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MCE Deepwater Development 2015
London, March 25th 2015
Lars Mehus, Chief Engineer, Steel tube Umbilicals
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Aker Solutions’ Power Umbilicals
General Features of Aker Solutions’ Power Umbilicals
Low helix lay-angle (~2deg)
Efficient utilisation of tension elements within the cross
section
Conduits forming channels where elements can move
relatively to each other
Long lay length gives low radial loads and low frictional
forces
Elements with high axial stiffness are bundled with the
same lay-length as the umbilical elements
No armouring
Axial stiffness is achieved by tension elements within the
cross section
No cable armouring gives smaller component diameter and less
splicing during umbilical manufacturing
Low weight with Aker Solutions design
Can be of advantage for installation and selection of
vessel.
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Power Umbilical in Deep Waters
Aker Solutions Power Umbilical installed and in operation at 2700m
wd
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Challenges
Key facts
Water depth @ host ~2700m
Length ~21 km
Design top tension = 985 kN
Umbilical (cable) strain @ top
< 0,15%
Umbilical axial stiffness = 6,9 *108
High top tension
Strain limits for cables
Load control in cables
Fatigue
Creep
Thermal expansion
Cable splicing
Mechanical properties – installation
loads
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Loads and strain
=
How to control the global strain within limits for the components
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© 2015 Aker Solutions
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Increased axial Stiffness – Lower strain
Traditional way of increasing axial
stiffness is by use of steel armoring
At deep water the effect of added
stiffness will at large degree be taken
by added weight
.
Carbon fiber rod technology is much
more efficient at deep water due to low
density compared to stiffness.
Steel rod:
=
CF rod:
=
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)
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= 0.0325
= 0.0038
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Typical strain of an umbilical vs. # carbon fibre rods
Umbilical strain vs. number of carbon fiber rods
0,25
strain %
0,2
Typical strain
limit for
power cable
0,15
0,1
0,05
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
carbon rods
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Force in cable vs water depth
The force in a cable is
dictated by the strain in the
cable1)
The strain in a cable is
governed by umbilical strain
Cable strain and load can be
the same at shallow water
as for deep water depending
umbilical axial stiffness
Note1) Thermal loads not considered
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Cable suspension and umbilical global configuration
Due to temperature, material creep and
frictional effects in the bend stiffener area,
it is difficult to predict loads in power
cores.
These effects complicate reliable
prediction of fatigue life
Hang off power cores below the dynamic
bending zone
Copper cores free to expand through
the dynamic zone.
No load effect from thermal
expansion.
No internal friction. Reduced stress
amplitudes
Copper cores exposed to bending
only, hence reliable fatigue life
estimates.
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Cable suspension for dynamic operation
Utilize the PVC conduits to
make channels where cables
can move axially
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Cable suspension system
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FE Model
Low axial strain in the section
with high dynamic movements.
Cables suspended in the
umbilical cross section
Full strain in the cable below
the bend stiffener
No axial load gives no radial
load and no friction
No load effects from thermal
expansion
No creep effects in the section
where the umbilical see high
tension combined with dynamic
bending
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USAP
Umbilical Stress Analysis Program
Stress control
Finite element approach
All linear and non linear effects
Program focus on longitudinal helix
behaviour
Bell-mouth and bend stiffener modelling
Full Catenary model
Actual material stiffness properties
Reeling with large Centre Tubes
Installation
Temperature impact
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USAP 3D vs. 2D programs, why 3D?
3D analysis includes effects in axial direction
Assumptions made by 2D programs:
• Constant curvature along the umbilical
• Beyond slippage, the frictional effect remains constant
Comparison
between 2D
and 3d
analysis
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Installing umbilical with cable splices in deep waters
Challenge:
Strain hardening due to conductor manufacturing
Strain hardening during conductor stranding
Conductor compacting
Water blocking and no further annealing
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MV Power Core- Factory splice
Cable splice weld
Proper strength in weld zone
Local annealing in the heat
affected zone
Change in mechanical properties
locally due to heat input
Welded conductor
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Tensile test of splice
Tensile properties of spliced
cable
‘All’ strain concentrated in a
local spot
No ductile behavior
More like a brittle fracture
due to local straining
Fracture just outside the
weld
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Reduce mechanical stresses in the splice
Splice reinforcement with a
stiff high strength fiber
sleeve
The strain is lower in the
splice than in the conductor
outside the splice area.
The forces is picked-up by
the sleeve.
Sleeve length is determined by:
Shear force capacity between the
different layers of materials.
Required strain reduction of the
splice section
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Summary
Power umbilicals installed and in operation @ 2700m water depth
Technology developed to improve service conditions for power
conductors
Better control of dynamic loads
Durability in service
Improve reliability for installation of power umbilicals including splices
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Copyright and disclaimer
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