Mud-Gas Rupture Disk Outlet / Starboard Diverter
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Figure 53. Gas Flow Rate (MMscfd) as a Function of Time from Each Release Point (Scenario B)
0
To tal G as R ele as e R at e ( M M sc fd )
Re le as e R at e at S pe cif ie d L oc at io n ( M M sc fd )
Time from Start of Event at 21:40:30 (sec)
Mud-Gas Rupture Disk Outlet
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5 FLACS M
ODELD
ESCRIPTIONFigure 54 shows an overview of the solid model as viewed from the starboard aft corner of the MODU. Figure 55 and Figure 56 show an overview of the solid model as viewed from the aft and forward ends respectively. The model includes the hull, the main deck structure, the key buildings in the vicinity of the drill floor, the helideck, the catwalk aft of the drill floor, the partial walls around the drill floor, the bottle rack forward of the drill floor, a simplified representation of drilling pipe and risers on the fore and aft decks, and a simplified representation of the derrick. The actual distribution of drilling pipe and risers and other equipment on the decks at the time of the event has not been fully established at this time, and hence a simplified representation was used in this model.
The model also includes a penetration through the drill floor from the moon pool to simulate releases through the actual arrangement of the rotary table. Figure 57 shows the underside of the hull looking port. The moon pool opening in the hull is shown in this figure, along with the extensions of the 2nd and 3rd decks into the moon pool. A closer view of the moon pool is shown in Figure 58. The cylinder shown in this figure is meant to represent the riser. The “break” in the riser which can be seen in this figure is the release point for the “slip joint” release scenario, and the termination of the riser just below the drill floor is the release point for the “riser bore”
release scenarios. Comparisons between solid model views from within the moon pool area and photographs of approximately the same area are shown in Figure 59 and Figure 60.
The solid model includes three large openings in the hull between the drill floor and the main deck on the forward wall of the moon pool. There are also two large doors in the BOP House:
one on the forward wall and one on the forward end of the port wall. All of these openings would allow gases released in the moon pool to flow onto the main deck of the ship. There are no openings on the starboard or aft sides of the ship. The openings included in the solid model are listed in Table 7. Opening O5_BOP_port was modeled in the “closed” position. Figure 61 shows each opening called out on a plan drawing of the main deck and Figure 62 and Figure 63 provide a comparison of the solid model with photographs of the exterior of the ship. Figure 64 and Figure 65 provide comparisons of the solid model with photographs taken from within the BOP house and the moon pool, respectively.
The monitor locations (monitor points are used within FLACS to record field parameter values, such as fuel concentration) included in the FLACS model are shown in Figure 66 and Figure 67.
Those monitor point locations corresponding to the engine room ventilation air intakes on the aft end of the ship are shown separately in Figure 68 and Figure 69. Also shown in Figure 68 are four monitor locations corresponding to supply fans at the port and starboard sides of the forward end of the ship.
The supply and exhaust fans discussed above and listed in Table 4 are included in the FLACS model as discrete momentum sources and sinks. In FLACS nomenclature these are identified as
“leaks” and labeled as “suctions” (momentum sinks) or “jets” (momentum sources). The ventilation system “leaks” included in the FLACS model are shown in Figure 70 (from above) and Figure 71 (from aft). Figure 72 and Figure 73 show the gas release point for the riser bore,
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Figure 77 show that gas release point for the mud-gas rupture disk outlet, Figure 78 and Figure 79 show the gas release point for the slip joint, Figure 80 and Figure 81 show the gas release point for the mud pit room exhaust fans, and Figure 82 and Figure 83 show the gas release point for the vacuum breaker vent.
