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October 7-8, 2008Return to Session Directory
OPERATIONS
The Use of Safety Moorings in DP OperationsNils Albert JenssenKongsberg Maritime
MTS DP conference 2008/ 2 /
Background
• Relative DP operation• Fixed structure• Floating structure
• Need to improve safety of operation
• Blackout• Drive-off• Drift-off
• Mooring alternatives• To sea bed• Via support vessel• Safety hawser
MTS DP conference 2008/ 3 /
Relative Positioning
Free floating vessel• ROV vessels• Platform Supply Vessels• Accommodation vessels• Diving vessels• Construction vessels
Vessel “connected”• Shuttle tanker loading• Heavy lifting
MTS DP conference 2008/ 4 /
Configurations
From bad … … to worse … … to disaster
Case #1 Case #2 Case #3
MTS DP conference 2008/ 5 /
Basic problem
• Mooring force is unknown• Mooring restoring stiffness unknown
• May be high
• Relationship restoring – damping violated• May be fixed if mooring known
R
Gx Gv
( )xv GRmG += ξ2• DP and mooring may “fight” each other• Relative DP
• Reference may move
• Special “anchor assist” functionality available• Can not change physics .
MTS DP conference 2008/ 6 /
Heavy lifting
-2000
-1000
0
1000
2000
3000
4000
5000
23:15 23:20 23:25 23:30 23:35 23:40 23:45
Time
Hoo
kLoa
dSB
[t]
-5
-4
-3
-2
-1
0
1
2
Surg
epo
sitio
nde
viat
ion
[m]
• Oscillations due to interconnection• Too large DP bandwidth
Relative position measurements do not help
• Special severe when lifting to moored structure
• Very different from free floating bodies
Load
Position
MTS DP conference 2008/ 7 /
Mooring and DP
0 200 400 600 800 1000 12000
20
40
60
80
100
Mooring stiffness
0 200 400 600 800 1000 1200-8-6-4-202468
Motion Time
G
G = DP
R = mooring
OK whenR ≤ G
R
MTS DP conference 2008/ 8 /
Case #1Fixed mooring
• Mooring must be pre-tensioned to assure stability and avoid collision if blackout
• Define minimum restoring stiffness
• Define maximum restoring stiffness
• Offsetting by thrusters (if needed)
• Stiffness is generally unknown
• Stability problem if mooring stiffness exceeds DP stiffness
• Anchor dragging severe problem
• Line break disaster
MTS DP conference 2008/ 9 /
0 500 1000 1500 2000 2500 3000 3500555
560
565
570
575
580
585
590
595
600
Time (s)
Pos
ition
(m)
VesselSetpointLine break
Case #1Single anchor moored - Disaster
• Wire mooring
• Diameter 66 mm
• Length 600 m
• Depth 200m
• Environmental forces 250 kN
• DP set-point at equilibriumLine breaks
• Explanation:
• Disturbance excite mooring
• DP pumps energy into the system
• Mooring stiffness too high, or needs much quicker DP control and thruster response
MTS DP conference 2008/ 10 /
Case #1Stiffness
556 558 560 562 564
50
100
150
200
250
300
350
400
Offset(m)
Tens
ion
(kN
), S
tiffn
ess
(kN
/m)
TensionStiffness
Environmentalload
• Mooring stiffness 100kN/m
• DP stiffness 30 kN/m
MTS DP conference 2008/ 11 /
Mooring and DP
0 200 400 600 800 1000 12000
20
40
60
80
100
Mooring stiffness
0 200 400 600 800 1000 1200-8-6-4-202468
Motion Time
G
G = DP
R = mooring
OK whenR ≤ G
R
MTS DP conference 2008/ 12 /
Case #1Single anchor mooring – On the limit
556 558 560 562 564
50
100
150
200
250
300
350
400
Offset(m)
Tens
ion
(kN
), S
tiffn
ess
(kN
/m)
TensionStiffness
• Mooring stiffness 20 kN/m• DP stiffness 30 kN/m
0 500 1000 1500 2000 2500 3000 3500560
561
562
563
564
565
566
Tme (s)
Pos
ition
(m)
VesselSetpointLine break
• Environment 150 kN
MTS DP conference 2008/ 13 /
Vessel to vessel mooring
R = 30 – 90 kN/m
R = ?
