on a tug's length as one of main dimensions representing the hull size, and often serving as an essential operational parameter.
The extreme length or the length over all (LOA), as usually published, is chosen and used throughout the study. This length affects the manoeuvring area – swept path – occupied by a moving/assisting tug and to some extent expresses its basic manoeuvring ability, comprising turning and stopping performance. The lower is the length the less nautical space has to be secured for safe tug assistance, but at the same time the tug's manoeuvring time is much shorter. In the aspect of analysing ship manoeuvrability the length between perpendiculars (LBP), and/or the waterplane length (LWP), seems to be much better but is hardly available and definable for unconventional tugs.
The manoeuvring time directly influences the time left to a tug's master to assess the navigational situation and make appropriate manoeuvring decisions/orders in view of fulfilling the pilot's instructions. The scale of time is more unfavourable for a tug's master than for a large ship's master. For example, to complete full turning a tug requires half a minute while a large ship takes more than ten minutes. So, the response of tug's master must be much faster.
However, using the standard dimensionless quantities the manoeuvrability of a ship is almost constant. Namely, one can find that merchant ships, independent of size, behave similarly in that the stopping distance or turning diameter, both measured in a ship's length units, do not change between ships. This is partly due to the fact that a ship's machinery (main engine, propeller, and rudder) increases in size and power in accordance with the increase of ship's hull dimensions. The case of harbour tugs is quite different, which exhibit the relatively high ratio of installed power to the hull displacement. This makes tugs superior in dimensionless manoeuvrability over merchant ships, though it causes some problems with a tug's stability under the hawser tension. Additionally, the same hull of a tug within the current shipbuilding practice can accommodate propulsion units of quite different power. Thus, the higher propulsion power is installed the higher towing forces are achieved, and the turning or stopping ability of a free-sailing tug is essentially improved. In this context the benefits of high propulsion power are unquestionable.
The research conducted hereafter is based on analysing the world fleet statistical data. The obtained results provide support for making operational and investment decisions by harbour authorities in the aspect of harbour modernisation or traffic expansion.
To solve the problem of availability and elementary properties of high power tugs, the internet database Sea-web [6] has been extensively used throughout the study, where suitable records were extracted and processed by the author. The Sea-web database is almost the largest, most complete, comprehensive, and accessible database. It records all sea-going ships with gross tonnage of 100GT and more.
A special attention in the paper has been devoted to unconventional tugs as often being the first choice option or the reference (starting) point for optimising the purchase decision. This can be explained by a technical attractiveness of this kind of tugs – their extraordinary manoeuvrability – though paid for with higher price, maintenance costs, and lower reliability, as compared to conventional tugs (particularly with fixed pitch propellers). But as the matter of fact, the unconventional tugs, mostly with azimuthing propellers, have reached in recent years a production level of the conventional tugs.
2. HARBOUR TUGS IN THE SEA-WEB DATABASE.
BOLLARD VS. EFFECTIVE PULL
The world fleet of tugs, including but not limiting to typical harbour tugs, on Oct 4th, 2012 [6] consisted of 14624 vessels. They are classified within the category B32A2ST ('tug'). However, due to a multi-purpose and flexible operation of some tugs, this group also encompasses crafts additionally engaged in coastal and/or ocean towing, and in selected offshore services. This extra employment, other than pure harbour towing, essentially affects the design and main particulars of a tug, especially its linear dimensions. In the aspect of this study's primary goals – i.e. harbour tugs – eliminating from the given population the multi-purpose tugs is technically difficult. However, all these cases are expected to be in minority and significantly not interfere with the final results.
Fortunately, the vessels/tugs strictly relating to the offshore industry, also comprising the deep-water towing service, of various types and names – PSV ('platform support/supply vessel'), AHT(S) ('anchor handling tug/supply'), OSV ('offshore service/supply/support vessel') – are incorporated into a separate resource of the Sea-web database and are assigned different alphanumerical marking. The total number of these crafts is more than 6800 at present. The last name or abbreviation – OSV – is also usually used to depict in general all these three types of offshore ships. The OSVs are obviously not investigated in this paper.
From the viewpoint of technical operation (mostly attributed to the propulsion type), the harbour tugs considerably vary, which implies some specific construction and arrangement features of a tug's hull. Therefore it is purposeful to investigate the fleet development trends in particular segments of propulsion type. The global tendencies are here useless.
