Pipeline Material and Grade Selection
Wednesday, January 20, 2016
A submarine/subsea/offshore pipeline is a pipeline that is
laid on the seabed or below it inside a trench.In some cases, the pipeline is
mostly on-land but in places it crosses water expanses, such as small seas,
straights and rivers.Submarine pipelines are used primarily to carry oil or
gas. A distinction is sometimes made between a flowline and a pipeline. The
former is an intrafield pipeline, in the sense that it is used to connect
subsea wellheads, manifolds and the platform within a particular development
field. The latter, sometimes referred to as an export pipeline, is used to
bring the resource to shore.Sizeable pipeline construction projects need to
take into account a large number of factors, such as the offshore ecology,
geohazards and environmental loading – they are often undertaken by
multidisciplinary, international teams.
***
Pipe Materials
There are some applications where plastic, concrete, or
other piping materials are both desirable and acceptable. Utility systems such
as those for water, sanitary or storm water, air, draining or low-pressure oil
or gas service applications often use the nonsteel piping material systems.
However, for the vast majority of the “pressure” piping systems encountered,
steel pipe is required.
For petroleum applications, pipe materials that meet
American Petroleum Inst. (API), American Soc. for Testing and Materials (ASTM),
ASME, and ANSI standards are used most often. All of these standards have very
rigid design, specification, chemistry, and testing standardization and
manufacturing requirements. Modern steel pipe manufactured to these exacting
standards assures both high quality and safety in design.
Material Selection
Material selection shall be optimized, considering
investment and operational costs, such that Life Cycle Costs (LCC) are
minimized while providing acceptable safety and reliability.
The following key factors apply to materials selection:
- Primary consideration shall be given to materials with good market availability and documented fabrication and service performance.
- The number of different material types shall be minimized considering costs, interchangeability and availability of relevant spare parts.
- Design life.
- Operating conditions.
- Experience with materials and corrosion protection methods from conditions with similar corrosivity.
- System availability requirements.
- Philosophy applied for maintenance and degree of system redundancy.
- Weight reduction.
- Inspection and corrosion monitoring possibilities.
- Effect of external and internal environment, including compatibility of different materials.
- Evaluation of failure probabilities, failure modes, criticalities and consequences. Attention shall be paid to any adverse effects material selection may have on human health, environment, safety and material assets.
- Environmental issues related to corrosion inhibition and other chemical treatments.
- For main systems where materials/fabrication represent significant investments and/or operational costs, an LCC analysis shall be basis for material selection (Ref. Annex A).
Deviation from Specified Material Selections
Where an overall evaluation of LCC, safety and reliability
shows that it is more cost effective to deviate from the specific materials
selections and corrosion protection principles given in this document and the
NORSOK standards referenced herein the alternative solution should be
implemented. Such deviations may include replacement of CRAs with carbon steel
where CRAs are specified in this standard.
Suppliers standard materials shall be used for equipment,
provided that:
- This is cost efficient.
- The supplier can document previous successful experience.
- The materials satisfy the general requirements given in this document with respect to corrosion resistance and/or maintenance and replacement.
Grade Selection
The steel applied in the offshore oil and gas industry vary
from carbon steels (taken from American Petroleum Institut standards- Grade B
to Grade X 70 and higher) to exotic steels (i.e. duplex). The following factors
are to be considered in the selection of material grades:
- Cost
- Resistance to corrosion effects
- Weigh requirement
- Weldability
The higher the grade of steel (up to exotic steels) the more
expensive per volume (weight). However, as the cost of producing high grade
steels has reduced, the general trend in the industry is to use these steel of
higher grades.
Fabrication, Installation, and Operating Cost Considerations
The choice of material used for the pipelines will have cost
implications on:
1. Fabrication of pipeline
The cost of steel increases for the higher grades. However,
increase in the grade may permit a reduction of pipeline wall thickness. The
result in the overall reduction of fabrication cost when using a high grade
steel compared with a lower grade steel.
2. Installation
It is difficult to weld high grade steels, and consequently
lay rate is lower compared to laying the lower grade steel. However, should the
pipeline be laid in very deep water and a vessel is laying at its maximum lay
tension, then the use of high grade steel may be more suitable, as the
reduction in pipe weight would result in lower lay tension. In general, from an
installation aspect, the lower grade steel pipelines cost less to install.
3. Operation
Depending on the product being transported in the pipeline,
the pipeline may be subjected to corrosion (internal), internal erosion, and
H2S induced-corrosion. Designing for no corrosion defect may be performed by
either material selection or modifying operation procedures (i.e. through use
of chemical corrosion inhibitors).
Material Grade Optimization
Optimization of material grade is rigiously applied today
based on experience gained from the past 20 years of pipeline design, and the
technical advances in linepipe manufacturing and welding. The optimization is
based on minimization of fabrication and installation cost while meeting
operating requrements. As the selection of material grade will have a
significant impact on the operating life of the pipeline, the operator is
normally involved in the final selection of material grade.
***
Source:
0 komentar