Pipeline Burial

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<p>An Integrated Approach to Pipeline Burial in the 21st Century</p> <p>Mark Finch and Rob Fisher, Coflexip Stena Offshore, UK Prof. Andrew Palmer and Alex Baumgard, University of Cambridge, UK</p> <p>DEEP OFFSHORE TECHNOLOGY 2000</p> <p>ABSTRACT The inter-relationship between trenching, backfilling and subsea pipeline system design is described and an integrated design approach developed. Risk areas for pipeline trenching and backfilling are discussed and practical advice on their mitigation given. Results from recent geotechnical research are presented to enable optimum design with respect to upheaval buckling and on-bottom stability. The benefits of an integrated design approach are discussed and the practical advantages illustrated. INTRODUCTION Subsea pipelines are trenched and buried to provide physical protection, on-bottom stability, thermal insulation or download to mitigate upheaval buckling. Any or all of these may be required for a given pipeline but all are inter-related and have a significant effect on the design of the pipeline system as a whole. The rapid increase in the number of small diameter pipelines (typically in-field tiebacks) transporting high pressure and high temperature hydrocarbons has significantly raised the importance of geotechnical engineering in the overall pipeline system design. The prediction of trenching and burial performance and the resulting geotechnical and geometric properties of the trench, pipeline and backfill have become critical. An integrated design approach is required. Over the past five years Coflexip Stena Offshore have sponsored a large number of geotechnical research programmes to investigate the issues related to pipeline burial and the resultant properties of trench and backfill. By combining the research with the practical experience of burying over 1000 kilometres of subsea flowlines, technical risks and issues have been identified and design guidelines developed to facilitate an integrated approach to pipeline burial design. The results are presented in this paper. This paper introduces the variables and risks that need to be considered for an integrated approach to pipeline burial. The paper illustrates how research programmes have allowed the inherent uncertainties of geotechnical design to be reduced and how these findings have been married to practical experience, resulting in a safe and efficient pipeline burial design.</p> <p>TRENCHING AND BACKFILLING PERFORMANCE Selection of appropriate trenching and backfilling plant has been well documented (Fisher, 1998). The main drivers are: Soil conditions Pipeline size Pipeline type Trenching specification Water depth Cost Availability</p> <p>There are three generic types of trenching method; ploughing, jetting and cutting. If the trenching specification is clearly defined and site specific soil conditions are known, selection presents few problems. However, in most cases the best trenching tool can only be defined in parallel with the pipeline design, thus there is a requirement for in integrated approach. Ploughing Ploughs are capable of trenching a wide range of soil conditions from very soft clay to fractured or weak rocks. The trenching and backfilling mechanisms of ploughs are have been described (Reese and Grinsted, 1986). Examples of a typical pipeline plough is shown in Figure 1. Traditional ploughing algorithms are widely used in the industry today. It should be noted, however, that the traditional algorithms introduce a significant level of uncertainty, particularly in sand soils. Recent consolidation of ploughing experience has reduced the level of uncertainty in the traditional ploughing equations. Ploughing in Clay Soils The mechanisms involved with cutting clay soils are relatively simple (Reese and Grinsted 1986) and are based on undrained shear failure. Rate effects are not significant. In general cutting clay soils does not present problems. For example Coflexip Stena Offshore have carried out trenching operations in very hard clay soils containing boulders at Gullfaks without incident at moderate speeds (150 m/hr to 400 m/hr) for relatively low tow forces (100 Te to 200 Te). The upper limit to ploughing in clay soils is defined by the capabilities of the trenching support vessel and stability of the plough system. Ploughing in Sand Soils Sand soils generally result in slow trenching speeds (&lt; 250 m/hr) at high tow forces (&gt; 200 Te). Rate effects are very significant (Palmer 1997). This is a function of the density and permeability, therefore the particle size distribution, of the soil. Models for the behaviour of ploughs in sand are available in the public domain (e.g. Reese and Grinsted, 1986):</p> <p>T = (W p tan ( )) + (k1 h ) + k 2 h 3 v3</p> <p>(</p> <p>)</p> <p>(1)</p> <p>Where:</p> <p>T Wp k1 h k2 v</p> <p>-</p> <p>Tow load (Te) Submerged weight of the plough + supported pipe (Te) Soil / steel interface angle (degrees) Dimensionless factor derived from