The main design parameter of a harbour tug is bollard pull (in tonnes), i.e. the pull in zero-speed conditions, while for ocean towing the pull at the towing speed (e.g. 6 knot) is more important. From the bollard pull, practically unequivocally (but within certain limits) and dependent on the propulsion/propeller type, one can establish the demanded propulsion power in kW or HP. However, because of various operational regime or profile of a tug, especially connected with auxiliary tasks to be performed, there are different operational power margins applied in setting the working point of the main engine. In this way, the rated bollard pull for the same propulsion type is achieved with different nominal power of the main engine. In many circumstances one should exercise caution and limited trust to the published value of BP, and assure oneself whether this BP refers to the engine's continuous output or to the so-called short 'peak value'. Anyhow, the classification society's certificates have to be studied in detail for reliable information.
If harbour tugs are planned to escort large ships in fairways, i.e. to assist their transit at comparatively high speed, more attention is put on the indirect method of towing (if the fairway width permits) and the related hydrodynamic lateral resistance force exerted on a tug's hull in its oblique movement [5], [3]. This hydrodynamic force, being proportional to the tug's hull underwater area and the square of transit speed, essentially augments the action of the pure propeller. One should remember that the indirect towing (push or pull) is possible and effective only at relatively higher speeds, and more connected with
unconventional tugs (of azimuthing or Voith-Schneider propulsion). In this way the escort tugs are differently rated and certificated by the classification societies, where the pure BP is supplemented and/or replaced with the effective pull. For example, a tug with Voith-Schneider propulsion of power 6800 HP (5000 kW) has a bollard pull of 65t, but its effective pull in the indirect towing (by means of the escort steering pull test) amounts to 125t at 8 knots, or 148t at 10knots.
3. STRUCTURE OF FLEET AND ITS TRIMMING
A general structure of the world fleet of harbour tugs, in terms of the aforementioned category LR/IHS- B32A2ST, is presented in Tab. 1.Table 1 World fleet of harbour tugs
(the state on 04.10.2012r., existing crafts or under construction, total 14624 tugs) no. of propellers CPP FPP azimuthing cycloidal (VS) no data
1 482 1805 36 11 267 2 698 5178 2926 466 2631 3 (typical) and more 1 49 39 3 32 TOTAL 1181 7032 3001 480 2930
For further considerations only vessels with twin conventional screw propellers (of fixed pitch – FPP, or controllable pitch – CPP) or with dual unconventional propellers (azimuthing or cycloidal) have been chosen. Having two propellers of either type is currently rather typical in the harbour manoeuvres i.e. consisting in assistance to berthing/unberthing, transit, and/or harbour entrance/exit. The conventional propellers often have nozzles as to increase their performance in bollard and astern conditions. However, these nozzles could be of fixed type, accompanied with normal stern rudders, or of movable type (traditionally referred to as Kort nozzles). Such additional data is however unavailable in the Sea-web database.
The conventional propellers have a fixed (longitudinal) direction, where the slipstream is diverted by means of rudder or nozzle. Of course, the range of deflection angles is limited here. Moreover, the physical direction of rudder or nozzle does not coincide with the direction of the effective outflow and thus the thrust. The analysis of the thrust vector is absolutely fundamental in the effective tug handling.
On the contrary, the unconventional propellers just on principle have ability to actively control the direction of thrust over 360q. In the case of azimuthing thrusters the direction of thrust corresponds to the direction of the device. The mechanism of the cycloidal propeller in this context is much more complex, but the effectiveness of rotating their thrust is similar to that of azimuthing propeller.
Though the Sea-web database does not provide appropriate information, it shall be mentioned that the azimuthing (or azimuth) propulsion is practically always mounted at the stern of a tug, popularly referred to as ASD tug ('azimuth stern drive'). On the other hand,
the cycloidal propulsion, also called the Voith-Schneider (VS, VSP, or just briefly Voith) propulsion, is installed mostly at the bow. In the latter case, a tug is named the Voith Water Tractor (abbreviated to VWT). By analogy, the ASD tug may be defined as a reverse tractor. However, one could find of course tugs with forward azimuthing propellers or with aft VSPs, but those should be in minority.
The location of dual propulsion – forward or aft – independently of its type, essentially changes the way of rendering assistance by a tug. ASD tugs operate in push/pull modes through the bow, i.e. directing the bow towards the assisted large ship, while VWTs work to the contrary by their stern. Details of operation of particular tugs, including the analysis of benefits and drawbacks, can be found e.g. in [5].
