Numerical risk evaluation, main deck doors, 127’/368 Passenger Dinner Boat, Graul Design
INTRODUCTION
This paper shows an application of the proposed use of risk indices leading to enhanced doorway access solutions for people with mobility impairments, similar to previous cases studies for a Casco Bay Line monohull by Seaworthy Systems and a Gladding Hearn/INCAT Designs catamaran. It is important to note that the risk guidelines are to be carefully applied, on a case-by-case basis, with sound technical judgment.
The particulars of the case are:
- This is a new design for a Subchapter K boat, capacity of 368 passengers, operating in protected waters but designed for partially protected waters (Figures 1 and 2).
- Hinged weathertight doors in the passenger cabin on the main deck, forward to starboard for weather deck access, and aft to port for embarkation and weather deck access.
- Doors have 6” coamings, designed for partially protected waters service per 46 CFR Subchapters K and S watertight integrity regulations for vessels less than 100 GT (Parts 116.1160 and 171.124, respectively).
- This exercise is to ascertain whether the coamings could be eliminated or reduced on a risk management basis as suggested by the proposed approach.
Figure 1. 127’/368 passenger dinner boat, outboard profile
Figure 2. 127’/368 passenger dinner boat, main deck plan
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APPLICATION
The selections for the risk factors appear below, shaded gray for the subject case and annotated where needed. The risk summation appears in Table 1, followed by a discussion of the possible solutions.
Note: Asterisk (*) indicates those provisions which apply to the dinner boat where more than one provision is listed, unless otherwise noted.
1. FORWARD DOOR
i. Purpose & use of door
- [0] – Open only for embarkation/disembarkation, always closed during voyages
- *[1] – Open during voyages for passenger access to weather deck, e.g., “promenade deck”, alternate access available
- [2] – Access to evacuation deck, required to be open in emergencies
ii. Door location
Note: “Position 1” is between the bow and the point 0.25L aft of the bow;
“Position 2” is between the point 0.25L aft of the bow and the stern
Risk scores for door position
Sill < 8 feet above WL
Sill >/= [8 feet] above WL
Position 1
Position 2
Position 1
Position 2
Facing outboard
[2]
[1]
[1]
[0]
Facing aft
[1]
[2], if < 0.25L from stern;
[1], if >/= 0.25L from stern
[0]
[1], <0.25L from stern
Facing Forward
[6]
*[4]
[3]
[2]
- For doors facing outboard, multiply score by [1.5] if the door is within [4 feet] of the deck edge.
- For all doors with low exterior exposure to the elements due to protective structural elements, multiply score by [0.67]. Discussion in 2.1 cites Subchapter S, K, and T language describing “exposed” locations. Such barriers would need to be in close proximity to the door, and preferably “upstream” in terms of the deck’s slope due to sheer and camber.
iii. Downflooding Potential
- Downflooding path to lower deck spaces:
Risk scores for downflooding path
X< [20 feet]
X>/= [20 feet]
Y < [2 feet]
Y >/= [2 feet]
Y < [2 feet]
Y >/= [2 feet]
Manholes only
[1]
NA
[0.5]
NA
Protected
[2]
[1]
[1]
[0]
Unprotected
[6]
[4]
[4]
[2]
- [0] – no pathway of any kind to watertight spaces below the passenger deck
- Manholes only. Watertight, bolted, flush manholes leading to void spaces, tanks, and unmanned spaces, closed during voyages.
- Protected: Watertight or weathertight closures (doors or hatchways) with coaming at downflooding point(s)
- Unprotected: Joiner doors, ventilation openings to spaces below
- X = distance from door to downflooding point
- Y = height of downflooding point above deck
- Size of accommodation space that the doorway leads to
- *[2] – more than [50%] of main deck area
iv. Area of operation
The aggregate scores for the above risk categories should be multiplied as follows for the OCMI designation of waters (that is, for the purposes of the stability regulations) in which the vessel is authorized to operate. Two cases are considered: 1) protected waters as for current operation; and 2) partially protected, per the weathertight door design in place.
