3.
DESIGN METHODS AND ACCEPTANCE CRITERIA
3.1
Inshore and Offshore Lifting Operations
The design method for the lifting operations shall be in accordance with 'Standard for
Insurance Warranty Surveyors in Marine Operations', DnV Marine Operations, June 1985 ref
/1/. The design method described by ref /1/ meets the limiting stress requirement given in
Statoil specification S-F-SS-138, section 8. Hence the stress method described below covers
Statoil requirements.
The Y-piece assembly, the protective steel cover and the reinforced concrete protection cover
shall be checked for the ultimate limit state (ULS) condition.
The ultimate limited state, related to the maximum load carrying capacity (yielding limit state
and buckling limit state) is calculated according to he partial co-efficient method as described
in ref /3/ part l, section 4.
The formal requirement is that the structure may reach but not exceed a defined limit state
when subjected to design loads expressed by
Sd # Rd
where
Sd = design loading effect
Rd = design resistance
In general the design loading effect may be written as
Sd = E (Yfi · Fki)
i
Yfi = load co-efficient
where
Fki = characteristic loading
The design resistance is written as
Rd = Rk / Ym
where
Rk = characteristic resistance
Ym = material co-efficient (= 1.15, see Section 5.1).
For steel structures the characteristic resistance is given by the yield stress (oy). The
equivalent stress according to von-Mises stress criteria for the design loading represents the
design loading effect that will be compared with the design resistance. Buckling of the pipe
will be checked as required.
The angle of tilt during lifting shall not exceed 1.5E in the sevceability limit state (SLS) (ref
Statoil Spec F-SS-139).
Maximum deflection of spool shall not exceed 2.0 m, during lifting operations including
environmental loads.
Final positioning of frames and covers shall be within 0.100 metres of its nominal position
as indicated on the Project Drawings (ref Statoil Spec F-SS-139).
3.2
Pipeline/Y-piece Alignment
For Y-piece and pipeline alignment on seabed, guidelines given in ref /4/ shall be used.
The formal requirement is that the structure may reach but not exceed a defined limit state
when subjected to design loads is expressed by
Functional Loads
Sd # 0.72 · Rk
(ref /4/)
Functional + Environmental Loads
Sd # ).96 · Rk
3.3
(ref /4/)
Transport of Concrete Covers from Fab site to CCB
Transportation of the concrete covers shall be performed in such manner that there is no
damage to the concrete.
3.4
Inshore and Offshore Lifting of Concrete Covers
Inshore the concrete covers will experience three lifting operations as follows:-
Lifting from fabrication site to barge
Lifting from barge to quay
Lifting from quay to Regalia
It is expected that none of these conditions are dimensioning for the concrete covers.
For offshore lifting the concrete covers will be designed as described in Section 3.1 and
according to Statoil Specification D027-S-F-SF-004 rev O.
4.
LOADS
4.1
Load Category Definitions
Loads are classified according to DnV (Ref /3/, part l) as
-
permanent loads (P)
variable functional loads (L)
environmental loads (E)
deformation loads (D)
accidental loads (A)
4.1.1 Permanent Loads (P)
Permanent loads are loads which will not be removed during the period considered, such as
-
weight of the structure
weight of permanent ballast and equipment which cannot be removed
external/internal hydrostatic pressure of permanent nature
buoyancy (permanent part)
4.1.2 Variable Functional Loads (L)
Variable functional loads are loads which can be removed or moved, such as
-
operational of cranes
loads from alongside vessels
mooring of vessels
differential ballasting
operational impact loads
stored materials, equipment and liquids, fluid pressure etc
4.1.3 Environmental Loads (L)
Environmental loads are loads caused by environmental conditions like
-
waves
current
storm surge
tide
wind
ice
earthquake
4.1.4 Deformation Loads (D)
Deformation loads are loads associated with imposed deformations, such as
-
differential settlements
-
temperature
4.1.5 Accidental Loads (A)
Accidental loads are loads that are not well defined with respect to intensity and frequency
and that may occur as a result of improper use or exceptional conditions like
-
collisions from vessels
dropped objects
loss of hydrostatic stability
loss of permanent internal pressure
4.1.6 Load Factors
The ultimate state is to be checked for two load combinations a and b, with load co-efficients
according to table 4.1 below
Load combinations
P
L
E
E
A
a. Ordinary
b. Extreme
1.3
1.0
1.3
1.0
0.7
1.3
1.0
1.0
-
Table 4.1
Load Factors for Ultimate Limit State
In addition load factors shall be included to cover effects like
-
dynamic amplification
contingency/inaccuracy
skew load
additional design factors
4.2.1 Dynamic Factors
The dynamic amplification factor (DAF) is a factor accounting for the global effects normally
experienced (ref /3/, RP5).