For both technical and to certain degree market-related reasons, the selection of azimuth propulsion will often lead to the aft propulsion of a tug, and vice versa. Similarly, deciding on VSP implies staying at the tractor (forward) propulsion.
However, some authors [5] adopt a wider definition of an ASD tug than the above given (simply meaning the aft azimuth propulsion), in which this tug in resemblance to a conventional tug (also having the aft propulsion) also can work in pull mode on its stern winch/hook. This is vital in the tug's operation at the bow of the assisted ship. An alternative name is sometimes used here to distinguish this 'universal' tug – a multi-tug.
Manoeuvring and reliability merits of the both unconventional propulsion types and/or tugs – azimuthing and cycloidal – are close to each other. This is also proved by a survey of expert and tug master opinions. However, the VSP tugs are widely used only in Europe [3]. In the world-wide global scale they are significantly dominated by the azimuth propulsion – see Table 1 and 2.
Within tugs with two propellers – see bolded figures in Table 1 – there is a considerable number of crafts with unspecified (unknown) type of propulsion. These have also been rejected from the analysis. Hence we have arrived at the most reliable population embracing more than 9200 crafts – refer to Table 2.
Table 2 Age structure of tugs with two propellers (total 9268 tugs)
no. of crafts CPP FPP azimuthing cycloidal (VS) TOTAL
to 1989 451 2271 707 177 3606
from 1990 247 2907 2219 289 5662
where
1990-1999 97 1170 588 145 2000
2000-2014 150 1737 1631 144 3662
In this set the oldest tugs were mostly built in the early sixties, but in the case of some conventional tugs with FPP we need to go back even to the turn of 40./50. (of the 20th century). Such scatter of age makes trouble in establishing latest trends in shipbuilding. Therefore the period of last 20 years, starting from 1990 (as included), has also been specially focused on in Table 2.
The set of tugs with two propellers outlined in Table 2 – 9268 items in total – constitutes the input data framework for all the subsequent figures and conclusions of this paper.
4. BOLLARD PULL AND MAIN PROPULSION POWER
AT PRESENT
The market of unconventional tugs has drastically changed over the last decades. According to Table 2, the ratio of ASD to VWT tugs built between 1990 and 1999 equals to nearly 4, while since the year 2000 this index has reached more than 11.
Such a disproportion is the direct effect of technological progress, the towing industry's acknowledgement, and a competition among manufacturers in the market of azimuthing propulsion. The Voith-Schneider propulsion is more monopolistic. The availability of high bollard pulls of order 80t and more is rather unique for VWTs, but has very strong position in the today's market of new ASD tugs [2]. However, the demand and supply in the range of medium bollard pulls for azimuth tugs (down to 40t) has been still maintained at least for the last ten years – see Fig. 1. In case of VS tugs, their overall fleet is so meagre that someone hardly can notice any clear trends.
0 50 100 150 200 250 1960 1970 1980 1990 2000 2010 2020 build year BP [t] CPP 0 20 40 60 80 100 120 1960 1970 1980 1990 2000 2010 2020 build year BP [t] FPP 0 20 40 60 80 100 120 1960 1970 1980 1990 2000 2010 2020 build year BP [t] azim. 0 20 40 60 80 100 1960 1970 1980 1990 2000 2010 2020 build year BP [t] cycl.
Fig. 1. Bollard pull as function of build year and propulsion type
The greatest disadvantage of VS propulsion is its low efficiency expressed as the ratio of bollard pull to the main engine power – MCR (maximum continuous rating). The VS tugs require 10-20% more power for given bollard pull than conventional and azimuth tugs – see Fig. 2 and 3. This means higher fuel consumption.
0 4000 8000 12000 16000 0 50 100 150 200 250 BP [t] MCR [kW] CPP 0 2000 4000 6000 8000 0 20 40 60 80 100 120 BP [t] MCR [kW] FPP 0 2000 4000 6000 8000 0 20 40 60 80 100 120 BP [t] MCR [kW] azim. 0 2000 4000 6000 8000 0 20 40 60 80 100 BP [t] MCR [kW] cycl.