interface angle and submerged weight of soil Trench depth (m) Dynamic factor dependent on grain size and density Plough speed (m/hr)</p> <p>Coflexip Stena Offshore research and experience suggests that such models can only be used accurately within the limits presented in Figure 2. For particle size distributions with a significant proportion coarser than the right hand limit, rate effects diminish and speeds increase accordingly, due to the increase in the permeability of the soil mass. Where particle size distributions have a significant fines fraction, lying to the left of the limits shown in Figure 2, experience demonstrates that undrained failure mechanisms, and therefore faster trenching speeds, govern. It must be stressed, however, that ploughing in soils close to the left hand limits in Figure 2, incorporates a significant level of uncertainty as the failure behaviour of the soil mass is very sensitive. Ploughing in Layered Soils Predicting plough performance in layered soils is very difficult because the resistance across the ploughshare is non-uniform. Experience confirms that the differences in static and dynamic resistance (see Equation 1) can be significant. This can lead to surging of the plough, which contributes to increased as trenched Out-Of-Straightness (OOS) of the pipeline. Sand over soft clay trench profiles represent a high risk of OOS in this regard. Reduction of this risk is achieved by slow, controlled, ploughing. Performance predictions in layered soils are usually based on extrapolation of previous experience. Analytical models have been found to be unrepresentative. Further investigation is planned to develop accurate models for layered soil profiles. Experience has indicated that it is the soils at the share tip which govern plough speed. This is a function of these soils being more constrained. Jetting Jetting tools, predominantly ROV based, are significantly smaller than ploughs and many cutting machines with typical in-air weights less than 15 Te. Jetting tools are used for trenching both rigid and flexible lines in cohesionless and soft cohesive soils. Natural backfill or trench collapse is relied upon for backfill cover. Jetting has become the primary trenching method in deep water where soft soils are prevalent (Finch and Fihn, 1999). Jetting in Sand Sand soils are trenched by fluidisation, which reduces the submerged weight of the soil in the trenching zone significantly. This allows a denser product to settle to the base of the trench. This typically results in a depression in the seabed where the jetter has passed and the product buried some way below this depression. The effectiveness of any given jetting tool in sand soils is a function of: i. ii. iii. Jetting pressure Input water volume Layback of the product (i.e. touchdown length)</p> <p>In sands, pressure only has a significant influence when the soil is dense. In such cases it may be difficult to erode individual soil particles from the soil mass at the trench face and thus high pressure (i.e. increased energy) is beneficial. The influence of water volume introduced into the soil mass is significant. As the water volume increases the submerged density of the fluidised soil mass decreases for a given trenching speed. A fluidised soil mass will begin to reconsolidate beyond the influence of the jetting system. In most cases this is seen to be approximately 15 m behind the jetting tool. If the layback length of the pipeline is greater than this, the achievable depth of lowering will begin to reduce significantly, assuming a cable depressor is not employed. Flexible products, umbilicals and cables, which have relatively short layback lengths, can normally be trenched effectively in a single pass. Rigid pipelines, with longer layback lengths, often require multiple passes to achieve the specified depth of lowering.</p> <p>Analytical models have been developed by Coflexip Stena Offshore to predict the degree of fluidisation, layback lengths and reconsolidation time. Jetting in Clay Jetting tools form a trench in cohesive soils by cutting and clearing the soil from the trench. Cutting is achieved by forward facing high pressure jets with clearing of the resultant debris facilitated by low pressure jetting or eduction, either to the rear or vertically through eduction tubes (Finch and Fihn, 1999). This typically forms an open, rectangular, trench profile. Consequently the layback length of the product does not significantly influence trenching performance, unless trench collapse occurs within this span. The risk of trench collapse within the time taken for the product to layback (i.e. the risk of increased as-trenched OOS) can be estimated by analysing the stability of the trench walls and the time taken for the product to reach the base of the trench. The cutting of clay soils has generally been considered to be a function of water jet pressure. Therefore, the majority of the large jet trenchers currently being commissioned have significant pressure potential. However, recent