River Crossing

(Jakarta, July 14, 2009)

Pada pekerjaan pipeline, khususnya onshore pipeline, sangat sering dan tidak bisa dihindari beberapa jenis crossing yang terjadi pada pipeline project.

Beberapa crossing yang sering tidak bisa dihindari adalah:

1. Road Crossing (Crossing Jalan),
2. River Crossing (Crossing Sungai)

Ada pula beberapa crossing yang tidak selalu ada dipekerjaan pipeline, seperti:

1. Rail Crossing

Yang ingin di bahas sekarang adalah masalah River Crossing. Sungai-sungai yang dilewati oleh pipeline ada yang begitu lebar dan adapula berupa sungai kecil atau malah hanya melewati selokan. Sehingga pada pekerjaan engineering ada yang harus dianlisa pakai pendekatan engineering adapula yang sudah cukup dengan typical crossing seperti project2 yang sudah pernah dikerjakan, biasanya setiap client/Oil Company yang besar sudah mempunyai standard masing2 atau mengacu pada Standard International yang sudah ada.

Kenapa perlu ada analisa pada River Crossing apabila melewati sungai yang cukup lebar?

Jalur pipa (pipeline) yang akan menyebrang sungai akan membutuhkan perlakuan khusus pada pekerjaan ini. Apabila pipeline hanya ditarik dan diseberangkan ke sisi sebalah sungai yang lain, pipa akan mengapung di air apabila daya angkat pipa lebih besar, pipeline-pun tidak mengikuti natural curvature –nya karena kita tidak memperhitungkan dan mendesainnya seperti itu, tentu ini tidak diinginkan, apalagi jika sungai ini aktif dan banyak kegiatan diperlintasannya.

Beberapa data yang harus dipersiapkan u/ mendesain River Crossing:

1. Contour /profile sungai,
2. Data tanah (berat jenis, jenis tanah dst),
3. Data Arus Sungai (kecepatan arus),
4. Data Pipa (mechanical data),
5. Data Coating,
6. Data Process
7. etc

Tahapan untuk mendesain, pertama kita harus mengetahui natural curvature dari pipa itu sendiri. Natural curvature bisa menggunakan dua alternative coating (pemberat/ additional weight). Apabila menggunakan additional weight dengan system perberian dibeberapa titik, maka calculation design berdasarkan natural curvature dari pipa itu sendiri (bare pipe).

Jika additional weight menggunakan coating disepanjang pipa, maka perhitungan berdasarkan natural curvature dengan pipa ber coating.

1. Bare pipe Radius Curvature

Formula untuk minimum radius curvature dengan pipa polos (bare pipe) adalah:

Dimana:

R = Radius Curvature

E = Young Modulus

D = Outside Diameter

SMYS = Specified Maximum Yield Strength

DF = Design Factor

1) Offshore Pipeline Design, Analysis and Method, A.H Mousselli

2. Coating Pipe Radius Curvature

Apabila pipa yang digunakan adalah pipa bercoating, maka rumus yang digunakan adalah:

1. Concrete Yield Strength (40% Concrete Compression Strength)

Dimana:

R1 = Radius Curvature

Ec = Young Modulus of Concrete

D = Outside Diameter

Yield c = 0.4 * fc

Fc = Concrete Compression Strength

DF = Design Factor

1. Concrete Tensile Strength (10% lesser than yield strength)

Dimana:

R1 = Radius Curvature

Ec = Young Modulus of Concrete

D = Outside Diameter

Tensile = Yield – (o.1*Yield)

DF = Design Factor

R=max(R1,R2) à mana yang terbesar ini yang menjadi acuan design.

Untuk design natural bending curvature bare pipe bisa juga dengan menggunakan API RP 1117.

Untuk perhitungan besaran concrete coating (additional weight) bisa mengikuti tahap2 seperti perhitungan di pushpull calculation.

Tentu perlu juga kita tambahkan perhitungan effect dari upheaval buckling yang mungkin terjadi pada saat pipeline sudah berada di bawah dasar sungai.

Semoga bermanfaat. Selamat mengambil hikmahnya.. kalau ada saran dan masukkan tolong disampaikan untuk koreksi dan perbaikan..

Salam persahatan

Kangen juga mau nulis masalah Pipeline, sudah lama dan sedang rame2nya koalisi partai yang nyambung dan putus.. memang kalau sesuatu yang ngantung bikin orang lain tidak enak dan nyaman. Laa.. kok arahnya ke politik.

