The Oceans In My Mind

Wednesday, 17 February 2016

Bagaimana Apabila Pipeline Melalui Permukaan yang Tidak Rata?

Ilustrasi analisis kerataan lereng

Pada saat pemilihan jalur untuk instalasi pipeline, tak jarang suatu pipeline terpaksa harus melewati suatu jalur / permukaan yang tidak rata. Pada kondisi tersebut, dibutuhkan analisis untuk menentukan apakah kondisi tersebut masih dapat ditoleransi dalam aplikasinya, yang dinamakan analisis bottom roughness.

Analisis bottom roughness adalah analisis yang diperlukan untuk mensimulasikan kondisi pipeline pada seabed apabila terdapat free span atau titik sepanjang pipa yang beresiko mengalami stress/tegangan yang tinggi dapat diketahui sejak awal sehingga dapat direncanakan perbaikan kondisi seabed sebelum pipa dipasang (pre-lay correction) atau pun setelah pipa dipasang (post lay correction). Analisis bottom roughness sangat penting karena tanpa analisis ini, jumlah support untuk pipeline seperti sand bag, grout bag, dan lain-lain, tidak dapat ditentukan.

Input yang dimasukkan dalam analisis ini adalah sebagai berikut.

Pipeline configuration.
Seabed roughness (profile).
Pipeline handling alignment.

Output yang dihasilkan dalam analisis ini adalah sebagai berikut.

Pipeline stress and stress response. Merupakan respon tegangan dan regangan yang diterima pada pipeline.
Span length and height. Merupakan panjang span yang terjadi, dan ketinggiannya diukur dari seabed dalam bentuk angka dan pipeline profiling.
Pre-lay and post-lay intervention assessment. Merupakan bahan penunjang saat akan melakukan pre-lay ataupun post-lay, karena berisi hasil analisis letak span yang dapat menjadi panduan bagi lokasi perbaikan.

Load cases yang diujikan dalam analisis ini adalah sebagai berikut.

As-laid.
Flooded.
Hydrotest.
Operations.

Berikut contoh output berupa pipeline profiling dengan berbagai load cases.

output bottom roughness.png — Contoh Pipeline Profiles Hasil Analisis dalam Beberapa Load Case (sumber:http://www.dyurindatama.files.wordpress.com)

Bagaimana Memilih Grade dan Material untuk Pipeline?

Pengecekan Material Pipa

(diambil dari www.youtube.com)

Before the installation of a pipeline system, it should be concerned if the pipeline material is compatible with the given environment and operating condition. Is the material alloy available in the size and thickness required? Is it the most economical choice? Will it withstand the working temperature? Pipeline material alloy used for one particular place may not be the same in other place, due to difference of the environment. For instance, material used for in-land pipeline will differ to offshore pipeline. At offshore, the circumstances is more corrosive than in-land.

Material selection shall be optimized, considering investment and operational costs, that are minimized while providing acceptable safety and durability. There are few criteria that should be considered in selecting the right material, such as:

The availability of the material required
Installation procedure
Operating condition (seawater: corrosive)
External and internal environment
Design life
Inspection and maintenance
The ability of the material to be treated

There are also reference that can be used in choosing offshore pipeline material, like:

API 5L – Specification for Line Pipe
API RP 17B – Recommended practice for flexible pipe
ASME B31.3 – Chemical plant and petroleum refinery piping
ASTM D 2992 – Practice for obtaining hydrostatic or pressure design basis for fiberglass pipe and fittings
DnV RP B201 – Metallic materials in drilling, production, and process system

SOURCE: HTTPS://NONERIESKA.WORDPRESS.COM/2013/01/30/PIPELINE-MATERIAL-AND-GRADE-SELECTION/

Apa yang terjadi ketika suatu free span ter mitigasi?

During pipeline routing evaluation, consideration has to be given to the shortest pipeline length, environment conservation, and smooth sea bottom to avoid excessive free spanning of the pipeline. If the free span cannot be avoided due to rough sea bottom topography, the excessive free span length must be corrected.

Free spanning causes problems in both static and dynamic aspects. If the free span length is too long, the pipe will be over-stressed by the weight of the pipe plus its contents. The drag force due to near-bottom current also contributes to the static load. To mitigate the static span problem, mid-span supports, such as mechanical legs or sand-cement bags/mattresses, can be used.

