Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 8, No. 3, 2023 9 Design of Riser System Based on Target Oilfield and Platform Chuan Wang*, Hai He, You Peng Southwest Petroleum University, Chengdu, China *Corresponding author: Chuan Wang Abstract: Abstract: Risers are an important component in the development of offshore oil and gas resources, but they are also one of the weakest components. Therefore, the safety of the design of deep-water riser systems plays a crucial role in the development of offshore oil and gas resources. This article focuses on the selection and initial design of riser systems based on the target oilfield and platform. Taking into account both the transportation capacity of oil and gas pipelines and the strength of casing, multiphase flow theory is used to calculate the pressure drop and transportation capacity of the riser. Based on this, the size of the oil pipe and casing is iteratively calculated. The riser design is based on the specifications and professional software Orcaflex, which is mainly used in the design process, is used for stress analysis of the riser, Taking into account the main scale of the FWPSO platform, reservoir and wellhead data, as well as the marine environment data where the riser is located, the selected oil and casing will be subjected to wall thickness design verification. Keywords: Riser design; TTR; Force Analysis; Orcaflex; the FWPSO platform. 1. Introduction Based on the sea conditions and platform characteristics of the target oilfield, and taking into account the production capacity of the offshore platform, the oil and gas characteristics of the oilfield are designed to be suitable for the riser system of the target oilfield and platform. At present, the most widely used standards are API RP 2RD developed by the American Petroleum Institute and DNV OS F201 developed by Det Norske Veritas. API RP 2RD is based on the allowable stress method, while DNV OS F201 is based on the reliability analysis load resistance coefficient method. This time, the riser system design will be carried out using API RP 2RD. This article uses the standard riser design process and comprehensively considers the relationship between the oil pipe and the casing. Once again, based on reservoir data, a TTR-SCR riser system was designed to be used in conjunction with the riser system. The multiphase flow theory[1] was used to calculate the vertical upward pipe flow pressure drop, and hydraulic analysis was performed on the tubing to calculate its corresponding pressure gradient and pressure drop at different flow rates. The inner diameter of the tubing corresponding to the minimum pressure drop was selected as the initial tubing design diameter, and based on API SPEC 5L and API SPEC 5CT, Select materials and determine the size of the oil pipe. Finally, based on the selected riser size and the requirements of API RP 2RD specification, a minimum wall thickness verification analysis of the riser will be conducted. 2. Target Oilfield Operating Conditions 2.1. Riser design The design of risers[2] needs to consider various factors such as production requirements, environmental conditions, and floating platforms. In order to standardize the design of marine risers, many classification societies and petroleum associations have conducted research on the standardization of risers. At present, the most widely used standards are API RP 2RD developed by the American Petroleum Institute and DNV OS F201 developed by Det Norske Veritas. API RP 2RD is based on the allowable stress method, while DNV OS F201 is based on the reliability analysis load resistance coefficient method. This time, the riser system design will be carried out using API RP 2RD. 2.1.1. Sea conditions For the target oilfield with a water depth of 1000m, FWPSO is used as the operating platform. Risers are subjected to various environmental loads during their service life. When conducting structural analysis of the riser, it is necessary to combine wind, waves, and currents for design to ensure that the riser meets conditions such as strength and fatigue during its design life[3]. Table 1. Design condition sea wave current wind design condition verification factor Extreme 100-yr cyclone 100-yr cyclone 100-yr cyclone 1.2 Survival 1000-yr cyclone 1000-yr cyclone 1000-yr cyclone 1.5 Operating 10-yr cyclone -- -- 10.0 Under extreme sea conditions, it is necessary to calculate the motion response of the riser to ensure that the response meets the specified requirements. In survival sea conditions, it is necessary to ensure that the riser components do not fail or be damaged. 