Maritime University of Szczecin
Akademia Morska w Szczecinie
2010, 20(92) pp. 33–40 2010, 20(92) s. 33–40
Risk based method admittance policy of large ferries
approaching to Ystad Port
Zasady wprowadzania dużych promów do portów oparte
o metody szacowania ryzyka na przykładzie Portu Ystad
Lucjan Gucma
Maritime University of Szczecin, Faculty of Navigation, Institute of Marine Traffic Engineering Akademia Morska w Szczecinie, Wydział Nawigacyjny, Instytut Inżynierii Ruchu Morskiego 70-500 Szczcin, ul. Wały Chrobrego 1–2, e-mail: l.gucma@am.szczecin.pl
Key words: navigational risk, under keel clearance, ship accident Abstract
The paper presents method of safety water depth evaluation for ship approaching to ports. The method utilizes two models: real time simulation model used for determination of ships speed approaching to given port, and Monte Carlo model for determination of probability of accidental collision with the bottom. Minimal safety depth has been calculated with use of probabilistic acceptance criteria presented in the paper. Słowa kluczowe: ryzyko nawigacyjne, zapas wody pod stępką, wypadek statku
Abstrakt
W artykule zaprezentowano metodę szacowania bezpiecznego zapasu wody pod stępką statków podchodzą-cych do portów morskich. Metoda składa się z dwóch osobnych modeli: modelu symulacji czasu rzeczywi-stego oraz modelu Monte Carlo do oceny zapasu wody pod stępką. Aby ocenić bezpieczeństwo, zastosowano metody ryzyka akceptowalnego.
Introduction
Marine Traffic Engineering (MTE) research team of Maritime University of Szczecin since 70- -ties is engaged in research works concerned with evaluation of navigation safety for port design and optimization of water areas. In this paper author’s intention is to present complex researches for de-termination of safety waterway depth for approach ships. It is widely known that squat of ships is most important factor affecting under keel clearance (UKC). Unfortunately the major factor of squat – ship’s speed, cannot be reduced especially in diffi-cult navigational conditions. This happens because it is necessary to maintain high speed to keep proper manoeuvrability. Only one way to determine ships speed during approach especially in non exist-ing solutions is to use simulation models (real time simulators) where navigators can freely adjust speed of ships to given conditions.
From the other side speed is not the only one factor of under keel clearance (UKC). To deal with such complex phenomenon as under keel clearance, Monte Carlo method is most suitable in author’s opinion and is applied in presented researches.
With use of probabilistic acceptable risk criteria it is possible to achieve the results of presented method: the minimal required depth for approach-ing ships in given area.
Modernisation of Ystad Port by simulation researches
The researches described in this paper are focused on Ystad Port modernisation which is one of the latest research studies of MTE team. The main aim of researches has been focused on [1]: 1. Determination of optimal parameters of:
– approach channels to reconstructed port of Ystad with respect to shape, width and depth;
– inner and outer port breakwaters with respect to its shape with respect to waving in port; – turning places with respect to its shape and
optimal depth;
– two new berthing places in inner port in respect to its shape, length, depth, maximal energy of ships contact, maximal speed of ships propeller and bowthruster streams on the bottom.
2. Determination of safety conditions of port operation in respect to:
– admissible meteorological conditions for given kind of ships and manoeuvres;
– other navigational conditions and limitations like presence of other ships on berths, use of position fixing systems on approach, naviga-tional markings, vessel traffic service.
3. Determination of manoeuvring procedures during berthing and unberthing for different kind of ships and propulsion systems.
4. Determination of under keel clearance by Monte Carlo method.
5. Determination of usage of main engine during entrance.
6. Determination of ferry distances to the most dangerous objects.
7. Carrying out most typical emergency runs and describe necessary emergency action for the captains.
Two characteristic Ro-Pax ships have been chosen as typical for development of Ystad Port. M/f “Wolin” is midi size ferry originally built as train ferry. The second ship m/f “Piast” is newly designed ferry build for Unity Line by Stocznia Szczecińska Shipyard. Most important parameters of ferries are presented in the table 1.
