Validation of OLGA HD against transient and pseudo-transient experiments from the SINTEF large diameter high pressure flow-loop G Staff 1, D Biberg 1, T Vanvik
1, N Hoyer 1, J Nossen
2, H Holm 3, P S Johansson
3 1 Schlumberger, SPT TC, Norway 2 Institute for Energy Technology (IFE), Norway 3 Statoil, Norway
ABSTRACT In this paper we have compared the OLGA HD stratified flow model against transient ramp-up and pseudo transient experiments from a Statoil funded experimental campaign performed at the SINTEF Multiphase Flow Laboratory. The experiments were designed to specifically target the transition point from low to high liquid holdup; the accumulation point. We have explained how the removal of the liquid, when the gas flow is increased to inside the multiple holdup solution region, can be described by a steady state fully developed flow approximation. OLGA HD showed excellent predictions compared to the accumulation point experiments. We have also compared the time needed to remove the liquid for a fully transient simulation of experiments with ramp-up into the multiple holdup solution region. A retuned version of OLGA HD, presented in [1], performed well also on the experiments where OLGA 2014.1 HD overpredicted the time needed to remove the liquid. 1 INTRODUCTION Multiphase flow simulations of gas-condensate pipeline transport are a challenging task. Important properties are the pressure drop for high rates and liquid accumulation at low rates; factors contributing to determining the operational envelope of the field. However, not only the liquid content and pressure drop at steady operating conditions are important but also behaviour during transient operations like rate changes and outlet pressure changes. The arrival time and the rate of the liquid after ramp-up may be critical factors when designing liquid receiving facilities and operational guidelines. The work described in this paper originates from a project funded by Statoil where the uncertainty of OLGA for a gas-condensate field offshore Tanzania has been evaluated. The Tanzania project is described in more details in [2]. The project consists of two parts: the “Large Scale Liquid Loading Two-phase Flow Tests” campaign carried out at the SINTEF Tiller large scale test facility, and the “Core Model Evaluation and Flow Assurance Risk Study” done by Schlumberger. The experiments are described in more detail in [3]. The OLGA HD stratified flow model is the next generation flow model for stratified flows, the predominant flow regime in a gas condensate pipeline. In this paper we will validate the OLGA HD 2014.1 flow model against some of the transient and pseudo-transient experiments done in the Large Scale Liquid Loading Two-phase Flow Tests campaign.
2 RAMP-UP AND THE CONNECTION TO MULTIPLE SOLUTIONS The new data from the SINTEF Multiphase Flow Laboratory consist of measurements for both two and three phase flow. For simplicity we will in this paper only consider the two-phase experiments. 2.1 The two fluid model For two-phase flow OLGA applies the two-fluid model. We assume isothermal conditions and zero mass transfer. Mass conservation is given by + = 0, (1) + = 0, (2)
where , are the gas/liquid densities, , are the gas/liquid (bulk) velocities and
, are the gas/liquid volume fractions. Momentum conservation is given by + = − − − − sin − cos ℎ , (3)
+ = − − + − sin − cos ℎ , (4)
where , , are the gas-wall, liquid-wall and gas-liquid interfacial shear stresses,
, , are the gas-wall, liquid-wall and gas-liquid interfacial wetted lengths, ℎ is the height of the gas-liquid interface, is the pipe inclination relative to the horizontal plane,
is the acceleration due to gravity and A is the pipe cross sectional area. Assuming steady state fully developed flow the mass balance equations (1)-(2) are trivially true and the momentum balance (3)-(4) reduces to − = + − sin , (5) − = − − sin . (6)
(5)-(6) are also approximately valid as long as the neglected dynamic terms in (3)-(4) are small compared to the gravity and friction terms. Eliminating the pressure drop between (5) and (6) gives the corresponding holdup equation: 1 − − g sin = − 1 − + (7) where the gravity term balances the friction forces.
