subsea pipeline tie-in

sumber: offshore magazine

video: www.youtube.com/watch?v=E4q3c5xBYGc

Technip was recently contracted by Burullus Gas Co. (Burullus) to tie in an expansion to its existing subsea West Delta Deep Marine (WDDM) facilities. To accomplish this, it was necessary to tie in a new 36-in. gas trunkline pipeline, which is part of the Phase VII project, to the existing system under pressure without shutting down production. To perform the tie-in, Technip retained T.D. Williamson S.A. (TDW) to carry out three subsea hot tap intervention operations.

Two traditional 16-in. hot tap operations would be completed on a 26-in. pipeline, and one innovative 20-in. hot tap on a 36-in. pipeline. To ensure that the hot tap interventions would be successful, it was necessary to engineer, install and pre-commission two hot tap assemblies, including one capable of cutting through a blind weld-neck "tappable flange" made of duplex stainless steel on the 36-in. line.

Hot tap machine

In preparation for the operation that would take place in depths to 95 m (311 ft), TDW worked with a Belgium-based engineering and construction specialist to produce the special hot tap tool known as a "cutter." This special tool would be used for the 20-in. hot tap and would need the ability to effectively cut the duplex plate. Since the duplex has a very high mechanical strength – meaning that it has a high elongation before reaching breaking point and a high level of hardenability – the cutting process employed must be very rigid and vibration-free while using the TDW Model 936D subsea tapping machine.

TDW's customized subsea tapping machine onboard the dive support vessel Wellservicer.

Working at TDW's facility in Nivelles, Belgium, a series of engineering, design and preliminary tests was performed. The first step involved engineering several alternative designs. The first alternative consisted of using either a proven cutter design; or that proven design updated with various teeth geometry. However, this option was not pursued because it could not penetrate the duplex stainless steel.

TDW's customized subsea tapping machines onboard the dive support vessel Wellservicer prior to the operation.

A second option involving removable teeth and welded teeth support was considered, but this was rejected due to its inability to resist vibration.

Ultimately, the design selected for fabrication featured a subsea electro-pump to supply adequate hydraulic power, a pilot drill with bronze plates to reduce vibration, and a specially manufactured set of cutters with removable cutting teeth that would be able to penetrate duplex stainless steel without breaking.

Preliminary trials

After the design was finalized, materials were procured and the prototype was fabricated and made ready for the first phase of testing: the internal preliminary trials.

A diver prepares for the vertical hot tapping operation.

During a period of eight weeks, the prototype was subjected to rigorous testing associated with a number of capabilities. The cutter's ability to make deep cuts on a plate of the same type of duplex stainless steel as the blind weld-neck "tappable flange" on the pipeline was an initial challenge.

A diver carries out the horizontal hot tapping operation.

The team made material and design improvements, ultimately achieving a prototype that could produce a smooth and satisfactory cutline. In addition, special bronze guides were developed and installed on the pilot drill to control vibration.

By the end of the four-month trial period, several renditions of the prototype had been used to complete four tapping operations. Before and after each cutting trial, visual and nondestructive examinations (NDE) of the cutters and pilot drills were carried out. The final prototype, which featured dual sets of cutting teeth and the pilot drill with the bronze guides, performed well. As the hot tap machine would be required to operate at an average pressure of 100 bar, pressure tests were undertaken to satisfy the requirements of the forthcoming factory acceptance test. The decision was then made to proceed to the second stage: the official trials.

Official trials

During the official trials three tapping operations were carried out with the custom machine. Two hot taps were completed on duplex plate, and one cold tap was executed through equal duplex tappable plate. These tapping operations revealed that the equipment endured the rigorous process, remained properly aligned and cut the duplex steel plate effectively. These operations took place as part of a requisite system integration test (SIT), which confirmed the following:

• The teeth accurately cut the duplex stainless steel
• The pilot drill remained rigid and vibration-free
• The design of the cutter was improved by adjusting the teeth support.

It also proved that the tapping machine could be unset in the middle of the cut and reset while reaching the cut back without causing the tapping machine to be misaligned or moved out of proper position.

With the official trials of the custom hot tap cutter successfully completed, the system received approvals from Burullus, Technip, and the Burullus Independent Verification Authority to perform the subsea operation well in advance of the project mobilization. In preparation for the impending operation, two hot tap machines were produced in order to guarantee 100% back-up of this critical piece of equipment.

