Soil friction along HDD s the influence of a refined model on expansion

Transcription

1 REPORTS Gas pipelines Soil friction along HDD s the influence of a refined model on expansion by Frigco Kwaaitaal In the article the aspects influencing the built up of soil friction along HDD s are introduced. Based on these aspects a proposal is done how to deal with soil friction modeling along HDD sections in a day to day engineering practice. After definition of this approach a quantitative assessment on the proposed model itself and its impact on longitudinal elongation (expansion) along the HDD section are presented. This is done for three typical HDD sizes: DN100, DN600 and DN1200. The assessment shows that applying soil friction significantly influences the expansion built up along the HDD and in the end leads to a reduction of stress in the upper bends. By applying soil friction, the need of taking mitigating measures to divert expansion (e.g. expansion cushions, expansion loops) is lower for some cases. 1. INTRODUCTION The reason for setting up the investigations on modeling of soil friction was that in the new revision of the Dutch pipeline code a friction value of zero was introduced as a worst case approach. A literature survey showed that research generally was done on friction during installation (of relevance for the pull back of the pipe) but not for the operational phase of the pipeline. For pipelines under elevated temperature axial displacements (as a result of expansion) in longer HDD configuration increase quadratically and thus the loads on the upper bends of the HDD section also increase. In specific cases this may lead to stresses above the acceptable limits. To mitigate these stresses either the pipeline routing can be changed by incorporating expansion loops or other expansion measures can be taken such as placement of expansion cushions behind the bends. Taking into account any friction (versus no friction) along the HDD section seems to be crucial for a sound and structural reliable design of the pipeline section. Next to this, minimizing or even preventing mitigating measures will lead to a reduction of material and construction costs. 2. CURRENT SOIL FRICTION MODEL To model the soil friction a linear elastic behavior of the soil springs along the HDD is assumed. The maximum friction (W max ) is reached at small relative displacements. Main aspects influencing the soil friction W max are in general:. intergranular pressure around pipe; adhesion between pipe and soil; angle of friction between pipe and soil (dependent of soil friction angle and pipe wall roughness). To determine W max the following basic relationship is available for pipelines in open trench: (1) With: W soil friction along the pipe N/m πd o outside pipe circumference m K ratio horizontal/vertical intergranular pressure, in case of neutral horizontal soil pressure K equals K o =1 sinφ' - φ' angle of internal friction of soil 42 gas for energy Issue 2/2012

2 Gas pipelines REPORTS Figure 1. Soil friction acting on pipes in open trench compared to pipes in an HDD. σ k vertical intergranular pressure at pipe axis level N/m 2 tanδ friction coefficient between soil and pipe wall (depending on internal friction of soil and pipe wall roughness) a adhesion (only for clay or peat, equals the undrained cohesion parameter, c u ) N/m 2 Q eg deadweight of the pipe N/m Q vul weight of medium in the pipe N/m Q op buoyancy capacity of pipe N/m ite shell is present around the pipe. The bentonite shell around the pipe will stiffen and will act as a highly compressible layer. The combination of this compressible layer and the thickness of the soil column above the pipe leads to arching of the soil. In compressible soil types such as clay the arching effect fades out after installation. For non-compressible soil types (sand) the arching effect stays present and leads to a reduced vertical intergranular pressure on the pipe (see Figure 2). The build up of friction in a HDD differs significantly from a pipe in open trench. In HDD crossings the borehole is filled with drilling fluid (bentonite slurry) which stiffens after installation. After a period of time the drilling fluid around the pipe acts as a shell. The friction distribution on this shell acts on two interfaces (shear planes): the pipe-bentonite interface and the bentonite-soil interface. Another difference is that due to the installation at large depth soil arching occurs (depending on the soil type) which can reduce the vertical intergranular pressure in the soil surrounding the pipe. The modelling of W max differs for a pipeline in open trench from a pipeline in a HDD section. In Figure 1 an overview is given how W max is built up in both cases. 2.1 Arching in soils and determination of intergranular pressure in HDD s Under normal circumstances the vertical intergranular soil pressure is determined by the weight of the soil column above the pipe. Just after drilling the pilot hole arching occurs and after installation of the pipe a benton- Figure 2. Arching around borehole. Issue 2/2012 gas for energy 43