2.2 The OLGA High Definition (HD) stratified flow model The OLGA High Definition (HD) stratified flow model was developed to meet the needs for more reliable predictions of pressure drop and liquid inventories in ultra-long gas condensate pipelines, by combining the consistency of a 3D model with the evaluation speed of a 1D model. The OLGA HD model represents a significant improvement in consistency over traditional 1D models for stratified pipe flows by accounting for the 2D velocity distribution over the pipe cross section, which in combination with the 1D (axial) conservation equations gives a 3-D flow description for a gradually evolving stratified flow. The velocity distribution determines the frictions and momentum flux terms leaving the closures of traditional 1D models redundant. Integrating the flow description over the pipe cross section gives the corresponding 1D result: a pre-integrated 3D flow description that executes with a similar speed as a 1D model, see Biberg et al. [1]. The OLGA HD development was initiated by the HORIZON Joint Industry Project sponsored by Chevron, Conoco, ENI, Exxon, Shell, Statoil and Total, see Biberg et al. [4]. 2.3 Multiple solutions for the steady state fully developed two-phase model The holdup equation for fully developed steady state flow (7) may exhibit multiple solutions depending on the input parameters. It is argued, e.g. by Barnea and Taitel in [5], that when multiple solutions exist, the low solution is always stable, the middle solution is linearly unstable and the high solution is non-linearly unstable for large perturbations. These results are supported by the results in Section 4.2. In OLGA-S, the steady state fully developed flow version of OLGA, it is always assumed that the lowest holdup solution is the correct solution. In this paper, OLGA-S will not refer to the commercial product OLGA-S, but the physical model inside it. Figure 1 shows the liquid holdup for different superficial gas velocities. For this case we notice a large region with multiple solutions. In the following, the abbreviations USG and USL are the superficial gas and liquid velocity respectively. USG1 exhibits only a single solution α1 which is a relatively high solution for the liquid holdup. If the gas rate is ramped up from USG1 to USG3, still only a single solution exists; however, α3 is a low solution for the liquid holdup. If the gas rate is ramped up from USG1 to USG2, the liquid holdup will, according to the theory that the low solution is the stable one, end up at the low solution α2
low. In this paper we will study ramp-up scenarios similar to increasing the gas rate from USG1 to USG2 and how the liquid is removed during the transition from one stable solution α1 to the other stable solution α2
low. We will call this situation a ramp-up into the multiple solution region. 2.4 A ramp-up into the multiple solution region The OLGA HD simulation of the liquid removal for a case where the gas rate is increased to inside the multiple solution regions is shown in Figure 2. The liquid is removed in three sweeps.
1. Sweep 1; A hydraulic gradient is propagating from the inlet to the outlet, at high speed (top plot Figure 2)
2. Sweep 2; A hydraulic gradient is slowly propagating from the outlet to the inlet (middle plot Figure 2)
3. Sweep 3; A hydraulic gradient is slowly propagating from the inlet to the outlet, but faster than in sweep 2 (bottom plot Figure 2)
Figure 1. Liquid holdup for different superficial gas velocities. Ramp-up from
USG1 to USG2 takes the solution into the multiple holdup solution region.
Figure 2. Ramp-up from outside the multiple solution region into the multiple
solution region. There exist three distinct propagating hydraulic gradients. H1 is the liquid holdup for the steady state solution prior to ramp-up. H4 is the liquid holdup for the steady state solution after the ramp-up.
There exist two quasi-stable liquid holdup solutions H2 and H3. By using a simple mass balance it can be shown that the average propagating speed of
the hydraulic gradient is given by = . Nossen et al argued in [6] that
in the SINTEF experiments studied here, the liquid rate at the outlet during the second and third sweeps is given by the liquid rate at the accumulation point for the current gas rate. For the example in Figure 2 the difference in the liquid holdups H2 and H3 (second sweep) is larger than the difference in the liquid holdups H3 and H4 (third sweep), explaining the difference in propagation speed of the hydraulic gradient for the second and third sweep. The discussion of sweep 1 is outside the scope of this paper.
The liquid holdup levels of the dynamic ramp-up corresponding to the quasi stationary solutions H2 and H3 can be related to the multiple holdup solutions for a fully developed steady state flow as given by (7), see Figure 3. We argue that H2 and H3 in Figure 2 are α2
high and α2acc in Figure 3, based on the results from [6] that the liquid outlet rate is the
liquid rate at the accumulation point. By ramping up to USG3 we are outside the multiple holdup solution region and all the surplus liquid is removed in a single sweep.
Figure 3. Multiple solution curve for the inlet superficial liquid velocity and the superficial liquid velocity at the accumulation point for superficial gas velocity
USG2. Liquid holdup for USG1 is given by α1. Liquid holdup for USG2 after steady state is reached is given by α2
low. The two intermediate holdup solutions during the liquid removal; H2 and H3 in Figure 2, are given by the high holdup solution α2
high for (USG2,USLinlet) and the holdup solution α2
acc at the accumulation for (USG2,USLacc).