Maintaining gas pressure

For three weeks, TDW worked from Technip's dive support vessel Wellservicer to carry out all three hot taps. Throughout the process, a prevailing pressure of 100 bar (1,450 psi) was successfully maintained in the existing gas export system. The innovative hot tap on the duplex tappable flange required just six days to complete.

In spite of the fact that the hot tap intervention was carried out subsea, making it more complicated to mobilize and install equipment than when working onshore, the operation was carried out by skilled divers as intended, with no lost time incidents or production downtime.

Much of the success was attributed to the investment in planning and pre-operational equipment trials and testing. TDW worked with the Technip and Burullus teams to ensure that the operation would proceed like clock-work, and that the customized cutting tool would operate effectively on duplex stainless steel. As a result, the operation provided three tie in points, preparing the way for Technip to successfully tie-in the new 36-in. gas trunkline for the WDDM Phase VII development.

The author

Michel Courbat is offshore project manager for T.D. Williamson S.A.

Pipeline Material and grade selection

Pemilihan material menggunakan logam ( metal ) sudah mulai diterapkan secara umum sejak tahun 1950-an berdasarkan standar API  Code 5L tentang pemilihan material pipa. Pada akhir 1980-an berdasar kode API pula, sudah ada beberapa macam tipe material pipa, yaitu A25, A, B, X42, X46, X52, X56, X60, X64, X70 dan X80. Setiap tipe material mempunyai karakteristik zat dan material penyusun masing-masing. Spesifikasi material baja yang digunakan tergantung pada komposisi kimiawi, kekuatan material, dan toleransi pipa dalam industri dan manufaktur.

Beberapa material harus ditentukan untuk mendapatkan material pipa yang tepat sesuai kebutuhan sistem perpipaan. Kriteria – kriteria dibawah ini dapat digunakan dalam pemilihan material untuk pipa :

• Mechanical properties, termasuk ketahanan untuk menahan static load, dynamic load, dan elastisitas dalam proses manufaktur
• Weld ability, kemudahan dan kekuatan material pipa dalam proses pengelasan.
• Corrotion resistance, kemampuan material dalam menahan korosi.
• Cost, berhubungan dengan biaya yang harus dikeluarkan per satuan ukuran material.
• Availability, terkait dengan ketersediaan dan suplai material pada pasaran, sebagai pertimbangan untuk volume cadangan dan biaya

Material yang yang sering digunakan dalam dunia migas, industri, dan manufaktur terdiri dari dua, yaitu :

• Carbon Steel

Material pipa jenis ini adalah yang paling banyak digunakan, spesifikasinya banyak ditemukan dalam ASTM ( American Society of Testing and Materials ) dan ASME ( American Society of Mechanical Engineering ).

Ada 3 jenis pipa material ini yang paling sering digunakan :

1. ASTM A106. Terbagi dalam 3 grade, tergantung Tensile Strengh nya; Grade A ( 48 ksi ), Grade B ( 60 ksi ), dan Grade C ( 70 ksi ).
2. ASTM A53. Material pipa ini yang biasanya dilapisi oleh zinc ( galvanized ), yang biasanya merupakan alternatif dari ASTM A106. Material ini juga terbagi dalam 3 Grade, A, B dan C, dan memilik 3 tipe; Tipe E ( Electrical Resistance Weld ), Tipe F ( Furnace Butt Weld ), dan Tipe S ( Seamless ). Grade A dan B pada ASTM 106 memiliki Tensile Strength yang sama dengan Grade A dan B pada ASTM A53.
3. ASTM A333.  Material ini biasa digunakan pada fluida yang memiliki temperatur rendah, mulai dari -50 derajat Fahrenheit.

• Stainless Steel

Material pipa ini dinamakan austenitic stainless steel. Namun secara umum biasanya disebut stainless steel. Stainless steel mempunyai grade 108, tetapi yang biasa digunakan adalah tipe 304L. Sesuai kode L dibelakang nama 304L, tipe ini mengandung cukup sedikit campuran karbon daripada tipe 304, tetapi memiliki kekuatan yang tinggi dan ketahanan terhadap korosi yang cukup baik.

