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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


Relative Positioning

Free floating vessel• ROV vessels• Platform Supply Vessels• Accommodation vessels• Diving vessels• Construction vessels

Vessel “connected”• Shuttle tanker loading• Heavy lifting


Configurations

From bad … … to worse … … to disaster

Case #1 Case #2 Case #3


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 .


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


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


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


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


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


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


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


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


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)


Case #2Moored to moored support vessel





• Stiffness is generally unknown, but high

• Stability problem

• Anchor dragging severe problem

• Anchor line break disaster


• Wire mooring between vessels• Diameter 66 mm

• Length 300 m


• 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


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


• Support vessel in equilibrium• Mooring as before

• DP set-points at equilibrium


• 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

)


Case #3Moored to DP support vessel





• Stiffness is generally unknown, but high

• Stability problem

• DP failure support vessel sever problem


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


• DP set-points at equilibrium

Line breaks



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

)


ConfigurationsSummary

From bad … … to worse … … to disaster

Case #1 Case #2 Case #3

Environment150 kN

Environment140 kN

Environment70 kN


Additional hazardsMoored to moored support vessel

Hold-back vessel• Drive-off• Black out• Positioning instability

Main vessel• Positioning instability

• Positioning instability


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?


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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