Transportation of petroleum products through pipeline presents considerable risks including wax formation and deposition as a result of heat loss of fluids, which is harmful to the flow due to the reduced inner diameter or totally blocked pipelines in extreme cases. The production interruption due to blocked pipelines can cause colossal financial loss. Therefore, in order to diminish those adverse effects, it is critical that pipeline design for flow assurance should be considered. Flow assurance is a relatively new field in oil and gas industry, it means that the flow of hydrocarbon stream from one point to another must be ensured successfully and economically. Although flow assurance is extremely diverse, encompassing many discrete and specialized subjects and bridging across the full gamut of engineering disciplines, our work concentrated principally on the study of wax deposit in the pipelines. The main purpose of this paper is to focus on the aspect of material in pipeline design and the selection of thermal insulation coatings. Furthermore, operating parameters such as pressure, temperature and flowrate will be examined to achieve optimum results. For the case study in this paper, the pipeline connecting Ca NguVang Oilfield’s Wellhead Platform (WHP) to the Central Processing Platform of Bach Ho Oilfield (CPP-3) in Vietnam will be studied. Hence, this work covers several aspects, namely the theoretical study, the modeling using Excel as well as using specialized software OLGA, and finally the application for a real case in the petroleum industry in Vietnam.
Oil and gas fields in Vietnam are located hundreds of kilometers offshore in which producing and processing procedures are separated and conducted on different platforms. These platforms are connected by a pipeline system lying on the seabed. Specifically, crude oil is transferred from the Wellhead Platform of Ca NguVang field to the Central Processing Platform of Bach Ho field, where crude oil is under processing, by the approximately-25-kilometer-length pipeline (
Ca NguVang field is operated by Hoan Vu Joint Operating Company. As a result of an average reserve, which ranges between 6200 - 20,000 barrels/day [
Crude oil is a mixture of waxes, aromatics, naphthenes, asphaltenes, and resins. At typical reservoir temperatures and pressure, wax molecules are dissolved in the crude oil. As the produced oil flows through a subsea pipeline lying on the ocean floor, its temperature drops below WAT (Wax Appearance Temperature) because of heat loss along the pipeline [
In fact, there were a number of accidents related to flow assurance which was attributed to wax deposition. From 1992 to 2002, over 50 cases of pipeline blocking due to wax deposition were reported in Gulf of Mexico [
In respond to the consequences of wax deposition, many studies of flow assurance are now focusing on remedy and prevention techniques. One of the solutions was to prevent the heat loss along the pipe, in which choosing thermal isolating materials and design optimum thickness for the coat was mentioned in
this article. Additionally, the amount of wax deposition also depends on the flow operational parameters such as flowrate, temperature, pressure. Therefore, examining the wax thickness and the wax deposition position by evaluating these parameters is critical to optimize the transportation of produced oil.
Crude oil is a liquid organic substance. It is made up of hydrocarbons, which are composed of hydrogen and carbon atoms, and some proportions of impurities such as CO2, H2S, etc. Components of hydrocarbon vary widely, ranging from the simplest one―methane (CH4), to the complex chemical substances in which the number of carbon atoms can reach over 60 [
The critical factor leading to the phenomenon of wax formation is the fact that fluid temperature flowing through the pipeline is lower than wax appearance temperature (WAT) [
The reason of heat loss during crude oil transport is due to heat transfer from the fluid to the environment surrounding pipeline. Particularly, heat transfer process is supposed to be divided into three patterns [
Heat transfer is primarily influenced by the following factors [
Formulas for computing total heat transfer coefficient [
The amount of heat transfer through unit length of pipeline is given by
In which k is thermal conductivity of a certain layer of the pipe (W/m・K). rjo, rji are respectively inner and outer radius of a certain layer of the pipe (m).
Heat transfer coefficient due to conduction heat h (W/m2・K) of a particular coating is given by
Total heat transfer coefficient U (W/m2・K) is related to three patterns of heat transfer according to the following relation
From a material selection aspect of pipeline design, it is acceptable to neglect the effect of convection and radiation of heat, leading to the simple form of total heat transfer coefficient equation presented below
Pipelines are usually manufactured from coating layers with their specific functions (
Fusion Bond Epoxy (FBE): layer protecting pipelines from corrosion, 3) Polypropylene Adhesive (PP Adhesive): gel layer connecting two adjacent layers, 4) Polypropylene Solid (PP Solid): layer supporting PU Foam layer, 5) Polyurethane Foam (PU Foam): thermal insulating layer and 6) Concrete Weight Coating (CWC): layer made up of concrete helping maintain stability and protect the whole pipeline. In addition, CWC may also perform a secondary function as the thermal insulation.