Pada dunia industri yang sebenarnya, ada dua jenis pipa stainless steel yang paling sering dipakai, yaitu:

1. ASTM A312, untuk pipa berukuran dibawah 8 inci.
2. ASTM A358, untuk pipa berukuran diatas 8 inci.

Selain 2 tipe material diatas (  Carbon Steel dan Stainless Steel ), masih banyak lagi material yang dipakai dalam dunia perpipaan, walaupun jarang digunakan, yaitu :

• Chrome-Moly Pipe ( Chromium-Molybdenum Alloy Pipe ), yang terdiri dari 10 grade, merujuk pada kode ASTM A335.
• Nickel and Nickel Alloy Pipe, contoh penggunaan secara luas adalah Inconel, Incoloy dan Monel.
• Piping Cast Iron ( pipa besi )
• Copper Piping ( pipa tembaga )
• Plastic Pipe ( pipa plastik )
• Concrete Pipe ( pipa beton ).

Sumber :

http://pipinganalysis.blogspot.com/2011/07/pipe-material-selection.html

http://pipinganalysis.blogspot.com/2011/07/frequently-used-pipe-material.html

http://pipinganalysis.blogspot.com/2011/07/pipe-material.html

http://oceanovolution.ocean.itb.ac.id/?p=252

Piping Stress Analysis

Piping Stress analysis adalah suatu cara perhitungan tegangan (stress) pada pipa yang diakibatkan oleh beban statis dan beban dinamis yang merupakan efek resultan dari gaya gravitasi, perubahaan temperature, tekanan di dalam dan di luar pipa, perubahan jumlah debit fluida yang mengalir di dalam pipa dan pengaruh gaya seismic. Process piping dan power piping adalah contoh system perpipaan yang membutuhkan analisa perhitungan piping stressnya yang dilakukan tentunya oleh pipe stress engineer untuk memastikan rute pipa, beban pada nozzle, dan tumpuan pipa telah dipilih dan diletakkan tepat pada tempatnya sehingga tegangan (stress) yang terjadi tidak melebihi limitasi besaran maksimal tegangan yang diatur oleh ASME atau peraturan lainnya (codes/standard) dan peraturan pemerintah (government regulations). Untuk melakukan sebuah pipe stress analysis biasanya para piping engineer memakai pendekatan finite element method dengan memakai beberapa software umum di dunia perpipaan yaitu CAESAR II, AutoPipe, ROHR2 atau CAEPIPE.

Tujuan utama dari piping stress analysis adalah untuk memastikan beberapa hal berikut:

• Keselamatan sistem perpipaan termasuk semua komponennya
• Keselamatan sistem peralatan yang berhubungan lansung dengan sistem perpipaan dan struktur bangunan pendukung sistem tersebut
• Defleksi pipa agar tdak melebihi limitasinya.

Ada beberapa macam mode kegagalan yang bisa terjadi pada suatu sistem perpipaan. Para piping engineer bisa melakukan tindakan pencegahan untuk melawan mode kegagalan tersebut dengan melaksanakan stress analysis berdasarkan ketentuan dan aturan dalam dunia perpipaan. Dua macam mode kegagalan yang biasa terjadi pada pipa adalah sebagai berikut:

• Kegagalan karena tegangan yield (material melebihi deformasi plastis):
• Kegalalan karena fracture (material patah/fails sebelum sampai batas tegangan yieldnya):

o    Brittle Fracture: Terjadi pada material yang getas (mudah pecah/patah)

o    Fatigue (kelelahan): Disebabkan oleh adanya beban yang berulang

Teori maximum principal stress adalah yang digunakan dalam ASME B31.3 sebagai dasar teori untuk analisa pipa. Nilai maksimum atau minimum dari normal stress bisa disebut sebagai principal stress. Selanjutnya tegangan (stress) dapat dikelompokkan menjadi 3 kategori yaitu:

• Primary Stresses

Terjadi karena respon dari pembebaban (statis dan dinamis) untuk memenuhi persamaan antara gaya keluar dan gaya ke dalam, serta gaya momen dari sebuah sistem pipa. Primary stresses are not self-limiting.

• Secondary Stresses

Terjadi karena perubahan displacement dari struktur yang terjadi karena thermal expansion dan atau karena perpindahan posisi tumpuan. Secondary stresses are self-limiting.

• Peak Stresses

Tidak seperti kondisi pembebanan pada secondary stress yang menyebabkan distorsi, peak stresses tidak menyebabkan distorsi yang signifikan. Peak stresses adalah tegangan tertinggi yang bisa menyebabkan terjadinya kegagalan kelelahan (fatigue failure).