Fig. 2. Main engine output vs. bollard pull and propulsion type – all years
0 4000 8000 12000 16000 0 50 100 150 200 250 BP [t] MCR [kW] CPP 0 2000 4000 6000 8000 0 20 40 60 80 100 120 BP [t] MCR [kW] FPP 0 2000 4000 6000 8000 0 20 40 60 80 100 120 BP [t] MCR [kW] azim. 0 2000 4000 6000 8000 0 20 40 60 80 100 BP [t] MCR [kW] cycl.
Over the last 20 years the relationship between bollard pull and delivered power for all types of propellers have not practically improved as compared to the previous years. This can be assigned to the technological barrier.
5. LENGTH OF HARBOUR TUGS
Analysing parameters of existing harbour tugs, new and older ones, it can be concluded that to a certain degree there is no strict relationship between bollard pull and hull size of a tug. Its dimensions, particularly the length over all, depend on a specific design of the tug, where a lot of design factors are involved that must be somehow optimised. Besides stability and manoeuvring aspects also autonomous life duration and comfort for the crew is accounted for. The designers and manufacturers wishing to maintain a flexible and profitable production, while simultaneously satisfying the safety and operational effectiveness requirements of a tug, frequently offer the same hull but with variable bollard pull (or main propulsion power). At this instance, the size of tug and its arrangement corresponds to the main engine and auxiliaries of the highest power.
One can also observe the opposite situation, though rather rare, where the same main propulsion power is combined with different size and arrangement of a tug for purpose of its better functionality.
As example of these practices, Table 3 and 4 report main particulars of some existing offers of tug sale [1]. A similar situation seems to exist amongst many other tugbuilders. However, more research is worth to do on this topic since the tug design process is nowadays often beyond shipyard in that it is externally ordered to independent naval architectural design offices. Surprisingly, there are only a few major ship design companies around the world specialising in tugs.
Table 3 Bollard pull vs. length. Case 1
Shipyard Pella (St. Petersburg/Russia, www.pellaship.ru)
parameter series Pella
80600
series Pella 16609
LOA[m] 25.4 28.5
B[m] - breadth 8.8 9.5 H[m] - hull depth (w/out skeg) 4.6 4.8 T[m] - extreme draft (with skeg) 3.5 4.5 m[t] - displacement 390 500
v[kt] - speed 12 12
BP[t] 20-36 40-60
MCR[kW] 1000-2400 2600-3800
propulsion type ASD ASD
Figures 4 and 5, based on Sea-web's data, display the length of a tug in dependence on bollard pull and type of propulsion. There is a meaningful statistical scatter of values. Vessels in length of 40m an more have to be treated as coastal or ocean/deep-water tugs. These are additionally optimised for long cruising range and seakeeping. In case of azimuth tugs, their length almost remains unchanged while increasing bollard pull in wide range from 50 to more than 80 tonnes.
Table 4 Bollard pull vs. length. Case 2
Shipyard Sanmar (Istanbul/Turkey, www.sanmar.com.tr) conventional tugs (2-screw)
Sanmar Nehir Dogancay
LOA[m] 18 22 25
BP[t] 30 30 30(42)
MCR[kW] 1618 1618 1618(2207)
ASD tugs
Ulupinar Sanmar Terminal Sanmar Escort 80
LOA[m] 24 28 32 BP[t] 40 60 80 MCR[kW] 2427 3678 4781 0 20 40 60 80 100 0 50 100 150 200 250 BP [t] LOA [m] CPP 0 15 30 45 60 75 0 20 40 60 80 100 120 BP [t] LOA [m] FPP 0 10 20 30 40 50 0 20 40 60 80 100 120 BP [t] LOA [m] azim. 0 10 20 30 40 50 0 20 40 60 80 100 BP [t] LOA [m] cycl.
0 20 40 60 80 100 0 50 100 150 200 250 BP [t] LOA [m] CPP 0 15 30 45 60 75 0 20 40 60 80 100 120 BP [t] LOA [m] FPP 0 10 20 30 40 50 0 20 40 60 80 100 120 BP [t] LOA [m] azim. 0 10 20 30 40 50 0 20 40 60 80 100 BP [t] LOA [m] cycl.
