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5. Passenger Vessel Operations and Damaged Stability Standards (Non-convention vessels) (2007) - TP 10943
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Appendix 1

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Risk-Based Methodology For Safety Assessment Of Enclosed And Semi-Enclosed Domestic Ro-Ro Ferries

1. Principles

1.1 This methodology can be used to demonstrate acceptable levels of safety on a route-specific basis, where an existing vessel is non-compliant with the current standard TP 10943 Part II.
1.2 Risk-based methodologies must consider the probability of an incident and the severity of its consequences. The methodology for assessing the safety of ferries following collision damage incorporates the following elements considering both aspects:

• Probability of occurrence of collision: traffic density on route
• Severity of collision damage: statistical treatment of location of damage and damage extent
• Probability of loss of ship: loading condition, residual stability properties, prevailing environmental conditions
• Probability of loss of life: numbers on board

1.3 This is not a comprehensive list. However, it provides a more complete treatment of safety than is offered by current national and international regulations and provides adequate assurance of safety without excessive complexity in analysis and application.
1.4 The application of the method is described below and in the supporting Annexes.

2. Overview

2.1 The overall application of the methodology is illustrated by the flow chart in Figure 2.1.
2.2 It permits Transport Canada to approve or endorse the outcomes at several stages in the process, to minimize the complexity for the majority of users. The Stage 1 approach corresponds to the standard approval process. Stages 2 and 3 represent alternative procedures under the voluntary MOU and are therefore endorsed rather than approved.
2.3 The Stage 3 approach is only required where some combination of loading and sea state restrictions are required to attain the target safety level. The Stage 3 approach will require to be supported by suitable operational procedures and associated documentation contained in a Risk Management Plan. The contents of such a Plan are described in Annex A.

Figure 2.1: Procedure flow diagram

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3. Stage 1 Approach

3.1 Any existing vessel covered by this standard that has been modified since its most recent inclining experiment, or whose last lightship survey is more than 5 years old, should have its weight and centre of gravity confirmed by a lightship survey and/or inclining experiment before proceeding with a Stage 1, 2 or 3 approach.
3.2 Stage 1 is the normal approval procedure for damage stability assessment under Transport Canada standard TP 10943.
3.3 In the event that the vessel is fully compliant with the standard TP 10943 criteria when assessed against two compartment damage then no further analyses are required and the calculations can be submitted to Transport Canada for approval.
3.4 In the event that a vessel is compliant when assessed against one compartment damage, then it will be subjected to future compliance requirements as described in Part II subsection 16(4) of the standard TP 10943. However, the Stage 1 analysis can be used as the basis for approval for operations in the interim period.

4. Stage 2 Approach

4.1 Where a vessel does not comply with the operation assumptions underlying the standard TP 10943, then it can be evaluated on a route-specific basis.
4.2 The wave climate for the route must be established, using data of acceptable quality. A majority of ferry routes in Canadian coastal waters are covered adequately by published data. Where data does not exist for the route, the operator may choose to use data for a more severe area nearby, or may initiate a data collection and monitoring program. Normally the data collection option will only be permissible as part of a Stage 3 approach (see below), as an acceptable statistical base is required for a Stage 2 endorsement.
4.3 The data is to be presented in the form of probability plots, as shown in Figure 4.1. The value of importance for the safety assessment is the significant wave height associated with a 10 percent probability of exceedence; i.e., less than 10 percent of the time will be spent in more severe conditions. The probability of exceedence is (100 – the cumulative probability of occurrence); i.e., a 90 percent probability of one is a 10 percent probability of the other. In this case, the 10 percent probability of exceedence value is approximately 3.4 m.