Changes in the heat along a pipeline are related to the variation of the level of thermal insulation of coatings, which means total heat transfer coefficient U needs analyzing [
Information and data about dimension of pipeline (
According to
Length (m) | Diameter (mm) | Thickness (mm) | Roughness (mm) | |
---|---|---|---|---|
OD | ID | |||
24.921 | 273.10 | 232.9 | 20.1 | 0.0457 |
Material | Heat conductivity (W/m・K) | Density (kg/m3) | Heat capacity (J/kg・K) |
---|---|---|---|
Steel (API5LX65) | 45.0 | 7850 | 460 |
FBE | 0.3 | 1450 | 1350 |
PP Adhesive | 0.22 | 900 | 1000 |
PU Foam | 0.04 | 165 | 1600 |
PP Solid | 0.215 | 900 | 1800 |
CWC | 2.0 | 2242.60 | 1000 |
(Extracted from MsiKenny Company).
Density (g/ml) | 0.8136 |
---|---|
Cloud point temperature (˚C) | 30 |
Wax content (%) | 16.8 |
WAT (˚C) | 59.8 |
It is precise that produced oil from Ca NguVangField is categorized as the light crude oil with high wax content. Besides, both cloud point temperature and WAT outnumber ambient temperature of the environment (30˚C and 59.8˚C respectively in comparison with 25.13˚C), which contributes to wax appearance and hinder restart operation after shut-in pipeline. Furthermore, Ca NguVang field is in the beginning stage of production with a relatively low value of water cut, which is the proportion of water produced to the total liquid (2 percent).
On the purpose of transporting crude oil through subsea pipeline safely from Ca NguVang field to CPP-3 of Bach Ho field, the thickness of each coating is computed in order to corresponds with previously determined U = 1.91 W/m2・K [
Oil production (barrel/day) | 6100 |
---|---|
Gas Oil ratio (ft3/barrel) | 2390 |
Water cut (%) | 2 |
Density (kg/m3) | 1025 |
---|---|
Seabed temperature (˚C) | 25.13 |
Surface temperature (˚C) | 27.57 |
Air temperature (˚C) | 27 |
Steam velocity (m/s) | 0.63 |
Wind velocity (m/s) | 12.1 |
Material | Inner radius ri (mm) | Outer radius ro (mm) | Thickness (mm) | Results of thickness (mm) | |
---|---|---|---|---|---|
Steel | 116.45 | 136.55 | 20.1 | 0.00056 | 20.1 |
FBE | 136.55 | 136.55 | 0 | 0 | 0.15 |
PP Adhesive | 136.55 | 136.55 | 0 | 0 | 0.35 |
PP Solid | 136.55 | 136.55 | 0 | 0 | 3.5 |
PU Foam | 136.55 | 136.55 | 0 | 0 | 25.21 |
PP Solid | 136.55 | 136.55 | 0 | 0 | 4 |
CWC | 136.55 | 136.55 | 0 | 0 | 48.22 |
H | 1775.70 | W/m・K | |||
U | 2426.90 | W/m2・K |
coefficient U using formula (II) and (IV), 6) use tool Solver in Excel for every thickness with pre-determined U (U = 1.91 W/m2・K). Results are shown in the last column in
From
To evaluate quantitatively influences of the existence of the coating layers, especially CWC and PU Foam, the following five cases are considered in
The wax deposition level after 30 days can be observed in
The thickness computed in
Case | Thickness (mm) | Total heat transfer coefficient (W/m2・K) | Wax thickness (mm) | ||||||
---|---|---|---|---|---|---|---|---|---|
Steel | FBE | PP Adhesive | PP Solid | PU Foam | PP Solid | CWC | |||
1 | 20.1 | - | - | - | - | - | - | 669.12 | 1.80 |
2 | 20.1 | 0.15 | 0.35 | 3.5 | 25.21 | 4.00 | 48.22 | 1.91 | 0.151 |
3 | 20.1 | 0.15 | 0.35 | 3.5 | - | 4.00 | 48.22 | 20.58 | 1.19 |
4 | 20.1 | 0.15 | 0.35 | 3.5 | 25.21 | 4.00 | - | 1.96 | 0.153 |
5 | 20.1 | 0.15 | 0.35 | 3.5 | - | 4.00 | - | 31.30 | 1.37 |
Case 1 is the case in which pipeline is simply made of steel (20.1 mm), with no other coating. In this circumstance, total heat transfer coefficient U rises abruptly (669.12 W/m2・K); consequently, heat loss in pipeline is so serious that wax molecules crystalize earlier and form a far thicker layer (1.80 mm) in the inner pipe wall than that in case 2.