Free spans are also subject to dynamic motions induced by current, which is referred to as a vortex induced vibration (VIV). The vibration starts when the vortex shedding frequency is close to the natural frequency of the pipe span. As the pipe natural frequency is increased, by reducing the span length, the VIV will be diminished and eliminated. Adding VIV suppression devices, such as strakes or hydrofoils, can also prevent the pipe from vibrating under certain conditions. The VIV is an issue even in the deepwater field since there exists severe near-bottom loop currents.

To prevent static and dynamic spanning problems, a number of offshore pipeline spanning mitigation methods in Table 3 have been identified. Based on soil conditions, water depth, and span

height from the seabed, the appropriate method should be selected. If the span off-bottom height is relatively low, say less than 1 m (3 ft), sand-cement bags or mattresses are recommended. If the span off-bottom height is greater than 1 m (3 ft), clamp-on supports with telescoping legs or auger screw legs are more practical. Graphical illustrations of each method are shown in below.

SOURCE: HTTP://WWW.JYLPIPELINE.COM/UKC2002.PDF,HTTPS://ARIFKL.WORDPRESS.COM/2013/02/03/FREE-SPAN-MITIGATION/

Apa yang Dimaksud Vortex Induced Vibration (VIV)?

Ilustrasi VIV

(diambil dari adietrarizky.wordpress.com)

Vortex-induced vibration is a major cause of fatigue failure in submarine oil and gas pipelines and steel catenary risers. Even moderate currents can induce vortex shedding. Pipelines from offshore petroleum fields must frequently pass over areas with uneven seafloor. One of the serious problems for the structural safety of pipelines is uneven areas in the seafloor as they enhance the formation of free spans. Route selection, therefore, plays an important part in design, Matteelli (1982). However, due to many obstacles it is difficult to find a totally obstruction free route. In such cases the pipeline may have free spans when crossing depressions. Hence, if dynamic loads can occur, the free span may oscillate and time varying stresses may give unacceptable fatigue damage. A major source for dynamic stresses in free span pipelines is vortex induced vibrations (VIV) caused by steady current. This effect is in fact dominating on deep water pipelines since wave induced velocities and accelerations will decay with increasing water depth. The challenge for the industry is then to verify that such spans can sustain the influence from the environment throughout the lifetime of the pipeline.

The aim of the present project is to improve the understanding of vortex induced vibrations (VIV) of free span pipelines, and thereby improve methods, existing computer programs and guidelines needed for design verification. This will result in more cost effective and reliable offshore pipelines when laid on a very rugged seafloor.The Ormen Lange field in the Norwegian Sea is one of the examples where the pipeline will have a large number of long spans even for the best possible route (see Figure 1). It was decided to evaluate two different strategies for field development; one based on offshore loading and the other on a pipeline to an onshore gas terminal. A key problem for the last alternative is that the seafloor between these fields and the coast is extremely rugged meaning that a pipeline must have more and longer free spans than what is seen for conventional pipelines. Today’s knowledge and guidelines are inadequate for obtaining a cost effective and reliable pipeline under these conditions, Det Norske Veritas (1998). Significant uncertainties are related to the assessment of fatigue from vortex induced vibrations caused by ocean currents. An extensive research program has therefore been initiated. The aim has been to improve the understanding of VIV for free span pipelines and thereby identify potential unnecessary conservatism in existing guidelines. Some changes have been proposed by Det Norske Veritas (2002), but improved analysis models have not been developed so far.

Two alternative strategies for calculation of VIV are seen today. Practical engineering is still based on empirical models, while use of computational fluid dynamics (CFD) is considered immature mainly because of the needed computing resources. Most empirical models are based on frequency domain dynamic solutions and linear structural models Larsen (2000), but the free span pipeline case has indeed important nonlinearities that should be taken into consideration. Both tension variation and pipe-seafloor interaction will contribute to non-linear behavior, which means that most empirical models will have significant limitations when dealing with the free span case. CFD models may certainly be linked to a non-linear structural model, but the needed computing time will become overwhelming. Then, one of the main focuses of the present research is investigation about time domain model for analysis of vortex induced vibrations for free span pipelines and the other is about multi free span pipelines where neighbor spans may interact dynamically. The interaction will depend on the length and stiffness of the pipe resting on the sea floor between the spans, and sea floor parameters such as stiffness, damping and friction. Each of them has important issues to investigate for improvement of our VIV knowledge.