Figure 4.1: Typical Wave Height Probability Plot

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4.4 The critical wave height for ship survival is to be calculated for the worst damage condition in which the vessel fails to meet the standard TP 10943 damage stability criteria. In the event that no single damage case provides the worst performance against all of the criteria (GM, GZArea, steady heel, freeboard), then all cases in which one or more criterion is at a minimum are to be evaluated.
4.5 Critical wave height is to be determined using the Static Equivalent Method (SEM) as described in Annex B. The damage condition is assessed to determine the maximum survivable accumulation of water on the vehicle deck, using any proven stability analysis software. This is matched to the significant wave height that will cause this accumulation using a simple formula derived from the SEM. The significant wave heights for each damage case are then compared with the 10 percent probability of exceedence value determined as outlined at 4.3.
4.6 In the event that all damage cases show critical wave heights in excess of the 10 percent value, then the vessel is considered to be adequately safe for unrestricted service on the route under consideration. The technical data, including stability analyses and wave climatology can be submitted to Transport Canada for endorsement.
4.7 At some operating draft (deadweight) and centre of gravity a vessel may have critical wave heights for all damage cases above the 10 percent exceedence values. In the event that the operator is prepared to keep the ship at or below this level of loading, then a technical data package and operating procedure based on a revised load line and a realistic worst centre of gravity can be submitted to Transport Canada for endorsement. This can be considered as a modified Stage 2 approach.
4.8 In the event that a vessel has been assessed against one compartment damage, then it will be subjected to future compliance requirements as described in Part II subsection 16(4) of the standard TP 10943. However, the Stage 2 analysis can be used as the basis for approval for operations in the interim period.

5. Stage 3 Approach

5.1 Where a Stage 2 analysis indicates that a vessel may not be adequate for unrestricted service on its intended route, it may still be operable using an acceptable operating envelope that matches loading to environmental conditions on a real time basis.
5.2 This Stage 3 approach combines loading, wave climate and risk management. The approach includes three sequential steps.

Step 1 – Definition of Operating Environment

5.3 In order to predict the level of operability that will be available under a Stage 3 approach, historical wave statistics can be used, constructing a plot similar to that of Figure 4.1. This can be done:

1. on an annualized basis (as in the Stage 2 approach);
2. on a seasonal basis (monthly or longer).

5.4 Wave data sources will normally present information for a variety of time intervals, both annual (as in Figure 4.1) and for shorter periods. Severe weather is more common during some months/seasons that often coincides with lower traffic levels on ferry routes. An operator can use historical data for both climate and traffic levels to assess the impact of a Stage 3 approach on future operations.
5.5 To implement the Stage 3 approach, the operator will be required to develop procedures to ensure that accurate predictions of wave climate are available, and that correlations between forecast and actual conditions are undertaken. Examples of acceptable procedures for this are provided at Annex A.

Step 2 – Calculation of Damage Survivability

5.6 The maximum permissible loading condition is to be determined for all wave heights in which the ship may be operated, using the SEM as described in Annex B as part of the procedure described below.
5.7 A minimum of three loading conditions must be analyzed, for example corresponding to full load (deep departure), no load (operational light) and an intermediate case with 50 percent load. The centre of gravity for this any condition should be for an unfavourable but realistic distribution of vehicle load.
5.8 A set of wave heights are selected, covering the range of conditions expected to be encountered at intervals no greater than 0.5 m.
5.9 For each wave height, loading condition, and one or two compartment damage case, the SEM is used to assess whether the damage will be survivable. Where a damage case gives an ‘s’ value of 1 under the MSC Circ 574 approach, it is not necessary to undertake the SEM calculation.
5.10 For each case where s < 1 survivability can be demonstrated using the SEM. For each loading condition a value can be found for the maximum survivable wave height.

Step 3 – Derivation of Permissible Operating Envelope

5.11 At any given wave height the load level at which all s = 1 can be found by interpolation. A permissible operating envelope curve can then be derived, as shown at Figure 5.1. When operating under a Stage 3 approach, a vessel’s loading for a given voyage will be restricted to the value associated with the actual and forecast wave conditions along the route.

Figure 5.1: Operating Envelope

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Risk Management

5.12 If the vessel is intended to remain in service (without modification) beyond the applicable compliance date, then it will need to be operated in accordance with a Risk Management Plan that defines how deadweight and/or environmental conditions will be monitored and controlled to achieve the required levels of safety.
5.13 The Risk Management Plan, together with all supporting stability calculations, etc., is to be submitted to Transport Canada for endorsement. Necessary components of such a Plan are outlined at Annex A.

6. Monitoring And Audit

6.1 When a Stage 3 approach is adopted, the operator must ensure that the Risk Management Plan includes monitoring and audit procedures as outlined in Annex A.