3 DESCRIPTION OF THE SINTEF EXPERIMENTS Through the Statoil funded experimental campaign at the SINTEF Multiphase Flow Laboratory Large Scale Loop at Tiller, several steady state, transient and pseudo transient experiments were conducted. Most transient experiments were ramp-up experiments into and beyond the multiple solution region. During the experiments it was noticed that the liquid outlet rate during sweep two and three (see Figure 2) was not influenced by the inlet rate. We argued in Section 2.4 that this corresponds to the accumulation point, see also Nossen et al. [6]. As a result of this discovery, experiments were performed where the liquid inlet rate was adjusted to keep the experiment in the second sweep situation (middle plot Figure 2). When the gas rate was increased, the liquid outlet rate quickly stabilized on the liquid rate determined by the accumulation point. This allowed for frequent increments of the gas rate and hence the measurement of the corresponding liquid outlet rate at the accumulation point at several gas rates. These experiments will be referred to as pseudo-transient experiments, or as screening experiments. The liquid holdups on both sides of the hydraulic gradient were measured in addition to the liquid outlet rate.
For more details on the SINTEF experiments we refer to Kjølaas & Holm et al. [3]. 4 COMPARING THE EXPERIMENTS TO OLGA HD STRATIFIED
MODEL 4.1 Comparing against the screening experiments We will compare both OLGA HD and OLGA-S HD against the screening measurements. The OLGA HD simulations are fully transient simulations. The details of the OLGA model can be found in Section 4.1.1. OLGA-S HD is a model for steady state fully developed two-phase and three-phase flow, similar to (7). The OLGA-S HD and OLGA HD comparisons are included to show the validity of the steady state fully developed assumption for the conditions in the SINTEF experiments. This is important for justifying the explanation of the ramp-up into the multiple solution region, given in Section 2.4. 4.1.1 OLGA model of the experimental setup The OLGA model is shown in Figure 4. The gas source is changed by a manual controller allowing the gas rate to be ramped up in several steps. To prevent the removal of all the liquid in the pipeline, another controller increases the flow rate of the liquid source when the liquid level at the centre of the pipeline drops below a critical value. The fluid file has been constructed based on the received fluid composition, and the liquid viscosity and gas-liquid surface tension have been adjusted to match the measured values.
Figure 4. OLGA model for the screening experiments.
4.1.2 OLGA HD comparison against the screening experiments The main comparison will be against the measured flow rates. For the screening experiments, all rates will be shown as the Froude numbers = − cos / (8)
where f is gas or liquid. As described in [3], 5 different gamma densitometers located along the test section allowed the holdup before and after the hydraulic gradient to be measured, equivalent to
α2high
and α2acc
from Figure 3. Early in the measurement campaign, the liquid holdup values were not considered important and no special care was taken to keep the hydraulic gradient in the middle of the pipe. Consequently, for some of the measurements the hydraulic gradient influenced the holdup at the inlet or the outlet depending on the position of the hydraulic gradient. The liquid holdup measurements with severe influence by the hydraulic gradient have been removed from the comparisons.
The error is presented as mean error, Root Mean Square error (RMS) and the percentage of comparisons within ±5,10 and 20%. We will show the errors both in superficial gas and liquid velocity. The error in the superficial gas velocity is considered to be the most relevant error since the gas volume rate is very close to the total volume flow rate in a typical gas-condensate field. Consequently, the uncertainty in the flow rate for a specific behaviour of the field is best represented by the error in the superficial gas rate. However, the error in the superficial liquid velocity will be important when comparisons against the liquid removal from the full ramp-up experiments are discussed in Section 4.2. Overall the predictions from OLGA HD are good compared to the measured superficial gas velocity. The error in the superficial liquid velocity is relatively larger due to the small values of the superficial liquid velocities and the non-linear increase in the superficial liquid velocity at the accumulation point as a function of the superficial gas velocity. The error statistics for the superficial velocities can be found in Table 1 and the comparisons are shown in Figure 5. Table 1 Relative error statistics for superficial gas (USG) and liquid (USL) velocity.