3 REPORTS Gas pipelines Based on the theory of Terzaghi it is considered that arching occurs when the thickness of the soil mass extending above the pipe is larger than 4 times the width of the soil column in shear (2B 1 ). The width B 1 is defined as: (2) With: B 1 is half the width of the soil column in shear m D o is the outside diameter of pipeline m φ is the internal angle of friction R is the radius of the borehole m A reduction of the vertical soil load due to arching results in a lower vertical intergranular pressure (σ k ) on the pipe and thus reduces the overall friction. For the friction model in HDD sections is assumed that when arching occurs the contribution of the vertical intergranular pressure is totally neglected. At soil covers H 8B 1 (when arching is assumed) the actual vertical intergranular pressure at top of pipe level is used. For larger soil columns arching will occur and the vertical intergranular pressure then is assumed to be zero. For pipes in multiple layer soils the approach for determination of the vertical intergranular pressure is similar. It should be noted that according to the theory arching will only occur in a sand layer at a depth of H 8B 1 below the layer separation. In the new approach arching is assumed to be effective as of the layer separation. Immediately taking into account arching over this layer and thus neglecting the build up of vertical intergranular pressure over the layer depth for H<8B 1 is a conservative approach. 2.2 Friction due to curvature of pipeline During pull back of the pipe along the HDD borehole the elastic bend is cold formed by applying a torque. Since the pipe has an axial stiffness which cannot be neglected mechanically equilibrium reaction forces will occur in the soil, see Figure 3. The distribution of the soil reaction forces is based on the theory for beams on elastic foundations by Hétenyi. The soil reaction forces will occur at the ends of each bend and induce additional frictional effect acting on the pipe. The additional normal force in the elastic bend as a result of this torque can be calculated by: (3) With: T 3b is the additional normal force for one bend in the borehole N q r is the maximum soil reaction near the end of the bend: N/m k v is the vertical modulus of sub grade reaction N/m 3 y is the maximum displacement m λ is the pipe-soil stiffness characteristic m- 1 EI is the bending stiffness of the pipe N m 2 D o is the outside diameter m R is the radius of the bend m f 3 is the frictional coefficient between the pipe and the borehole wall f 3 = 0,2-2.3 Proposal new friction model along HDD s Parallel to the friction formula for pipelines in open trenches and based on the elaborations above a slightly adjusted friction formula for HDD techniques can now be introduced: (4) Formula 4 represents the new approach to model friction along HDD sections. Compared with basic friction formula (1) two new parameters are introduced W T3b and f 2 : W T3b is introduced to take into account the friction as a result of the curvature of the pipe in the elastic bends. f 2 replaces the adhesion component and takes into account the friction between pipe and drilling fluid (bentonite). The other parameters are identical with formula 1 but might have a different value because of the application on HDD sections. 2.4 Frictional effect pipe-drilling fluid interface After installation of the pipe in the borehole the drilling fluid layer (bentonite) will stiffen (see Figure 4). The adhesion at the pipe-bentonite friction plane will as a result develop to a certain level. Under open trench conditions (see formula 1) the adhesion contribution is 0,6a and only applicable for cohesive soils (e.g. clay and peat); for granular soils there is no adhesion. Taking into account a pipe section installed in a borehole and surrounded with bentonite, the adhesion is set equal to the dynamic friction of the bentonite. This value f 2 is irrespective of the stiffness of the bentonite. The shear stress in the bentonite reaches a static threshold value (τ y,static ) which for increasing displacements will drop to a dynamic value (τ y,dynamic ). This 44 gas for energy Issue 2/2012

