Numerical risk evaluation, main deck doors, 399 passenger ferry, Casco Bay Line
INTRODUCTION
This paper shows an application of the proposed use of risk indices leading to enhanced doorway access solutions for people with mobility impairments. 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 risk values appearing below increase in magnitude with increasing risk and are absolute pre-weighted numbers. The particulars of the case are:
- This is a new design for a Subchapter K boat, capacity of 399 passengers, operating in protected waters (Figure 1: Profile, and Figure 2: Main Deck Plan).
- Sliding weathertight doors on the main deck, port and starboard, for embarkation access, located forward (approximately 0.25L from the bow) at the deck edge.
- Doors have 3” coamings (designed per Subchapter S watertight integrity regulations for vessels less than 100 GT).
- This exercise is to ascertain whether the coamings could be eliminated on a risk management basis as suggested by the Phase 2 report (footnote #1). The designer states that elimination of the coamings at those points would improve embarkation access and simplify the design of the gangways from the shoreside piers used by Casco Bay Line.
APPLICATION
The descriptive language for the risk factors appears verbatim, with gray shading showing the particulars for the subject case and annotations 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 ferry where more than one provision is listed.
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
Figure 1. Casco Bay Line boat – Profile
Figure 2. Casco Bay Line boat – Main Deck Plan
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
Per definition of International Load Line Convention and the Load Line Technical Manual
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] X [1.5]
|
[1]
[1]
[0]
- *For doors facing outboard, multiply score by [1.5] if the door is within [4 feet] of the deck edge.
- For doors with low exterior exposure to the elements due to protective structural elements, multiply score by [0.67].
*NOTE: For Casco Bay Line board, doors are at 0.25L point. The Position 1 score is assigned as the conservative choice.
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
- [0] – less than [25%] of main deck area
- [1] – between [25%] and [50%] of main deck area
- *[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.
- *Protected :: [0.75]
- Partially protected :: [1.0]
- Exposed :: [1.5]
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 row shows the door as designed, in its forward position at the deck edge. The second and third rows show the scoring for alternate door arrangements. Discussion of the solutions for these cases appear following Table 1.
Table 1
| 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])
Current configuration
Forward embarkation doors, port and starboard, as designed
Passenger accomm. space
Embarkation only, closed otherwise
(0.0)
Door sill less than [8 feet] above the main deck, on deck edge, facing outboard in Position 1 (2.0 X 1.5 = 3.0)
Protected DF pathway; separation of downflooding point of at least [20 feet] from the door (1.0)
Passenger accommodation space, more than 50% of main deck area (2.0)
Protected waters (0.75)
(0 + 3 + 1 + 2) * 0.75 = 4.5
Conservative approach: door with 3” coaming, as designed.
Reconfigurations
Forward embarkation doors, port and starboard, recessed inboard
Passenger accomm. Space
Embarkation only, closed otherwise
(0.0)
Door sill less than [8 feet] above the main deck, inboard of deck edge, facing outboard in Position 1 (2.0)
Protected DF pathway; separation of downflooding point of at least [20 feet] from the door (1.0)
Passenger accommodation space, more than 50% of main deck area (2.0)
Protected waters (0.75)
(0 + 2 + 1 + 2) * 0.75 = 3.75
Sliding weathertight door with no coaming
Amidship embarkation doors, port and starboard, at deck edge
Passenger accomm. space
Embarkation only, closed otherwise
(0.0)
Door sill less than [8 feet] above main deck, on deck edge, facing outbd in Position 2 (1.0 X 1.5 = 1.5)
Protected DF pathway; separation of downflooding point of at least [20 feet] from the door (1.0)
Passenger accommodation space, more than 50% of main deck area (2.0)
Protected waters (0.75)
(0 + 1.5 + 1 + 2) * 0.75 = 3.4
Sliding weathertight door with no coaming
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 or protective structural features against ingress of exterior water - Aggregate risk score = [4 ? R ? 8]
- Weathertight door with removable regulation height coaming - 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]
NOTE: Subchapters K and T (46 CFR 116.1160 and 179.360, respectively) allow for substituting a watertight door with a minimal height sill for a weathertight door with a coaming. Such would be appropriate for a door with any risk score, if operation of the door is by crew only (as currently interpreted by Coast Guard) as for use in embarkation/disembarkation only, or if industry develops a watertight door appropriate for operation by passengers.
Doors as located
The embarkation doors, as designed, scored 4.5, identical for the port and starboard doors. The indication is that a weathertight door without coaming (similar to the type found on the Gladding-Hearn/Incat/Harbor Express boats) would be a suitable solution. However, there would be no exterior drainage or water barrier protection available, given the deck edge location. The conservative approach (or a very conservative OCMI’s approach) might dictate retention of the 3” coaming as structural protection and a strong gasketing surface against the unlikely event of wave slapping loads on the door.
Alternately, a very well designed sliding weathertight door with no coaming might avail, if it had the confidence of the designer, inspector, and operator. This prospect is brightened by the facts that 1) the score is close to 0 – 4 threshold, and 2) the doors are in fact AT the 0.25L longitudinal point and therefore at the safer, aft end of the forward, exposed zone.
Door reconfiguration
Two possible reconfigurations would lower the risk score to below 4.0 and allow installation of a no coaming weathertight sliding door. In the first case, the doors would remain at Frame 7 with a 48” recess, protective bulkhead forward, and possibly a portable protective coaming at the deck edge while the door is closed. As shown in Table 1, the overall risk score would be 3.75 and the solution would be a weathertight door with no coaming, and the added safety of protective structural features against ingress of exterior water. The impact on the internal arrangement would be the loss of a small bench seat on the aft side of the bulkhead at Frame 5 (6 seats total) and approximately 30 square feet of interior space on each side.
The second would be to move the doors aft, to Frame 13 or so (forward of the engine room vent on the port side), resulting in a score of 3.4, due to the doors’ locations further aft. The internal space arrangement modification would be minimal. Bench space lost amidships would be regained forward at the former position of the door.
SCORING METHODOLOGY COMMENTS
The case illustrates how the two “downflooding” subfactors, “distance to downflooding point” and “area of accommodation space”, work against each other. Long distances to the downflooding point are more common in large accommodation spaces. At first blush, it seems that rethinking this contradictory linkage is necessary. However, the space area metric also protects against large volumes of entrapped water, should the worst situation occur, that is, a failed door allowing ingress of large amounts of water from waves abeam or heavy spray.
Figure 2. Door reconfigurations.