10 Table 2. Current Current (m/s) Water (m) 1-yr cyclone 10-yr cyclone 100-yr cyclone 1000-yr cyclone 0 1.57 1.85 2.49 2.80 55 1.17 1.45 2.09 2.40 166 0.86 1.01 1.36 1.58 239 0.73 0.85 1.15 1.34 313 0.68 0.81 1.09 1.26 Sea bed above1m 0.49 0.58 0.78 0.91 2.1.2. Riser material One of the initial steps in riser design is to choose the materials used for the riser, and commonly used materials[4] for marine risers include carbon steel, alloy steel, etc. The strength and fatigue characteristics of different steels vary. When selecting steel, it is necessary to consider its cost, weight, corrosion performance, strength, fatigue characteristics, etc. Generally speaking, the higher the grade of steel, the better the mechanical properties of the material, but the higher the cost. Therefore, the selection of steel should be based on factors such as cost, riser working environment, and work requirements. For top tensioned risers, the commonly used steels are X65 steel and x80 steel[5]. Table 3. Material parameters Material type Minimum yield limit (MPa) Maximum yield limit (MPa) Minimum ultimate tensile strength (MPa) Maximum ultimate tensile strength (MPa) X80 552 690 621 827 X65 448 600 531 758 2.2. Stress verification method The wall thickness of a deep-water riser has a significant impact on the force it bears under internal and external loads during installation and operation, and also affects the cost of the riser. Choosing the appropriate wall thickness[6] is of great significance in ensuring the strength of the riser structure and achieving safe installation and operation, while reducing production and installation costs. For the design of riser wall thickness, it is sufficient to follow the specifications. At present, the main specifications for deepwater risers include DNV-OS-F201 and API RP 2RD. According to regulations, the wall thickness of deep-water risers should ensure that the verification results of the combined stress and buckling and crushing stress of the riser meet the requirements. Firstly, calculate the Von Mises combined stress as 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 /√2 Among them: 𝜎 is the Von Mises equivalent stress, 𝜎 is the radial stress, 𝜎 It is the circumferential stress, and 𝜎 is the axial stress. Verification criteria: 𝜎 𝐢 𝜎 𝜎 𝜎 1.5𝐢 𝜎 Among them, 𝜎 is the membrane stress, corresponding to the average stress of the cross-section, 𝜎 is the stress caused by bending, 𝜎 = 𝐢 𝜎 is the allowable stress parameter, 𝐢 =2/3, 𝜎 is the yield strength of the material. 𝐢 is the design operating condition parameter, which can be obtained by looking up the table (API RP 2RD). For ordinary circular pipes, as lateral shear and torsion can be ignored, they only need to be checked according to Namely: 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 /2 𝐢 𝜎 Among them: 𝜎 𝑃 𝐷 𝑃 𝐷 𝐷 𝐷 𝜎 𝑃 𝑃 𝐷 2𝑑 𝑃 𝜎 𝑇 𝐴 𝑀 2𝐼 𝐷 𝑑 𝑃 is the pressure inside the pipe, 𝑃 is the pressure outside the pipe, 𝐷 , 𝐷 is the outer diameter and inner diameter of the riser, 𝑑 is the wall thickness of the riser, 𝐴 is the cross- sectional area of the riser, 𝑇 is the axial force of the riser, 𝑀is the section bending moment, 𝐼 is the moment of inertia of the cross-section. 2.3. Wall thickness design For the design of TTR production riser systems, the first step is to clarify their functions, which helps deepen the designer's understanding of the entire design process and helps them better complete the design. The TTR production riser system mainly has the following functions[7]: 1) Transport liquids produced from oil reservoirs from underwater wellheads, Christmas trees, or manifolds to floating production equipment. 2) Transport completion fluid, repair fluid or kill fluid, and other repair fluids from floating production systems to underwater production equipment. 11 3) Provide and maintain necessary full control over the wellhead and production fluid under all operational and emergency conditions. 