Numerical risk evaluation, main deck doors
35 meter long catamaran 149 passengers
Gladding-Hearn Shipbuilding and INCAT Designs - Sydney
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 particulars of the case for this Gladding-Hearn and INCAT Designs (GH-ICD) boat (deck plan, Figure 1) are:
- This is a design type for a Subchapter T boat, capacity of 149 passengers, intended for operation in protected waters. According to the builder, two important design features are optional depending on the particular service and client:
- The deck may or may not “float”. The floating deck is structurally separate from the catamaran hulls’ watertight envelope and affords no direct access or downflooding through the passenger cabin deck; otherwise the passenger cabin deck is integral with the hull tops.
- Embarkation access may be via the bow to the forward deckhouse doors, or via bulwark gates near the stern to the aft deckhouse doors. Figure 2 shows GH-ICD boats of similar, but not identical, design to the 35-meter boat considered herein, one each with the bow and stern embarkation access points.
- GH-ICD specifies watertight or weathertight doors, facing forward from the deckhouse forward bulkhead, port and starboard (0.35L aft of bow); and weathertight doors facing aft, port and starboard, from the aft deckhouse bulkhead, all on the main deck.
- Doors are presumed to have 3” coamings (designed per Subchapter S watertight integrity regulations for vessels less than 100 GT).
This case study is to ascertain whether access through the doors could be improved, possibly by the elimination of the coamings, on the basis of the risk management approach. This example is instructive because it uses a “type” boat design wherein a number of options for design and operation may be selected. This allows for examination of several scenarios involving the two design options shown above, deck structure types and bow vs. stern embarkation. The study includes hypothetical consideration of operation in partially protected waters as well, recognizing that the builder intends the design for protected water operations.
**Figure 1. General plans, Gladding –Hearn/INCAT 35 meter catamaran
**
Figure 2. Gladding-Hearn INCAT Design boats 331 and 332 at dock.
NOTE: These boats are not identical to the design chosen for this example and shown in Figure 1. The photograph shows how bow doors and bow loading reconfigure the design (boat at bottom of photograph) of the passenger cabin. The configuration of the side loader (at top) has different doors into aft area of the passenger cabin, which are not part of the configuration for this example, as shown in Figure 1.
APPLICATION
The descriptive language for the risk factors developed in the Volpe Center report appears verbatim. The risk values appearing below increase in magnitude with increasing risk and are absolute un-weighted numbers.
The forward doors and aft doors are treated separately (sub-headings i and ii), each with scenarios for the two deck construction techniques. Gray text shading of text show the particulars for the subject case, with annotations added where appropriate. Annotations show differences between the cases of the separate floating deck and the structurally integral deck..
The risk summation for all scenarios follows the detailed scoring for the forward and aft doors, appearing in Table 5, and with a discussion of possible doorway access solutions.
Note: Asterisk (*) inidcates those provisions which apply to GH/INCAT vessel where more than one provision is listed.
1. FORWARD DOORS
i. Purpose & use of door
- *[0] – Open only for embarkation/disembarkation, always closed during voyages. NOTE: GH-ICB state that there is no passenger access to the forward weather deck 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
Per definition of International Load Line Convention and the LL 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]
[1]
[1]
[0]
Facing aft
[1]
[2], if < 0.25L from stern;
[1], if >/= 0.25L
[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 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.