Static Stress Analysis

Setiap sistem perpipaan pasti mempunyai basic stress yang nantinya secara kumulatif bisa disebut sebagai static stress. Basic stress terdiri dari:

(a) Axial Stress : σ = F /A

(b) Bending Stress : σ = Mb / Z

(c) Torsion Stress : σ = Mt / 2Z

(d) Hoop Stress : σ = PD / 2t

(e) Longitudinal Stress : σ = PD / 4t

(f) Thermal Stress : σ = ΔT x α x E

Static stress analysis adalah sebuah analisa perhitungan pada pipa untuk memastikan nilai dari semua tegangan (stress) akibat beban statis tidak melebihi dari limitasi yang diatur oleh aturan atau standard tertentu. Biasanya, pada piping engineer menggunakan aturan (standard) yaitu ASME B31.3 sebagai panduan untuk melakukan dan menganalisa static stress. ASME B31.3 mengatur semua masalah perpipaan mulai dari limitasi propertis yang dibutuhkan, sampai pada pembebanan yang memperhitungkan kondisi pressure, berat struktur dan komponennya, gaya impact, gaya angin, gaya gempa bumi secara horizontal, getaran (vibrasi), thermal expansion, perubahan suhu serta perpindahan posisi tumpuan anchor.

ASME B31.3 mengklasifikasi beban menjadi 2 macam:

• Primary Loads

o    Sustain Loads

Beban yang muncul terus menerus dan berkesinambungan selama masa operasi dari sistem perpipaan. Contoh: gaya berat dari struktur pipa sendiri, pressure fluida yang mengalir di dalamnya.

o    Occasional Loads

Beban yang muncul tidak berkesinambungan, atau munculnya tiba-tiba selama masa operasi dari sistem perpipaan. Contoh: gaya angin, gaya gempa bumi.

• Expansion Loads

o    Beban yang muncul karena adanya perubahan displacement dari system perpipaan yang bisa diakibatkan oleh thermal expansion dan perubahan letak tumpuan.

Sedangkan dalam ASME B31.3 limitasi dari masing-masing besaran pembebanan adalah sbb:

·         Stress karena Sustained Load, limitasinya adalah:

SL < Sh

Dimana:

SL = (PD/4t) + Sb

Ketebalan dari pipa yang digunakan untuk menghitung SL haruslah merupakan tebal nominal setelah dikurangi tebal lapisan korosi dan erosi yang diijinkan.

Sh = Tegangan yang diijinkan pada suhu maksimum dari suatu material

·         Stresses karena Occasional Loads

Jumlah beban longitudinal karena pressure, weight dan sustain loads lainnya kemudian ditambah oleh tegangan yang diakibatkan occasional load seperti gempa bumi dan gaya angin, nilainya tidak boleh melebihi 1.33Sh.

·          
Stresses karena Expansion Loads, limitasinya adalah:

S_E < S_A

Dimana:

S_E   = (S_b² + 4S_t²)^1/2

S_A   = Allowable displacement stress range = f [(1.25(Sc + Sh) – SL]

S_b   = resultant bending stress,psi = [(IiMi)² + (IoMo)²] / Z

Mi = in-plane bending moment, in.lb

Mo             = out-plane bending moment, in.lb

Ii    = in-plane stress intensification factor (appendix B31.3)

Io   = out-plane stress intensification factor (appendix B31.3)

St   = Torsional stress ,psi = Mt / (2Z)

Mt = Torsional moment, in.lb

SC             = Basic allowable stress at minimum metal temperature

Sh = Basic allowable stress at maximum metal temperature

f     = stress range reduction factor (table 302.2.5 of B31.3)

Dynamic Stress Analysis

Dynamic stress (tegangan dinamis) adalah tegangan (stress) yang ditimbulkan oleh pergerakan berulang dari pembebanan atau vibrasi (getaran). Pembebanan seperti ini bisa ditimbulkan oleh beberapa eksitasi seperti:

• Flow Induced Turbulence
• High Frequency Acoustic Excitation
• Mechanical Excitation
• Pulsation

Analisa Vibrasi dapat didefinisikan sebagai studi dari pergerakan osilasi, dengan tujuan mengetahui efek dari vibrasi dalam hubungannya dengan performance dan keamanan sebuah sistem dan bagaimana mengontrolnya. Vibrasi secara sederhana dapat dilihat dari gambar 3. Seperti terlihat pada gambar 3, ketika massa kita tarik ke bawah lalu dilepaskan, maka pegas akan meregang dan selanjutnya akan timbul gerakan osilasi sampai periode waktu tertentu. Hasil frekuensi dari gerakan osilasi ini bisa disebut sebagai natural frekuency dari sistem tersebut dan merupakan fungsi dari massa dan kekakuan.

dengan

EI        = kekakuan pipa (stiffness), lbs-ft²

L          = panjang bentangan bebas pipa, ft

M         = kombinasi massa pipa dan massa tambah disekitar pipa persatuan panjang, slug/ft

C         = konstanta yang tergantung dari kondisi ujung bentangan bebas pipa.

