Kuliah 7_Jacket Bracing Systems
Transcript of Kuliah 7_Jacket Bracing Systems
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Dr. Eng. Rudi W. Prastianto
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Outline
1. Jacket Bracing Configurations
2. Jacket Brace Size Selection3. Calculation Example
4. Faktor Sekunder dalam Frame Jacket
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1. Jacket Bracing Configurations
• There is a wide variation of platform bracing
patterns, each with its advantages and some
shortcomings.
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Type 2 and 5, V brace Patterns:
• Fewer brace connections at a joint,
• Lack of redundancy and symmetry.
• Lack continuity of load flow from one bracing level to the
other, resulting in larger horizontal brace dimensions.• These patterns are seldom used and are not recommended.
Type of Bracing Pattern
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Type 3, N-bracing Pattern:• Fever braces connecting to joints.
• Lacks symmetry and redundancy.
• All diagonal braces would be under compression or tension load
depending on the horizontal load direction.
Type of Bracing Pattern
• Due to lack of tensile brace backup,
buckling under compressive loading
of one highly loaded diagonal brace
can rapidly propagate to other bracescausing platform collapse.
• Type 3 bracing pattern is seldom
used and is not recommended.
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Type 4, V plus X braced Pattern:
•
In common use in most offshore locations.• Braces run along the shortest diagonals of their bays with reduced buckling
lengths.
• Adequate symmetry, redundancy and ductility are available.
• Disadvantage higher number of brace connections at joints and the V braces at
the transverse directions framing into horizontal braces.
Type of Bracing Pattern
• V braces in vertical plane carry high
loads and would have larger diameters
than the horizontal braces. Such a joint
intersection would either require
enlarged joint cans or larger thannecessary horizontal brace dimensions.
• Replacing the V braces in the
transverse direction by X braces
(similar to the transverse direction of
Type 6) results in higher ductility and
better seismic resistance.
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Type 6, Fully X-braced Pattern:
• Provides high horizontal stiffness, ductility, and redundancy.
• The joints are crowded and high volume of welding is present.
• This bracing pattern is popular in deepwater jackets where
stiffness is needed to reduce sway periods and in seismically
active regions where ductile behavior is important.
Type of Bracing Pattern
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2. Jacket Brace Size Selection
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2.1 Petunjuk Menentukan Ukuran Brace
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Lanjutan 2.1 …
c) For diameter sizes (OD)
18 in. use the wall thickness for seamless
standard pipe as a starter.
For sizes 29 in. would most likely be rolled from plate and seam and
butt-welded try 1 in.
For sizes 30 36 in. start with 5/6 in.
For sizes > 36 in. start with a wall thickness that satisfies D/t > 31
requirement.
If the brace is at the splash zone after selecting the brace size that satisfies
all structural strength requirements consider adding 1/8 in. 1/4 in. to
the wall thickness as corrosion allowance.
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Lanjutan 2.1 …
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2.2. Geometry and Stiffness Parameters
of a Single X-braced Jacket Bay
P a r a m e
t e r - p a r a
m e t e r
S i s t e m B
r a c i n g
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2.2.1. The Horizontal Stiffness Parameters
• The horizontal stiffness (k L) is represented bythe force required to cause an average unitdeflection at its top two joints.
• The most prominent parameters that control thisstiffness (Kumar et al., 1985):
Ratio of the cross-sectional areas of the diagonalbraces (A3) and the jacket leg area (pile plus leg
area, if grouted), A1 ( = A3/A1 ) The aspect ratio, = (a + b)/2h,
Batter, S, and
Height, h.
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• Low A3/A1 values the system stiffness
would be low and high horizontal deflections
would be experienced.
• Higher A3/A1 values the diagonal braces
would become highly effective in transferring
the shear forces from one jacket leg to theother.
2.2.1. The Horizontal Stiffness Parameters
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2.2.2. A limiting (A3/A1)0 Ratio [Kumar, et al., 1985]
• Similar behavior
with variation of S
value.
• At commonly
jacket aspect
ratio ( ) 0.1
A3/A1 < 0.2
assure rigid truss
behavior.
• A3/A1 > 0.2 :
Structural strength acceptable
Inefficient steel
use.
Note: K-braced Trusses A3/A1 = 0.2 –
0.4
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2.2.3. The Horizontal Stiffness Parameter (k L) vs.
the Aspect Ratio ( ) [Karsan, 1986]
• X-braced Truss, A3/A1 =
0.2 , S = 12:
= 2 (k L) max,
correspond to
= 36o
1 < < 2 (k L) values
stationary
= other ranges (k L)
values rapidly decreased• Other common S values
similar behavior.
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Note on A Full Jacket Structure
• A full jacket structure a stacked assembly
of the truss modules similar conclusions
apply to the full jacket structure.• Jacket horizontal stiffness becomes
increasingly important for Dynamic Response
reasons as the water depth increases.
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3. Example 1
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3. Example 1
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3. Example 2
Tentukan
dimensi awal
dari brace
untuk
konfigurasi
bracing X
seperti
nampak
padagambar.
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3. Example 2
Diketahui:
• a = h = 45 ft, S = 12; diameter luar kaki jacket
(OD) = 54 in. dgn tebal dinding (T) = 1,0 in.
• Diameter luar pile (OD) = 48 in. dgn tebal
dindingnya = 1,25 in. Bagian pile di dalam kaki
jacket adalah ungrouted .
• Diasumsikan brace paling bawah berada pada
level 80 ft di bawah MWL (Mean Water Level).
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3. Example 2
Solusi:
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3. Example 2
Untuk konfigurasi bracing X:
• Dalam kondisi terkena beban horizontal paling
kritis satu bagian brace X mendapat beban
tekan, sedang satu lainnya beban tarik.
• Panjang buckling = L2- L (dimana: L= faktor
reduksi panjang brace, karena
kenyataannya brace disambungkan pada
dinding kaki jacket-nya.
• Diasumsikan koefisien buckling k = 0,8.
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3. Example 2
•
Jari-jari girasi pipa dinding-tipis r
0,35.OD.• Sehingga OD brace dapat ditentukan:
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3. Example 2
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3. Example 2
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3. Example 2
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4. FAKTOR SEKUNDER DALAM
FRAME JACKET
• Beberapa faktor yang harus dipertimbangkan
dalam penentuan elemen struktur sekunder
dari jacket adalah:1. Selama proses analisis tegangan struktur jacket
perlu mensimulasikan kekakuan dan besar beban
dari elemen sekunder. Perancang perlumemastikan kekuatan dan stabilitasnya dalam
perhitungannya.
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FAKTOR SEKUNDER DALAM
FRAME JACKET
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FAKTOR SEKUNDER DALAM
FRAME JACKET
AK O S KUN ALA
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FAKTOR SEKUNDER DALAM
FRAME JACKET
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Refferences
• Karsan, D. I. (1986). “Design of jackets indeepwater Gulf of Mexico waters”, ASCEJournal of Waterway, Port, Coastal and Ocean
Engineering, Vol. 112, No. 3, pp. 421-446,May.
• Kumar, A., Nair, V. V. D., and Karsan, D. I.(1985). Stiffness properties of fixed and guyed
platforms, ASCE Journal of StructuralEngineering, Vol. 111, No. 2, p. 239, February.