The main points of the risk-based approach are:
- Numerical risk scoring of each door by factors for service and route, door location, purpose and use, and downflooding potential;
- Correspondence of the aggregate risk score to a range of doorway access options, from full, no-coaming access to full compliance with the coaming regulations; and
- Reconfiguration to lower risk and improve access, through:
- Relocation of the door
- Mitigation of downflooding exposure
- Protective structural and drainage features against exposure to water.
It is very important to properly characterize these risk-based guidelines as just that: a tool to be carefully applied, case by case, by the naval architect and the inspection authority, with sound technical judgment.
3.1 Terms of reference
The weathertight doors addressed herein are the following:
- Those doors providing access onto the boat, i.e., from the dock/gangway to passenger accommodation spaces
- Those doors providing access to/from passenger accommodation spaces from/to weather decks where passengers are allowed access (sun decks) or are required to have access (evacuation stations)
The reader should note that this methodology does not address interior doors, including fire zone doors, joiner doors, stairway access, and doors that are restricted to crew access. It may turn out, however, that this methodology can offer reconfiguration solutions that can be applied in these kinds of cases as well.
3.2 Characterization of risk factors
The proposed risk factors follow, with annotations showing the technical basis and supporting sources, for example, the Load Line Technical Manual or the Code of Federal Regulations:
- Purpose and use of the door. The type and frequency of use, such as evacuation, embarkation/disembarkation only, or passenger access to weather deck (e.g., “promenade deck”). There is no explicit reference to this factor in either the Load Line regulations or the CFR Title 46. However, the Coast Guard has in many cases considered the operational use, including restrictions on use, in assessing the safety of a particular door, for example, the Harbor Express boats Flying Cloud and Lightning (see case study in Phase 1 report). In that case, the bow passenger loading doors have no coamings and one of the reasons for allowing that design is that the bow doors are closed at all times except for embarkation and disembarkation.
- Door’s location. Its exposure to or protection from waves, spray, and precipitation.
- Height above design waterline. The Load Line Technical Manual (LLTM) allows coaming height reductions for added height above the “freeboard deck”, with increasing reductions allowed for increasing height; the allowances are presented in tabular format. The table is not prescriptive, and the reductions are allowed based upon the judgment of the inspector; the “Concept” discussion lists the factors to be taken into account. The reader should note that there is no case where the coamings are eliminated altogether.
- Proximity to bow or stern. The LLTM identifies “Zones 1 and 2” as the forward 25% and the aft 75% of the ship, respectively. The table allows greater coaming height reductions in Zone 2.
- Proximity to deck edge. The LLTM identifies “athwartships location” and “area of open deck around or adjacent to the opening” as mitigative factors to consider. These factors are not quantitatively addressed in the table.
- “Exposure” of door. Protection from water by bulwarks, bulkheads, overhangs, and other structural barriers can reduce the hazard and risk.
CFR antecedents include the Subchapter S (“Stability”) regulation § 171.124 (“Watertight integrity above the margin line in a vessel less than 100 gross tons”) requires a coaming for an “exposed location on a flush deck vessel”. It requires interpretation by the designer and inspection authority to determine the extent to which the door’s location is “exposed”. In addition, Subchapters K (§ 114.400) and T (§ 175.400) both define “weather deck” in terms of exposure, specifying “partially or completely exposed from above or from at least two sides”.
- Downflooding potential through the interior space accessed and protected by the door.
- Downflooding path to lower deck spaces. The Load Line Manual height reduction table specifically differentiates between doors having “direct access below” (“Category A”, 23.5” required standard height) and “no direct access below” (“Category B”, 15” required standard height), with greater height reductions also allowed for the “no direct access below” case. The Title 46, Subchapter S “Watertight integrity…” regulation (§ 171.124) specifies coamings for doors that “Give access in to the hull” and allow for no coaming in the converse case. The Phase 1 report described the Incat/Gladding-Hearn catamaran with an allowed coaming-less door into a large passenger accommodation space, in part, because there was no downflooding path to spaces below.
This methodology also allows for lowering the risk score based upon protective design features within the space, that is, interior doors and coamings, drainage arrangements, or other structural barriers interdicting flow to the downflooding point(s). Distance between the weather deck door and the downflooding point is also a consideration in the model. Neither the Load Line Manual nor the CFR address these matters directly.
The downflooding paths may be generally categorized as follows:
- Direct access to space below the “bulkhead”, or main, deck, by an unprotected downflooding path, e.g., a stairway
- Indirect access to space below the bulkhead deck, that is, protective design features isolating the downflooding point(s)
- No access to lower deck
- Size and configuration of the immediately affected space – The Load Line Manual cites the “type and volume of space the opening leads to” as a factor to be considered in the judgment of the inspector. Figure 75 of the LLTM shows two similar spaces with similarly oriented weathertight doors and indicates that the larger space requires a higher coaming. For these purposes, therefore, larger passenger spaces served carry more risk.
