Copyright 2001, Offshore Technology Conference This paper was prepared for presentation at the 2001 Offshore Technology Conference held in Houston, Texas, 30 April–3 May 2001. This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented.

Abstract Backfill is frequently required on small diameter, steel pipelines for thermal insulation as well as a measure to mitigate upheaval buckling. Current industry practice for mitigating upheaval buckling is to rock dump on susceptible sections of pipeline routes. This process can lead to excessive and unnecessary remedial costs due to conservatism in backfill parameters.

Methods of increasing the download on pipelines will reduce or even eliminate the amount of rock dump required. Improvement of backfill material and optimisation of current upheaval buckling models provide a number of options for increasing pipeline uplift resistance. Coflexip Stena Offshore has undertaken extensive research into upheaval buckling of pipelines and has carried out associated studies into backfill improvement techniques. The research incorporated a full-scale laboratory-testing programme that assessed the uplift resistance behaviour of backfill soils and rock dump material. The backfill improvement study reviewed traditional land based ground improvement techniques and their potential transfer to subsea applications. Potential innovative solutions were identified. Improvement of backfill material and optimisation of upheaval buckling models will result in cost-effective solutions for the mitigation of upheaval buckling. Introduction Small diameter rigid steel pipelines are frequently used as in-field tiebacks to transport high pressure and high temperature hydrocarbons. Since the distances over which these tiebacks are placed are relatively short, in comparison to export pipelines, the problem of upheaval buckling can be a significant concern

for pipeline designers, installation contractors and field operators alike. Measures to mitigate upheaval buckling typically consist of trenching and perhaps backfilling the pipelines. One purpose of backfill material is to provide download over the pipeline to resist the vertical forces associated with temperature and pressure induced upheaval buckling. In areas where the backfill cover is not of sufficient thickness to resist upheaval buckling, additional rock dump is typically placed over the pipeline. In cases of highly sensitive pipelines blanket rock dump is normally required. However, it should be noted that whilst trenching is often required to provide physical protection and perhaps also on-bottom stability, the additional of backfill and rock dump inevitably leads to increased cost. In order to reduce such costs and to fully understand the failure mechanisms associated with upheaval buckling and cyclic ratcheting, the Coflexip Stena Offshore Group (CSO), has undertaken extensive research and development programmes. This paper describes typical trenching and backfilling tools, summarises the findings of the detailed upheaval buckling research programmes and describes potential backfill improvement techniques, which have been investigated by CSO, that could be employed to improve the download provided to pipelines. Trenching Small diameter rigid pipelines are usually trenched by ploughs or ROV based jet-trenching tools. Each of these methods has benefits and restrictions with respect to reliably providing backfill cover for mitigating upheaval buckling. Ploughs Modern pipeline ploughs are capable of trenching rigid pipelines post-lay in all soil types and even rocks where they are weak or heavily fractured. Internal review of CSO's extensive trenching experience, including a database of over 900 km of plough trenches using the CSO Multi-Pass Plough (Figure 1), has demonstrated that ploughs, when operated correctly, produce smooth profiled trenches with minimal as-trenched out-of-straightness (OOS) imperfections. This review has also allowed CSO to accurately predict the frequency and wavelength of OOS imperfections for any geotechnical conditions and pipeline size (2).

OTC 13142

Backfill Improvement Techniques – Optimising Pipeline Burial Solutions M.Cowie and M. Finch, Coflexip Stena Offshore, UK