4 Gas pipelines REPORTS dynamic value is assumed to be equal to the dynamic friction of bentonite: f 2 =50 N/m 2 and is used in formula 4 since displacement under operational conditions are relatively large. 2.5 Frictional effect pipe-borehole wall interface Parallel to the pipe-drilling fluid interface a friction distribution at the pipe-borehole interface plane must be taken into account (see Figure 4). The generated friction at this plane depends in case of no arching on the vertical intergranular pressure (vertical soil load Q n ). Independent of the occurrence of arching another part of the friction is also generated by the deadweight/buoyancy effects on the pipe in the borehole. Results on direct shear tests show that at low levels of bentonite mixed with sand the internal frictional angle δ (and thus friction value) drops significantly. I.e. for 100% sand mixture δ=22 and for a 10% sand-bentonite mixture δ=12. Since no empirical values of other types of mixture are available and the internal friction angle seems to be relatively independent of the soil surrounding the bentonite shell this value is assumed to be generally applicable for pipes in HDD sections. also identical for all typicals: sand with a moderate density with a ground water table of 1 m below surface level. Based on the formula 4 the soil friction is calculated for each typical per critical cross section along the HDD: at the upper bend (R=40D o ); at the arching tipping point (soil cover = 8B 1 ); at the start of the elastic bend; at the center of the floor pipe (symmetrical point of HDD). To assess the differences in friction, contribution for each typical diameter in a plot is made of the friction along the borehole per section for each typical pipe diameter. u 3. ASSESSMENT OF NEW APPROACH Based on formula 4 and the approach and assumptions above for determining the overall friction a break down for each component contributing in the friction along 3 typical HDD configuration is assessed in this paragraph (see Figure 5). By this the effect of each friction component can be evaluated per typical. The three typical HDD configurations are based on realistic engineering cases in the Dutch gas grid. Considered are three typical diameters, namely DN100, DN600 and DN1200. The DN100 typical consists of a pipeline with a diameter of 114,3 mm with a wall thickness of 6,0 mm. The total length of the HDD is 200 m and the elastic bend radius is 115 m. The exit and entry angles are 11. The DN600 typical consists of a pipeline with a diameter of 610 mm with a wall thickness of 11,1 mm. The total length of the HDD is 500 m and the elastic bend radius is 610 m. The exit and entry angles are 11. The DN1200 typical consists of a pipeline with a diameter of 1219,1 mm with a wall thickness of 19,1 mm. The total length of the HDD is 1000 m and the elastic bend radius is 1700 m. The exit and entry angles are 7. The upper bends in all typicals have a radius of 40 times the external diamater (40D o ) and the same wall thickness as the rest of the HDD. The soil conditions are Figure 3. Additional friction due to elastic curvature in bends. Figure 4. Distribution planes for friction in HDD. Issue 2/2012 gas for energy 45