4) Minimize pressure drop and throttling between underwater equipment and floating production systems. 5) Capable of withstanding the maximum operating pressure of underwater equipment and floating production system process equipment 6) Provide complete monitoring and visual inspection methods during installation and service 7) Capable of withstanding design loads under normal operations, emergency situations, and survival conditions 2.3.1. Basic Specification for Riser Design The main design specification is APIRP 2RD, while other specifications and standards are used for design reference and material and size selection[8]: APIRP 2RD "Design of Risers for Floating Production Systems (FPSs) and Tension LegPlatforms (TLPs)", First Edition, 1998. ABS Guide for Building and Classing Undersea Pipelines and Risers, 2001. APISpec 5L: "Specification for Line Pipe", 2010. APISpec 5CT: "Specification for Casting and Tubing", 2011. (1) Equivalent stress verification method The axial stress of the riser is, 𝜎 𝑇 𝐴 𝐴 𝜎 𝑇 𝐴 𝑀 𝐷 𝑑 /2𝐼 In the formula, 𝐴 is the area of the outer diameter circle of the riser; 𝐴 is the area of the inner diameter circle of the riser; 𝐴 the cross-sectional area of the riser; 𝜎 is the bending stress of the riser. 𝜎 𝑀 𝐷 𝑑 /2𝐼 𝑇 is the true tension of the pipe wall considering the end effect of effective tension and internal and external pressure. 𝑇 𝑃 𝐴 𝑃 𝐴 𝑇 𝑃 and 𝑃 represents internal and external pressures respectively; 𝑇 is the effective tension of the riser. The circumferential stress of the riser is, 𝜎 𝑃 𝑃 𝐷 2𝑑 𝑃 The radial stress of the riser is, 𝜎 𝑃 𝐷 𝑃 𝐷 𝐷 𝐷 According to the yield criterion, the equivalent stress of these three principal stresses is, 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 𝜎 2 Verify according to the coefficients given in API RP 2RD, 𝜎 𝐢 𝐢 𝜎 In the formula, 𝜎 is the allowable stress; 𝐢 is the design factor, taken as 0.667; 𝜎 is the minimum yield limit. 𝐢 is the allowable factor, with an air traffic control state of 1.2, a production state of 1.0, and a static water test state of 1.35. (2) Collapse verification method API RP 2RD provides a simplified evaluation method for collapse verification, which is applicable to risers installed on general floating structures to ensure that they do not collapse under external pressure[7]. Permissible net external pressure for crushing verification: 𝑃 𝐷 𝑃 In the formula, 𝐷 is the safety factor given by the specification, generally taken as 0.75. 𝑃 is the minimum crushing pressure, 𝑃 𝑃 （gf s/𝑠 οΌ‰ In the formula, 𝑃 Pressure for crushing circular pipes. 𝑃 𝑃 𝑃 𝑃 𝑃 In the formula, 𝑃 is the yield stress considering the effect of uniform tension. 𝑃 2π‘Œ 𝑑 𝐷 𝑃 is the elastic yield stress, 𝑃 2E/ 1 𝑣 t/𝐷 (3) Equivalent method for riser The riser in this design is a double-layer casing structure, which includes: production tubing, inner casing, and outer casing. Once the inner casing fails, the outer casing is designed to maintain the flow of the internal medium, thus maintaining the pressure balance of the oil well. Therefore, in the minimum wall thickness design stage, it is necessary to perform equivalent conversion on the double- layer casing structure. According to the requirements of API RP 2RD, the equivalence principle for overall analysis is[9]: The outer diameter of the equivalent tube is the same as the outer diameter of the outer tube, 𝐷 𝐷 The cross-sectional area of the equivalent tube is equal to the sum of the cross-sectional areas of the three layers of tubes, 𝐴 𝐴 𝐴 𝐴 The inertia moment of the equivalent pipe is equal to the sum of the inertia moments of the inner and outer sleeves, 𝐼 𝐼 𝐼 The weight of an equivalent tube is equal to the sum of the weights of three layers of tubes, π‘Š π‘Š π‘Š π‘Š 2.3.2. Riser design results According to the above method, the design parameters of the riser based on the sea conditions of the target oilfield and the target platform are shown in the table below. Table 4. Riser parameters Function Type Diameter mm Wall Thickness mm Material Production Inner TTR 224.5 10.16 API 5L X80 Outer TTR 323.8 12.7 API 5L X80 Tubing TTR 114.30 101.60 API 5L X80 Meg SCR 356 20.7 API 5L X65 Based on the annual sea conditions in the South China Sea, a circular FPSO was designed using the TTR riser for inter riser collision analysis. The analysis software used was Orcaflex11.2, which has been widely used in engineering. 