Sebagai contoh, jika kedua ujung bentangan bebas pipa diasumsikan berbentuk tumpuan sederhana maka C adalah p/2 atau 1.57. Jika kedua ujung pipa diasumsikan diklem, C adalah 3.5. Dalam praktek, cukup sulit untuk menentukan modeling terbaik kondisi ujung bentangan bebas untuk mensimulasikan kondisi ujung yang diasumsikan.

Dibutuhkan sedikit energi untuk menimbulkan frekuensi natural dari sebuah system, seperti halnya sebuah struktur yang ingin merespon frekuensi tertentu. Jika ada damping force maka ini akan menghilangkan energi dinamis dan mengurangi respon vibrasi.

Hasil dari vibrasi dapat berupa:

• Displacement
• Velocity
• Acceleration

Displacement tergantung dari frekuensi yang manadisplacement akan mempunyai nilai yang besar apada frekuensi yang kecildan sebaliknya jika frekuensi besar,displacement cenderung kecil padasatuan energi yang sama. Sebaliknya acceleration dipengaruhi padakeadaan amplitude tertinggi yang terjadi pada frekuensi tertinggi pula.Velocitymemberikan pengaruh sejenis yang lebih dari yang dibutuhkan, biasanya terkaithasil tegangan dinamis dan oleh karenanya biasa digunakan alat ukur untukmenghitung vibrasi. Ini yang menjadi alasan kenapa observasi secara visualuntuk vibrasi pipa tidak diijinkan sebagai metode untuk mengatasi beberapamasalah vibrasi.

Setiapsistem struktur, contohnya pipa, akan mengalami bermacam-macam frekuensinatural tergantung distribusi massa dan kekakuan dari system tersebut.Distribusi massa dan kekauan dipengaruhi oleh diameter pipa, materialproperties, tebal pipa, lokasi valve dan support, dan juga massa jenis fluida.Sebagai catatan, support pipa didesain pada kondisi statis yang pastinya akanberperilaku beda pada keadan dinamis.

Setiapfrekuensi natural akan mempunyai bentuk defleksi yang unik yang sesuai denganfrekuensinya masing-masing, biasa disebut mode shape. Respon pipaterhadap eksitasi yang terjadi tergantung pada hubungan antara frekuensieksitasi dengan frekuensi natural sistem tersebut, dan lokasi dari terjadinyaeksitasi tadi berhubungan dengan mode shape.

Salahsatu penyebab vibrasi pada pipa adalah flow dari fluida di dalam pipa itusendiri. Fenomena ini biasa dikenal dengan istilahFlow Induced Vibration(FIV). FIV bisa disebabkan karena peningkatan flowrate (debit) fluidayang menyebabkan kecepatan fluida di dalam pipa bertambah sehingga jenis aliranberubah dari laminar menjadi turbulen. Aliran turbulen ini yang menyebabkanpipa bergetar (vibrasi).

http://vladvamphire.wordpress.com/2008/12/27/sebuah-pengantar-piping-stress-analysis/

Pipeline Routing

The pipeline route selection process focuses on achieving the optimal location for a pipeline.  Even though this is the desired process outcome, in reality it is counter to data and information collection efforts that are concentrated on identifying and inventorying locations for pipeline avoidance.

Changing and aligning the routing mindset from output (pipeline route) to input (discipline information) is what truly allows for optimization of route selection, reduction in costs and risks, improvement in decision consensus, and reuse of valuable information throughout the pipeline lifecycle. Through discussion of an enhanced pipeline route selection process, this paper will cover a range of benefits including establishing data collection corridor widths, dynamic routing

considerations, and methodologies for managing and using multi-purpose and multi-discipline information.

