This work is a thermo-fluid numerical case study to investigate the size and performance of a system that eliminates needs for insulating, heating and inhibiting chemically the deposition of wax in subsea tie-in flowlines. For short, we call this type of systems “Cold Flow”. The particular system analyzed in this study consists of a reactor unit at the inlet to the flowline, where the thermal solubility of the wax-creating molecules is reduced by cooling. Subsequently, solid wax is deposited in the reactor piping and wax free crude is entering the flowline. The reactor is regenerated periodically. The reactor-pipeline system was modelled using a commercial flowline simulator, with transient, thermal, multiphase and deposition capabilities. The basic layout used was a transportation pipeline of 8 km and 6.69 in ID with a mass flow rate of 17.51 kg/s, a water cut (WC) of zero and an inlet temperature of 70 °C. The wax appearance temperature (cloud point) of the crude is 22 °C and the seabed temperature is 4 °C. Three types of reactors have been simulated: a non-insulated pipe section, a passive cooler with a bundle of parallel pipes and an active cooler. Sensitivity analyses have been performed for all three cases varying the external convective coefficient, the reactor pipeline diameter and the WC. For a non-insulated pipeline section cooler, the required length is of the same order of magnitude as the main flowline, implying that such a solution is impractical for short flowline distances or when a compact deployment is desired. For the passive cooler case, the required length was half of that in the previous case ; thus it is still significant. For the active cooler reactor, the required cooling duty was 2.2 MW. In all three cases, the pipe-flow dynamics were analyzed, and the pigging arrangement complexity has been qualitatively addressed. However, the detailed design falls out of the scope of this study.
Tie-in of satellite subsea fields to offshore processing facilities or tie-back of subsea fields directly to shore is very attractive solutions to save costs and offshore complexities. However, the untreated streams of oil, gas and water impose severe flow assurance challenges for the pipe flow. Wax deposition, which increases pressure drop and can ultimately cause pipe blockage, is one of the challenges addressed in this work. One of the approaches to overcome wax deposition is to extract, from the fluid stream, the wax-creating material before entering the flowline, a concept commonly known as “Cold Flow”.
To the authors’ knowledge, the idea of Cold Flow comes from the work of Coberly [
・ The wax eater (Kellog, Brown and Root, Halliburton) [
・ Cold seeding (NEI, Calgary, Canada and Marathon Oil, Houston, Texas) [
・ High-shear heat exchanger (Kellog, Brown and Root, Halliburton) [
・ Pressure surge (Kellog, Brown and Root, Halliburton) [
・ Flash cooling (Shell Western E&P Inc., Houston, Texas) [
・ Oil or solvent injection (C-Fer Technologies, Edmonton, Canada);
・ Magnetic conditioning (Magwell. Boerne, Texas, and Halliburton).
Cold flow can also be applied to handle gas hydrates and several technologies have been developed [
Wax is a solid formation and deposition in production flowlines that can cause flow assurance issues. Wax crystals appear in the production bulk flow as a result of temperature and pressure changes along the production system. When oil is transported, there is a temperature gradient between the pipe wall and the bulk flow, which can make the fluid temperature drop below the so-called Wax Appearance Temperature (WAT). This WAT, also known as Cloud Point, is the temperature below which wax precipitates. After precipitation, wax may deposit in the pipe walls, leading to increase in the pressure drop, decrease in flow rate and clogging of pipelines.
Conventional wax and hydrate mitigation methods are chemical, thermal and mechanical management. The chemical management consists in the injection of inhibitors into the pipeline [
Due to the problems associated with wax deposition in production pipelines, the oil industry makes important efforts in the understanding of this phenomenon. One way of understanding, is through numerical models that can predict where and how wax builds up, how the pressure drops and how often wax has to be removed. These models are based on the most relevant wax deposition mechanism, usually molecular diffusion and shear dispersion. Transient multiphase flow simulators, like OLGA [
This paper is focused on studying a wax cold flow concept [
The main contribution of this study is the simplified technical validation of a wax cold flow concept in a synthetic case of a subsea oil flowline. The validation consisted of performing a numerical estimation of required cooling distances and performance of three (3) cooling arrangements. Observations are then provided regarding the applicability of the method in a real system.
This work will hopefully contribute to the further maturing of cold flow concepts that are proposed by the industrial and scientific community to deal with wax accumulation problems in subsea flowlines.
There are several models proposed in the literature to predict the deposition of wax in a pipeline [
When the wall reaches the wax appearance temperature, the wax starts to precipitate in the region closest to the wall. Thus, a concentration gradient is created between the wax dissolved in the bulk flow and the wax in solution close to the wall. A molecular diffusion process then occurs when the wax dissolved in the bulk flow is diffused toward the pipe wall where it precipitates. Based on the results of Bern et al. and Brown, Niessen & Erickson, molecular diffusion is the principal responsible for wax deposition [
While the wax particles are suspended in the flowing oil, they tend to move in the same direction and speed of the flow. Due to the velocity gradient, shear is greater in the proximities of the pipe wall, so the particles tend to move towards the center of the pipe. This effect usually causes the transport of the precipitated wax to be predominantly far from the wall. Brown, Niessen & Erickson [
In this section the models that are available in the commercial software OLGA to predict wax deposition are described.