2 M. COWIE, M. FINCH OTC 13142

Following post-lay lowering of the pipeline into a "V" shaped open trench by the pipeline plough, backfilling is achieved using a backfill plough (Figure 2). Typically 80 % to 100 % of the trenched spoil is returned back into the trench. There are limitations, however, in using ploughing equipment to trench rigid pipelines. Where backfilling is to be carried out in soils sensitive to liquefaction pipelines should sufficient specific gravity to resist the uplift forces generated by the backfilling process (2). If liquefaction is expected, the pipelines specific gravity should also be checked in all installation and operational states to ensure the risk of floatation is minimised. It should be noted that in sensitive soils, such as silty sands and sensitive very soft to soft clays, where liquefaction is more likely the backfill being returned to the trench may suffer significant loss of strength, and thus reduced uplift resistance. In this case the main advantages of the backfill material will be it's thermal insulating properties and provision of additional physical protection. In practical terms the ploughing spread can also be very expensive, requiring a dedicated trenching support vessel and significant deck space. Jetters ROV based jet trenching tools are limited to trenching rigid pipelines in sand and very soft to soft clay. Any soil conditions that are outside these limits are likely to cause significant problems for jet trenching equipment, resulting in reduced trenching performance or limited pipeline lowering. However, ROV jet trenchers are generally less demanding on vessel specification than pipeline ploughs, and are often deployed from vessels of opportunity rather than dedicated spreads. As a result, jet trenching is cheaper than ploughing in operational terms. In granular sediments, the jetting action fluidises the soil and the pipeline settles over its natural span length through this material until touching down at the base of the trench. Backfill is achieved by a combination of re-constitution of the particles suspended in the fluidised soil matrix and natural trench collapse. The amount of backfill achieved is a function of: • Product stiffness. • Soil particle size distribution. • Direction and magnitude of jetting power imparted to

the trench profile. • Number of passes of the jetting tool. • Trench depth. • Environmental conditions. Typically 40 % to 80 % of the soil within trench depth remains as cover following the jet trenching operation in granular soils.

In clay soils CSO has analysed jetting performance on many of it’s projects and has demonstrated that the amount of backfill is likely to vary between an open trench state and up to approximately 50 % of trench depth. Any backfill remaining in the trench after jet trenching in clay is likely to be in the form of slurry with limited strength. As such, this material is unlikely to be of benefit in providing uplift resistance. The effect this material has on the mobilisation displacement behaviour of the pipeline when rock dump is placed should also be considered carefully. The backfill slurry can also pose a floatation risk for light pipelines. Induced trench collapse is a technique that has been employed to obtain more reliable backfill (1). In this case an additional pass of the jetting tool to under-reams the base of the trench walls is required. The notch formed under the trench wall then initiates collapse of the surrounding soil over the pipeline. This technique has only been performed in very soft clay material. It is considered unlikely to provide satisfactory results in soft to firm clay soils with higher undrained shear strengths as these materials may remain stable, even with a notch cut under the base of the trench walls. Trenching Tool Selection The selection of the most appropriate trench tool for a particular application has been highlighted by CSO (4). Whatever the trenching and backfilling mechanism employed, it is frequently required to provide additional download to any backfill achieved to mitigate upheaval buckling. Additional download may take the form of rock dump material and/or some form of backfill improvement. Backfill improvement incorporates any technique that increases the download resistance acting on a pipeline, be it from enhancement of industry standard upheaval buckling models or improvements to the physical properties of the backfill material itself. Uplift Resistance Models and Parameters CSO has sponsored a number of research programmes to investigate uplift resistance provided to pipelines by a representative range of backfill soils including sand, fine grained cohesive material and rock dump. The purpose of the research programmes were to investigate all of the four key areas of geotechnical engineering which influence the interface between the pipeline and the surrounding soil. An integrated approach to upheaval buckling analysis combining these four elements forms the basis of CSO's ongoing commitment to reducing costs for client's (2). The research projects are outlined below. Axial Friction Representative samples of actual pipeline coating material were tested against both sand and fine-grained soils at the University of Oxford and University of Newcastle in the UK. The testing programme yielded relationships which can be used to compare the soils particle size distribution and the pipelines coating roughness in order to obtain accurate pipeline / soil interface friction coefficients (2,3).