Table 4.2 below shows the values for DAF which shall be used for lifts in air.
Weight of the
lifted object (W)
# 100 t
DAF Offshore
DAF Inshore
100-1000t
1.30
1.20
1.15
1.10
Table 4.2
Dynamic Amplification Factors (DAF)
4.2.2 Contingency/Inaccuracy
The weight inaccuracy factor (WIF) shall according to Ref /3/, RP5, be WIF = 1.10
4.2.3 Skew Load
The skew load factor (SKF) is a factor to include the extra loading on slings caused by the
effect of inaccurate sling lengths and other uncertainties with respect to force distribution in
the sling arrangement.
SKF1 shall be 1.25 for single hook lifts.
For multi hook lifts an additional factor to include yaw (YEF) and tilt effects (TEF) shall be
included.
For multi hook lifts the skew load factor (SKF2) will be
YEF = 1.05
TEF = 1.05 (assumed)
SKF2 = SKF1 · YEF · TEF = 1.38
4.2.4 Design Factors
For design of padeyes and other structural elements, additional design factors shall be
included to account for load inaccuracies, local dynamic and consequences of failure (ref /3/,
Sec 6). See table 4.3 below.
DESIGN FACTOR
Item
Part coeff
method
Perm stress
method
Spreader frames,
spreader beams with lifting
points etc
1.75
1.35
Lifting points(padeyes
and their attachments
to the structure)
1.75
1.35
Load transferring
members supporting the
lifting points
1.50
1.15
Other members of the
lifted object
1.30
1.00
Table 4.3
Design Factors
4.3
Load Definitions and Calculation Methods
4.3.1 Permanent Loads
The weights and centre of gravity will be calculated according to accurate specific weights
and volumes and/or weighted or estimated weights of parts of the object, equipment, etc. The
calculated or estimated weights will be multiplied by a weight inaccuracy factor (WIF) equal
to 1.1, ref Section 4.2.2.
4.3.2 Variable Functional Loads
The variable functional loads are equal to permanent loads multiplied by the dynamic
amplification factor (DAF).
4.3.3 Environmental Loads
For loadout and installation the following load cases shall be considered
I
Lift inshore
permanent loads
variable functional loads (inshore DAF)
wind
tuggers
II
Lift Offshore in Air
permanent loads
variable functional loads (offshore DAF)
wind
tuggers
III
Lowering through Wave Zone
permanent loads
variable functional loads (offshore DAF)
wave load
tuggers
guide wires
IV
Lowering through water
permanent loads (submerged weight)
wave load
current load
guide wires
V
Load on Seabed
permanent loads (submerged weight)
variable functional loads (offshore DAF)
wave
current
For the Y-piece and the lifting gear, phase III is found to be more critical then phase I, II, IV
and V because
-
In-phase II there is no relative motion between the lifted object and the crane, which
could give rise to large dynamic hook loads. The wind loads acting on the structure
will be perpendicular to the main loading direction which is vertical, and the main
design wind speed is low.
-
In Phase IV the wave loads acting on the spool will be reduced as the spool is lowered
towards the sea bed.
a.