Fig. 1. General arrangement of m/f “Piast” [2] Rys. 1. Ogólny projekt promu m/f „Piast” [2]
Fig. 2. Photo of m/f “Wolin” ferry [2] Rys. 2. Fotografia promu m/f „Wolin” [2]
Table 1. Main parameters of “Wolin” and “Piast” as typical ferries operated on the Baltic Sea area [2]
Tabela 1. Główne parametry typowych promów pływających na obszarze Morza Bałtyckiego na przykładzie promów „Wolin” i „Piast” [2]
Parameter “Piast” “Wolin”
Operator
(route) (Świnoujście–Trellborg) Unity Line Unity Line Building year 1986 / 2002 rebuild expected 2009 Length –
LOA 207 m 188.9 m
Breadth 27 m 23.7 m
Draft 6.3 m 5.9 m
DWT 8000 t 5143 t
Machinery total 21.600 kW at 500 rpm total 13.200 kW at 600 rpm Propeller
2 variable pitch propellers turning inside
at 145 rpm
2 variable pitch propellers turning inside at 150 rpm
Speed approx. 21 kn 18 kn
Rudder 2 70 deg. active 2 70 deg. active Bowthrusters 2 2.300 kW 2 1100 kW Sternthruster 1 1.200 kW 1 736 kW Lateral wind
area approx. 3000 m2 approx. 2700 m2
The most important aim of Ystad Port moderni-sation is to provide access to the port by ferries up to 210 m length and enable future port development in the future to serve ships of 240 m length [1]. Three most important changes are planned (Fig. 3): 1. Building two instead of three ferry quays in
inner port,
2. Design of new turning place in outside port, 3. Shortening of inner western breakwater,
4. Lengthening of outer breakwater to provide shelter in western winds of turning place.
Fig. 3. Proposed changes in berths, turning place and breakwaters arrangements in port of Ystad [2]
Rys. 3. Proponowane zmiany w nabrzeżach, obrotnicy i falo-chronach planowane w Porcie Ystad [2]
400m
B4
B3 B1
Line of previous berths
Additional breakwater
Breakwater to shorten
Real time simulation methods applied
Real time simulation the interactive method with captains and pilots engaged in ships manoeuvring trials have been applied. This method is assumed as most reliable and suitable in this kind of research studies [3]. MTE research team possesses several kinds of manoeuvring simulators: form self made limited task with 2D display to modern commercial full mission simulator with 3D and real control systems. Both simulators have been applied in presented researches.
Real time simulation method – limited task simulator
Two classes of hydrodynamic models in MTE team own limited tasks simulators are utilized. First class of models are used when only limited parame-ters are known (usually when non existing ships or general class of ships are modelled). The second class models are used when detailed and exact characteristics of hulls, propellers and steering devices are known. Additionally real manoeuvring characteristics are used for validation of models. In present researches the second model has been
used (m/f “Wolin” exists and sea trials are avail-able, m/f “Piast” trial parameters has been extrapo-lated).
The model used in researches is based on modular methodology where all influences like hull hydrodynamic forces, propeller drag and steering equipment forces and given external influences are modelled as separate forces and at the end summed as perpendicular, parallel and rotational ones.
The model is operating in the loop where the input variables are calculated instantly (settings and disturbances) as the forces and moments acting on the hull and momentary accelerations are evaluated and speeds of movement surge, sway and yaw. The most important forces acting on the model are: – thrust of propellers,
– side force of propellers,
– sway and resistant force of propellers, – bow and stern thrusters forces, – current,
– wind, – ice effects,
– moment and force of bank effect, – shallow water forces,
Fig. 4. The main diagram of simulation model [2] Rys. 4. Schemat modelu symulacyjnego [2]
– mooring and anchor forces,
– reaction of the fenders and friction between fender and ships hull,
– tugs forces,
– other depending of special characteristics of power and steering ships equipment.