- *Protected :: [0.75]
- *Partially protected :: [1.0]
2. AFT DOOR
i. Purpose & use of door
- [1] – Open during voyages for passenger access to weather deck, e.g., “promenade deck”, alternate access available
ii. Door location
Risk scores for door position
Sill < 8 feet above WL
Sill >/= [8 feet] above WL
Position 1
Position 2
Position 1
Position 2
Facing aft
[1]
*[2], if < 0.25L from stern;
[1], if >/= 0.25L from stern
[0]
[1], < 0.25L from stern
- For all doors with low exterior exposure to the elements due to protective structural elements, multiply score by [0.67]. *Note: The aft door is protected overhead by the upper deck’s overhang, and from the outboard and aft directions by bulwarks at the deck edge. Use of the multiplier is appropriate. The score for the aft door is therefore: 2.0 X 0.67 = 1.33.
iii. Downflooding Potential
- Downflooding path to lower deck spaces
Risk scores for downflooding path
X< [20 feet]
X>/= [20 feet]
Y < [2 feet]
Y >/= [2 feet]
Y < [2 feet]
Y >/= [2 feet]
Manholes only
[1]
NA
[0.5]
NA
Protected
[2]
[1]
[1]
[0]
Unprotected
*[6]
[4]
[4]
[2]
- [0] – no pathway of any kind to watertight spaces below the passenger deck
- Manholes only. Watertight, bolted, flush manholes leading to void spaces, tanks, and unmanned spaces, closed during voyages.
- Protected: Watertight or weathertight closures (doors or hatchways) with coaming at downflooding point(s)
- Unprotected: Joiner doors, ventilation openings to spaces below
- X = distance from door to downflooding point
- Y = height of downflooding point above deck
- Size of accommodation space that the doorway leads to
- *[2] – more than [50%] of main deck area
iv. Area of operation (two cases)
- *Protected :: [0.75]
- *Partially protected :: [1.0]
Table 1 summarizes the analytical framework for characterizing the design technical risk factors associated with the location and use of weathertight doors. The first two columns describe the pathway served by the door, and its purpose and operational function. The next four are individual risk factors, which are to be scored as specified above, with ranges defined by relative severity of the hazard.
Table 1’s first two rows show the doors as designed. The third and fourth rows show the scoring for alternate door arrangements. Discussion of the solutions for these cases follows Table 1.
SOLUTIONS
The roster of possible access enhancement solutions appears below, tied to total risk scores as shown:
- Weathertight door with no coaming :: Aggregate risk score = [0 ? R ? 4]
- Weathertight door with no coaming with deck drainage arrangement (e.g., “Concept A” or “Concept B” (Appendix A), or Gladding – Hearn drainage detail on Flying Cloud), or with exterior water barrier protection :: Aggregate risk score = [4 ? R ? 8]
- Removable regulation height coaming, similar to American-Canadian-Caribbean Line boats :: Aggregate risk score = [8 ? R ? 12]
- Reduced height coaming [50%] with sloped1 deck ramp (grated) and landing at sill height :: Aggregate risk score = [8 ? R ? 16]
- Regulation height coaming with sloped deck ramp and landing at sill height :: Aggregate risk score = [16 ? R ? 20]
- Regulation height coaming, no sloped deck due to water “runup” risk :: Aggregate risk score = [20 ? R ? 24]
Table 1
Current configuration (doors as designed)
| Pathway | To & From |
Purpose and use of door ([0 – 2])
Door Location ([0 – 9])
Downflooding Potential
Area of Operation multiplier
Total risk “R” ([0-30])
Solution(s)
DF path ([0 – 6])
Size of space doorway leads to ([0 - 3])
Forward weather deck door, starboard, as designed
Passenger accomm. space
Weather deck access and embarkation
(1.0)
Door sill less than [8 feet] above the waterline, facing forward, in Position 2 (4.0)
Unprotected pathway to watertight spaces below, separation of DF point more than [20 feet] from the door, less than [2 feet] above the deck. (4.0)
Passenger accommodation space, more than 50% of main deck area (2)
Protected waters (0.75)
(1 + 4 + 4 + 2) * 0.75 = 11 * 0.75 = 8.25
Scores are very close for the two doors. The lower score for the aft location is nearly offset by its proximity to the downflooding point. Scores are of course higher for partially protected waters. Weathertight protection for these doors as designed is appropriate according to the model.