*Note: For the GH/INCAT, there is no reduction credited here because the protective features forward of the door are relatively short in height and are about 8 meters away.
iii. Downflooding potential
- Downflooding path
The plan provided shows no downflooding path in the passenger accommodation space. Manholes giving access to engine room spaces appear aft of the deckhouse on the weather deck. The engine rooms’ air supply plenums (frames 9 – 10, port and starboard) are in the aft corners of the passenger accommodation space; the intake ducts would be in the aft or outboard bulkheads of the plenum, NOT in the passenger space. However, it is assumed that there are bolted manhole covers elsewhere in the space giving access to fuel tanks and void spaces. The conservative view would be that there is a “protected” downflooding path in the space for integral deck construction. Such is not the case for the “floating” deck construction, where there is no downflooding path.
| 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], structurally integrated deck
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, for “floating 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
- [0.0] – less than [25%] of main deck area
- [1.0] – between [25%] and [50%] of main deck area
- *[2.0] – more than [50%] of main deck area (nearly exactly 50% in this case)
iv. Area of operationThe 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] (both areas of operation considered)
- Exposed :: [1.5]
2. AFT DOORS
i. Purpose & use of door
- [1] – Open during voyages for passenger access to weather deck, e.g., “promenade deck”, alternate access available. NOTE: Access through these doors to the aft weather deck is required whether the forward or aft doors are the chosen embarkation path.
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 aft
[1]
[2], if < 0.25L from stern;
*[1], if >/= 0.25L from stern
[0]
[1], <0.25L from stern
For doors with low exterior exposure to the elements due to protective structural elements, multiply score by [0.67]. *NOTE: For the GH/INCAT vessel, there is no reduction credited here because there are very limited protective structural features nearby.
iii. Downflooding potential
- Downflooding path
*See discussion for forward doors. Score is 1.0.
- Size of accommodation space that the doorway leads to
- [2.0] – more than [50%] of main deck area
iv. Area of operation
- Protected ::[0.75]
- *Partially proected ::[1.0]
3. Risk summary and solutions
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. Aggregated risk scores are in the seventh column.
The table shows four scenarios for both the forward and aft doors. Moving from the leftmost column rightward, the cells first divide to indicate the deck construction options, and then divide again for service in “protected” versus “partially protected” waters.
Table 1 (forward doors)
| 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 - 2])
Forward doors
Forward doors, port and starboard
| Weather | passenger accomm. space |
Embarkation only, closed otherwise
(0.0)
Door sill less than [8 feet] above WL, facing forward in Position 2 (4.0)
Manholes only (1.0)
Passenger accomm. space, more than 50% of main deck area (2.0)
Protected waters (0.75)
(0 + 4 + 1 + 2) * 0.75 = 5.25
Passenger embarkation only for a “bow loader”: The 4 scores are in the low range. “Floating” deck yields modestly lower risk scores because of DF through manholes is low risk. Additional protective features against water on deck would improve the case for an accessible doorway solution, especially for partially protected water service.
Partially protected waters
(1.0)
(0 + 4 + 1 + 2) * 1.0 = 7.0
“Floating” deck, no DF pathway (0.0)
Protected waters (0.75)
(0 + 4 + 0 + 2) * 0.75 = 4.5
Partially protected waters
(1.0)
(0 + 4 + 0 + 2) * 1.0 = 6.0
Aft doors
Aft doors, port and starboard
| Weather | passenger accomm. space |
Embarkation and passenger access to weather deck during voyage
(1.0)
Door sill less than [8 feet] above WL, facing aft in Position 2 (1.0)
Manholes only (1.0)
Passenger accomm. space, more than 50% of main deck area (2.0)
Protected waters (0.75)
(1 + 1 + 1 + 2) * 0.75 = 3.75
Accessible embarkation and deck access via the aft doors are appropriate for all scenarios. The low scores in protected waters indicate that with or without a “floating” deck a coaming-less door would be appropriate. For service in partially protected waters, scores are low as well. Without the floating deck, more protection would be needed for an accessible door solution.
Partially protected waters (1.0)
(1 + 1 + 1 + 2) * 1.0 = 5.0
“Floating” deck, no DF pathway (0.0)
Protected waters (0.75)
(1 + 1 + 0 + 2) * 0.75 = 3.0
Partially protected waters
(1.0)
(1 + 1 + 0 + 2) * 1.0 = 4.0
|
Doorway Solutions
The roster of possible access enhancement solutions appears below, tied to sub-ranges of 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.
