Pipelines Expansion Loops

Pipelines Expansion Loops

A pipeline will expand or contract when temperature and pressure vary from installation conditions. Conditions during construction, therefore, become the reference temperature and pressure. For present purposes, discussion will be limited to expansion, but similar issues may require addressing for contraction.

As the pipeline expands, it will follow the path of least resistance, which leads directly to the risers to the platform.

If the lower riser span is insufficiently flexible to absorb expansion within the permissible stress limit, expansion loops are typically placed just before each riser. When the loops do not function properly, as when buried, they overstress at the first bend experiencing the pipeline expansion.

The several methods readily available to reduce the expansion in the pipeline result in build-up of axial forces in the pipeline and lead to upheaval buckling, a mode of failure in trenched and buried pipelines. Pipelines resting on the seabed may also buckle but tend to buckle laterally where there is insignificant resistance.

Upheaval buckling results from the axial force generated from the expanding pipeline combined with an uneven trench bottom which results from the trenching process, undulations in the seabed, a rock formation, or an area of denser soil. Trench unevenness is generally referred to as an "imperfection."

Axial force and imperfection are related. In general terms, the more uneven the trench profile, the lower the axial force required to produce an upheaval buckle.

Methods to control expansion and upheaval buckling, discussed presently, were investigated for the design of a high pressure and temperature, buried sour gas flow line offshore in Mobile Bay, Ala., for a major operator.

Controlling expansion

There are several conventional methods used to handle expansion which, along with some unconventional methods, were investigated and include the following:

• Anchor flanges and concrete and/or rough fusion-bonded epoxy (FBE) coatings each performs the same function of increasing the friction, thus reducing or eliminating expansion in the pipeline. This increase in friction, however, results in the build-up of axial forces in the pipeline contributing to increased risk of upheaval buckling.
• Cold springing the riser has proven successful for many pipelines. Cold springing prestresses the riser during construction, resulting in splitting the difference between the prestress and the expansion loads to be encountered during operation. This allows the riser to accommodate larger amounts of expansion. Cold springing allows for the natural relief of the pipeline stresses through the riser. Even allowing for cold springing, the middle of the pipeline may be anchored because of friction and experience the maximum axial force possibly resulting in upheaval buckling.
• The riser bend may be reinforced with a brace, distributing the forces over more of the riser. Reinforcing is limited in the amount of expansion it may accommodate, however, and does not alleviate the forces in the middle of the pipeline.
• Pipe-in-pipe construction is an option that has been utilized when other solutions fail. It is costly because the product carrier pipe is inside a jacket pipe normally two sizes larger. The two pipes are mechanically connected with bulkheads that transfer the loads from the carrier pipe to the jacket pipe.

While the carrier pipe expands, the jacket pipe resists the expansion loads. The spacing and size of the bulkheads are determined to eliminate buckling of the carrier pipe within the jacket pipe to minimize installation expense.

The structural design of the pipe-in-pipe virtually eliminates expansion and the potential of upheaval buckling if the system is properly designed. The cost of extra materials, however, and the lengthy process of fabricating the pipe-in-pipe warrant investigation of other options.

• Expansion loops and doglegs serve the purpose of acting as a spring to accommodate expansion at the risers. Several expansion loop and dogleg configurations were investigated for the installation in Mobile Bay, including conventional U-loops of various dimensions, and various angles for the doglegs.

It was found that buried expansion loops and doglegs experience high lateral resistance that in turn creates localized high stresses at the first bend. Only a short length of the expansion loop was buried, with minimal effectiveness.

To illustrate the relative effectiveness, it was found that a conventional expansion loop 40 x 40 ft was effective only over 4 ft when buried and had a much-reduced capacity for expansion. Similar to other methods, the expansion loop placed at the riser would not relieve the stresses in the middle of the pipeline.

• A zig-zag shaped pipeline has been employed to accommodate expansion and relieve axial forces throughout the pipeline. The pipe joints were bent in a zig-zag configuration and double jointed for installation. Each joint accommodated some of the expansion and relieved some of the axial force.

Although this solution has applications, the additional cost and concerns of fabrication and installation limit its applicability.

• Snaking the pipeline during installation appeared promising in principal. The pipeline would act as its own spring throughout the length, accommodating the expansion and alleviating the axial forces.

Several configurations that were considered easily layable were investigated for a range of soil conditions. It was determined that the pipeline follows the path of least resistance, which is axially.

Snaking the pipeline requires additional pipe which contributes to more costs and more expansion at the ends.

source :

http://www.ogj.com/articles/print/volume-96/issue-31/in-this-issue/pipeline/expansion-loop-enclosure-resolves-subsea-line-problems.html