Partially protected waters (1.0)
11.0 * 1.0 = 11.0
Aft weather deck door, port, as designed
Passenger accomm. space
Weather deck access and embarkation
(1.0)
Door sill less than [8 feet] above the waterline, facing aft, within 0.25L from the stern, with structural protection from water (1.33)
Ditto, except DF point is less than [20 feet] from the door
(6.0)
Passenger accommodation space, more than 50% of main deck area (2)
Protected waters (0.75)
(1 + 1.33 + 6 + 2) * 0.75 = 10.3 * 0.75 = 7.75
Partially protected waters (1.0)
10.3 * 1.0 = 10.3
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Doors as designed
The forward doors, as designed, score 8.25 and 11.0, for protected and partially protected waters service, respectively. The aft door likewise scores 7.75 and 10.3. The indication from the model is that some form of weathertight protection is appropriate especially for partially protected waters service, for which the doors are designed. The deck arrangement precludes relocation or reconfiguration of the doors without serious impact. The reasonable approach must then be to examine reconfiguration or modification of use of other features contributing to the risk score, and perhaps identifying the single best accessible pathway of the two.
Reconfiguration
The best candidate is better protection of the downflooding point and reduction of the score for that risk factor. Replacing the non-weathertight door with a weathertight door with a coaming would significantly reduce the risk. The score for each door would drop from 6.0 to 2.0, as per the revised table below:
Risk scores for downflooding path
X< [20 feet]
X>/= [20 feet]
Y < [2 feet]
Y >/= [2 feet]
Y < [2 feet]
Y >/= [2 feet]
Manholes only
[1]
NA
[0.5]
NA
Protected
*Aft door: [2]
[1]
*Forward door: [1]
[0]
Unprotected
[6]
[4]
[4]
[2]
See Table 2 for resulting calculations (modified cells are highlighted gray). The aggregate risk scores for the aft door are lowered considerably (4.75 and 6.3), and a weathertight, accessible coaming-less door aft with a protective drainage arrangement appears to be appropriate for both protected waters and partially protected waters service.
The forward door has significantly lowered scores (6.0 and 8.0), but remains in need of protection against water entry. The conservative approach for safety might dictate retention of at least a reduced height (3”) coaming as structural protection and designation of the aft door only as accessible for the mobility-impaired. This would provide the embarkation pathway and the accommodation of access to the weather deck. The fore deck would remain available to other passengers for embarkation and access during voyages. There would be benefit to the operator here as well in the reduction of the barrier for able-bodied passengers.
Table 2
**Reconfiguration**
Note: Asterisk (*) indicates changes made due to reconfiguration.
| Pathway | To & From |
Purpose and use of door ([0 – 2])
Door Location ([0 – 9])
Downflooding Potential
Area of Operation multiplier
Total risk “R” ([0-30])
Solution(s)
DF path ([0 – 6])
Size of space doorway leads to ([0 - 3])
Forward weather deck door, starboard, as designed
Passenger accomm. space
Weather deck access and embarkation
(1.0)
(4.0)
*Protected pathway to watertight spaces below, separation of DF point more than [20 feet] from the door, less than [2 feet] above the deck (1.0)
Passenger accommodation space, more than 50% of main deck area (2)
Protected waters (0.75)
*(1 + 4 + 1 + 2) * 0.75 = 8 * 0.75 = 6.0
Scores are very close for the two doors. The lower score for the aft location is nearly offset by its proximity to the downflooding point. Scores are of course higher for partially protected waters. Weathertight protection for these doors as designed is appropriate according to the model.
Partially protected waters (1.0)
*8.0 * 1.0 = 8.0
Aft weather deck door, port, as designed
Passenger accomm. space
Weather deck access and embarkation
(1.0)
(1.33)
*Ditto, except DF point is less than [20 feet] from the door.
(2.0)
Passenger accommodation space, more than 50% of main deck area (2)
Protected waters (0.75)
*(1 + 1.33 + 2 + 2) * 0.75 = 6.3 * 0.75 = 4.75
Partially protected waters (1.0)
