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MEETING REPORT

Pipeline protection

Report on the BGS meeting 'The geotechnics of subsea pipelines' held at the Institution of Civil Engineers on 11 November 1998, by Neil Mitchell, Maunsell.

Introduction

Dr Bill Craig, the then chairman of BGS, introduced the speakers, Mark Finch of Coeflex Stena Offshores, Trevor Jee of Trevor Jee Associates, Gillian Sills of the University of Oxford and Malcolm Bolton of the University of Cambridge.

Mark Finch said the presentation would include recent advances in subsea pipelines both in design and installation from the perspectives of consultants, contractors and academics.

Background

There are four main reasons why pipelines should be buried within the seabed, Finch explained:

To give stability against wave loads

For mechanical protection

To negate the effects of uplift

For additional thermal protection

Currently, there are three ploughline ploughs installing some 1,000km of pipelines in the North Sea. The ploughs weigh between 100t to 150t in air and can form trenches up to 2.1m deep. They are pulled by ships called Bullard Pulls which can weigh in the region of 250t. At present there are only two multiple pass ploughs working in the North Sea but this is expected to increase to three in the near future. The ploughs can use imported backfill in the trench or replace excavated material.

Pipelines can fail in a number of ways but there are two main mechanisms, upheaval buckling and flotation. Upheaval buckling is due to the 'out of straightness' and thermal expansion of the pipe. Flotation is a function of the specific gravity of the pipe and the backfill used and is a particular problem in soft clay and sandy silt.

Increasing emphasis is being placed on the accurate design of pipelines because lighter materials are being used and the oil is being transferred at higher temperatures and pressures.

Why bury pipelines?

Trevor Jee described the reasons why pipelines are buried. As it costs around £100,000/km to place a pipe in a trench and an extra £50,000 to bury it, there must be good engineering reasons for doing this.

For mechanical protection

The main cause of impact damage to pipelines is by trawl-net fishing. Trawl gear can weigh up to 2t, which at a speed of 2m/s can impart around 2kJ of energy, creating a 5t to 10t impact. Burying pipelines offers protection, although it will not prevent damage from the impact of a sinking ship or ships' anchors. Accurate charts will reduce the likelihood of this type of damage.

For additional thermal protection

As the unrefined oil passes through the pipe, it cools, and heavier particles form a wax that solidifies on the inside of the pipe, reducing its flow capacity. While the pipe can be cleaned using a device called a pig, this is often costly. Burying the pipeline insulates it and reduces heat loss.

There are two main ways to insulate a pipe. Backfill alone can be used as an insulator or a foam coating can be applied, although when this is used the pipe is generally placed in a trench. The relative conductivity of each of the materials is shown in Table 1. Although foam is least con- ductive, a thick layer of backfill can provide more effective insulation.

To give stability against wave loads

An unburied pipeline placed on the seabed will be subjected to hydrodynamic forces, including drag and lift, caused by the sea and the pipeline's reaction with the seabed. This results in friction with the seabed from lateral movement of the pipe. If the pipeline is not buried, additional weight, either concrete or steel, can be added to reduce the effects of these forces.

Flotation

The drawbacks to burying a pipeline are the risk of flotation and upheaval buckling. Upheaval buckling is where the pipeline experiences axial loading due to thermal expansion and the pipe buckles, while flotation generally occurs during installation.

Gillian Sills presented her research findings into the causes of flotation. When pipe is empty, it is buoyant. As backfill is generally excavated material, it is disturbed, relatively weak soil, with low shear strength, giving little resistance to uplift of the pipe. Figure 1 shows analysis techniques for designing pipes against flotation, where uplift is compared to the shearing resistance of the backfill material. If it is assumed that the difference in the density of the pipe and the material above it is 2kN/m3 then the required shear strength of the material to prevent uplift is 0.1kPa.

Sills hypothesised that flotation could occur when there are loose soils and a low confining stress.

Sills adapted a shear vane apparatus to measure the torque and the subsequent pore water pressure of different soils during shearing (Figure 2). Three samples of a soil in the experiments, with initial results shown in Figures 3 and 4.

Figure 3 shows, after an initial rotation of the vane, that the normalised pore water pressure increases to around 1. A notable feature is that the soil with a higher liquidity index has the highest pore water pressure, encouraging low shear strength and flotation. This is supported by the torque required to turn the vane for the three soils (Figure 4).The material with the highest liquidity index, after the initial shearing period, shows the lowest torque. The required residual torque equates to an undrained shear strength of around 0.1kPa.

Figure 5 shows the particle size distributions of the analysed soil as well as soils from other locations where liquefaction is known to occur.

Sills concluded that liquefaction is more likely to occur when:

the soil is in a loose state with low confining stress

there is a presence of up to 50% non-plastic fines in a medium, clean sand

the soil has a comparatively low plasticity index

It is known that static liquefaction can occur in some soils and is particularly likely where there are low confining stresses, where con- fining stresses around the trench are low and where the soil is sheared twice, once on removal from the trench and again on being replaced.

Sills concluded that to assess the causes of flotation more thoroughly, more research is required into some of the processes that cause soil liquefaction, namely:

examining the shear behaviour of the soil during excavation and refilling of the trench with a series of model studies

examining the soil types and the conditions which are most susceptible to static liquefaction

developing a process to allow easy identification of areas susceptible to liquefaction

adjusting design practices so the uplift caused would not be greater than the available shear resistance.

Tests for upheaval resistance

Malcolm Bolton presented the findings of his research for a new test into the risks of upheaval resistance. The conventional method for checking for upheaval buckling is to assume a failure mechanism, find the soil strength and then use a theoretical formula to check but this approach has some disadvantages says Bolton:

tests to define soil parameters are inaccurate for the insitu soil samples taken

samples of the material are difficult to make the failure mechanism is assumed

The new approach is to test a model of the pipeline within a centrifuge to determine the correct stress levels to be applied. The uplift and consolidation data can then be obtained directly. The test equipment is a mini-drum centrifuge (Figure 6). The model is constructed at a twentieth of the actual size and the centrifuge runs at 20g, with a six hour test relating to six months on site.

Tests were carried out on pelletised and reconsolidated clay and various samples of sand with varying buoyancy weights. Within the samples the pipe was buried to different depths beneath the sample surface. Pelletised clay was used as this is often the state of the clay once it has been replaced as backfill (Figures 7 to 13) .

The uplift resistance and the effects of pull-out rate of the pipe were measured in clay. Tests included consolidated liquefied clay through reduction of the porewater pressure.

Tests show the centrifuge produces the correct stress levels required to simulate actual situations. The test equipment has been constructed to provide essential data on settlement and consolidation of the model. The method is essential as it has been found that the failure mechanism varies from sample to sample and as such the theoretical failure mechanism chosen could be inaccurate, giving false safety factors.

Comparison of static cone and T-Bar testing

In a contribution from the floor, Helen Kinlock described recent innovations by Fugro into the use of the T-bar in preference to the cone in certain situations for insitu soil testing (Figure 14).

Major recent developments have been into deep water and softer soils testing as the T-bar produces more accurate insitu test data than the static cone. Results for the static cone are based on semi-empirical results whereas test data from the T-bar are based on exact solutions.

But although the T-bar has a larger surface area and the measured results tend to be more accurate than the static cone, it can only be used in very soft to firm homogeneous clay.

Kinlock said the T-Bar was a valuable new tool for insitu testing of undrained shear strength of very soft clays.

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