Pipelines can stretch for thousands of miles across varying terrain. Some of the areas can be relatively flat, posing little threat of geologic events to a pipelines’ structural integrity.
However other regions may be more susceptible to ground displacement, subsidence and landslides. Pipeline corridors that cross mountainous terrain are especially vulnerable. Midstream and downstream oil and gas operators have determined that to protect their investments and maintain public safety, assessment and management program are needed. Proactively dealing with such threats is an integral part of asset integrity management. Preparing for and perhaps preventing a pipeline rupture is better than having to react to one.
“Significant landslides could cause catastrophic damage.”
Even in areas where landslide occurrence may be frequent, the threat may not be significant. According to the U.S. Geological Survey, many pipelines are buried to depths of about one meter. Shallow landslides or those that cause little erosion damage aren’t a particular threat to those pipelines. Yet, significant landslides brought on by an excessive amount of precipitation could cause catastrophic damage.
Quantitative vs. Qualitative
According to the USGS, there are generally two approaches a company may take to assess regional landslide threats: quantitative and qualitative.
A qualitative method often employs maps and either relies on the expertise of a geologist, engineer or technician to interpret them and give a probability of whether a landslide will happen in a specific location. Types of qualitative analysis are geomorphic, in which an expert will determine the likelihood of a landslide based on aerial imagery or observation.
Weighted parameter analysis is another type of qualitative method in which an expert uses weighted parameter maps. In a publication for the USGS entitled “Landslide Hazards to Pipelines – Regional Hazard Mapping,” Edwin L. Harp wrote that experts choose and map parameters, such as geology, drainage density and slope, that determine geologic stability in a particular area. Then, a weight is assigned to each parameter to determine its potential contribution to a landslide.
Harp stated the maps are overlaid upon each other to create an overall hazard map. The entire process could be automated through a Geologic Information Systems (GIS) platform.
A quantitative method takes more of a data-driven approach. “Quantitative methods of depicting probability of landslides vary from the purely statistical correlation of various factors to physical process-based models that attempt to replicate the failure and, in some cases, the movement of slope materials,” Harp noted.
The USGS survey indicated there are statistical techniques and geotechnical models for assessing landslide probability. Statistical analysis allows the different weights of each topographical and geographic parameter to be compared directly with a landslide map.
“Factor of safety is the ratio of forces that encourage slope movement to those that resist.”
Geotechnical models, however, use engineering slope stability analysis. Physical properties of materials in a specific location are quantified and implemented in mathematical models to determine a factor of safety, or FS. FS is the ratio of forces that encourage slope movement to those that resist.
Regional vs. site-specific
In a report from May 2015, The Washington Utilities and Transportation Commission suggested a three-phase approach to a landslide assessment program that moves from regional assessment to that of specific sites. The commission’s report stated active landslide hazards are often “re-activations” of past landslides. So much of the program emphasizes studies of previous landslides.
The first phase of the commission’s plan is a qualitative approach in which engineers conduct a geomorphic analysis of areas where landslides occurred. Each analyzed area is issued a landslide hazard threat level of “low,” “moderate” or “high.” Aerial photos are taken, and data is managed by GIS.
Phase two evaluates specific sites and delves into each previous landslide more fully. Experts use non-intrusive methods to investigate to confirm threat levels. Phase three then uses data collected from landslides to develop a mitigation plan. The program team also develops a plan to monitor such sites so it can react accordingly in case a landslide occurs.
During phase three, teams also use intrusive methods to study the ground’s surface and subsurface to develop a model that will further enhance landslide mitigation techniques.
In addition to evaluating for geological stresses and threats, operators can utilize geotechnical monitoring/instrumentation techniques to assess the overall performance capabilities of the asset in question.