Nonetheless, case 1 exists only in theory analysis due to the fact that constructing the pipeline without any protection is impossible. Besides, evaluating the effects of two thermal isolating coatings (PU Foam and CWC) requires to only change the thickness of these coatings, the rest layers are remained unchanged.
Maximum wax thickness after 30 days results in case 3, 4, 5 (changing PU Foam and CWC thickness) are presented in
Among case 3 and 5, the fact that PU Foam (primary thermal isolating material) is omitted seriously affects the amount of wax deposition. Wax thickness has risen dramatically (1.19 mm and 1.37 mm respectively). In addition to case 5, CWC is also excluded; thus, wax thickness is higher than case 3. However, due to high heat conductivity of CWC layer (2 W/m・K, just lower than that of steel), eliminating this layer do not affect wax thickness significantly.
Taking concern on circumstance 4 which only gets rid of CWC layer; because CWC heat conductivity is relatively high, wax thickness in this case increases slightly (0.153 mm compare with 0.151 mm in case 2) and can be negligible. However, it is undesirable to exclude CWC layer just for economic reason. The most important function of CWC layer is to aggravate and protect the pipeline from any mechanical impact; in other words, CWC layer strengthens and helps the pipeline win over buoyant force. Moreover, optimum thickness of CWC layer has always been considered so that it is as economical as possible.
In order to determine the optimum thickness of CWC layer, the problem relating to the force balance is considered. The whole pipeline rests on the seabed; the buoyant force, according to Archimedes, is equal to the product of the seawater density and the volume occupied by the pipeline. The weight of the pipe is the sum of the crude oil weight currently in the pipe, the steel weight and all the weights of coating. It is necessary to meet the following condition to stabilize the pipeline on the seabed: gravity force of the pipeline system is greater than Archimedes buoyant force in the case of the pipeline without crude oil. Computing Archimedes force is followed as: 1) consider CWC layer thickness as variable, 2) compute total weight of the pipeline system, 3) compute the volume of the pipeline occupying the water and water density, 4) solve the force balance equation and compare with outsourcing criteria of company. Results are presented in
From
Knowing the CWC outsourcing cost per ton, it is possible to calculate the total saving cost. In conclusion, as can be seen from the above calculation and analysis, PU Foam layer has the biggest effect on thermal controlling procedure; which is the reason why hindering wax deposition problem requires focusing on this layer.
After material choosing and thickness calculating, the upcoming section is to examine operational parameters (flowrate, temperature and pressure).
To evaluate impacts of variations of flowrate, temperature and pressure, OLGA software is utilized. In particular, the amounts of wax deposition after a 30-day period is plotted and compared to the others in each case of flowrate, temperature and pressure using the function named Parametric Study and the wax modeling of Matzain.
The variations of wax deposition thickness correspond to the changes in the flowrate of fluid. It is obvious from
Parameter | Unit | Value |
---|---|---|
Weight of pipe per unit length | kg/m | 136.41 |
Weight of sea water per unit length | kg/m | 92.79 |
Minimum thickness of CWC layer | mm | 40* |
Maximum thickness of CWC layer | mm | 150* |
Case | Q (stb/d) | Maximum wax deposition thickness (MWDT) (mm) | Position at which wax starts to deposit (PWD) (m) |
---|---|---|---|
1 | 2000 | 0.192 | 741.5 |
2 | 3000 | 0.171 | 1988 |
3 | 4000 | 0.160 | 3234 |
4 | 5000 | 0.156 | 5726 |
5 | 6000 | 0.152 | 6972 |
6 | 7000 | 0.148 | 8218 |
time for fluid to flow along the pipeline, which is inversely proportional to the flowrate. Therefore, the position at which fluid temperature is smaller than WAT tends to be closer to the input of pipeline if the larger value of flowrate is considered. Finally, it is clear that the pipeline with higher flow rate has more capability to transport fluid in comparison with the one with lower flow rate, resulting in smaller chance of wax deposition.
The input temperature of the pipeline, donated as TWHP, affects the distribution of temperature along the pipeline, resulting in the dependence of position where wax appearance occurs onto input temperature. The appropriate value of TWHP can be obtained using equipment called Heat Exchanger. The results of how TWHP has an impact on wax deposition are given in
In the first case, wax deposits at the very beginning of the pipeline with the noticeable thickness of wax (2.674 mm), compared to the other cases (0.151 mm). This is attributed to the fact that the value of WAT outnumbers the input temperature (59.8˚C and 50˚C respectively). The fundamental difference amongst the other cases is the position where wax starts to deposit. The lower TWHP is the closer to WHP wax would deposit. The similarity amongst these four cases is the equivalent wax deposition thickness when reaching stable condition (approximately 0.151 mm). The reason of this occurrence is due to the similar pipeline structure and flowrate considered; therefore, all four cases experience no difference in the wax deposition after fluid flow achieves stability.