Design Conditions
It is the intention that maximum significant wave height which the installation can be
performed in shall be the one corresponding to either maximum allowable stresses in the
structure or to maximum allowable stresses in the structure or to maximum capacity of
installation vessel cranes.
The start off condition for the lifting operation shall be described by the following seastate
Significant wave height
Average wave period
Hs = 3.0 (m)
T = 8.7 (sec)
The limiting conditions shall then be calculated.
The characteristic wave height (Hk) (maximum expected waveheight in a seastate) described
by Hs = 3.0 m is taken as
Hk = Hs · % (0.5 · 1n N)
where
N=
number of waves passing during the operation. N is estimated for a 3 hour period.
This gives a characteristic wave height
Hk = 3.0 · %(0.5 · 1n (3600 · 3/8.7)) = 5.66 (m)
Based on wave records from Veslefrikk, recorded in an 11 year period from 1975 to 1986,
Tk is taken as
Tk = 7 (sec)
We now have the following probabilities
Prob (T < Tk) = 0.084 (Based on wave records from Veslefrikk)
Prob (H > Hk) = 1 - exp {-1/T·N·exp[-2(Hk/Hs)²]}
Prob (H>5.66) = 1-exp{-1/8.7·3·3600·exp [-2(5.66/3)²]} = 0.63
Joint probability = 0.084 · 0.63 = 0.05 (5%) which satisfies demands in Ref /3/.
The wave loading per meter fixed pipe may now be expressed by Morison's equation.
dF = dD + dM
where dD represents the drag force and dM represents the mass forces.
dD = 0.5 · Ro · CD · D · *V* ·V (per amplitude)
and
dM = 0.25 · Ro · B · D² · CM · a (per amplitude)
where
V = 2B/T · Hk/2 ·sin wt = 2.54 ·sin wt (m/s)
(water particle velocity)
a = (2B/T)² · HK/2 = cos wt = 2.28 cos wt (m/s)
(water particle
acceleration)
Ro = 1.025 (tonn/m3)
(water density)
D
(outer diameter)
CD = 1.0 · 1.1 = 1.1
(drag coefficient)
CM = 2.0 · 1.1 = 2.2
(mass coefficient)
g = 9.81 (m/s²)
(acceleration of gravity)
Additional forces, A, due to vessel motion shall be included
A = Ro · BD²/4 · g · (DAF-1)
+ Ro · BDi²/4 · g · (DAF-1)
+ Wp · (DAF-1)
(added mass)
(mass of water trapped
inside)
(mass of pipe)
where
Di
(inner diameter)
Wp
(weight of pipe)
Drag forces due to vessel motion is assumed to be negligible.
The basic drag and mass co-efficients are assumed to be 1.1 and 2.2 respectively. This
includes a 10% increase due to clamps etc. For Hk $ B · CM · D/(2 · CD) the total vertical
force amplitude may be expressed as
FW = Ro/2 · CD · D · (2B/T)². (Hk/2)2 . (1+(BD/4 ·CM/CD ·
b.
Dynamic hook load (DHL), one hook lifts.
Pipe submerged
2/Hk)²) + A
DHL = (W+RW) · DAF-WB) · WIF · Yfp + (FW-WP(DAF-1) · Yfe
where
W
WB
RW
WIF=1.1
DAF=1.2
Fw
Weight of lifted object
Buoyancy of flooded pipe
Weight of rigging
Weight inaccuracy factor
Dynamic amplification factor
Wave loads and load effects on structure due to vessel motion
The below given factors are included in the crane design.
Yfp (=1.0)
Yfe (=1.0)
Load factor, permanent loads
Load factor, environmental loads
Pipe emerged
DHL = ((W+RW) · WIF + Aw) · DAF · Yfp
where
As - mass of water trapped inside pipe
Yfp = 1.0
The skew load factor (SKF1) will not have any influence on the dynamic hook load for the
single hook lifts.
c.