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Table 7. Moon Pool and BOP House Openings and Corresponding Dimensions Side Section
(Structure) Opening Dimension (Width x Height)
Forward
Moon Pool
O1_MP_stbd 7 m x 4 m
O2_MP_mid 4.5 m x 4 m
O3_MP_port 4.5 m x 4 m
BOP O4_BOP_fwd 2.4 m x 10 m
Port O5_BOP_port 5 m x 10 m
Starboard None
Aft None
Figure 54. Solid Model, View from Starboard Aft to Port Forward
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Figure 55. Solid Model, View from Aft
Figure 56. Solid Model, View from Forward
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Figure 57. Solid Model, View from Bottom of Hull (looking port)
Figure 58. Solid Model, View from Bottom of Moon Pool (looking starboard)
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Figure 59. Comparison of Solid Model Moon Pool (looking starboard) with Photograph
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Figure 60. Comparison of Solid Model Moon Pool (looking up towards drill floor) with Photograph
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Figure 61. Openings between the Main Deck and Drill Floor (HRBS-058-000-P0607, rev.5)
O1_MP_strbrd:
7 m x 4 m O2_MP_mid:
4.5 m x 4 m O3_MP_port:
4.5 m x 4 m O4_BOP_fwd:
2.4 m x 10 m O5_BOP_port:
5 m x 10 m
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Figure 62. Comparison of Forward Openings in Solid Model with Photograph
O1_MP_stbd:
7 m x 4 m
O3_MP_port:
4.5 m x 4 m O4_BOP_fwd:
2.4 m x 10 m
O2_MP_mid:
4.5 m x 4 m
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Figure 63. Comparison of Forward and Port Openings in Solid Model with Photograph
O3_MP_port:
4.5 m x 4 m O4_BOP_fwd:
2.4 m x 10 m
O5_BOP_port:
5 m x 10 m
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Figure 64. Comparison of Solid Model BOP House with Photograph (looking forward)
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Figure 65. Comparison of Forward Openings in Solid Model Moon Pool with Photograph (looking forward)
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Figure 66. Solid Model, View from Above Derrick with Monitor Locations
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Figure 67. Solid Model, View from Starboard Aft with Monitor Locations
Figure 68. Solid Model, View from Above Derrick with Selected Monitor Locations
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Figure 69. Solid Model, View from Aft with Selected Monitor Locations
Figure 70. Solid Model, View from Above Derrick with Ventilation System
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Figure 71. Solid Model, Viewed from Aft with Ventilation System
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Figure 72. Solid Model , Riser Bore Release Point (looking port)
Figure 73. Solid Model, Riser Bore Release Point (looking forward port)
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Figure 74. Solid Model, Mud-Gas Separator Release Point (aft view)
Figure 75. Solid Model, Mud-Gas Separator Release Point (starboard view)
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Figure 76. Solid Model, Mud-Gas Rupture Disk Outlet Release Point (aft and above view)
Figure 77. Solid Model, Mud-Gas Rupture Disk Outlet Release Point
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Figure 78. Solid Model, Slip Joint Release Point (looking forward)
Figure 79. Solid Model, Slip Joint Release Point (looking starboard)
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Figure 80. Solid Model, Mud Room Exhaust Release Point (starboard and above view)
Figure 81. Solid Model, Mud Room Exhaust Release Point (aft and above view)
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Figure 82. Solid Model, Vacuum Breaker Vent Release Point (aft view)
Figure 83. Solid Model, Vacuum Breaker Vent Release Point (starboard view)
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6 R
ESULTS ANDD
ISCUSSION 6.1 Scenario AThe Scenario A release described previously in Section 4.4.7 was evaluated with the wind from port (to starboard) at 2 m/s. The simulation was first run for 60 seconds without gas release to establish a steady-state flow field due to the wind. The ventilation sources and sinks listed in Table 4 and shown in Figure 70 and Figure 71 were assumed to be in operation for the duration of the simulation, including this 60 second “start-up” period. A vector plot representing the wind field immediately prior to the gas release in the xy-plane located three meters above the main deck (representative of the engine room supply air louvers), is shown in Figure 84. Note that the supply and exhaust fans as well as the buildings and obstructions on the main deck influence the wind field significantly. In particular, large recirculation zones are formed behind the control building under the helipad, at the forward side of the drill floor, and aft of the drill floor at mid-ship due to the combination of the mid-ship’s geometry and the location of the supply and exhaust ventilation. Also note that air flows from within the moon pool outward through the doors in the BOP house and through the openings on the forward side of the moon pool.