Kurang lebih satu bulan yang lalu, ada pekerjaan di Handil, Balikpapan yang menggunakan methode push pull untuk daerah rawa (swampy) kurang lebih 2 km. Daerah rawa ini cukup unik, pada saat musim hujan terlihat seperti danau besar dan kebalikkannya pada saat musim kemarau kering dan bersemak, tapi daya dukung tanah sangat rendah, dinaikin orang aja akan jeblosss.. apalagi membawa alat berat untuk mengangkut pipa sampai ke seberang.

Dapat kita bayangkan pipa berdiameter 24”, ketebalan 1” ditambah coating dan concrete weight, sehingga total berat antara 8.5 – 9 ton akan diseberangkan di daerah seperti ini. Bisa bikin pusing para site manager dan field engineer bila kurang persiapan dan tidak mempunyai methode untuk mengatasi masalah ini.

Push pull di swampy area method yang dipakai hampir sama dengan shore pull di offshore method. Kalau shore pull ditambah dengan laying analysis-nya, sedangkan pada push pull method murni tarik dan tekan sampai ke seberang.

Dengan tenaga ahli dan kecerdasan teman2 dilapangan dan perhitungan yang matang maka semua proses ini bisa dilaksanakan dengan lancar tanpa masalah, walaupun sempat terjadi ketegangan karena pipa belum mengapung juga.

Pipa disaat masih dalam kekakuan penuh, akan mengikuti kemiringan yang disediakan sampai batas tekuknya baru akan lurus dan bisa naik ke atas. Sebenarnya menurut saya pribadi, kita tidak perlu sampai batas tekuk pipa, kita bisa melakukan penambahan daya angkat pipa terhadap air di depan pipeline yang diseberangkan. Karena kalau kita menunggu tekuk pipa membutuhkan kedalaman yang lebih dan pekerjaan yang membuat kita khawatir juga.. J resiko pipa kandas di dasar swampy adalah resiko terbesar, karena akan butuh teknik tersendiri untuk mengangkat pipa tersebut.

Analisa push pull di swampy area sudah merupakan perpaduan antara field engineering dan engineering consultant, posisi saya di engineering consultant sempat diingatkan kalau ini bagian pekerjaan site. Karena pernah bergabung dan sering bantu di Contractor menjadikan saya ingat, paling tidak methode analisanya engineering consultant membantu, sedangkan real di lapangan harus dicek dan disesuaikan.

Pada pelaksanaan ini kita menggunakan drum yang ada dipasaran, sehingga mudah mendapatkan di lokasi pekerjaan.

Steps analysis of Puss pulls in swampy area.

1. Buoyancy of pipe
2. Weight of pipe with coating – water absorb
3. Total Weight = point 2 – point 1
   
4. Buoyancy of tank (drum) à disesuaikan dengan keinginan dilapangakan, apakah kita ingin drum nonggol ¼, 2/3 atau penuh.

1. Specific Gravity

Pipa dinyatakan Stabil berdasarkan [DNV RP E305 Sec.3.2.2] adalah

Specific Gravity > 1.1

Apabila SG sesuai seperti yang disyaratkan maka tidak perlu melakukkan tambahan pemberat untuk pipa atau additional weight. Apabila perlu dibuat design pemberat pipa dari concrete weight seperti specification client atau dapat diambil dari design yang sudah ada sampai pipa dinyatakan stabil.

Langkah perhitungan ini hanya untuk membantu analisa dilapangkan, sedangkan pelaksanaan dilapangan harus dilakukan perhitungan ulang. Banyak data2 dilapangkan kadang berbeda dengan data analisa di meja.

Perlu juga diperhitungkan wire rope yang mampu menarik pipa2 tersebut sampai keseberang. Kalau dihitung pipa di udara tentunya akan ketemu wire rope yang besar, karena pipa akan meluncur di air maka ini akan tereduksi significant. Pondasi tempat peluncuran pipa, design seefektip mungkin, atau kita bisa menggunakan skid yang dibuat sedemikian mungkin sehingga mampu menahan dan melancarkan peluncuran pipa selama pelaksanaan. Pelaksanaan juga membutuhkan winch sebagai penarik pipa sampai sepanjang 2 km ini, kalau kita menghitung satu pipa di udara, minimal 3 pipa sehingga total berat adalah 3 * 9 ton = 27 ton, secara kasat mata kita menggunakan winch dengan kapasitas 30 ton. (wow.. gede sekali!!). Tetapi dengan menggunakan winch yang lebih kecil 15 ton, dengan dibikin kemiringan pada skid peluncur ditambah ballast roda-roda, ditarik dengan swampy backhoe berkapasitas 7 ton, pelaksanaan ini berjalan sampai selesai dan Alhamdulillah lancar.