12 Figure 1. Platform riser layout The target oilfield is located at a water depth of 1000m, with an environmental load of once in a year production condition, wave data, and ocean current environment. The platform drift distance is set to 7% of the water depth, which is 70m. The TTR top tension is set to 1.5 times the weight of the riser, which is 1461KN for a single riser[4]. Based on the annual sea conditions in the South China Sea, the circular FPSO was subjected to stress analysis using the designed TTR riser. The analysis software used was Orcaflex11.2, which has been widely used in engineering. Figure 2. TTR riser spacing variation chart with water depth According to the analysis results, it can be seen that the minimum distance between the risers is greater than 1.63m and there will be no collision, and vortex induced vibration will not occur at this distance, which can ensure that a safe distance is always maintained between the risers. Figure 3. Time variation curve of TTR riser top load and bending stress Figure 4. TTR riser axial stress and total stress variation chart with water depth Vertical pipe size verification, in accordance with the requirements of API RP 2RD specification, checks the combined stress and crushing stress of TTR vertical pipes in three states: empty pipe, production, and static water pressure testing. The results show that the combined stress of each layer of casing and tubing meets the specification requirements, and the utilization coefficient is less than 0.8. Therefore, this design can be used for the target oilfield and corresponding working conditions. 2.4. Impact of TTR spacing The various risers used in the process of offshore oil and gas development are prone to vortex induced vibration under the action of ocean currents, causing structural damage. Vortex induced vibration is an important factor in riser failure. The flow field around the two columns was divided into four regions based on the parallel rigid cylindrical wake structure[10, 11]. When the single vortex street area is 1.04.0 in the undisturbed zone, the wake flow field of the two cylinders is completely the same, there is no interference between the two cylinders, and the release form of the vortex is no different from that of an isolated cylinder. When the spacing T is 1.2~2.0D, although the parallel risers are symmetrical in structural space, the wake flow field exhibits asymmetry. This flow that occurs in a completely symmetrical structure and leans towards one side is called a bistable phenomenon, and the smaller the spacing, the more obvious this asymmetry is. The wake range of one side of the riser is narrower, and the wake range of the other side of the riser is narrower. 2.5. Analysis of riser simulation results From the above simulation results, it can be seen that the maximum stress of the riser always occurs at the top and bottom of the riser. This is because the top of the riser is connected to the platform, and the platform undergoes movements such as heave and roll with the waves, resulting in large stress fluctuations at the top of the riser. The large stress fluctuations at the bottom of the riser are due to the connection between the bottom of the riser and the wellhead, and the bottom of the riser cannot move. The simulation results of the riser include analysis results of stress, tension, riser spacing, etc. It can be seen from the results that in this design result, the TTR riser spacing meets the design requirements, there will be no collision, and vortex induced vibration will not occur at the minimum spacing. The stiffness and strength of TTR and SCR risers can meet the requirements for use. Table 5. Three Scheme comparing Working Condition Scheme 1 Riser Max von Mises stress Max Blending stress Minimum spacing/m ProductingCondition 100-yr cyclone TTR 324MPa 293MPa 1.63 SCR 308MPa 268MPa -- 3. Conclusion This article designs a riser system suitable for the target oilfield based on its sea conditions and platform parameters, including the layout and size of SCR/TTR risers. 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