kuliah 7_jacket bracing systems

8/18/2019 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.


• 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.


• 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.



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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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FRAME JACKET

AK O S KUN ALA


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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.

kuliah 7_jacket bracing systems

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