Annex A

1. Risk Management Plan

1.1 An existing vessel that does not comply with the standard TP 10943 damage stability requirements may have its fitness for continued operation using an operational envelope on a route-specific basis endorsed on the basis of compliance with a suitable Risk Management Plan.
1.2 The vessel will be provided with a Safety Certificate, endorsed by Transport Canada, when Transport Canada is satisfied that the operator has made adequate provisions from the point of view of safety generally, and the following matters specifically:

1. the suitability of the craft for the service intended, as defined in the route operational manual;
2. the arrangements for obtaining weather information on the basis of which the commencement of a voyage may be authorized;
3. the designation of the person responsible to cancel or delay a particular voyage, e.g., in the light of the weather information available;
4. communications arrangements between the vessel, coast radio stations, emergency services and other ships, including frequencies to be used and watch to be kept;
5. the keeping of records to allow Transport Canada to verify:
   1. that the craft is operated within specified parameters;
   2. the loading of passengers and vehicles;
   3. the observance of safety procedures.
6. the existence and use of adequate instructions regarding:
7. safe loading of the vessel;
8. emergency and contingency plans.

2. Vessel Documentation

2.1 Documentation available on board the vessel should contain at least the following information:

1. load limitations, general and related to given environmental conditions;
2. loading procedures, including control of both load and centres of gravity;
3. damage control procedures;
4. evacuation procedures;
5. procedures for obtaining weather information;
6. identification of the person responsible for decisions to cancel or delay voyages, and/or to select load limits;
7. communications arrangements between the vessel, coast radio stations,
8. emergency services and other ships, including frequencies to be used and watch to be kept;

3. Training

3.1 Prior to issuing a Safety Certificate, Transport Canada will require evidence that the Master(s) of the vessel have been trained in the use of the risk management approach, and that procedures are in place for the training of future operators.

4. Record Keeping

4.1 Sufficient records of all voyages are to be made and retained to allow for future audit of the actual operating practices. Such audits should form part of all reendorsements of a vessel’s safety certificate, and may also be undertaken at intervals within a certificate’s period of validity. The information should be transferred/copied to a shore site at regular intervals.
4.2 The records should include not only ‘results’ (e.g., sea state, displacement, KG) but also notes on the data sources from which the information has been derived.

5. Data Sources

Wind and Wave Data

5.1 Wave data is a critical element of the risk management system. Historical wave climate data can be obtained from sources including Canadian Wind and Wave Climate Atlas, TP10820E. Data is unlikely to be available for the exact location of any ferry service. However, it can and should be used to give a general characterization of the route.
5.2 Current and forecast wave data can be obtained from meteorological offices and from sensors such as wave buoys. Where data from remote stations or sensors is used, measures should be taken to calibrate and audit the accuracy of predictions. The quality of data should be taken into account in the operating procedures, and also in the frequency of audits.
5.3 Local wind data should always be available from anemometers at the terminals and on the vessel. Current and forecast wind speeds and directions can be obtained from meteorological services.

Ship Loading

5.4 A record should be kept of the number of passengers and vehicles loaded for each voyage. Vehicles can either be weighed individually, or their probable weight (and that of the passengers) can be estimated from historical averages. Centres can be assigned based on the vehicle decks in use.
5.5 Other deadweight items (fuel, water, etc) should be monitored at a frequency appropriate to their importance and to the variability in consumption rate.
5.6 Loading information should be combined to provide overall displacement and KG estimates for each voyage. The displacement and LCG should be checked against the draft marks or other draft sensors.

6 Monitoring and Control

6.1 Responsibility for the safety of the vessel and its operations is vested first in the Master through his actions and decisions, and second in the operator through its policies and procedures.
6.2 Transport Canada Marine Safety must be satisfied that the procedures and policies are adequate, and that they are being correctly implemented on a day-to-day basis. This should involve a formal reporting system by the operator, complemented by an audit process by TC inspectors on a regular or random basis.

Annex B

Stability Analysis Methodology

1. Introduction

1.1 The approved stability analysis methodology for use in the risk assessment approach is known as the Static Equivalent Method (SEM). Additional information on the method can be found in references [1] and [2].
1.2 The SEM predicts that capsize will occur when the accumulation of water on deck is sufficient to overcome the residual stability of the ship in its damaged condition, as defined by its maximum righting arm, GZmax at a heel angle qcrit.
1.3 The accumulation of water can be related to the wave climate by a simple empirical formula:

h = 0.085.Hs^1.3    (1)

where:
h is the elevation of the water surface inside the ship above the mean sea level outside;
Hs is the significant wave height.

1.4 The total depth of the water on deck includes not only this elevation of the water surface, but also the depth of flooding on deck and the further sinkage of the ship due to the accumulation of water. These components are illustrated in Figure 1.