Relative damping ≈ 1
Environment 100 - 250 kN
R = 30 – 90 kN/m
Relative damping ≈ 1
MooredSupport vessel
DP operatedSupport vessel
MTS DP conference 2008/ 14 /
0
10
20
30
40
50
60
70
80
90
100
125 130 135 140 145 150 155
Vessel to vessel mooring
• Catenary
0
10
20
30
40
50
60
70
80
90
100
125 130 135 140 145 150 155
( )h
hLwF
hLhL
hhLd
x 2
ln2
22
22
−=
⎟⎠⎞
⎜⎝⎛
−+−
=
Ø =36mmDistance (m)
Force (kN)
Ø =66mm
Stiffness kN/m
L = 300 m
Distance (m)
MTS DP conference 2008/ 15 /
Case #2Moored to moored support vessel
• Mooring must be pre-tensioned to assure stability and avoid collision if blackout
• Define minimum restoring stiffness
• Define maximum restoring stiffness
• Offsetting by thrusters (if needed)
• Stiffness is generally unknown, but high
• Stability problem
• Anchor dragging severe problem
• Anchor line break disaster
MTS DP conference 2008/ 16 /
• Wire mooring between vessels• Diameter 66 mm
• Length 300 m
• Environmental forces 250 kN
• Support vessel in equilibrium• Four point mooring (symmetric)
• Diameter 66 mm
• Length 800 m
• Depth 200 m
• DP set-point at equilibrium
Case #2Moored to moored support vessel - Disaster
0 500 1000 1500 2000 2500 3000 3500-15
-10
-5
0
5
10
15
20
25
30
35
Time (s)
Pos
ition
(m)
Main vesselSupport vessel
0 500 1000 1500 2000 2500 3000 35000
500
1000
1500
2000
2500
Time (s)
Tens
ion
(kN
)
• Explanation:• Same story
Line breaks
MTS DP conference 2008/ 17 /
Case #2Moored to moored support vessel – On the limit
0 500 1000 1500 2000 2500 3000 3500-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Time (s)
Pos
ition
(m)
Support vesselMain vessel • Wire mooring
• As before
• Environmental forces 140 kN
• Support vessel in equilibrium• Mooring as before
• DP set-points at equilibrium
• Explanation:• Same story
• Mooring stiffness matches DP• DP 30 kN/m
• Vessel-vessel mooring 25 kN/m
• Additional body
0 500 1000 1500 2000 2500 3000 3500105
110
115
120
125
130
135
140
145
150
155
Time (s)
Tens
ion
(kN
)
MTS DP conference 2008/ 18 /
Case #3Moored to DP support vessel
• Mooring must be pre-tensioned to assure stability and avoid collision if blackout
• Define minimum restoring stiffness
• Define maximum restoring stiffness
• Offsetting by thrusters (if needed)
• Stiffness is generally unknown, but high
• Stability problem
• DP failure support vessel sever problem
MTS DP conference 2008/ 19 /
0 500 1000 1500 2000 2500 3000 3500 40000
500
1000
1500
2000
2500
Time (s)
Tens
ion
(kN
)
Case #3Moored to DP support vessel - Disaster
0 500 1000 1500 2000 2500 3000 3500 4000-40
-30
-20
-10
0
10
20
30
40
Tim e (s )
Pos
ition
(m)
M ain vesselS upport vessel
• Wire mooring• Diameter 66 mm
• Length 300 m
• Environmental forces 250 kN
• DP set-points at equilibrium
Line breaks
• Explanation:• Same story
MTS DP conference 2008/ 20 /
Case #3Moored to DP support vessel – On the limit
• Wire mooring• Diameter 66 mm• Length 300 m
• Environmental forces 70 kN• DP set-points at equilibrium
• Explanation:• Same story• Mooring stiffness matches DP
• DP 30 kN/m• Mooring 15 kN/m• Additional body
0 500 1000 1500 2000 2500 3000 3500-1
-0.5
0
0.5
1
Time (s)
Pos
ition
(m)
Main vesselSupport vessel
0 500 1000 1500 2000 2500 3000 35000
5
10
15
20
25
30
Time (s)
Stif
fnes
s (k
N/m
)
MTS DP conference 2008/ 21 /
ConfigurationsSummary
From bad … … to worse … … to disaster
Case #1 Case #2 Case #3
Environment150 kN
Environment140 kN
Environment70 kN
MTS DP conference 2008/ 22 /
Additional hazardsMoored to moored support vessel
Hold-back vessel• Drive-off• Black out• Positioning instability
Main vessel• Positioning instability
• Positioning instability
MTS DP conference 2008/ 23 /
Additional hazardsMoored to moored support vessel
Hold-back vessel• Drive-off• Black out• Positioning instability
Main vessel• Positioning instability
• Positioning instability
MTS DP conference 2008/ 24 /
Alternative risk control measuresIMCA M 185 (2007)Before considering attaching a hold-back vessel:• Reschedule operation to a time when principal
hazard does not exist• Wait for environmental conditions• Use sea bed anchor• Connect to another moored vessel• Moor to a fixed facility
Considerations About the Use of Hold-Back VesselsDuring DP Diving Operations
Good enough?
MTS DP conference 2008/ 25 /
Alternative risk control measuresGood enough advice?
Before considering attaching a hold-back vessel:• Reschedule operation to a time when principal
hazard does not exist• Wait for environmental conditions• Use sea bed anchor• Connect to another moored vessel• Moor to a fixed facility
Considerations About the Use of Hold-Back VesselsDuring DP Diving Operations
YOU CAN’T BEAT THE PHYSICS
KNOW WHAT YOU ARE DOING
Thank you for the attentionQuestions?
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