Simon Impey, MEng AIGEM, Civil Engineer, DNV GL Utilities Specialist Services, Loughborough, UK 1 of 5

Dam Failure: Assessing the Risks to Pipeline Infrastructure

For a 2013 Civil Contingencies exercise, the potential

failure of a large reservoir dam was investigated.

Directly within the predicted flow path of the

reservoir deluge were two National Transmission

System high pressure gas pipelines and a compressor

station.

Our client National Grid Gas Transmission required an

assessment of the impact of the dam failure to

evaluate and appreciate the hazards, to inform the

development of their emergency action plans. This

investigation was to be a ‘desk-top’ study of available

information, to identify the sources of risk and

recommend possible mitigation measures.

Modelling Methodology

There were three considerations used to determine

the hazard to the pipelines from the potential dam

failure.

1. Is there a risk to the pipelines of buoyancy failure

from the initial inundation by the flood waters?

2. Is the subsequent flow capable of causing

erosion, and to what extent may this change the

pipeline cover depth?

3. If the cover depth to the pipeline has been

eroded, does this exacerbate the risk of buoyancy

failure to the pipelines?

Following from previous studies of flooding risks to

pipelines, our approach to answer these questions

was to use Geographic Information Systems (GIS) to

collate and geo-reference different datasets from

various institutions. The combined data sets would

then facilitate the application of empirical equations

and make location specific calculations possible.

Sources of Information

The Client

The British Geological Survey (BGS)

The Environment Agency (EA)

The client provided the pipeline properties, including

material, dimensions and pipeline routing information

in the form of a GIS data layer (drawn by GPS data

taken by route surveys). The client also provided as-

built construction information describing the cover

depth at locations along the pipeline.

Geology data was purchased from the BGS, this

comprised GIS polygons of the superficial soil types

identified. This enabled us to understand what soil

types are likely to be encountered, such as sands and

gravels, or clays, etc.

The EA provided the predicted flood event data

comprising predicted maximum extent, maximum

depth and velocity.

Cross Referencing of Data sets

Figure 1 shows how the geo-referenced data sets for

superficial geology, pipeline route and flood water

extents overlap within the GIS software [1]. The

coloured shapes represent different soil types

present, e.g. Alluvium (yellow), River Terrace Deposits

(Orange). The purple lines represent the two

Transmission Pipelines, A & B, and the blue outline

represents the flood water extent.

Figure 1: Cross referenced data sets for superficial geology, pipelines and flood water extent.

Figure 2 shows a visual representation of how the

flood water depth profile at peak conditions interacts

Simon Impey, MEng AIGEM, Civil Engineer, DNV GL Utilities Specialist Services, Loughborough, UK 2 of 5

with the pipelines, which varies by location as a

gradient colour field.

Figure 2: Cross referenced data sets highlighting a velocity profile for the predicted flood water at peak flow conditions.

Buoyancy Assessment

Pipeline Buoyancy Theory

Pipelines that are buried wholly or partially below a

water level are subject to a buoyancy uplift force (as

gas is less dense than the water it displaces). A

downwards force to counteract the uplift is therefore

required. The downwards force is typically provided

by the soil overburden and pipeline self-weight (see

Figure 3). The parameters chosen which define the

downwards force represent the minimum that the soil

could resist uplift, as typically shear forces within the

soil structure would also provide resistance as well as

a more prismatic column shape.

Figure 3: Forces associated with Buoyancy.

If the pipeline is buried in soft, low density organic

soils such as peat, the soil overburden load may be

insufficient to counteract the uplift force, leading to

breakout failure and exposure of the pipeline – see

Figure 4. Note that failure could also be caused by

insufficient pipeline cover depth and low self-weight

provided by the pipe material.

Figure 4: Buoyancy failure of a High Pressure natural gas pipeline in 2007 floods. The pipeline was buried in low density Peat soils.

Acceptance Criteria

Pipelines that are known to be constructed through

areas with potential to cause buoyancy uplift

(floodplains and river crossings) are generally

designed to be negatively buoyant. T/SP/CE/2 [2]

provides a target factor of safety of 1.2 for buried

pipelines; this is described in the relationship below,

using the forces as described in Figure 3.

Factor of Safety = 2.1Buoy

soilpipe

F

FF

Analysis Assumptions

For each of the main soil types identified on site

through which the pipeline is buried, a range of

estimates for soil bulk density was produced. The

purpose of having a range of estimates was to reflect

the fact that this assessment is a desk-top study and

covers large sections of pipeline.

The cover depths applied to the pipeline were based

upon as-built survey records and as such were

relatively sparse in some locations and potentially out

of date.

Weight of the soil column,

(product of pipe diameter, cover depth

and soil density)

Self-weight of pipeline,

(product of pipe diameter, wall thickness

and pipe density)

Buoyancy force,

(product of pipe void area and density of

soil & water)

Simon Impey, MEng AIGEM, Civil Engineer, DNV GL Utilities Specialist Services, Loughborough, UK 3 of 5

Erosion Assessment

Erosion to the host soils around natural gas pipelines

is a serious issue normally encountered at pipeline

river crossings and is why they are typically designed

to be at depth in excess of 2m. Figure 5 shows a high

pressure pipeline exposed by erosion.