INTRODUCTION

Discussions, documentation, and development of the uses and merits of the least cost path analysis in decision support are readily available (Husdal 2001) within the analytic Geographic Information System (GIS) community. The applicability of this analytical method to the pipeline routing process is somewhat well known and there are cases where this analysis proves to save on overall pipeline construction costs by nearly 30% (Delavar 2003). Yet there remains an obvious reluctance to adopt these techniques as commonplace within the overall pipeline routing process even with such incredible cost savings and cost avoidance opportunities.

TRADITIONAL ROUTE SELECTION PROCESS RELUCTANCE

The traditional pipeline route selection process typically begins with “Point A to Point B” plan. An owner company gathers high-level data, almost overview information, and the feasibility of progressing the plan further is rapidly determined. If the plan is progressed, then the plan becomes a project, contracting companies (Engineering and Environmental) become involved, and the pipeline route becomes the pivot around which personnel begin honing data, information, opinion, and decisions.

In early stages of the route selection process a battle rages between those deeply rooted in fieldbased approaches (traditional) and those who strongly believe in a desktop approaches (partially traditional and partially enhanced). Most often, the field-based approaches win out over the desktop approaches not based on technical merits but on rather on “project panic”. “Project panic” is an intense fear experienced by personnel involved with projects stemming from process change or any movement away from the status quo. The intensity is due to the pace of the project and the fear manifests itself in a myriad of phrases similar to “it will affect the project schedule and/or budget”.

Heavy reliance on the way things were done (successfully?) in the past is what has caused companies to avoid process or technology enhancements. Even where technologies such as GIS have made it into the routing process it is not utilized to its full potential, for example, use of GIS to make prettier maps is not much higher on the value chain, although it may save some costs, than use of CAD or manual drafting to accomplish the same goal. Another example of technology underutilized occurs when GIS is used to make paper maps for disciplines to mark-up, measure against, take-off quantities, perform analysis, or any other manual task that is actually repeatable within a GIS environment.

TRANSITIONING TO AN ENHANCED PROCESS

Personnel and software are key parts of any geographic information system and the critical components of an enhanced pipeline route selection process. The software or technology component of this enhanced process is mature; in fact, little has changed with least-cost-path algorithms in recent times. So what is the holdback on adopting least-cost-path analysis as a fundamental portion of the route selection process?

In a non-technology sense, one reason for avoiding a least-cost-path approach is that there has been little to drive the contracting community to improve upon their processes – the old way remains sufficient. Only with a strong push from owner companies will there exist enough momentum to invoke process change. When owners set and enforce expectations, contracting companies (and the GIS community in general) will begin to explore the technology and educate themselves.

Owner and contracting companies are beginning to realize this very fact and some are taking the correct steps towards technology edification. Others remain, however, with only a general understanding or genuine lack of understanding of the technology or choose to stay with what is familiar especially in project situations.

Changing and aligning the routing mindset from output (pipeline route) to input (discipline information) is what truly allows for optimization of route selection, reduction in costs and risks, improvement in decision consensus, and reuse of valuable information throughout the pipeline lifecycle. Enhancing the route selection process will only come from a concerted effort on behalf of personnel to understand the technology.

CAPABILITIES CREATION THROUGH ENHANCED PROCESS

Data Collection Corridor

One the most important aspect of the route selection process is proper identification and inventory of impassible or possible problematic places. A vast majority of sites requiring identification is available from public domain data sources; however, to fill any potential data gaps and to improve data quality the project will undertake a field data collection program.

This approach certainly has its purpose but initial data collection programs performed in early stages of a project can be done in such haste that many crucial items may be overlooked, minor items like the data collection corridor width. Regardless of the data collection corridor width, if not set properly there can be additional costs incurred by the project. A collection width too narrow means revisiting the corridor whereas a corridor set too wide becomes logistically unwieldy and involves many personnel and many chargeable hours.

Using an enhanced route selection process does not eliminate the need for field data collection but it is able to help curb the amount of fieldwork necessary. The least-cost-path analysis can effectively establish an optimal data collection corridor. Defining the corridor with more rigor than just stating “1 mile either side of the alignment” is a different approach but has its advantages in its ability to focus efforts on realistic routing options. Without sensible identification of these options prior to a field program wastes financial, personnel, and time resources. Careful planning of collection corridors allows for better project controls.

In the simplest context of a least-cost-path analysis, a data collection corridor begins with two cost accumulation surfaces: one surface running from “Point A to Point B” and the other from “Point B to Point A”. Adding these two surfaces together generates a new surface that indicates the total cost of positioning a pipeline alignment though any given location. The values generated in the output surface are most useful for establishing data collection corridors when the values at or near the minimum of the surface are extracted.