The functional idea of the ship manoeuvring simulation model is presented in figure 4.
Interface of the model is typical 2D chart interface (Fig. 5). The interface covers information of ships state (position, course speed, yaw etc), quay and shore line location, navigational markings, soundings, external conditions, tug and line control and control elements of the model. The model is implemented in Object Pascal with use of Delphi™ environment and Visual C™ with use of C++ language.
Fig. 5. Interface of simulation model ferry “Wolin” turning at outer turning place of Ystad Port (limited task simulator) [2] Rys. 5. Interfejs modelu symulacyjnego promu „Wolin”, manewrującego przy obrotnicy zewnętrznej Portu Ystad („limited task”) [2]
Real time simulation method – full mission simulator
Kongsberg Polaris™ simulator located at Ma-rine Traffic Engineering Centre (MTEC) premises in Maritime University of Szczecin comprises (Fig. 6):
– one full mission navigation bridge simulator with 270° visual projection and live marine ship equipment (DNV class A);
– two part task navigation bridges with 120° visual projection and mix of real and screen- -simulated ship-like equipment including one Voith-Schneider tug console (DNV class B); – two desktop PC simulators with one monitor
visual projection and one monitor screen- -simulated ship-like equipment.
All hardware and software forming the Polaris ship manoeuvring simulator has been granted DNV certificate for compliance or exceeding the training regulations set forward in STCW’95 (section
A-I/12, section B-I/12, table A-II/1, table A-II/2 and table A-II/3).
In order to create own ship models a hydro-dynamic ship-modelling tool is available. This tool enables creating almost any ship type (controls for at least two engines with propellers’ controls for fixed propeller, adjustable pitch propeller and azimuth; rudder controls adequate for various types of conventional rudders, active rudders, Z-drive / azimuth and thrusters) with very high fidelity hydrodynamics in 6 DOF (surge, sway, heave, yaw, roll, pitch). The hydrodynamics comprise all known to state of the art external effects like squat, bank and channel effects.
Assumptions to safety depth evaluation
The main aim of this part is to determine safety depth of Ystad waterway. The probabilistic criteria of acceptable risk is applied and together with simulation results (real time ships manoeuvring) used to obtain minimal acceptable (under given safety conditions) depth of waterway (Fig. 7).
Fig. 6. Bridge A at MTEC (with 270 visual projection) and captain “at work” [2]
Rys. 6. Mostek A w CIRM (z wizualizacją w zakresie 270°) oraz jeden z kapitanów podczas pracy [2]
The minimal depth on approach to sea ports is very important factor which influence safety of navigation in respect to under keel clearance which is together with horizontal area the most important factor of navigational safety.
Following assumptions have been taken into consideration:
1. Swedish water levels for design: HHW +1.65
MHW +0.87
MW +0.03
MLW –0.97 LLW –1.47;
2. Water level for calculations – MLW 3. Ship lifetime – 15 years
4. Waterway lifetime – 50 years 5. Ships draught – 6.3 m
6. Ships speed – variable according to simulations results
7. Wave – 0.4 0 m.
Fig. 7. Method of safety depth evaluation [2] Rys. 7. Metoda określania bezpiecznej głębokości [2] Navigational risk
The navigational risk can be defined as proba-bility of certain losses during expected period of time (one year / lifetime of ships or waterway):
C P
R A (1)
where: PA – probability of serious grounding
accident, C – consequences of accident.
With assumption that accidents consequences are similar (grounding of passenger ship without losses) we can expressed risk as probability of accident only.
Acceptable risk criterion
Probabilistic acceptance criterion is proposed in this study. Such criteria are widely used in Marine Traffic Engineering (Dutch, England, Denmark, and Poland).
Monte Carlo simulations performed in stage II of researches enabled to find probability of accident in single passage assumed when UKC < 0 is expressed as PUKC < 0.