OTC 13142 BACKFILL IMPROVEMENT TECHNIQUES – OPTIMISING PIPELINE BURIAL SOLUTIONS 3

Uplift Resistance Testing Both monotonic and cyclic ratcheting uplift tests were carried out using a full-scale model test tank at the University of Cambridge, UK, on sand, fine grained soil and rock dump material. A preliminary programme of centrifuge model tests was also carried out at C-CORE in Newfoundland, Canada. The findings have challenged industry accepted uplift resistance models and geotechnical parameters (2,3,7). The programmes have been accepted by certifying authorities and resulted in significant cost savings to clients. Soil Thermal Conductivity Consolidation of laboratory testing data and correlation with empirical relationships has allowed CSO to developed a comprehensive database for the assessment of soil thermal conductivity, both for pipeline coating design and upheaval buckling assessment. As-trenched Out of Straightness (OOS) Statistics A unique in-house database has been established by drawing on trenching experience from over 900 km of pipelines trenched using both ploughs and jetting tools. This database indicates that OOS frequency is primarily a function of soil conditions, whilst OOS wavelength tends to be governed by pipeline stiffness, in the case of pipelines trenched with ploughs (2). Backfill Improvement As well as optimising the models for use in upheaval buckling analysis CSO has undertaken a research programme to investigate possible improvements that could be made to backfill material remaining after trenching operations. Uplift resistance models such as that proposed by (6), show that improvements in the submerged unit weight and uplift resistance coefficients, i.e. increased density and strength of backfill soils. The effect of such improvements is to increase the available uplift resistance, thus reducing the requirement for additional rock dump. The research was based primarily on techniques that are commonly employed on land based civil engineering, with the object being to review the potential practical transfer of this knowledge to the offshore environment. The techniques investigated all involve additional offshore operations, which to an offshore contractor mean more offshore time and therefore greater cost. However, it is important to compare the requirements of the likely operations to identify if in fact they are feasible and can add a beneficial effect to the backfill of a pipeline in terms of uplift resistance and therefore the economics of a pipeline project. The backfill improvement techniques reviewed are summarised in Figure 3 and outlined below.

Compaction This is one of the most common methods employed to improve geotechnical conditions onshore. The main purpose of this technique is to increase the density and strength of a soil by packing the particles/grains closer together. There are a number of compaction processes (Figure 3). Dynamic and mechanical compaction involves either dropping or rolling weights over the ground. Other methods employ vibrations to achieve the same effect. Many compaction techniques have been developed specifically for land and it is unlikely that any of them would be suitable for offshore pipeline applications. The main reason being that operating compaction/vibrating plant over pipelines is likely to involve significant time increases to a project and potentially represents a high risk of pipeline damage. If dedicated offshore tools could be developed, either as stand alone equipment items, similar to those used in onshore highway applications, or as attachments to existing large scale trenching equipment, the risk of pipeline damage may be mitigated. However, all options would involve either additional time or equipment, the cost of which is likely to be prohibitive in comparison to rock dump placement for upheaval buckling mitigation. Consolidation This process involves the removal of water from the soil structure, thus increasing it’s strength and density. The only feasible technique highlighted for possible offshore applications is that of electo-osmosis. This technique induces migration of water molicules that in turn accelerates consolidation. Theoretically, the procedure could be used for consolidating backfill clay material but extensive research and development is needed. Topics for study may include: • Potential improvement to marine soils • Efficiency in saline conditions • Efficiency in backfill materials • Development of anode/cathode materials and power

sources • Effect on cathode protection • Environmental effects Ground Improvement These techniques involve physically adding a material or substance into the backfill soil to increase its strength. This method is commonly used in land applications and includes the injection of grouts, mixing of lime, cement or reinforcing elements into the soil. All of these techniques are theoretically possible but the practicality of installing such elements limits its use as a feasible technique for backfill improvement. The use of geotextiles has previously been identified as a potential method of improving upheaval buckling resistance (5). Concerns were raised over the long-term stability of backfill soil reinforced using geotextile mattresses. Problems could include creep and structural degradation of the geotextile itself. However, geotextiles have been used in the marine environment for many years and this is not considered a problem for the design life of a pipeline, provided the correct materials are selected.