5 REPORTS Gas pipelines on the system stiffness of the total HDD crossing: pipe diameter, wall thickness, bend configuration, material. The soil parameters are schematized and calculated according to the formulas and spring models of the soil as defined in the Dutch design code NEN : 2003, except for friction. The external loads taken into account in the calculation models are: DN100: p d =40 barg and ΔT=35 C DN600: p d =80 barg and ΔT=45 C DN1200: p d =80 barg and ΔT=45 C Figure 5. Distribution of friction along the borehole per typical diameter. In the full study two load combinations have been taken into account: p d + ΔT (load case 4) and ΔT (load case 3). The results for load case 4 show similar results as for load case 3. For this publication only the results of load case 4 are presented. Load cases based on NEN. 4.2 Expansion results As can be noted from Figure 5 the friction for the DN100 typical is very low and has a uniform distribution along the HDD. For the larger diameters the built up of friction occurs between cross section A and C. The top soil load component over this part of the HDD is the governing factor. The contribution of the curvature component in the total friction becomes larger for smaller diameters. The friction along the floor pipe is largely determined by the buoyancy or deadweight effect of the pipe in the borehole. In general the influence of the adhesion component (between pipe and drilling fluid) on the overall friction is very limited. 4. EXPANSION EFFECTS OF NEW FRIC- TION SCHEMATIZATION ALONG HDD To determine the expansion effects of applying friction along a HDD the three typical crossings are assessed by means of a dedicated finite element software package for subsoil pipelines (PLE 1 ). 4.1 Calculation model HDD typicals In PLE the pipeline is modeled as an elastically supported beam in which the soil surrounding the pipe is described with elastic springs. The reactions (displacements, cross sectional reaction forces) resulting from the movements of the pipe due to the soil and external loads (e.g. temperature and internal pressure) are calculated and depend In the next paragraphs the expansion results for each typical is assessed based on the PLE calculation. The results for the DN100 typical show that the reduction of axial force in cases when friction is applied is negligible compared to the case without friction. Increasing the diameter influences the distribution of forces and the absolute value of forces significantly. To show the influence of the modeled friction the resulting axial forces over the HDD are plotted in graphs and presented in Figure 6 for the DN600 and DN1200 typical. The axial force at the upper bends for the DN600 typical is about 111% (1,1x) lower in case of friction compared to no friction. The axial force at the upper bends for the DN1200 typical is 1200% (12x) lower in case of friction compared to no friction. The force in the floor pipe increases for both typicals: about 29% for the DN600 and about 28% for the DN1200. The increase of force in the floor pipe is caused by the applied friction which increases the displacements of the total HDD and as a result reduces the forces at the upper bends. 5. CONCLUSIONS Based on the assessment of the proposed friction model and the calculations the following conclusions can be drawn: Neglecting friction in HDD s is not realistic especially not for pipes with larger diameters. At an increase of diameter application of friction seems 1 PLE software is developed and distributed by EDS, Rijswijk, The Netherlands and is generally used for assessment of pipeline crossings according to the Dutch national pipeline code: NEN 3650 series. 46 gas for energy Issue 2/2012

6 Gas pipelines REPORTS to progressively reduce the forces on the critical parts of the HDD (the upper bends). For smaller diameters the difference between application or neglecting of friction is negligible. The proposed schematization is a practicable method to refine the soil friction based on existing theories. In general it can be concluded that by applying friction in the proposed way will lead to more structural reliable pipeline section and can in some cases lead to prevention or limitation of mitigating measures such as expansion cushions or loops. To verify the proposed theoretical calculation model it is recommended to do further investigations by monitoring on real-time HDD s and observing if the noticed effects in this study are actually occurring. axial friction F = 0 N/m axial friction F = calculated REFERENCES: [1] Stability of borehole during Horizontal Directional Drilling, Thomas Viehöfer et al. [2] Soil deformations due to Horizontal Directional Drilling Pipeline Installation, G.M. Duyvestyn and M.A. Knight, Proceedings NORTH AMERICAN NO-DIG 9-12 April [3] Experimental Investigation of Borehole and Surface Friction Coefficients During HDD Installations, G. El-Chazli et al., Proceedings NASTT NO-DIG April [4] Modeling the soil pipeline interaction during the pull back operation of horizontal directional drilling, J.P. Pruiksma and H.M.G. Kruse. [5] Dutch Standard NEN A1 Requirements for pipeline systems part 1 General Quire 1 to 6, distributed by NNI, Delft, August [6] Tebodin report Berekenen van de wrijving voor leidingen gelegd in open ontgraving d.m.v. HDD techniek, Hengelo, November axial friction F = 0 N/m axial friction F = calculated Figure 6. Distribution of resulting axial force for DN600 and DN1200 typical. Soil friction, horizontal directional drillings, underground pipelines, expansion AUTHOR Frigco Kwaaitaal Engineering Manager Tebodin Middle East Ltd. Dubai United Arab Emirates Phone: fkwaaitaal@tebodinme.ae Issue 2/2012 gas for energy 47