Figure 5: A High Pressure natural gas pipeline where the host soils have become eroded within the river bank in 2013.

In this project we made use an empirical technique

featured in the United States Department of

Transportation, Federal Highway Administration,

Hydraulic Engineering Circular 18 (HEC-18),

‘Evaluating Scour at Bridges’ [3]. This method used

the concepts of ‘stream power’ and ‘erodability’.

Stream Power

This value is representative of the ability of the flow to

generate turbulence at the boundary, and hence the

erosive capability of the flow. It is a function of bed

friction, the inherent energy within the flow and the

shear force acting on the bed. It is therefore possible

to quantify the erosive capability, given a flow depth

and velocity (as provided by the EA) and assumptions

about the bed conditions (estimations based upon

BGS data and land usage inferred from Aerial

photography).

Erodability

The methodology for determining the threshold

power required to dislodge particles, and hence cause

scour, is based upon a product relation of four key

factors. The four factors are determined using unique

relationships featuring a host of factors unique to site

conditions.

Where Will Erosion Occur?

By direct comparison of the applied stream power at

the bed and the erodability index (both expressed in

kW/m2), it is possible to determine where erosion is

likely to occur.

Results & Discussion

Initial Buoyancy Assessment

The results of the factor of safety calculation for

pipeline A are shown as an example in Figure 6. This

graph shows the surveyed soil type against the FOS

against buoyancy values calculated at specific position

along the pipeline. The graph for Pipeline A shows

that the predicted FOS for this pipeline is generally

acceptable; however at a short section (chainage

2140m) with minimum cover depth and worst case

soil density, the FOS is below 1.2.

Erosion Assessment

The results of the calculated applied stream power at

bed and the worst case erosion power thresholds

(vegetated and non-vegetated) for Pipeline A are

shown in Figure 7. These results indicate that the

predicted applied stream power to the bed in general

does not exceed the predicted erosion power

thresholds assigned to the soils across the route of the

pipeline.

There is however some evidence of the applied

stream power at the bed exceeding the threshold for

erosion in the vegetated case within the alluvium

soils. This occurs within the section of Pipeline A at

the deepest point within the flow profile (circa

ch.5500m from compressor). This then indicates that

erosion is predicted to occur within the shallow extent

of the soils which contain the roots of the vegetation

(i.e. top-soil), but would not erode the soils

underneath.

Simon Impey, MEng AIGEM, Civil Engineer, DNV GL Utilities Specialist Services, Loughborough, UK 4 of 5

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Simon Impey, MEng AIGEM, Civil Engineer, DNV GL Utilities Specialist Services, Loughborough, UK 5 of 5

Secondary Buoyancy Assessment

Only a small section of Pipeline A is predicted to

experience erosion and this is predicted to be limited

to the depth of soil used to cultivate crops, which can

be assumed to be with the top-soil. Top-soil is not

included within the factor of safety against buoyancy

failure calculations, and so the removal of this will not

impact on the initial factor of safety against buoyancy

calculation.

Further Study

To determine the consequence of the pipeline

becoming exposed due to localised scour, an

additional assessment was performed. This

assessment took into account the drag force of the

flow and the vortex induced vibrations on the

pipelines. This study concluded certain lengths of

exposure of each pipeline which would be critical to

pipeline integrity. These conclusions were then

included in a bespoke risk assessment of both

pipelines.

Recommendations

A number of recommendations were made to the

client following the results of the assessments. These

were grouped into Proactive mitigation measures and

Reactive; i.e. mitigation that could take place now to

reduce the risks and measures that could happen

should the dam fail (with approximately 1h15mins

warning).

Why This Project Was Significant

This project was important to the client as it helped

them to participate with the Government exercise

with some confidence about the risk of buoyancy and

erosion and what they could do about it should it

happen.

The project was special to me as it allowed me to fully

explore the use of technology to solve real

engineering problems in a creative way. In this

project I learnt how to use GIS to collate and geo-

reference datasets from the different institutions, and

then split and export the collated data to apply

empirical calculations. The application of these

empirical methods also helped to develop my

engineering knowledge providing useful background

information for further work.

It was a very enjoyable project and an example of the

challenging engineering problems I am solving with

my team every day.

SIMON IMPEY, MEng AIGEM

References

[1] ArcMap v10 Build 3600, ArcGIS Desktop 10 Service

Pack 4, ESRI corporation

[2] T/SP/CE/2. Specification for the Design, Construction

and Testing of Civil and Structural Works. Geotechnical

Works and Foundations. National Grid June 2009.

[3] HEC-18 (Hydraulic Engineering Circular No. 18).

Evaluating Scour at Bridges. U.S. Department of

Transportation Federal Highway Administration,

Publication No. FHWA-HIF-12-003, Fifth Edition, April

2012.