Figure 84. Wind Field 3 m. above the Main Deck Immediately Prior to Gas Release In the simulation, the release of flammable gas began at 60 seconds and followed the flow rate profile shown in Figure 52. This figure has been duplicated below on a mass flow rate basis as Figure 85 for the convenience of the reader. The total duration of the release was 510 seconds (8.5 minutes). As noted previously, a total of 1.2 MMscf of gas was released. The simulation was run through another 60 seconds beyond the termination of flammable gas release.
Approximate boundary of ship
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Figure 85. Scenario A Gas Flow Rate (kg/s) as a Function of Time from Each Release Point The flammable gas cloud developed due to the release profile shown in Figure 85 is shown at 1, 100, 120, 140, 300, and 360 seconds after the release (i.e., at 61, 160, 180, 200, 360, and 420 seconds) in Figure 86 through Figure 91. These figures provide a cut plane through the well center along the starboard-port plane looking aft and a cut plane through the well center along the forward-aft plane looking aft-starboard. The upper and lower end of the contours shown in the figure correspond to the fuel mixture’s lower flammability limit (LFL) and upper flammability limit (UFL) for the prescribed composition (i.e., 1.5% and 9.9%, respectively), so that the edge of the contours corresponds to the edge of the flammable gas cloud.
At 1 second after the start of the release (i.e., at 61 seconds), Figure 86 indicates that a small gas cloud is forming just aft of the mud room and another immediately starboard of the starboard overboard mud-gas rupture disk outlet. At 100 seconds after the start of the release (i.e., at 160 seconds), gas is flowing from all five release points. Figure 87 indicates that the moon pool and BOP house contain flammable gas clouds and that the aft-starboard portion of the main deck is engulfed by a flammable mixture by this time. 120 seconds after the release (i.e., at 180 seconds), the volume of the BOP house containing a flammable gas cloud has increased and a significant flammable gas cloud has developed on the forward deck, as seen in Figure 88. The size of the flammable cloud on the aft-starboard region of the main deck has also increased in size. Figure 89 shows that at 140 seconds after the release (i.e., at 200 seconds), the majority of the forward side of the main deck is engulfed by a flammable gas cloud. Figure 90 indicates that at 300 seconds after the start of the release (i.e., at 360 seconds), the BOP house and moon pool contain large flammable vapor clouds, the mid-ship regions of the aft and forward decks have
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Total Gas Release Rate (kg/s)
Release Rate at Specified Location (kg/s)
Elapsed time from 21:39:30 (sec., release begins at 60 sec.)
Mud-Gas Rupture Disk Outlet / Starboard Diverter
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on the starboard-aft region of the main deck. This time corresponds to one minute after the gas flow rate through all five release points has peaked. At 360 seconds after the release began (i.e., at 420 seconds), Figure 91 shows that the gas cloud over the main deck has begun to disperse, as can be seen by comparing Figure 90 and Figure 91; however, a large portion of the main deck is still within the flammable range. To better illustrate the extents of the flammable gas cloud at the late stages of the release, Figure 92, Figure 93, and Figure 94 show 3D contours viewed looking aft from port-forward and forward from aft-starboard at 300 seconds, 360 seconds, and 450 seconds after the start of the release (i.e., at 360 seconds, 420 seconds, and 510 seconds, respectively). The outer surface of these contours represents the lower flammability limit of the gas mixture (1.5%).