Itu dulu yang bisa saya sharing kan, semoga bermanfaat untuk teman-teman semua, yang butuh bantuan atau bertanya silahkan kontak via imel dan blog ini.

Tiada yang lebih indah selain berbagi, semoga amal dan ibadah kita menjadi berkah buat kita semua dan keluarga. Amin..

Road Crossing

1.1. Cover

In this design of pipeline at road crossings we have adopted a depth of cover of 1.2 meters minimum from the top of the pipe to the travelled surface of the road, in accordance with API RP 1102.

1.2. Load

1.2.1. General

The pipeline at road crossing will be subjected to both internal loads from pressurization and external loads from earth force (dead load) and highway traffic (live load). An impact factor should be applied to the live load. The tandem axles were use in this design for the live load from highway traffic.

Pipe stress calculation was design to assure that sufficient pipe strength is provided at the road crossing to anticipated design loads. In this design calculation, the AASHTO HS20-44 truck used as a design load with a safety factor of 1.25.

Total Effective Stress of the pipeline at road crossing shall be less than or equal to the Design factor, F times the pipe specified minimum yield strength.

Pipe stress calculation was performed for pressurized conditions at internal operating pressure equals to maximum allowable operating pressure (MAOP).

The pipe horizontal deflections (ovality) due to combined effect of the external loads shall not exceed 3% of pipe outside diameter.

1.2.2. External Load

1.2.2.1. Earth Load

The earth load is the force resulting from the weight of the overlying soil that is conveyed to the top of pipe.

1.2.2.2. Live Load

It is assumed that the pipeline is subjected to t

he loads from two trucks travelling in adjacent lanes, such that there are two sets of tandem or single axles in line with each other.

The crossing is assumed to be oriented at 90 degrees with respect to the highway and is an embankment-type crossing.

1.2.2.3. Internal Load

The internal load is produced by internal pressure, p, in pounds per square inch. The maximum allowable operating pressure, MAOP, or maximum operating pressure, MOP, was used in the design.

1.3. Stresses

The detailed information of stresses to be used in the design approaches can be described as below.

1.3.1. Stress Due to External Loads

External loading on the pipeline will produce both circumferential and longitudinal stresses. It is assumed that all external loads are conveyed vertically across a 90 degree arc centred on the pipe crown and resisted by a vertical reaction distributed across a 90 degree arc centred on the pipe invert.

1.3.1.1. Stress Due to Earth Load

The circumferential stress at the pipeline invert caused by earth load, S_He (psi), is determined as follows:

Where:

K_He = stiffness factor for circumferential stress from earth load.

B_e = burial factor for earth load.

E_e = excavation factor for earth load.

g = soil unit weight, in psi.

D = pipe outside diameter, in inches.

1.3.1.2. Stress Due to Live Load

Stress due to live load can be classified in two items:

Surface Live Loads

The live external highway load, w, is due to the wheel load, P_t, applied at the surface of the roadway. For design, only the load from one of the wheel sets needs to be considered. In this design calculation, the maximum wheel load from a truck’s tandem axle set was used.

The applied design surface pressure, w (psi) is determined as follows:

P = Design tandem wheel load, Pt (lbs)

A_p = The contact area over which the wheel load is applied;

A_p is taken as 144 in².

Impact Factor It is recommended that the live load be increased by an impact factor, F_i, which is a function of the depth of burial, H, of the pipeline at crossing.

Highway Cyclic Stresses

The cyclic circumferential stress due to highway vehicular load, DS_Hh (psi) can be determined as follows:

Where:

K_Hh = highway stiffness factor for cyclic circumferential stress.

G_Hh = highway geometry factor for cyclic circumferential stress.

R = highway pavement type factor.

L = highway axle configuration factor.

F_i = impact factor.

w = applied design surface pressure, in psi.

The cyclic longitudinal stress due to highway vehicular load, DS_Lh (psi) can be determined as follows:

Where:

K_Lh = highway stiffness factor for cyclic longitudinal stress.