For example, Figure 1 was generated by first constraining movement between “Point A and Point B” to occur only where the longitudinal slope is less than seven percent. The two output surfaces (cost accumulation surface from “Point A to Point B” and vice versa) were then added together and the minimum value of this grid determined. To derive the data collection corridor shown, the minimum value (representing a least cost) was increased slightly by a factor and then used to extract the potential data collect corridor shown. One key observation is that the data collection corridor is no longer a constant value but instead is quite variable. This variability in width can be interpreted as “collecting more information where there are more routing options”.

Figure 1 Data Collection Corridor

Dynamic Routing Variables

In pipeline applications, the amount of terrain undulation along the pipeline is a measure of several costs throughout the pipeline lifecycle; costs that are evidenced in terms of “cut and fill” operations, the type of equipment usable in clearing and construction, potential geotechnical issues, or site remediation. Remember, the aim of enhancing the routing process is to reduce the overall costs of the pipeline and not just the front-end phases (i.e. field data collection).

In a traditional routing process, the longitudinal slope, or slope along the pipeline alignment, is not derivable until a preliminary alignment exists. The reason for this is that longitudinal slope is measurable only in a frame of reference that is relative to the pipeline alignment. Once the longitudinal slope is determined, segments of the pipeline alignment with slopes exceeding a predetermined threshold undergo an assessment and then tweaks made to the pipeline alignment. The longitudinal slope is then derived again and the iterative process continues until

the alignment meets the project requirements.

In the enhanced routing process, the predetermined threshold for longitudinal slope becomes part of the routing criteria and is used to establish (not post-appraise) the alignment location. This is accomplished by setting up a least-cost-path type analysis that is capable of calculating costs over an elevation surface while restricting movement over the surface to areas under the slope criteria (Tomlin 1990). Important to note that the slope criteria is not evaluated prior to the least-cost-path analysis but rather it is calculated during the analysis thus making the slope a dynamic property of the alignment location.

Multi-Use and Multi-Discipline

The pipeline routing process is a multi-discipline process involving such disciplines as engineering, environmental, regulatory, commercial, and more. Each discipline has its own needs in terms of data inputs and, as is often the case, this results in data sets having multiple uses. Data management is as much (if not more) a part of an enhanced routing process as the software required to run a least-cost-path analysis.

In working across multiple disciplines, it is quite important to establish boundaries with respect to data ownership and data usage constraints. The boundaries help in managing change across the disciplines especially in the case of multi-use data. As an example of multi-use data, a digital elevation model suitable for pipeline hydraulics will likely not have an adequate extent for air and/or noise modeling. Similarly, a digital elevation model suitable for air and/or noise modeling will not likely have adequate resolution for pipeline hydraulics.

In this case, what discipline owns the digital elevation model? There is no correct answer just a

business decision to be made by the owner company with input from the disciplines. The point of setting and enforcing these controls is for the project to benefit from reduced data expenditures through better data management and data acquisition approaches.

Another awareness issue in working in a multi-discipline environment is that there are now more personnel available “touching” the same datasets. This can positively affect data quality since more personnel using data will lead to more opportunities to identify any potential data issues or gaps. In addition, integration of datasets across disciplines presents itself as another means of data validation.

SUMMARY

The software or technology behind the least-cost-path analysis routing is very mature yet it remains underutilized within the pipeline routing industry. People and software are key parts of any geographic information system and are just as important to a successful enhanced routing process.

Enhancing the route selection process will only come about from efforts of personnel to enhance their understanding of GIS fundamentals. The fundamentals are not that difficult but without self-edification then the potential process improvements and possible cost savings will remain unrealized.

REFERENCES

Delavar, M.R., 2003, Pipeline Routing Using Geospatial Information System Analysis, ScanGIS’2003 – The 9th Scandinavian Research Conference on Geographical Information Science, 4-6 June 2003, Espoo, Finland – Proceedings, pp. 203-213

Husdal, J., 2001, Corridor Analysis – A Timeline of Evolutionary Development http://www.husdal.com/gis/corridor.htm

Tomlin, C.D., 1990, Geographic Information Systems and Cartographic Modeling, pp. 97-153

Sumber :

http://www.gisdevelopment.net/proceedings/gita/oil_gas2004/papers/09gos2004.pd