Dynamic hook load, two hook lifts
Total loading, pipe submerged
F = (((W+RW) · DAF-WB) · WIF · Yfp+(FW-WP(DAF-1) · Yfe) · SKF2
where
SKF2 - skew load effect
Yfp
Yfe
= 1.O
= 1.0
The total force is then distributed to the two cranes according to the rigging arrangement.
d.
Structural components
The design loading for the lifted object is then written
Submerged
Sd = (W · DAF - WB) · WIF · Yfp + FW · Yfe
Emerged
Sd = (WIF + Aw) · DAF · Yfp
For lifts with skew load factors exceeding 1.0 the design loading effect calculated according
to the above loadings should then be adjusted to account for uncertainties in the internal force
distribution in the slinging arrangement.
The maximum dynamic forces including skew load effects shall be multiplied by a design
factor according to table 4.4 below (ref /3/, Rp5, Section 6). This effect is included by
reducing the design resistance accordingly.
Item
I
II
III
DF (design factor)
Spreader frame, spreader
beams, cantilevers with
lifting points, padeyes, etc
1.75
Load transferring members
supporting the lifting points
1.50
Other members of the lifted
object
1.30
Table 4.4
Design Factors (DF)
(i)
Y-piece/spool piece (Item II), one hook lifts
The effects of increasing the loading in one or more slings with SKF1 shall be
investigated. The case giving the largest spool piece stresses represents the
design loading effect to be checked according to
Sd # Rd = Rk/(Ym · DF) = 0.58 · Rk
(ii)
Y-piece/spool piece (Item II), two hook lifts
The effects of tilting the system both ways will be investigated. For the worst
loading condition the skew load effect from SKF1 will be investigated as for
the one hook lift. The yaw effect factor will be included. The case giving the
largest spool piece stresses represents the design loading effect to be checked
according to
Sd # RD = Rk/(Ym · DF) = 0.58 · Rk
(iii)
Item I, one and two hook lifts
The loads from the design loading (using the appropriate skew load factor)
give the design loading effect. The capacity will be checked according to
Sk # MBL/NSF
for slings
(ref /3/, RP5, Section 4)
Sd # SWL
for shackles
(ref /3/, RP5, Section 5)
Sd # Rk/(Ym · DF) for lifting points
= 0.50 · Rk
spreader beams etc
where
MBL
Minimum breaking load
SWL # 0.25 · MBL Safe working load
NSF
Nominal Safety Factor
(ref /3/, RP5, Section 4)
e.
Alignment Analysis
The pipeline will be moved on the seabed using pipeline handling frames. The frames
will be used for lifting the pipe ends from the seabed, moving and holding them at the
required level for fit-up alignment and welding.
Waves and Current
The design condition for the alignment analysis operation is taken as Hs = 5.0 m. The
characteristic design wave height (Hk) is then given by
Hk = Hs · (0.5 · 1n N) 0.5
where
N = number of waves in the seastate
The characteristic wave height is calculated based upon a 24 hour seastate duration with an
average period of 7.5 seconds giving
Hk = Hs (0.5 · 1n(3600 · 24 / 7.5) 0.5 = 10.8 (m)
As the wave forces at seabed increase with increasing wave period, the calculation of wave
forces shall be performed using a long period. The wave period to be used is set to 15
seconds.
The current with a 5 years recurrence period is assumed to act together with the above
described wave. This gives at seabed
Uc = 0.45 (m/s) (ref/7/)
The forces from simultaneously acting waves and current will be calculated according to
Morsion's equation.
dF = 0.5 · Ro · CD · D · *V+Uc*· (V+Uc) + Ro · (BD²/4) · CM · a
where
V = water particle velocity at seabed
a = water particle acceleration at seabed
Uc = current velocity
Forced displacement
Necessary forced displacements for the alignment operation will be applied. Friction forces
acting between the pipeline and the seabed will be included.
4.3.4 Deformation Loads
None
4.3.5 Accidental Loads
None
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