4 M. COWIE, M. FINCH OTC 13142

The main difficulty seen in the use of geotextiles is subsea handling and placement. New equipment and procedures would have to be developed for handling and laying the reinforcement before, or during, the backfilling operation. The lightweight nature of the material would also have to be overcome. New developments such as fibre reinforcement may have greater potential and could possibly be used in conjunction with electro-osmosis. Mechanical Restraint Whilst mechanical restraint does not fall into the category of ground improvement, it may be used in conjunction with one or more of the techniques summarised above. The most common form of this technique is rock dump. Other methods that would provide mechanical restraint may include: 1. Soil nails 2. Anchors 3. Pins 4. Grout bags 5. Precast concrete covers or mattresses Options one to three would have to be employed with girdles around the top of the pipeline or synthetic sheets spread over the top as a load transfer mechanism. It is unlikely that these techniques would be suitable for an entire pipeline route due to placement time, but may be cost effective if used for a localised problem. A simple example of which is the placement of concrete mattresses for localised upheaval buckling mitigation where rock dump is not available. Improvement Versus Rock Dump Rock dump can be compared against the improvement techniques highlighted in terms of added benefit to uplift resistance. This is demonstrated in Figure 4, where the backfill consists of typical North Sea sand. For this example, it is observed that ground improvement only becomes effective as the thickness of the existing soil cover increases. Where existing soil cover is limited, the required uplift resistance may not be achieved by ground improvement alone and rock dumping would still be required. Conclusions Small diameter rigid pipelines are used frequently as tie-backs for in-field developments. These lines are generally sensitive to upheaval buckling and require to be trenched and backfilled and/or rock dumped to resist buckling. CSO has undertaken extensive research and development programmes into upheaval buckling and ground improvement techniques.

Industry standard uplift resistance models have been enhanced, reducing the conservatism in upheaval resistance modeling. Significant cost savings to clients has resulted from this research for a number of offshore pipeline projects. Potential backfill improvement methods have been identified. Many are unproven in marine conditions and require new equipment and technologies to be researched and developed in order to make them operationally and commercially attractive. Backfill improvement techniques have been shown to increase uplift resistance. The techniques only become efficient when there is sufficient existing backfill cover to allow the use of improved backfill alone without additional rock dump. It is considered that some of the improvement methods highlighted may be suitable for localised improvements to uplift resistance where rock dumping is not available, or is commercially unattractive. For most pipeline routes, rock dump is likely to remain the main measure for mitigating upheaval buckling. Acknowledgements The authors wish to acknowledge the contribution to this paper of Rob Fisher (CSO) and SETech Limited. References 1. Finch, M. and Fihn. E., “Deepwater Pipeline Burial”,

Deep Offshore Technology Conference, Stavanger, Norway, 1999.

2. Finch, M. et al.: “An Integrated Approach to Pipeline

Burial in the 21st Century”, Deep Offshore Technology Conference, New Orleans, 2000.

3. Finch, M., “Upheaval Buckling and Floatation of Rigid

Pipelines: The Influence of Recent Geotechnical Research on the Current State of the Art”, paper OTC 10713, 1999.

4. Fisher, R. “The Right Tool for the Job – Selection of

Appropriate Trenching and Backfilling Equipment”, Proc. Subsea Geotechnics Conf., Aberdeen, 1998.

5. Palmer, A.C., Ellinas, C.P., Richards, D.M. and Guijt, J.,

“Design of Submarine Pipelines Against Upheaval Buckling”, OTC, 1990.

6. Pedersen, P. and Jensen, J., "Upheaval Creep of Buried

Heated Pipelines with Infill Imperfections", Dept. of Ocean Engineering, Technical University of Denmark, 1988.

7. Schaminee P.E.L., Zorn, N.F. and Schotman, G.J.M.,

“Soil Response for Pipeline Upheaval Buckling Analyses: Full Scale Laboratory Testing and Modeling”, paper OTC 6486, 1990.

OTC 13142 BACKFILL IMPROVEMENT TECHNIQUES – OPTIMISING PIPELINE BURIAL SOLUTIONS 5

Figures

Figure 1 - CSO Multi-Pass Plough

Figure 2 - CSO Backfill Plough

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Figure 3 - Backfill Improvement Techniques

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Comparison Of Ground Improvement With Rock Dump For Typical North Sea SAND

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