Most of the accidents are not serious. Proba-bility of serious accident can be calculated with
assumption that serious accidents are 10% of all of total number of accidents: PSA = 0.1 (so called
Heinrich factor usual assumption in restricted water areas validated by real accidents statistics see [4]). Under above assumptions probability of serious accident PAS can be calculated as:
0
SA UKC
A P P
P (2)
Intensity of all accidents in given time (ex. one year) can be calculated as:
A
NP
(3)
where: N – ship movement intensity per 1 year. Typical probabilistic criterion for risk of colli-sion with the bottom is based on Poisson process the collisions with the bottom are random with intensity [collision / time] and expected number n during given time t:
! n e t n P t n (4)where: n – expected number of collision with bottom in given time period, – intensity in given time.
No accident probability in given time period t can be calculated with assumption that n = 0 as:
n
e tP 0 (5)
The opposite to above safety factor can be expressed as occurrence at least of one accident in given time t and expressed as:
n
e tP 1 1 (6)
Typical probabilistic safety criterion is proba-bility of no accident in given time. For example Dutch criterion on approach to Rotterdam (with tides consideration) is 10% probability of more then one accident in 25 years of waterway operation which is expressed as:
n1
1e t 0.1P (7)
(where t = 25 years) which gives t = 0.105.
Assuming that t = 25 years of operation we obtain
= 0.0042 of all accidents per year which lead to following criterion: one accident in 238 years period (=1/
). The criterion comprise all accidents so with assumption that serious accidents are 10% of all accidents we can calculate criteria value of yearly intensity of serious accident as:00042 . 0 1 . 0 S (8) Real time simu- lations Ship speed Monte Carlo method Probability of collision with bottom Minimal depth Acceptable risk
Polish criterion that is used in Marine Traffic Engineering is slightly less restrictive due to low traffic intensity, and nature of the bottom in ports (sand, mud). In Poland it is assumed limit accident rate per year at the level = 0.007 of all accidents or = 0.0007 for serious accidents (serious acci-dent is such acciacci-dent where special rescue action should be undertaken) as criterion value (the crite-rion is based on acceptance of one serious accident per ships lifetime which equals 15 years, because ships during this time are not likely to be rebuild in opposite to waterway which during 50 years of operation will be rebuild few times most likely).
In further step taking into consideration the passages of ferries which are around 10 per day in Ystad Port (N = 10365 = 3650 passages / year) it is possible to calculate limited probability of collision with the bottom (accident) in single passage as:
6 accept 3650 1.1510 0042 . 0 N PA
Monte Carlo method of under keel clearance on ferry approach
The stochastic model of under keel clearance evaluation has been presented in [3]. It is based on Monte Carlo methodology where overall ships under keel clearance is described by following mathematical model (Fig. 8):
N Swi Swa Ti Hoi T H UKC
) ( ) ( ) ( 0 (9) where: Hoi the uncertainties concerned withdepth and its determination, Ti the uncertainties
concerned with draught and its determination, Swi –
the uncertainties concerned with water level and its determination, N navigational and manoeuvring
clearance.
Fig. 8. Concept of probabilistic under keel clearance determi-nation [2]
Rys. 8. Metoda probabilistyczna określania rezerwy wody pod stępką [2]
The final model takes into account depth measurement uncertainty, uncertainty of draught determination in port, error of squat determination, bottom irregularity, tides and waves influence are deciding factors for under keel clearance of ships. Program is capable to consider above mentioned uncertainties using distributions and their para-meters. The following parameters are randomly selected from their distributions:
1. Depth – hi (averaged in following sections –
100 m, 0 m, +100 m, 200 m, 300 m from the breakwater),
2. Sounding error – BSi,
3. Mudding component clearance –
i
Z
,
4. Draught determination error – Ti, 5. Ship’s heel error –
i
P
.
Random draught module. User’s entered
draught is corrected for draught determination error value and ship’s heel error. Iterated draught (Ti) is
calculated as follows: i i P T i T T (10)
where: T – ships draught [m] assumed as 6.3 m,
i
T
– draught determination error (assumed
as 0.05 m),
Pi– ships heel error (assumed
as 3 degrees).
