The Situation

Sometimes natural gas transmission pipelines are constructed in regions that are susceptible to significant geohazards such as mud flow, soil creep or landslides. Although it’s possible to design a pipeline to withstand these ground movements, it isn’t always cost-effective to install kilometres of high-grade, thick wall steel pipe to traverse a mountainous region. Further, at the time of construction some areas might not appear to be prone to soil movement, with visible signs of slope instability only appearing after installation.

The 344 kilometres (214 miles) Mariquita to Cali natural gas transmission pipeline traverses mountain ranges that are typical of the challenging geological conditions found in Colombia. The pipeline is critical as it is the sole source of residential, vehicular and industrial natural gas supply for more than four million people.

Landslides caused by rainfall and occasional seismic activity are common across the steep terrain. As a result, multiple sections of the pipeline have been damaged due to ground movements caused by extended periods of elevated rainfall. In early 2012, wrinkles were noticed in the pipe bridge near the city of Manizales. Soon after, the pipe bridge underwent a significant lateral deflection as a result of land movement in the hill above the bridge.

Two interventions were successfully carried out to repair the affected pipeline. The pipe across the bridge was cut and returned to its original alignment. About 400m up the hill from the pipe bridge, a portion of the buried pipe was exposed, cut and reconnected, in order to relieve the pipe tensile strain caused by the displaced soil mass.

After remediating the pipe, owner TransGas endeavoured to implement a system that monitors movement of the hillside and along the buried pipe. Through this monitoring it was hoped that early intervention could be made to mitigate further land movement and provide greater reliability of the gas distribution system. A number of actions were proposed in the pipe integrity management plan including conducting regular ground movement surveys of the slope, measuring pipe stress at selected locations, analysing the monitored data and setting the trigger levels for pipe mitigation.

Advisian performed numerical analyses to advise the strain gauge locations for measuring pipe stress changes as well as locations for inclinometers to measure soil movement. We also determined the trigger level at which the inferred pipe stress exceeds the allowable limit and undertook other mitigating measures.

Approach

After reviewing the available information, it was decided the project would be carried out in two stages:

  1. Slope stability and deformation analysis to estimate the potential slip surfaces and the characteristic of the soil deformation along the buried pipe affected by the landslide
  2. Pipe stress analysis using the soil deformation estimated in the slope deformation analysis

Both analyses were performed using the general-purpose nonlinear finite element software, Abaqus. The computed pipe stresses were assessed against the design criteria as per ASME B31.8-2010.

The critical soil movement that caused the pipe stress to exceed the design limit was also determined so a suitable soil movement trigger point could be established. The computed slip surfaces provided information on where and at what depth to install inclinometers.

The stress/strain distribution along the pipe revealed how the pipe behaved when subject to landslide movement, and the locations to monitor large stress changes were assigned.

Results

The application of numerical analyses enabled TransGas to understand the pipe response when subject to potential landslide scenarios. The novel use of a shear strength reduction technique in a nonlinear finite element model allowed different potential slip surfaces, and their corresponding deformation, to be estimated.

The results assisted and confirmed the most effective locations to monitor pipe stresses, ground and pipe movements. The landslide monitoring system provides an early warning to the pipeline operators to take appropriate mitigation measures thus avoiding potential pipe damage and increased maintenance costs, as well as the loss of gas supply for affected communities.

The pipeline model was further utilised to explore the feasibility of a pipe loop concept which represents a more permanent mitigation solution going forward.