Page 88 - Pipeline Pigging Technology

Pigging for flexible pipes

fatigue lifetime. Due to this lack of slip between layers there is little heat build-
up or internal friction in this construction.

Non-bonded pipe construction

Non-bonded pipes are also made up from alternating layers of polymers,
steel reinforcement, and textile tapes. The individual polymer layers are
extruded over steel structural elements, but no adhesives are used. Separa-
tions of layers allows for individual layer slip. Lubricating media or interme-
diate sheaths are installed to reduce internal friction. The inner polymer
sheath is designed to serve as a leak-proof fluid conduit, whereas the outer
sheath serves to keep the reinforcement steel together while protecting the
inner structure from abrasion forces. This superposition of polymers and steel
can induce residual volume variations (due to pressure effects). As layers are
separated, settling will occur. As a result of component variations and relative
motions due to pressurization, there will be flexible elastic deformations.

Polymers and gas permeation

The polymer (plastics and elastomer) components in flexible pipe largely
serve as fluid conduits or chemically-resistant structures. As such, ageing and
resistance to hydrocarbons and gases are important. Plastics or polymers are
composed of long-chain molecules which form a network structure. Al-
though intermolecular distances are extremely small, molecular chains per-
form continual thermal vibrations, and it is these vibrations which permit the
passage of gas molecules through the structure [Makino et at, 1988].
When gases or fluids containing gas are passed through a polymer pipe, gas
molecules permeate through the polymer layers as a result of absorption,
solution, and diffusion mechanisms. Consequently, gases can accumulate in
interstitial spaces of the metallic armour and between the inner and outer
polymer layers. This accumulated gas gradually increases over time and as a
result of increases in pressure. Gas migration through the structure is an
operational concern, but becomes very important when considering en-
trapped gas behaviour during rapid pipeline depressurization(s). During such
an occurrence, entrapped gas volumetrically expands, exerting significant
forces on inner polymer sheaths. Should such forces overcome the shear
strength of the polymers, permanent deformations or even collapse could
result; this is known as ED (explosive decompression). For most gas pipe
designs, a stainless steel inner carcass or corrugated tube is used to prevent
such deformations from occurring as the steel liner is not affected by such

69