Water level module. Water level PWi can be
automatically load from online automatic gauges if such exists (Szczecin solution). In these researches the level has been modelled as normal cut distribution with parameters (0, 0.1 m).
Depth module. Depth hi has been assumed as
constant in given sections (it varies from 9 before and near breakwater to 8.5 m inside the port see figure).
Squat module. Squat (ship sinking due to
decrease of water pressure during movement) is calculated in three stages. First module calculates squat with analytical methods used to obtain moving vessel squat (Huusk, Milword 2, Turner, Hooft, Barrass 1, Barrass 2). Next standard errors of each method are applied. Squat model selection and their standard errors have been verified by GPS-RTK experimental research [5]. As a result of the experiment uncertainty of each model has been assessed and each squat method assigned weight factor. Method’s weights and statistical resampling bootstrap method are used later on to calculate final ship's squat.
Under keel clearance module. Under keel
clearance Zi is determined by using draught, depth, Minimal dredged depth
Grounding probability Water level Draught Depth Squat V V = 0 fs(s) fh(h)
water level and squat results which have been calculated before. Under keel clearance is defined as:
i Z BS
i i N WP F
i h i i T O i
Z (11)
where: hi – up-to-date depth in each iteration in
sections (sounding from October 2007),
i
Z
–
mudding component clearance (normal cut distribution with 0 and 0.1 m),
i
BS
– sounding error (normal cut distribution with 0 and 0.1 m),
Ti – ships draught with its uncertainty (0, 0.05 m),
Oi – iterated squat (bootstrap model), N –
navigational clearance (0 m),
i
WP
– height of tide error (0 m), F – wave clearance (wave height
assumed as h = 0.4 m before breakwater, h = 0.2 m in the breakwater and h = 0 m inside, direction from ferry traverse).
Results
As the result of combined method Monte Carlo with simulations results in sections of waterway (breakwater = 0 m) we obtain parameters of distri-butions of UKC in distance from breakwater for real mean depth existing in Ystad Port (Fig. 9).
Fig. 9. Mean actual depth in given sections (sounding from fall 2007) [2]
Rys. 9. Średnia aktualna głębokość w poszczególnych prze-krojach (sondaż z jesieni 2007) [2]
Speed of approaching ships has been determined from real time simulations (extreme conditions E20 m/s wind) have been applied (Fig. 10). Speed applied in Monte Carlo model has been calculated with 95% probability level.
In next step Monte Carlo model described in Ystad development study has been applied to determine histograms and parameters of distributions of UKC in function of ships position on the waterway. Wave influence has been taken into account.
In the further step on the basis of Monte Carlo results the UKC on 95% and squat has been
calculated (Fig. 11). Important for the probability calculations is mean UKC and standard deviation of UKC presented in figure 12. Due to lack of distribution or probabilities of given water levels the water level assumed in this study is equal to MLW = –0.97 m.
Fig. 10. Speed of ferry “Piast” in knots on approach with E20 m/s wind (x = 0 outer breakwater) obtained by mean of real time researches [2]
Rys. 10. Prędkość promu „Piast” w węzłach na podejściu do portu przy wietrze wschodnim o prędkości 20 m/s (przyjęto
x = 0 jako główki portu) [2]
Fig. 11. UKC on 95% and 5% level of confidence of m/f “Piast” approaching with E20 m/s wind (x = 0 outer break-water) [2]
Rys. 11. Zapas wody pod stępką na poziomie ufności 95% i 5% promu m/f „Piast” wchodzącego do portu przy wietrze wschodnim o prędkości 20 m/s (przyjęto x = 0 falochron zewnętrzny) [2]
Fig. 12. Mean UKC and standard deviation of UKC of “Piast” ferry (T = 6.3 m) entering to Ystad Port [2]
Rys. 12. Średni UKC i odchylenie standardowe UKC promu „Piast” (T = 6,3 m) wchodzącego do Portu Ystad [2]
7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25 9.5 9.75 10 -200 -100 0 100 200 300 400 actual depth breakwater [m] [m] -6 -4 -2 0 2 4 6 8 10 12 14 -600 -400 -200 0 200 400 600 800 Serie1 Serie2 Serie3 Serie4 Serie5 Serie6 Serie7 Serie8 Serie9 Serie10 Serie11 Serie12 Serie13 Serie14 Serie15 Serie16 Serie17 Serie18 mean 1wej1E20_2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 -150 -100 -50 0 50 100 150 200 250 300 350 UKC_95% squat UKC_5% breakwater [m] [m] 1wej1E20_2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 -200 -100 0 100 200 300 400 mean ukc st.dev. ukc breakwater [m] [m]
Final calculation of required depth (H) for distribution with parameters m and to fulfill assumed Dutch criterion is based on following formula:
H A m A f x x P P 6 accept ) , ( ( )d 1.15 10 1 (12)The results as required depth on approach to Ystad Port are presented in figure13.
