Use Of Drag Embedment Anchor For Floating Wind Turbines

Use Of Drag Embedment Anchor
For Floating Wind Turbines

By Roderick Ruinen
Vryhof Anchors BV

Contents Presentation

• Introduction and use of anchors
• Anchor design
• Conventional drag embedment anchor - Stevpris Mk5
• Vertical load anchor - Stevmanta VLA
• Conclusion

Anchoring options

gravity anchor pile

suction pile drag embedment anchor

Anchor types

Holding capacity of anchors depends on:

• type of soil
in sands and hard clays higher holding capacities are generated
than in soft clays.

• type of anchor
modern high holding capacity generate significantly more
holding capacity than older types. In the range of 100 to 150
times the anchor weight compared to 7 to 20 times the weight
for older anchor types.

• size of the anchor
using an anchor with a larger surface area (generally also a
higher weight) generates more holding capacity.

Use of drag embedment anchors

Drag embedment anchors
are used on :
• semi-submersibles
• SPM buoys
• floating production systems
(FPSOs)

Two main types of mooring

Catenary mooring
• generally used in shallow to

deep water (<1000 m)
• use of chain and / or wire rope

mooring lines
• significant length of mooring

line laying on the seabed
• anchor is loaded in a

horizontal direction
• conventional drag embedment

anchors are typically used

Two main types of mooring

Taut leg mooring
• typically used in deep and ultra

deep water (>1000 m)
• mooring line enters seabed at a

significant angle, no line laying
on seabed
• use of light weight mooring
lines (synthetic rope / wire
rope)
• anchor is loaded in horizontal
and vertical direction
• vertical loaded anchors (VLA)
can be used

Anchor design requires the following information:

• soil data
the type of soil will determine the type of anchor that can be
used and how it will be set

• load data
typically anchors are designed based on the calculated
maximum intact and damaged design loads

• mooring line configuration
catenary or taut leg mooring
chain or wire mooring line (forerunner)

• classification society
determines the appropriate safety factors

Required anchor holding capacity

• Required holding capacity = design load times factor of safety

• different factors of safety for intact and damaged (one line
failure) load conditions

• maximum value governs

• anchor installation load (embedment load) typically equal to
80% to 100% of the maximum intact load
this corresponds to 50% to 70% of the required holding capacity

Factors of safety for drag embedment anchors

(API RP 2SK, 1996 {section 6.6.1 drag anchors})

permanent mooring quasi-static dynamic
intact condition
damaged condition 1.8 1.5
1.2 1.0

temporary mooring 1.0 0.8
intact condition

Factors of safety for VLAs

(dynamic loads @ 45°)

permanent mooring ABS BV
intact condition
damaged condition 2.0 2.15
1.5 1.55

Factors of safety

• Higher factors of safety required for VLAs

• This is caused by the difference in failure mode of the anchor

• Drag embedment anchor is pulled horizontally through the soil
and keeps resistance

• VLA is slowly pulled to the surface due to the vertical load
component

Drag Embedment anchor - Stevpris

Stevpris Mk5 anchor

• Suitable for allow types of soil conditions. Anchors have been
installed in soils varying from very soft clays to dense sands and
cemented soils

• high holding capacities possible. Anchors have been supplied
with holding capacities exceeding 1500 mT

• limited uplift is possible in very soft clay soil conditions

Stevpris Mk5 holding capacity

10000
typical ultimate holding capacity [t]
1000

100

10 10 100
1 Stevpris Mk5 size [t]

very soft clay medium clay hard clay and sand

Fluke/shank angle

The correct fluke/shank angle is important for optimal anchor

performance.

• Mud (very soft clay) : 50°

• Medium and hard clay : 32°

• Sand : 32°

• Intermediate or layered soil : 41°

Stevpris Mk5 anchor and uplift

• In soft clay soil conditions the Stevpris Mk5 anchor is capable of
taking some uplift loads

• Tests have shown that below 20 degrees uplift the effect is
marginal on anchor performance

• Test have been witnessed by classification societies

• API allows uplift, mayor classification societies on a case by
case basis

Stevpris Mk5 and uplift

Stevpris Mk5 and uplift

API RP 2SK, 1996 (Appendix B, section B.6) : 5°
: 10°
Allowable uplift angle on Stevpris anchors:
maximum intact load condition
maximum damaged load condition

Drag-in plate anchor (VLA) - Stevmanta

Stevmanta VLA - principle

• Installed with a low angle
between mooring line and
fluke

• when the required
installation load is reached,
anchor is triggered to the
normal loading mode

• load arrives perpendicular to
fluke giving a high pull-out
resistance;

Features of the Stevmanta VLA

• suited for taut leg moorings
• deep penetration in soft clay soil conditions
• installed like a conventional drag embedment anchor
• VLA to be set in “normal mode”
• normal mode allows for vertical loading
• ratio ultimate holding capacity to installation load

: 2.5 to 3.5
• can be easily recovered
• loading in all directions possible

Typical effect of changing the anchor mode

300

250

200

line load [%] 150

100 ⇑ Change from pull-in 40

50 to normal mode

0 5 10 15 20 25 30 35
0 line length pulled [m]

Stevmanta VLA installation

Two different methods:

• Single line installation
shear pin angle adjuster

• calibrated shear pin triggers
anchor to normal loading
mode

• one line connected to the
anchor

• tail used for orientation and
recovery

Stevmanta VLA installation

Two different methods:

• Double line installation
fixed angle adjuster

• anchor is triggered by
reducing tension on the
installation line and
increasing tension on the
mooring line

• two lines connected to the
anchor

Stevmanta VLA typical holding capacity

1800 600
500
1600 Su = 4 + 1.5 × d [kPa] 151 mm dia. 400
1400 Nc = 12 300
Ultimate Pull-out Capacity [mT] 200
Installation load [mT]1200100
0
1000 121 mm dia. 30
800

600 120 mm dia. K4
400 76 mm dia.

200

0
0 5 10 15 20 25
Stevmanta VLA fluke area [m2]

six strand and spiral strand spiral strand chain

Conclusion

• Stevpris Mk5 anchor (conventional drag embedment anchor)
can be used in all types of soil conditions. In soft clays limited
uplift loading is possible. Anchor is used in catenary and semi
taut mooring systems in shallow to deep water.

• Stevmanta VLA (vertical load anchor) is suitable for use in soft
clay soil conditions were deep penetration below the seabed is
possible. High uplift angles are possible. Anchor is typically
used in (semi) taut mooring systems.

• Both anchor types have successfully been used world wide for
the temporary mooring of semi-submersible drilling rigs and for
the permanent mooring of SPM buoys and floating production
systems (FPSO / FSO / FPU)