Figure 1 : SEM Variables

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2. Basic Calculation Approach

2.1 The application of the SEM uses a sequence of standard hydrostatic analyses.
2.2 The ship is modelled with all compartmentation necessary to define the extent of damage and flooding in all damage conditions to be analyzed.
2.3 The residual stability of the ship is defined by its properties when the relevant compartments are flooded. The critical heel angle is the angle at which GZmax is reached in this flooded condition.
2.4 The total amount of water on the vehicle deck is modelled by treating it as a tank. At the critical heel angle, the vessel will be at the point of capsize when the total weight of water in the tank has an overturning moment equal to the total restoring moment at this angle. The compartment should be ‘filled’ until this balance is obtained.
2.5 The same result will be obtained by comparing the moment due to the elevated water to the restoring moment for the flooded ship at the same angle. The calculation approach can work with whichever quantities are easier to calculate.
2.6 This method can be used to determine the survivable wave height for the damage condition by inverting equation 1, i.e.:

Hs = (11.76.h)^0.77

The wave height can then be matched to historical or forecast conditions to assess the survivability of the damage condition.

3. Application To Risk Assessment

3.1 The survivable wave height for any standard damage condition can be determined and matched to target wave data. The wave data can consider historical averages, on an annual or seasonal basis. It can also be used for real time decisions when the current or forecast wave height on the route is known.
3.2 If the target survivable wave height is known, the survivable load condition can be found by interpolation, as shown in Figures 2 and 3. In this example, the worst damage condition under the standard TP 10943 Part II analysis has survivable wave heights of 0.83, 2.49, and 3.32 metres at 100%, 50%, and 0% load. The target wave for this route (10% probability of exceedence) is 2.34 metres (Figure 2). By interpolation the allowable load level for the ship will be approximately 70% of deadweight (Figure 3).

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Figure 3: Capsize Wave Height Derived From SEM For Worst Case Damage

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3.3 For a Stage 3 approach it is necessary to develop a comprehensive list of single and twin compartment damage cases at full load, as per part II of the standard TP 10943
3.4 Test each damage case at full load for the degree of compliance (0 – 1) with the ‘s’ equation given in Circ 574 as follow:

where:
GZmax is the maximum positive residual lever (m) within the range of 15° beyond the angle of equilibrium, but not more than 0.1 m;
Range is the range of positive righting levers beyond the angle of equilibrium, in degrees, but not more than 15°;
Area is the area under the righting lever curve (m-rad), measured from the angle of equilibrium to the lesser of the angles at which progressive flooding occurs, or 22° (measured from the upright) in the case of a one-compartment flooding, or 27° (measured from the upright) for the flooding of two or more compartments, but not more than 0.15 m-rad.

In respect of the Area, please note that the allowable area is up to a heel angle, measured from the upright 22°/27°, depending on whether flooding of a single or two adjacent compartments is concerned.
and c is determined according to the following:
c = 1 where the final angle of equilibrium is not more than 7°;
c = 0 where the final angle of equilibrium is more than 20°;
else
In all case where the margin line is immersed in the equilibrium condition, s is to be taken as 0.
3.5 Apply the SEM method to derive the survival wave height for each of the above damage cases at full load where the ‘s’ equation test result is < 1.
3.6 Apply the SEM method to derive the survival wave heights for each of the above damage cases at progressively reduced loads down to zero cargo.
3.7 Use the results of 5 and 6 to construct the operational envelope for the vessel covering the complete load range, as follows:

• For one-compartment designs, use the lowest survival wave heights for all single compartment damage cases at each load level.
• For two-compartment designs, use the lowest survival wave heights for all single and twin compartment damage cases at each load level.

3.8 When applying the SEM method to a damage case, the deck may be immersed in the equilibrium condition before the effect of the water accumulated on deck is taken into account provided that:

1. the lower edge of openings through which progressive flooding may take place and such flooding is not accounted for in the calculation of factor s are not immerged. Such openings shall include air-pipes, ventilators and openings which are closed by means of weathertight doors or hatch covers; and
2. no part of the bulkhead deck considered a horizontal evacuation route is immerged.
3. the requirements of the standard TP 10943, part II subsection 8(2) are also met in the final stage of flooding.

3.9 The Stage 3 operational envelope derived deterministically is shown below.

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References

[1] “Flooding Protection of Ro-Ro Ferries, Phase III”, TP13216, March 1998.
[2] “Dynamic Stability Assessment of Damaged Passenger/Ro-Ro Ships and Proposal of Rational Stability Criteria”; Vassalos et.al., Marine Technology Oct.97.

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Date modified:

2010-01-19

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