SOURCE: HTTPS://DWINIRESTU.WORDPRESS.COM/2015/02/05/VORTEX-INDUCED-VIBRATION-ON-PIPELINE/, HTTP://WWW.DIVA-PORTAL.ORG/SMASH/GET/DIVA2:217873/FULLTEXT01.PDF

Apa yang Dimaksud Upheaval Buckling pada Pipeline?

When production starts through a pipeline, internal temperature and pressure will rise. The temperature increase will lead to thermal expansion of the steel. A pipeline will be restrained variously along the routing due to soil friction, and the temperature rise will result in axial compressive forces in the pipe. As a response to the longitudinal compressive force interacting with local curvature of the pipe, global buckling may occur.

A pipeline can buckle downwards in a free span, sideways on the seabed or upwards for buried pipelines. Vertical buckling of a pipeline is called upheaval buckling, and the direction of the buckle is upwards because this is the way of least resistance. If a vertical buckle leads the pipe into exposure on the seabed, this is a severe problem. An expensive and time consuming operation is needed to re cover the pipe at this location. If the buckle damages the pipeline, this part must be replaced before re covering takes place.

For upheaval buckling to occur, the pipeline must first have an initial imperfection. Imperfections are typically due to the pipeline being laid over a boulder or due to irregularities in the seabed profile.

Figure below illustrates a sequence of events which initiates buckling in a buried pipeline:

The pipeline is laid across an uneven seabed (a) and later trenched and buried (b). The trenching and burial operations modify the profile of the foundation on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance, a lump of bottom soil falls under the pipe.

The occurrence of an upheaval buckle is highly depending on the smoothness of the seabed profile. According to the DNV-RP-F110 (Global Buckling of Submarine Pipelines), it gives criteria to avoid upheaval buckling from occurring by designing sufficient cover providing enough resistance for pipelines to remain in place. Therefore, upheaval buckling is considered as an ultimate limit state (ULS) in the RP.

SOURCE: HTTPS://NONERIESKA.WORDPRESS.COM/2013/02/01/UPHEAVAL-BUCKLING-OF-OFFSHORE-PIPELINES/

Apa yang dimaksud Elbow dan Bend pada Pipa?

Piping Elbows and Bends are very important pipe fitting which are used very frequently for changing direction in piping system. Piping Elbow and Piping bend are not the same, even though sometimes these two terms are interchangeably used.A BEND is simply a generic term in piping for an “offset” – a change in direction of the piping. It signifies that there is a “bend” i.e, a change in direction of the piping (usually for some specific reason) – but it lacks specific, engineering definition as to direction and degree. Bends are usually made by using a bending machine (hot bending and cold bending) on site and suited for a specific need. Use of bends are economic as it reduces number of expensive fittings.An ELBOW, on the other hand, is a specific, standard, engineered bend pre-fabricated as a spool piece (based on ASME B 16.9) and designed to either be screwed, flanged, or welded to the piping it is associated with. An elbow can be 45 degree or 90 degree. There can also be custom-designed elbows, although most are catagorized as either “short radius” or long radius”.

In short “All bends are elbows but all elbows are not bend”

Whenever the term elbow is used, it must also carry the qualifiers of type (45 or 90 degree) and radius (short or long) – besides the nominal size.

Elbows can change direction to any angle as per requirement. An elbow angle can be defined as the angle by which the flow direction deviates from its original flowing direction (See Fig.1 below).Even though An elbow angle can be anything greater than 0 but less or equal to 90°But still a change in direction greater than 90° at a single point is not desirable. Normally, a 45° and a 90° elbow combinedly used while making piping layouts for such situations.

Elbow angle can be easily calculated using simple geometrical technique of mathematics. Lets give an example for you. Refer to Fig.2. Pipe direction is changing at point A with the help of an elbow and again the direction is changing at the point G using another elbow.

In order to find out the elbow angle at A, it is necessary to consider a plane which contains the arms of the elbow. If there had been no change in direction at point A, the pipe would have moved along line AD but pipe is moving along line AG. Plane AFGD contains lines AD and AG and elbow angle (phi) is marked which denotes the angle by which the flow is deviating from its original direction.

SOURCE: HTTP://WWW.WHATISPIPING.COM/PIPING-ELBOWS-AND-BENDS