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October 7-8, 2008Return to Session Directory
OPERATIONS
The Use of Safety Moorings in DP OperationsNils Albert JenssenKongsberg Maritime
On the use of ”safety mooring” in DP applications
Nils 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?