Case | TWHP (˚C) | MWDT (mm) | PWD (m) |
---|---|---|---|
1 | 50 | 2.674 | 0 |
2 | 60 | 0.151 | 741.5 |
3 | 70 | 0.151 | 6972 |
4 | 80 | 0.151 | 11,956 |
Pressure is an easily changed parameter at WHP by controlling flowrate or at CPP by selecting operating pressure of processing equipment. Furthermore, with a specific flowrate, these two values of pressure have effects on each other. For the sake of simplicity, the pressure at CPP, donated as PCPP, will be evaluated in the case of Q = 6100 stb/day and TWHP = 70˚C. The simulated results are shown in
From
Case | PCPP (psia) | MWDT (mm) | PWD (m) |
---|---|---|---|
1 | 100 | 0.148 | 4480 |
2 | 200 | 0.150 | 5726 |
3 | 300 | 0.153 | 5726 |
4 | 400 | 0.153 | 6972 |
of heat loss tends to increase when PCPP is low. In four circumstances, the values of wax thickness are nearly the same (about 0.150 mm). The reason to this is similar to the case of TWHP.
With the result of analysis operational parameters such as flow rate, temperature and pressure, it can be concluded that: reducing wax thickness and maintaining wax in liquid form as long as possible require increasing the flow rate, temperature at the inlet of the pipe (TWHP) as well as the pressure at the outlet of the pipe. In general, three mentioned parameters have similar tendency in affecting wax depositional process.
In this research, two aspects are carried out for study. Firstly, as engineers of Cuu Long-Hoan Vu JOC are in charge of designing pipeline system in term of geometry and material, therefore, the primary goal of this paper is choosing thermal isolating materials and determining optimum thickness of coating layers, especially CWC and PU Foam. Thus, issues relating to geometry of pipeline system will not be concerned. In pipeline design and material selection issue, PU Foam and CWC coatings have the largest thickness values. Amongst these coatings, PU Foam is the one performing thermal isolating function; thus, in order to minimize wax deposition as well as its thickness, the thickness of this coating must be carefully computed. In addition, CWC coating’s primary role is to stabilize and protect the whole pipeline in the marine environment so it is designable to adjust the thickness of these coatings so that issues relating to economy are improved.
Secondly, apart from the aspect of material, operational parameters namely flowrate, temperature and pressure will be considered to know how variation of these parameters may affect wax deposition. These operating parameters are vital in flow assurance facet. To illustrate how to apply this paper to a certain circumstance, authors consider the pipeline connecting WHP-CNV and CPP-3 Bach Ho Oilfield. All the information about production, ambient environment, characteristics of coating materials and properties of fluid transported through this route is necessary in progress of evaluating the degree of wax formation and deposition. For example, production data consists of the value of flowrate which in turn influences the position where wax starts to deposit as well as thickness of wax deposition. In addition, the ambient temperature also has the similar influences on this problem despite coating materials are thermal isolating. Moreover, the paraffin content of produced fluid has a profound impact on the level and the period of wax deposition. These data were provided by Cuu Long-Hoan Vu JOC, hence we can consider the results reasonable. It is recalled that the crude oil is produced at WHP (Well Head Platform) and flows through the pipeline resting on the ocean floor then reaches the end point at CPP (Central Processing Platform). Therefore, to avoid wax deposition occurring near the inlet of the pipeline, which leads to undesired pressure loss, and minimize maximum wax thickness, it is recommended that flowrate, temperature at WHP or pressure at CPP should be decreased. However, changes in these parameters must correspond to technical and economic availability.
Supporting the calculation in this subject, Excel and OLGA are utilized. How- ever, because of specialized function, OLGA software is used to examine effects of operational parameters in addition to determining optimum thickness of coating layers which can be achieved simply using Excel. Overall, the results derived from Excel are similar with those obtained from OLGA software.
The method mentioned above just partially tackles wax deposition problem and do not completely eliminate it. Therefore, this is the limitation of the approach. Eventually, if wax thickness increases to a particular value, PIG (pipeline inspection gauge) must be sent into the pipeline to scrape off the wax to maintain the flow.
Pham, S.T., Tru- ong, M.H. and Pham, B.T. (2017) Flow Assurance in Subsea Pipeline Design for Transportation of Petroleum Products. Open Journal of Civil Engineering, 7, 311-323. https://doi.org/10.4236/ojce.2017.72021