OLGA has a wax deposition module to model wax precipitation and deposition. The calculation is made based on a pre-calculated table that contains information of each wax-forming component [
The RRR model is a wax deposition model for multiphase flow in pipelines. Rygg, Rydahl and Rønningsen created this model in 1998 [
l max = V o l w a x d i f f + V o l w a x s h e a r ( 1 + ϕ ) 2 π r L (1)
where l max , is the rate of increase in thickness for the wax layer (m/s). ϕ , is the wax porosity, it can be used as a tuning parameter in OLGA [
The two volume rates of wax deposition terms are calculated with the following equations:
By molecular diffusion (function of composition)
V o l w a x d i f f = ∑ i N max D i ( c i b − c i w ) S w e t M W i δ ρ i 2 π r L (2)
where c i b , c i w are the molar concentration of the wax component i , dissolved in the bulk oil phase and at the wall, respectively (mol/m3). S w e t , is the fraction of wetted circumference. N max , is the number of wax components. M W i , is molar weight of wax component i (kg/mol). ρ i , is density of the wax component i (kg/m3). D , is the diffusion coefficient (m2/s). δ , is the thickness of the laminar sublayer (m).
In OLGA, the diffusion coefficient D is calculated with Hayduk-Minhas correlation [
By shear dispersion (Burger, Perkins & Striegler correlation, 1981 [
V o l w a x s h e a r = k * C w a l l γ ˙ A ρ w a x (3)
where k * , is the shear deposition rate constant (kg/m2). C w a l l , is the volume fraction of the precipitated wax in the oil at the inner wall temperature. γ ˙ , shear rate at the wall (s−1). A , surface area available for deposition (m2). ρ w a x , average wax density (kg/m3).
The RRR model does not take into account any removal mechanism. It assumes that all the wax transported to the wall sticks to the surface when the temperature is below the WAT.
The Matzain is a semi-empirical kinetic model that predicts wax thickness based on experimental tests conducted on South Pelto oil in the Gulf of Mexico [
This is a proprietary model used in OLGA for wax deposition in pipelines. The model takes into account molecular diffusion, shear dispersion and shear stripping as mechanism of deposition. No further details of the model are provided in the manual of the software [
Three Cold Flow concepts were studied to assess their feasibility as flow assurance methods for control of wax deposition. They are: a non-insulated pipe section, a passive cooler with a bundle of parallel pipes and an active cooler. They consist on inducing, by cooling, the formation, precipitation and deposition of wax in a dedicated section of the production system. This section is then periodically “cleaned” and the inert wax particles are sent together with the production fluids. The criterion used to determine the length of the section is that the thickness of wax deposited at the end of the section has to be less than 1% of the total pipeline diameter. Practically, this occurs when the fluid reaches thermal equilibrium with the surroundings.
The non-insulated Pipe Section concept,
and the residues are sent together with the flow in the main transportation line.
This concept consists in installing a passive cooler, made of parallel pipe segments, cooled by sea currents (
This concept consists in connecting an active heat exchanger at the beginning of the system. The goal is to find the minimum duty needed to cool down the fluid until it reaches the temperature of the surroundings. This concept is the most difficult to pig, since the typical active heat exchangers are shell and tubes and the geometry is significantly more complex than the other two cases.
The study was divided in 3 cases, each one representing one cold flow concept. Additionally, a base case representing the original production system was created. The original production pipeline has a horizontal length of 8 km, with an internal diameter of 6.69 inches. The fluid enters the pipeline with a mass flow of 17.51 kg/s, which represents a standard flow rate of 14 × 103 Sm3/d for 32 API, a gas specific gravity of 0.95, a temperature of 70˚C, a water cut of zero, a GVF of 0.085 and a WAT of 22˚C. The outlet pressure was 25 bara. The surrounding temperature was 4˚C. The external coefficient of convective heat transfer with seawater is 500 W/m-K.
Three layers compose the pipeline wall: steel, concrete and polypropylene. Their properties are given in
The wax deposition model used was RRR. The simulation time was 90 days, with a maximum time step of 1000 s and a minimum of 1 s. The information given above was used for all the cold flow cases.