research on behalf of Coflexip Stena Offshore has indicated that increased pressure above certain thresholds may prove inefficient (Machin, 2000). Water jet cutting of a cohesive soil is strongly influenced by jet nozzle orientation, resulting in bearing or slope failure mechanisms. In order to carry out analytical performance predictions, both the efficiency of the cutting and eduction systems must be assessed. This was carried out in detail during the development of the Perry Tritech TXL 500 jetting system (Finch and Fihn, 1999). Cutting Cutting tools form a trench by using penetrating cutting teeth mounted on wheels or chains, which rotate to remove the spoil. Cutting tool performance is governed by: i. ii. iii. Trench depth Soil conditions Available mechanical power and rotation speed</p> <p>The interrelation of these parameters is complex. In general performance prediction is carried out based on previous experience. Allowances should be made for abrasion and replacement of worn parts that can be very significant where granular soils are anticipated. Cutting tools can also experience difficulty in very plastic clays that may clog the cutting mechanism and significantly reduced performance. AS-TRENCHED OUT OF STRAIGHTNESS As-trenched Out-Of-Straightness (OOS) describes the residual imperfections in a pipeline profile following a trenching operation. The as-trenched pipeline profile is primarily driven by the undisturbed seabed profile, method of trenching and in-situ soil conditions. Critically, OOS events are potential initiation points for upheaval buckling (Richards, 1990). For a given wavelength, the larger the imperfection is the more backfill or remedial rock dump is required to mitigate upheaval buckling. Therefore, it is important to trench in a manner to reduce OOS. The accurate prediction of OOS at an early stage in pipeline system design is critical.</p> <p>OOS imperfections are a result of: i. ii. iii. iv. Original seabed profile, which may already contain imperfections Trench collapse before pipeline touchdown Debris falling under the pipeline during trenching Surging and change in pitch of trenching tool</p> <p>Virtually no OOS data is available in the public domain, however, Coflexip Stena Offshore have developed an extensive database based on high resolution survey data from over 150 km of ploughed rigid pipelines. Analysis of this database shows that for ploughing, the frequency of OOS is a function of local soil conditions. Figure 3 shows the typical trend for OOS frequency per kilometre for a large number rigid pipelines trenched using a pipeline plough. It should be noted that seabed features, such as sandwaves, have not been removed from these data. In order to assess upheaval buckling and predict download requirements (i.e. required backfill and/or rock volumes) at an early stage, both the frequency and the wavelength of any imperfection are required. Traditional predictive upheaval buckling assessments adopt the natural, or theoretical, wavelengths of the pipeline. This is very conservative. Review of in-house data clearly demonstrates that the wavelengthsof as-trenched OOS imperfections are greater than the corresponding natural wavelengths for a given pipeline. The data allows more accurate prediction of OOS at an early stage in a tender and therefore allows the early quantification of mitigation requirements and price. It must be stressed, however, that such analysis is predictive and the as-built mitigation can vary from those predicted at tender stage. BACKFILLING Backfill is placed over pipelines to provide download to resist upheaval buckling, to enhance a lines thermal insulation properties or to provide additional protection from physical impact. Backfilling Methods Burial of a trenched pipeline is achieved by either mechanical or natural backfill. Backfill over a trenched pipeline can be achieved in a number of ways. After ploughing a separate mechanical backfill plough is used to return deposited spoil heaps back into the trench. Cutting tools either create a vertical slot which is left to collapse naturally or open 'V' shaped trenches above which a second pass is carried using augers to screw deposited spoil back into the trench. Jetting tools normally rely on natural backfill. However, with the increasing need for thermal insulation, particularly for deepwater applications, tools have been developed to initiate trench collapse of vertical sided trenches in clay soils. For example, an undereaming method was developed for the TXL 500 jetting system (Finch and Fihn, 1999) and was successfully used for the Shell Angus pipeline project in the Gulf Of Mexico.</p> <p>Backfilling Issues There are two phenomena that must be considered before any backfilling operation: i. i. Uplift Floatation</p> <p>These issues have often been treated as a single problem called pipeline floatation (e.g. Cathie et.al. 1996). Recent research and backfilling experience has shown that the two phenomena must be considered separately. Uplif...</p>