Figure 95 and Figure 96 show the fuel concentration as a function of time at the supply fan locations on the forward end of the main deck and at the engine room ventilation air intake locations on the aft end of the main deck, respectively. Figure 95 shows that the gas concentration at the starboard forward supply fans (SF-36 and SF-37 reaches the flammable range about two minutes after the start of the release. The gas concentration at SF-35 on the port side of the forward deck reaches the flammable range about 2.5 minutes after the start of the release. Figure 96 shows that the gas concentration at the engine room #5 and #6 supply air intakes (SF-05 and SF-06, respectively) enters the flammable range between 60 and 90 seconds after the start of the release. At 300 seconds after the start of the release, the gas concentration at these locations peaks at over 20%, well above the upper flammability limit of the gas mixture (9.9%). This corresponds to roughly one minute after the gas flow rate through all five release points has peaked. After about 50 more seconds, the gas concentration at SF-05 and SF-06 re-enters the flammable range. At this time, the concentration at the ventilation air intakes for engine rooms #3 through #6 (SF-03, SF-04, SF-05, and SF-06) are within the flammable range and remain so for over two minutes.
Figure 97 shows a column graph depicting the maximum gas concentration recorded at monitor locations 1.5 meters above the main deck. This vertical plane is one to two meters below the engine room supply air intakes on the aft side of the main deck. The x,y location (0,0) on this plot corresponds to the center of the riser bore. Note that flammable concentrations are achieved over almost the entire deck. High concentrations, exceeding the upper flammability limit are achieved at mid-ship and at the aft-starboard region of the deck.
These results demonstrate that the release profile modeled in scenario A outlined in Table 5 and depicted in Figure 85 is capable of developing a flammable gas cloud throughout the moon pool and BOP house, over the drill floor, and over the vast majority of the main deck. In particular, flammable gas concentrations are obtained at the engine room #3 through #6 ventilation air intakes. It is feasible that given this release scenario, a flammable gas mixture could have formed within these four engine rooms.
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Figure 86. Flammable Cloud at 61 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
(UFL=0.099)
(LFL=0.015)
Flammable Concentration
Flammable Concentration
(LFL=0.015)
(UFL=0.099)
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Figure 87. Flammable Cloud at 160 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015) (LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 88. Flammable Cloud at 180 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015)
(LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 89. Flammable Cloud at 200 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015) (LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 90. Flammable Cloud at 360 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015)
(LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 91. Flammable Cloud at 420 seconds (Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015)
(LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 92. Flammable Cloud at 360 seconds (3D, Scenario A) FWD
FWD
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
Flammable Concentration
Flammable Concentration
(LFL=0.015) (LFL=0.015) (UFL=0.099)
(UFL=0.099)
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Figure 93. Flammable Cloud at 420 seconds (3D, Scenario A) FWD
FWD
Flammable Concentration
Flammable Concentration
(LFL=0.015)
(LFL=0.015) (UFL=0.099)
(UFL=0.099)
(Release initiated at 60 sec.) (Release initiated at 60 sec.)
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Figure 94. Flammable Cloud at 510 seconds (3D, Scenario A) FWD
FWD
Flammable Concentration
Flammable Concentration
(LFL=0.015) (LFL=0.015) (UFL=0.099)
(UFL=0.099)
(Release initiated at 60 sec.)
(Release initiated at 60 sec.)
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Figure 95. Forward Deck Ventilation Intake Flammable Gas Concentration History (Scenario A)
Figure 96. Engine Room Ventilation Intake Flammable Gas Concentration History (Scenario A)
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Figure 97. Maximum Gas Concentration 1.5 m above Main Deck (Scenario A)
6.2 Scenario B
The Scenario B release described previously in Section 4.4.8 was evaluated with the wind from port (to starboard) at 2 m/s, as was the case for the Scenario A release evaluation. As before, the simulation was first run for 60 seconds without gas release to establish the flow field due to the wind. The ventilation sources and sinks listed in Table 4 and shown in Figure 70 and Figure 71 were in operation for the duration of the simulation, including this 60 second “start-up” period.
The wind field immediately prior to the gas release in the xy-plane located three meters above the main deck (representative of the engine room supply air louvers) is exactly as it was for Scenario A, as shown above in Figure 84.
The release of flammable gas was again taken to begin at 60 seconds and followed the Scenario B flow rate profile shown in Figure 53. This figure has been duplicated below on a mass flow rate basis as Figure 98 for the convenience of the reader. The total duration of the release was 360 seconds (6 minutes). As noted previously, a total of 0.33 MMscf of gas was released.
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