Design considerations for new pipelines and rehabilitating pipelines with linings

Irrespective of whether a pipeline is a gravity or a pressure system – and whether it is new or one due to be rehabilitated – there are four basic requirements that must be met, namely hydraulic performance, strength, water-tightness and durability.

By Alaster Goyns, Pr Eng*

The primary function is hydraulic capacity and performance. This is determined for anticipated future demand within 20 or 30 years. Other requirements ensure that the primary one is met and can continue to do so even after the capacity demand has been reached, making an additional pipeline necessary to meet the required future capacity.

Pipeline failure can be either functional, when it is unable to meet the demand, or more serious when one of the other requirements is not met. Functional failure occurs when the demand is greater than the capacity and is generally a temporary condition that occurs with increasing frequency as a pipeline ages. A nonfunctional failure is not temporary and results in deteriorating pipeline condition and eventual structural failure if no remedial measures are taken.

When considering how to cope with the failure of a pipeline, a clear distinction must be drawn between a functional failure due to the inability to meet its required hydraulic performance, and failure to meet one of the secondary requirements when its deteriorating condition will eventually lead to a structural collapse and an inability to perform at all. With a functional failure, the pipeline can be upsized or an additional pipeline installed.

When one or more of the secondary requirements deteriorate, the primary requirement may still be met for some time, but a structural collapse will occur eventually due to a combination of these factors. Hence it is essential to assess the performance and condition of pipelines to identify any problems, determine the underlying causes, and take the necessary remedial action to ensure that they can continue performing satisfactorily.

Gravity versus pressure systems

Trenchless techniques for the rehabilitation of pipelines using linings to reinstate their structural, watertightness and durability requirements are similar, whether gravity or pressure systems. However, differences need to be considered when doing the structural design of the lining to be used. The major difference is due to the loading cases to be considered. With gravity systems, the lining needs to be designed for the groundwater pressure that develops between the liner and host pipeline due to leaks into the system. It needs to be designed so that it does not buckle under the maximum value of this pressure when the pipeline is empty. For pressure systems, the liner must be designed for the maximum internal pressure, as well as for external pressure that may occur when the pipeline is empty.

With gravity and pressure systems, the pressure between the host pipe and the lining differs. For the gravity system, this pressure will depend upon the maximum water level above the pipeline invert, which may be at the groundwater level or, under certain circumstances, at the level of flood waters above ground level. For the pressure system, there may be a leak in the pipeline at a high point. When a lining is placed, this water can then flow down to a low point and result in significant pressure between the host pipe and the lining. Under these circumstances, it may be necessary to take additional measures to reduce the magnitude of this pressure.

Preventing buckling or bursting

There is a significant difference in the structural requirements for a pipeline or a lining to handle external forces to prevent buckling and those required to handle internal pressures to prevent bursting. The buckling resistance is dependent upon the wall thickness to the power of 3, whereas the internal pressure is directly related to the wall thickness. This is shown in the formulae to be used for the two different loading cases.

For external loading, it is the pipe ring stiffness (PRS) that resists the buckling and this is calculated from:

Pipe subject to internal pressure expands around whole circumference

Pipe subject to external load is squashed vertically and expands horizontally

PRS = EI/D3 = (E/12) x (t/D)3 where PRS is pipe ring stiffness

E is Young’s modulus of pipe material

I is moment of inertia of pipe wall = t3 /12 t is pipe wall thickness

D is mean pipe diameter

For internal pressure, it is the direct tensile stress, called the hoop stress, that resists the internal pressure and this is calculated from: σ = (pD)/(2t) where σ is circumferential hoop stress in pipe wall p is internal pressure in pipe

D is the mean pipe diameter t is pipe wall thickness

To determine the structural requirements of a pipeline to handle internal pressure, the same approach will be taken irrespective of the pipe material being used; however, when determining these requirements to handle the external loads, there is a clear distinction between the approach taken depending on whether the pipes are rigid or flexible. Differences in the performance of the pipes, or linings when subject to internal pressures and external loads, will depend upon the pipe, and lining material properties.

Installation and founding conditions

With a new pipeline, its external load carrying capacity depends upon the installation and founding conditions, as well as the relationship between the pipe stiffness and soil stiffness. With rigid pipe materials, such as vitrified clay, concrete or fibre cement, the load will be sensitive to the actual installation that could be in a trench, under an embankment, or in a tunnel, and the founding conditions, which could vary from yielding to unyielding.

The bedding support will enable the installed pipe to carry up to 2.5 times the load that it could carry in a factory test. The combination of these factors can make a significant difference to the strength of pipe required for a given situation.

With flexible pipe materials such as polypropylene (HDPE), polyvinyl chloride (PVC), glass-reinforced plastic (GRP) or steel, the support from the embedment around the pipe enables it to carry as much as 12 times the load that it could carry in a factory test. In addition, a pipe made from a flexible material will deform under load and shed the load onto the columns of earth on either side of it.

The combination of these factors means that the rigid pipe will carry between 40% and 90% of the external load imposed on it and the bedding will carry between 10% and 60% of this load. This combination of factors will result in the flexible pipe carrying only between 5% and 20% of the external load imposed on it, while the embedment will carry between 80% and 95% of this load. This explains why the load carrying capacity of rigid pipes is dependent mainly upon their installation and founding conditions, whereas the load carrying capacity of flexible pipes is dependent mainly upon the strength and support provided by the surrounding embedment material.

Rigid or flexible

With the structural design of a new pipeline to carry external loads, the procedure depends upon whether the pipe is rigid or flexible. In both cases, the load carrying capacity depends upon the pipe/soil interaction but, in the case of the rigid pipe, the pipe strength is the critical factor. With flexible pipe, the pipe deflection is critical. In simple terms, this can be expressed for the rigid pipe as:

Pipe strength = Load on pipe/Soil strength

For a flexible pipe this can be expressed as:

Pipe deflection = Load on pipe/(Pipe stiffness + Soil stiffness)

There are differences and similarities in the way that pipes, and linings within pipes, handle these loads and pressures.

When comparing a new pipeline being installed versus one being relined, the interaction between the pipe and the soil is critical with a new pipeline; with a relined pipeline, it is the interaction between the host pipe and the lining. When an existing pipeline is being rehabilitated with a lining, the external loading on the lining will depend upon the deterioration of the host pipe. If the host pipe is still structurally sound, but is leaking or starting to show signs of durability problems, cleaning and lining the pipeline will rectify these problems, but there will be an accumulation of water pressure between the host pipe and the liner. The pipeline would also be classified as partially deteriorated. Under these conditions, the liner will just be designed to handle the water pressure between it and the host pipe. However, if the host pipe is structurally unsound and could be expected to collapse in the near future, it is then classified as fully deteriorated. Under these conditions, the liner is designed to handle both the external water pressure, plus the earth and traffic loading being carried by the host pipe.

In both cases, the lining may receive some radial support against buckling due to water pressure between the host pipe and the liner. The liner in a partially deteriorated host pipe will fit tightly within the host pipe and this will enhancement its ability to resist the groundwater pressure. However, with a fully deteriorated host pipe, the liner is designed to take the full external load, consisting of water pressure, earth loads and traffic loads, with no assistance from the host pipe. Over time, the loading on pipelines may change from those when they were first installed depending upon surrounding soil conditions. This should be checked when evaluating the condition of a pipeline to be rehabilitated.

*Individual member of the Southern African Society for Trenchless Technology (SASTT) and owner of PIPES cc