- Vessel’s route. The Coast Guard Officer in Charge of Marine Inspection (OCMI) designation of waters in which the vessel may operate, as specified in the stability letter, that is, exposed, partially protected, or protected waters. These designations denote varied severity of wind and waves, as well as proximity to harbor safe refuge. The CFR watertight integrity and coaming regulations are closely tied to these designations. The LLTM does not address route and service. The presumption therein for ships with load lines is that they operate on oceanic voyages.
3.3 Numerical valuation of risk factors
The proposed risk categories and factors appear below, with numerical values in square brackets (i.e., subject to review and revision). Again, it is important to properly characterize the risk guidelines as just that: a guiding tool to be carefully applied, case by case, with sound technical judgment. The risk values appearing below increase in magnitude with increasing risk and are absolute pre-weighted numbers.
i. Purpose & use of door (scoring range: 0 - 2)
-  – Open only for embarkation/disembarkation, always closed during voyages
-  – Open during voyages for passenger access to weather deck, e.g., “promenade deck”
-  – Access to evacuation deck, required to be open in emergencies
ii. Door location (scoring range: 0 - 6)
*Table 1. Door location risk scoring
**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 LLTM*
|Height of deck at door < [8 feet] above waterline (WL)||Height of deck at door >/= [8 feet] above WL|
, if <0.25L from stern; , if >/= 0.25L from stern
, <0.25L from stern
- 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.
ii. Downflooding Potential
- Downflooding (DF) path (scoring range: 0 - 6)
Table 2. Downflooding path risk scoring
X = distance from door to downflooding point
Y = height of downflooding point above deck
|X< [20 feet]||X>/= [20 feet]|
Y < [2 feet]
Y >/= [2 feet]
Y < [2 feet]
Y >/= [2 feet]
- 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
-  – no pathway of any kind to watertight spaces below the passenger deck
- Size of accommodation space that the doorway leads to (scoring range: 0 - 2)
-  – less than [25%] of main deck area
-  – between [25%] and [50%] of main deck area
-  – 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]
Total scoring range
The range of possible aggregate scores (“R”), before multiplying for the area of operation, is 0 – 16. The range of possible aggregate scores, after multiplying for the area of operation, is 0 – 24.
3.4 Doorway access solutions
The final step in the process is to identify the potential access solutions indicated by the total risk scores. In application, a solution may be selected for any risk score in or below its designated range. A high aggregate risk score may indicate the need to relocate the door and/or incorporate more protective features, to lower the score and consider the selection of a door with improved access.
The proposed menu of solutions follows:
- 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]
- “Concept A” or B” exterior drains (see figures, Appendix A)
- Gladding – Hearn exterior drainage detail on Flying Cloud (Appendix B, Figure 3)
- Bulwarks, bulkheads, deck overhangs, etc. preventing passage of water to the door, especially from the direction of exposure, for example, from the bow for forward facing doors or from the stern for aft facing doors.
- Weathertight door with removable regulation height coaming -
Aggregate risk score = [8 ? R ? 12]
This solution is for embarkation access only, that is, where the crew operates the door at known times and places only.
- 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 ramp or sloped deck due to water “runup” risk -
Aggregate risk score = [20 ? R ? 24]
3.5 Other doorway solutions
It may be fruitful to explore the possibilities for substitution of a watertight door with a minimally sized coaming, allowed in both the Subchapter T and K regulations (46 CFR 116.1160 and 179.360, respectively; see discussion, Phase 1 report). The currently available interpretation from Coast Guard safety personnel is that such substitution is meant specifically for vessels with licensed crew and other capable personnel (e.g., offshore drilling rig workers) aboard, who know how to operate a watertight door and can do so in emergency egress situations. This interpretation may be ripe for re-examination.
The use of such doors would raise other accessibility questions (hardware configuration and opening force); however, the result of the inquiry could be a set of new design requirements for consideration by watertight door manufacturers.
3.6 Embarkation doors at the deck edge
Many passenger boats have embarkation doors in the deckhouse side, at or very close to the deck edge and offering no weather deck access. Coamings are not the access barrier in these cases, but poorly designed gangways are. The problem most commonly seen is the double slope of the gangway and a short interior ramp meeting in an apex at the coaming (there are non-specification variations like the “whaleback” arching over the coaming). Gangways designed to provide proper slopes on both sides of the doorway and a proper landing over the coaming are the access solution, not removal or reduction of the coaming. A concept design appears in Appendix A, Figure 3.