Figure 5. Computed vs Measured for USG and USL. The dashed lines are
±20% error. The measurements have been sorted for different pipe diameters and inclinations in order to better show how they affect the predictions. The results can be found in Figure 6-Figure 11. The continuous lines are the results from the OLGA-S HD simulations, the small dots are the results from the transient OLGA HD simulations while the large dots are the measured values. The left plot contains the liquid Froude number at the accumulation point for different gas Froude numbers. The right plot contains the liquid holdup upstream and downstream the hydraulic gradient, corresponding to α2
high and α2acc
from Figure 3. α2high is the upper line while α2
acc is the lower line in the right plots. Notice that for increasing gas Froude numbers the low holdup solution, α2
The explanation is that at the liquid accumulation point the liquid Froude number increases for increasing gas Froude number, increasing the liquid flow rate. The agreement between the quasi steady state results from the transient OLGA HD simulation and the steady state fully developed approximation from OLGA-S HD is very good. This supports the claim that the steady state fully developed model (7) is a good approximation and hence a good explanation model. However, the deviation in the high liquid holdup solution predicted by OLGA HD compared to OLGA-S HD increases for lower gas rates. This is because a lower gas rate gives a lower liquid rate and hence a larger difference in the holdup across the hydraulic gradient, α2
high and α2acc respectively,
and consequently the influence of the hydraulic gradient increases for this relatively short pipeline. When the length of the pipeline is increased, the deviation between OLGA HD and OLGA-S HD is reduced. The predictions of the liquid holdup are good overall, considering the uncertainty in the holdup measurements due to the influence of the hydraulic gradient. The deviation compared to the measurements is larger for the 8'' 5 degrees inclined pipe and the 12'' 1 degree inclined pipe. We have no good explanation of why the holdups for these two configurations are predicted less accurately than the remaining configurations. Note also that for the very low holdups, the measurements are in the order of the measurement uncertainty which in [3] has been estimated to 0.001 volume fraction.
Figure 6. Comparisons against the 8 inch 1◦ inclined pipeline experiments.
Figure 7. Comparisons against the 8 inch 2.5◦ inclined pipeline experiments.
4.2 Comparing against transient ramp-up scenarios The OLGA cases for the ramp-up experiments were similar to the OLGA case for the screening experiments presented in Figure 4, except for the controller on the liquid flow rate. The time series for the outlet pressure, the gas and liquid source flow rate were extracted from the measurements and applied to the OLGA cases. All cases were initialized using the OLGA HD steady state pre-processor and then simulated with constant properties till the flow was fully developed, before the flow rates and outlet pressure were changed according to the measurements. We will compare the computed liquid holdup against the liquid holdup measurements from the 5 different gamma densitometers. The instruments were positioned at 10, 28, 47, 66 and 85 m measured from the start of the inclined pipeline. For more details on the experimental setup we refer to [3]. By looking at the change in liquid holdup as a function of time we are measuring the time it takes to remove the liquid, given by = −− , (10)
where L is the length of the pipe from which the liquid is removed, and are the liquid holdups before and after the sweep, and , are the superficial liquid velocities into and out of the pipeline. Consequently, the three computed values contributing to the difference in predictions compared to the measurements are
, and . From these three computed values, is normally the most sensitive value and the sensitivity increases with the reduction of − . In Figure 13 the error in the time required to remove the liquid is shown as a function of the error in the prediction of the superficial liquid velocity at the accumulation point. The left plot shows an example where the gas is increased from USG1=3 m/s to USG2=3.09 m/s. For this example the inlet superficial liquid velocity is set to the superficial liquid velocity at the accumulation point of USG1=3 m/s. For a predicted close to
, the error in the liquid removal time goes to infinity as shown in the right plot in Figure 13. If is predicted to be lower than , the pipe will accumulate liquid instead of removing the liquid. For this example we have assume no error in the predicted liquid holdup values. To make the results more readable, both the measurements and the OLGA HD results have been averaged using a moving average algorithm to remove noise and high frequency waves. We will show comparisons for three different ramp-up cases using two OLGA HD versions; the 2014.1 HD version and a retuned HD version described in [1]. In Figure 12 the three different cases are illustrated. Case 1 is outside the multiple holdup solution region while both case 2 and 3 are ramped up into the multiple holdup solution. Case 2 has the lowest inlet liquid rate while case 1 has the highest.
Figure 12. Illustration of the ramp-up situation for the transient three cases.
Figure 13. (Left) The superficial liquid velocity at the accumulation point as a
function of the superficial gas velocity. (Right) The relative error in the time needed to remove the liquid, as a function of the error in the superficial liquid velocity.
We assume no error in the predicted liquid holdup. 4.2.1 Results using OLGA 2014.1 HD 4.2.1.1 Case 1: Ramp-up for a case outside multiple solution region, 8'' 2.5 degrees
inclined pipe The gas rate is ramped up to a high rate as shown in the left plot in Figure 14. Case 1 has a higher liquid loading and does not exhibit multiple solutions as seen from the right plot in Figure 14. In Section 2.4 we argued that when the case is not ramping up to inside the multiple solution region, all the liquid is removed in a single sweep. This is confirmed from Figure 15 where both the OLGA HD simulation and the experiments show only a single sweep. We notice that OLGA HD underpredicts the liquid holdup. The measurements show different liquid holdup values for the different gamma densitometers, however, it is believed that for this case some of the difference is due to measurement uncertainty since there is a non-consistent behaviour of the liquid holdup measurements along the pipeline.