G_Lh = highway geometry factor for cyclic longitudinal stress.

R = highway pavement type factor.

L = highway axle configuration factor.

F_i = impact factor.

w = applied design surface pressure, in psi.

1.3.2. Stress Du to Internal Load

The circumferential stress due to internal pressure, S_Hi (psi) can be determined as follows:

Where :

p = internal pressure, taken as MAOP or MOP, in psi.

D = pipe outside diameter, in inches.

t_w = wall thickness, in inches.

1.4. Limit of Calculated Stresses

1.4.1. Check for Allowable Stress

The circumferential stress due to internal pressurization, as calculated using Barlow formula, S_Hi (Barlow) in psi must be less than the factored specified minimum yield strength. This check for natural gas is given by the following:

Where :

p = internal pressure, taken as MAOP or MOP, in psi.

D = pipe outside diameter, in inches.

t_w = wall thickness, in inches.

F = design factor = 0.6, for road/river crossing and bend

E = longitudinal joint factor.

SMYS = specified minimum yield strength, in psi.

1.4.2. Check for Principal Stresses

The principal stresses S1, S2, and S3 are used to calculate S_eff. The principal stresses are calculated from the following:

Where:

S₁ = maximum circumferential stress.

DS_H = DS_Hh, in psi, for highways.

Where:

S₂ = maximum longitudinal stress.

DS_L = DS_Lh, in psi, for highways.

E_s = Young’s modulus of steel, in psi.

a_T = coefficient of thermal expansion of steel, per °F.

T₁ = temperature at time of installation, in °F.

T₂ = maximum or minimum operating temperature, in °F.

n_s = Poisson’s ratio of steel.

Where:

S₃ = maximum radial stress.

1.4.3. Total Efective Stresses

The total effective stress, S_eff (psi) can be determined as follows:

Based on API RP-1102 Steel Pipelines Crossing Railroads and Highways, the total effective stress limit can be determined as follows:

1.4.4. Check for Fatigue

The check for fatigue is accomplished by comparing a stress component normal to a weld in the pipeline against an allowable value of this stress, referred to as a fatigue endurance limit.

1.4.4.1. Girth Weld

The cyclic stress that must be checked for potential fatigue in a girth weld located beneath a highway crossing is the longitudinal stress due to live load. The design check is accomplished by assuring that the live load cyclic longitudinal stress is less than the factored fatigue endurance limit.

The fatigue endurance limit of girth welds, S_FG, is taken as 12,000 psi for all steel grades and weld types.

The general form of the design check against girth weld fatigue is given by the following:

Where :

DS_L = DS_Lh, in psi, for highways.

S_FG = fatigue endurance limit of girth weld = 12,000 psi.

F = design factor = 0.6, for road/river crossing and bend

1.4.4.2. Longitudinal Weld

The cyclic stress that must be checked for potential fatigue in a longitudinal weld located beneath a highway crossing is the circumferential stress due to live load.

The check may be accomplished by assuring that the live load cyclic circumferential stress is less than the factored fatigue endurance limit. The fatigue endurance limit of longitudinal welds, S_FL, is dependent on the type of weld and the minimum ultimate tensile strength.

The general form of the design check against longitudinal weld fatigue is as follows:

Where:

DS_H = DS_Hh, in psi, for highways.

S_FL = fatigue endurance limit of longitudinal weld, in psi.

F = design factor = 0.6 for road/river crossing and bend

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Routing Criteria

The pipeline route to be selected by taking into account the following criteria:

• The most direct route (the shortest length) from start point to end point of pipeline.

• When installed adjacent to any existing pipeline, the minimum distance between parallel pipelines may not be less than 3 m (check with Owner general specification or Local Regulation) respectively which parallel to existing crude pipeline and existing gas pipeline.

• Adequate radius of curvatures. Minimize the use of bends (hot/cold) when the natural curvature does not applicable.

• Pipeline crossings to be minimized.

• Consideration for offset required for expansion spools.

• Installation clearance and access for maintenance.

• Burial requirement to a minimum distance 1 m (check with Owner general specification or Local Regulation) to the top of pipe.

Minimum Radius Curvature)*

Where:

E = Young Modulus of Steel

D = Pipe Outer Diameter

SMYS = Specified Minimum Yield Strength

DF = Design Factor

)* Offshore Pipeline Design Analysis and Method, A.H. Mousselli