Fig. 13. Minimal depth in Ystad Port with criterion
PA ≤ PA–accept = 1.1510–6 [2]
Rys. 13. Minimalna głębokość w Porcie Ystad z założeniem
PA ≤ PA–accept = 1.1510–6 [2]
Conclusions
Minimal depths for ferry Piast (T = 6.3 m) in function of distance to outside breakwater heads are presented in figure 13. Minimal safety depths to fulfil criterion are varied from 9.2 m before breakwater heads to 8.3 m inside the avanport.
Probabilistic safety criterion (so called Dutch criterion used in Rotterdam Port) has been applied (acceptable level of 0.0042 accidents per year). On the basis of this the limited probability of hitting the
bottom accident have been evaluated and used for minimal depth in Ystad Port.
References
1. Computer Simulation for Port Design and Safe Manoeu-vring of ships Simulation researches of m/f “Piast” con-ducted by means of PC Ship Manoeuvring Simulator and Full Mission Simulator. Stage I and II. Research Work Maritime University of Szczecin, Szczecin 2008.
2. GUCMA L.: Wytyczne do zarządzania ryzykiem morskim.
Wydawnictwo Naukowe AM. Szczecin 2009.
3. GUCMA L.: Risk Modelling of Ship Collisions Factors with
Fixed Port and Offshore Structures. Maritime University of Szczecin, Szczecin 2005.
4. SAVENIJE R.PH.: Probabilistic Admittance Policy.PIANC Bulletin, Bruxelles, 1996, No. 91.
5. Determination of squat of m/f “Śniadecki” by RTK method in Świnoujście Port. MUS Research work 2006.
Others
6. ARTYSZUK J.: Towards a Scaled Manoeuvring
Mathemati-cal Model for a Ship of Arbitrary Size. Scientific Bulletin, Maritime University of Szczecin, Szczecin 2005.
7. GUCMA L., GUCMA M., TOMCZAK A., PRZYWARTY M.:
Experimental determination of squat and trim of sea ferry “Jan Śniadecki” on approach to Świnoujście port by means of RTK method (in Polish) Proc. of 15 NavSup Confe-rence, Gdynia 2006.
8. GUCMA L.,JANKOWSKI S.: Method of determining probabi-listic models of propeller streams speed at the bottom of manoeuvring ships. Proceedings of the 9 International Sci-entific and Technical Conference on Marine Traffic Engi-neering, Szczecin 2001.
9. GUCMA L.,SCHOENEICH M.: Probabilistic model of under
keel clearance in decision making process of port captain. Proc of TransNav Conference, Gdynia 2007.
10. IRIBARREN J.R.: Determining the horizontal dimensions of
ship manoeuvring areas. PIANC Bulletin, Bruxelles, 1999, No. 100.
11. VASCO COSTA F.: Berthing manoeuvres of large ships. The
Dock and Harbour Authority. 1969.
7 7.25 7.5 7.75 8 8.25 8.5 8.75 9 9.25 9.5 9.75 10 -200 -100 0 100 200 300 400 minimal depth breakwater [m] [m]