Fig. 5. Length over all as function of bollard pull – from 1990 till present
6. OTHER DECISIONAL FACTOR. CONCLUSIONS
In general, as already stated in the Introduction section of this paper, high bollard pull is also crucial for safety and effectiveness of the tug itself. In a critical/emergency situation, having at its disposal high propulsion power, it can directly avoid a danger. In routine operations, especially in case of high linear and angular velocities of towed ships, the tug can easily change to a new position and direction. The latter is important when a tug on a hawser is supposed to hold constant the pull force while assuming the new direction. The operational margin of propulsion power, connected with the available higher bollard pull than required for towing, improves the dynamics of a tug, also in that the tug applies the force ordered by a pilot much faster if it's lower than the maximum bollard pull.Choosing either ASD or VWT tug, both of comparable manoeuvring capabilities, should also take into account the actual experience of local harbour towing companies in handling and maintenance of particular type of tug. A certain role in decision-making might be played by the type of towing assistance to be rendered in a given harbour and by the applied manoeuvring tactics. Here, the size and speed of assisted ships, shape of waterways, and hydro-meteorological conditions have some influence.
Tug prices, leading to investment (capital or initial) costs, are often rated in USD/1kW or USD/1t BP. Dependent on exchange rates and the state of the tug market, the price of
azimuthing tug can oscillate within 1-2 mln USD/10t BP if the total BP is approximately 60t [7]. Another aspect are the operational and maintenance costs, which are difficult to be estimated, as well as the prediction of towing fees. This calls for the economic cost and benefit analysis for a certain horizon of time.
One should also be aware that determining the required bollard pull of a tug is not an easy matter. We can settle it by means of analytical methods or ship manoeuvring simulation methods. The former deal with the static conditions of towing, where the basic problem consists in evaluating the hydrodynamic and aerodynamic drag of a towed ship (in both longitudinal and transverse direction) under given towing speed, water current and wind conditions. The latter methods, much better and involving human (operator) interaction, are capable of making an estimate of bollard pull in dynamical/variable conditions.
A very disputable in any design practice is always the so-called safety margin or safety factor. This denotes an arbitrarily increased demand on parameter values of the real process, system, or object over what was directly obtained in the course of analysis with their models and necessary assumptions. The safety margin is thought to provide a reserve for unexpected/emergency real situations and/or possible uncertainty (errors) of the analysis. Different values of safety margins exist in various industries, where they depend to some extent on the final application of the designed object. For example, safety margins are higher if protection of human life and health must be preserved.
The probabilistic approach, outlined in [4], seems to ease making the best decision from the viewpoint of the required tug's bollard pull. However, it does not replace the traditional, very convenient safety margin.
Summarising, due to the market share and wide commercial availability of high bollard pull tugs with azimuthing propulsion, these tugs are worth being purchased or being considered for a purchase. This solution increases the safety and effectiveness of the whole system harbour-ship-tugs, at least in the technical aspect.
Bibliography
1. ABR: A-Z Guide to Tug Builders 2008. Supplement to International Tug&Salvage, Jan/Feb, 2008. 2. ABR: magazine International Tug&Salvage (from June 2012 title changed to International Tug & OSV),
vol. 16 i 17, 2011/12.
3. Allan R.G.: Tugs and Towboats. Ship Design and Construction, vol. 2, ed.: Lamb T., SNAME, Jersey City, 2003.
4. Doorn N., Hansson S.O.: Should Probabilistic Design Replace Safety Factors?. Philosophy & Technology, vol. 24, no. 2 (Jun), pp. 151–168, 2011.
5. Hensen H.: Tug Use In Port (A Practical Guide). Ed. 2, The Nautical Institute, London, 2003. 6. LR/IHS: Sea-web. Internet database: www.sea-web.com (visit on Oct 4th, 2012), 2012. 7. MARCON: www.markon.com (visit on Oct 10th, 2012), 2012.
TYPY I MOC HOLOWNIKÓW PORTOWYCH - WSPÓCZESNE TENDENCJE
Streszczenie: Celem niniejszej analizy jest okrelenie dostpnoci oraz wasnoci manewrowych
holowników portowych w zakresie maksymalnego mo liwego ucigu na palu BP (ang. bollard pull). Szczególna uwag zwrócono na typ napdu holownika i dugo cakowit jednostki LOA (ang. length over all), od której w du ym stopniu zale y przestrze manewrowa zajmowana przez holownik i trudno
w manewrowaniu. Uzyskane wyniki stanowi pomoc w podejmowaniu decyzji operacyjnych i inwestycyjnych wadz portowych w aspekcie modernizacji portu lub nowych uwarunkowa eksploatacyjnych, gdzie podstawowym zadaniem jest zapewnienie bezpieczestwa operacji manewrowych.