Figure 14. Case 1, 8'' 2.5 degrees inclined. (Left) Gas/liquid Froude numbers and
outlet pressure taken from the experiment. (Right) The liquid holdup solution curve shows no multiple solution region. Consequently the ramp-up from gas Froude
number Frg1 to Frg2 removes the surplus liquid ( − ) in a single sweep.
Figure 15. Case 1, 8'' 2.5 degrees inclined. Liquid holdup values for 5 different
positions in the pipeline. The surplus liquid is removed in a single sweep which is in agreement with the right plot in Figure 14 showing no multiple solution region.
(Top) OLGA HD simulation. (Bottom) Measured values. 4.2.1.2 Case 2: Ramp-up to medium high gas rate, 8'' 2.5 degrees inclined pipe The gas rate is ramped up to a medium rate inside the multiple holdup solution region as shown in the left plot in Figure 16. The right plot in Figure 16 shows the multiple liquid holdup solution curve for the liquid Froude number at the inlet and the liquid Froude number at the accumulation point for the ramp-up gas Froude number, Frg2. From Figure 17 we notice that OLGA HD severely overpredicts the time needed to remove the liquid. The two main contributing factors are; OLGA HD underpredicts the liquid outlet rate and
overpredicts the liquid holdup ahead of the second sweep (equivalent to H2 in Figure 2). Both are consistent with the results in Figure 7. From the experiments in
Figure 17 we recognize the four different holdup values, , , , , as shown in the right plot of Figure 16. The OLGA HD simulation has not reached the third sweep inside the timeframe in Figure 17 and consequently we only see the first three holdup
values , , .
Figure 16. Case 2, 8'' 2.5 degrees inclined. (Left) Superficial gas/liquid rate and outlet pressure taken from the experiment. (Right) The multiple holdup solution
curve for the inlet liquid Froude number (Frlin) and the outlet liquid Froude number (Frlout) determined by the accumulation point for the ramp- up gas
Froude number (Frg2). The dash-dot line is the liquid Froude number at the accumulation point for the different gas Froude numbers.
Figure 17. Case 2, 8'' 2.5 degrees inclined. Liquid holdup values for 5 different positions in the pipeline. (Top) OLGA HD simulations. (Bottom) Measured values.
4.2.2 Results using retuned OLGA HD In [1], Biberg et al. described a retuned version of OLGA HD and showed the improvements on several datasets. We will here show the performance of the retuned OLGA HD version on some of the ramp-up cases. 4.2.2.1 Case 2: Ramp-up to medium high gas rate, 8'' 2.5 degrees inclined pipe We re-ran Case 2 using the retuned HD version from [1]. The gas rate is ramped up to a medium rate as shown in left plot in Figure 18. The right plot in Figure 18 shows an increase in prediction of the superficial liquid velocity at the accumulation point compared to the results from OLGA 2014.1 HD (ref Figure 16) which suggests that the time to remove the liquid is reduced. This is confirmed by the results in Figure 19. The simulations are qualitatively very similar to the measurements and the time needed to remove the liquid is less overpredicted. 4.2.2.2 Case 3: Ramp-up to low gas rate, 12'' 5 degrees inclined pipe The gas rate is ramped up to a rate just inside the multiple solution region. The right plot in Figure 20 shows the multiple liquid holdup solution curve for the liquid Froude number at the inlet and the liquid Froude number at the accumulation point for the ramp-up gas Froude number, Frg2. The small difference between the liquid rate in and out of the pipeline makes it challenging to correctly predict the time needed to remove the liquid. From Figure 21 we see that retuned OLGA HD very accurately predicts the time needed to remove the liquid. The liquid holdup is slightly underpredicted.
Figure 18. Case 2, 8'' 2.5 degrees inclined using retuned HD. (Left) Superficial
gas/liquid rate and outlet pressure taken from the experiment. (Right) The multiple holdup solution curve for the inlet liquid Froude number (Frlin) and the outlet
liquid Froude number (Frlout) determined by the accumulation point for the ramp- up gas Froude number (Frg2). The dash-dot line is the liquid Froude number at
the accumulation point for the different gas Froude numbers.