A sensitivity study to determine the required length of the pipe section (cooling length) was performed for three different subcases:
Case 1.1: Influence of the external convective coefficient of heat transfer. The required cooling length of the pipe section was found for several values of convective coefficient of heat transfer. The values used were within the range of expected free convection coefficients for water [
Case 1.2: Influence of pipe diameter. The required pipe section length was determined for different values of pipe diameter (
Case 1.3: Influence of watercut. Three values of watercut were studied in this subcase (
Since OLGA is a 1D software, the pipe section was modelled as an equivalent pipeline section attached at the beginning of the original production system described on the base case. This means that the length of the equivalent pipe section is two times the length of the loop (
Steel | Concrete | Poly-Propylene | |
---|---|---|---|
Thickness (cm) | 1 | 0.6 | 0.30 |
Conductivity (W/m-K) | 50 | 1.7 | 0.12 |
Density (kg/m3) | 7850 | 2250.0 | 960.00 |
Capacity (J/kg-K) | 485 | 880.0 | 1675.00 |
Case 1.1 | Case 1.2 | Case 1.3 |
---|---|---|
Convective coefficient (W/m2-C) | Pipe diameter(in) | Watercut (−) |
500 | 6.69 | 0.00 |
1000 | 8 | 0.15 |
2000 | 9 | 0.30 |
3000 | 10 | |
11 | ||
12 |
In this case, the equivalent 1D pipe section does not have insulation. This means that its wall consists only of one material, steel, with a thickness of 1 cm. The rest of the pipeline has the configuration mentioned in the base case.
As mentioned earlier, for this case two sensitivity variables were taken into account. The first one was the number of cooler parallel segments and, the second one, their length. In the same way as case 1, this case was divided into two sub-cases, the first model the cooler with two parallel segments (case 2.1), and the second use three parallel segments (case 2.2).
In both subcases, the fluid is distributed evenly through all the parallel segments and the distance between segments is 1 m (FLOWPATH_2 and FLOWPATH_3 in
Here, a heat exchanger was added to the system, and the minimum duty to avoid wax deposition in the pipeline was determined. Unlike the other cases, in case 3, external forced convection is used to cool down the flow to ensure the fluid leaves the heat exchanger at thermal equilibrium with the surroundings.A sketch of the model is shown in
The criteria used to determine the loop length for Case 1 was that the wax thickness at the end of the loop had to be at least 1% of the pipeline diameter after a production of 90 days.
AddedPressureDrop [ % ] = Δ p basecase − Δ p new Δ p basecase (4)
Changes in the outer convective coefficient and stream watercut do not have much influence in the required cooling length, but the watercut has a smaller influence in the added pressure drop than the convective coefficient. An increment in the convective coefficient of 2500 W/m2 C, decreases the required cooling length in 4.65%, but increases the pressure drop of 1.2%. In the other hand, a water cut of 30% gives a decrease of 4.63% in the required cooling length and of 0.6% in the pressure drop.
An important conclusion from the feasibility study performed on Case 1 is that this technology is more attractive to long transportation distances (>50 km). For example, comparing with the system used here (8 km pipeline), the minimal cooling length obtained (Case 1.2, using a 12 inches diameter pipeline) represents 38.28% of the original production pipeline, while for a pipeline of 50 km, would represent only 6.12%.
The results shown in this section might depend strongly on the fluid characteristics, rates and pipeline dimensions. Thus, direct extrapolation of these observations to other subsea production systems is not recommended.
For Case 3, a heat exchanger was connected at the beginning of the production system. Here the required duty that guarantee thermal equilibrium between the production fluid and the surrounding was 2.2 MW. To compare, a subsea heat exchanger with forced cooling advertised by the company NOV have a typical cooling capacity of 10 to 20 MW per unit [
heat exchanger. This case is the more efficient to control wax deposition, however is the less friendly for pigging due to the complex structure of heat exchangers.
・ Three wax cold flow concepts were successfully studied for installation at the upstream end of the main transportation pipeline. The three concepts are: a non-insulated pipe section, a parallel pipe heat exchanger with natural cooling and a heat exchanger with forced cooling. The analysis was performed with a commercial 1D mechanistic multiphase flow simulator.
・ For the non-insulated pipe section concept, the required length of the loop was at least 3 km, which can be impractical for some production systems. For most cases, the added pressure drop due to the presence of the loop was negligible (around 10%).
・ When using a passive cooler with a bundle of parallel pipes the required length for cooling was reduced considerably (30% - 50%) compared with the case 1. However, the lengths and diameters required might still be too large for placing them inside a compact subsea structure. Another drawback of this concept is that the cooler uses smaller pipe diameters, which means that the pressure drop will increase and the pipe will get block in shorter times, needing to be cleaned often.
・ In the active cooler case, the minimal duty obtained was comparable with subsea heat exchangers advertised by a manufacturer. The disadvantage of this concept is that it is difficult to pig due to the complexity of the equipment and the added pressure drop. The authors didn’t perform a review or studied methods to remove wax in such equipment. However, the following reference [
González, D., Stanko, M. and Golan, M. (2018) Numerical Feasibility Study of a Wax Cold Flow Approach for Subsea Tie-In Flowlines Using a 1D Mechanistic Multiphase Flow Simulator. Engineering, 10, 109-123. https://doi.org/10.4236/eng.2018.103008