Based strictly upon the risk scores, the aft doors would appear to be the better choice for passenger access. The scores in both cases are low, however, and an accessible pathway via the bow doors would also be appropriate for the right combination of design features and operation. Note that access through the aft doors to the aft weather deck is required wherever the embarkation point and pathway are sited.
Forward doors
If the desired configuration were a “bow loader” with the forward deckhouse doors used for embarkation only, the choice of a “floating” deck would reduce the risk scores by 0.75 and 1.0 to 4.5 and 6.0, for protected and partially protected waters, respectively. The benefit is modest and does not substantively change the outcome, because the downflooding risk from the closed manholes in the passenger cabin is low to start with.
With or without the floating deck, in protected water service, the solution indicated would be a coaming-less weathertight door (possibly similar to the GH-ICB sliding doors found on the Flying Cloud) or a watertight door with minimal height sill, as allowed by the CFR for operation by able-bodied crew. The risk for this design could be further reduced with protective drainage features against water on deck (e.g., the GH-ICB drainage detail for Flying Cloud, Figure 3).
Similarly for partially protected water service, the risk reduction due to the floating deck does not significantly change the outcome. It may be appropriate to replace the weathertight door with coaming with an improved access doorway, for example, a weathertight door with a coaming of reduced height, and the Flying Cloud drainage detail. Addition of protective structure forward of the door (against water on deck) to reduce the risk appears infeasible because of the access route from bow doors to the deckhouse.
In this example case, the operator and builder would have to consider accessible paths onboard in overall context of the desired operation for the boat (market served, waters served, and shoreside infrastructure and loading mode), the added cost of the floating deck and drainage features. An accessible pathway through the forward doors appears to be feasible, given the correct operational procedures and protective features forward of the doors.
Alternatively, the desired approach in this case may be retaining the forward weathertight doors as designed and providing passenger access via the aft bulwark gates and aft deckhouse doors. In this scenario, the boat would be configured for aft loading and the forward doors would be for crew only access. An improved access doorway could still be installed for future operational flexibility, or the conservative approach taken, that is, a weathertight door with 3” coaming, as designed.
Aft doors
Use of the aft doors for access to the weather deck is required whichever embarkation path is chosen. The floating deck results in scores of 3.0 and 4.0 for the protected and partially protected waters. A coaming-less door would be appropriate in either case, possibly similar to the aft sliding doors on the Flying Cloud. For partially protected waters, the score of 5.0 is on the cusp and it might be appropriate to include a protective drainage feature to be on the safe side or a reduced height coaming with ADA-compliant short ramps on either side.
Without the floating deck, the protective drainage feature or a reduced height coaming with ADA-compliant short ramps on either side would be appropriate, especially for partially protected waters.
Scoring methodology comments
The case shows how the builder and operator may have several design and operations decision points in which mobility access is one element in the consideration of cost. In this case, it turns out that the scoring bandwidth for all scenarios is relatively narrow at the low risk end of the range, but that there are several possible solutions.
As for the particulars of the scoring, it is evident that the door location and the size of the accommodation space drive the aggregate score in all scenarios, especially for the forward doors. The floating deck eliminates downflooding paths, but the benefit is modest because the only potential downflooding points in this case are bolted manhole covers leading to tanks and voids; the likelihood of any of those manholes being open during a voyage is slight. The scoring methodology now includes an added field “bolted manholes closed during voyages”, scored as [1.0] in this case study. The risk is lower than for a tight door leading to a workspace below but does not equate to the “no pathway” score of [0].
The exposure of the forward doors and the size of the passenger space alone raise the risk score above the lowest category, even for protected water service. The hazard of the forward door location is clear in the science and the regulations. The affected space area metric may bear more scrutiny, but the idea is that it 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 or heavy spray.
**FIGURE 3. DRAINAGE DETAIL, GH-ICB BOAT FLYING CLOUD, FORWARD EMBARKATION DOORS
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