Figure 19. Case 2, 8'' 2.5 degrees inclined using retuned HD. Liquid holdup values
for 5 different positions in the pipeline. (Top) Retuned OLGA HD simulations. (Bottom) Measured values. The time needed to remove the liquid is reduced
compared to OLGA 2014.1 HD (ref Figure 17).
Figure 20. Case 3, 12'' 5 degrees inclined using retuned HD. (Left) Superficial
gas/liquid rate and outlet pressure taken from the experiment. (Right) The multiple holdup solution curve for the inlet liquid Froude number (Frlin) and the outlet
liquid Froude number (Frlout) determined by the accumulation point for the ramp- up gas Froude number (Frg2). The dash-dot line is the liquid Froude number
at the accumulation point for all the gas Froude numbers.
Figure 21. Case 3, 12'' 5 degrees inclined using retuned HD. Liquid holdup values
for 5 different positions in the pipeline. (Top) Retuned OLGA HD simulations. (Bottom) Measured values.
5 CONCLUSIONS The behaviour of the liquid removal during a ramp-up scenario into the multiple holdup solution region can be approximated by a steady state fully developed model, indicating that for this scenario the dynamic terms from the momentum equation (3)-(4) are negligible compared to the friction and gravity terms. This is supported by the agreement between the transient solver OLGA HD and the steady state fully developed model in OLGA-S HD, and the agreement with the transient ramp-up and the pseudo-transient screening experiments done by SINTEF, [3]. This shows that the friction model is crucial for correctly predicting the arrival time and rate of the liquid at the receiving facility. The results in this paper do not indicate that a ramp-up scenario for an entire pipeline can be well predicted using a steady state fully developed approximation. In a real pipeline the pressure and fluid properties will change with time and the gas and liquid rate will not be constant, which will potentially move the system in and out of the multiple holdup solution region, making the scenario impossible to predict except by using a full transient solver. Still, the newly acquired understanding of the physics behind the liquid removal should be useful. The OLGA 2014.1 HD stratified model reproduces the SINTEF liquid accumulation experiments well, both for the liquid holdup H2 upstream and H3 downstream of the hydraulic gradient (Figure 2), and the liquid rate out of the pipeline. By looking at the
error relative to the gas rate, 79% of the experiments are reproduced within ±10% and 99% within ±20%. The prediction of the time needed to remove the liquid for a ramp-up scenario is influenced by the computed liquid holdups, and in particular the liquid rate at the accumulation point. OLGA 2014.1 HD predicts the liquid removal time well for some cases but overpredicts for cases where the gas rate is just inside the multiple holdup solution region. The retuned version of OLGA HD, presented in [1], predicts also these challenging cases well. 6 ACKNOWLEDGMENTS The authors would like to thank Statoil for funding parts of this work and for the permission to publish the results. We would also like to Chris Lawrence from Schlumberger for many useful discussions. In addition we would like to thank Jørn Kjølaas and Marita Wolden from SINTEF for their valuable help in getting the details of the experiments correct. REFERENCES [1] D. Biberg, G. Staff, N. Hoyer and H. Holm, "Accounting for flow model
uncertainties in gas-condensate field design using the OLGA High Definition Stratified Flow Model," in 17th International Conference on Multiphase Production Technology, 2015.
[2] H. Holm, "Tanzania Flow Assurance Challenges," in 17th International Conference on Multiphase Production Technology, 2015.
[3] J. Kjølaas, M. Wolden, T. E. Unander, H. Holm and P. S. Johansson, "A comprehensive study of low-liquid-loading two- and three-phase flows," in 17th International Conference on Multiphase Production Technology, 2015.
[4] D. Biberg, H. Holmås, G. Staff, T. Sira, J. Nossen, P. Andersson, C. Lawrence, B. Hu and K. Holmås, "Basic flow modelling for long distance transport of wellstream fluids," in 14th International Conference on Multiphase Production Technology, 2009.
[5] D. Barnea and Y. Taitel, "Structural and interfacial stability of multiple solutions for stratified flow," International journal of multiphase flow, pp. 821-830, 1992.
[6] J. Nossen, T. Sira, T. Vanvik and H. Holm, "Analysis of hydraulic gradients in large scale experiments," in International Conference on Multiphase Production Technology, 2015.
[7] D. Biberg, The HD Stratified Flow Model, in Design og drift flerfasesystemer for olje og gass. Tekna - Teknisk-naturvitenskapelig forening, Oslo, 2012.
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