Dynamic Approach to Quasi-static Nonlinear Problems for Sub-Sea Applications

Transcription

1 Dynamic Approach to Quasi-static Nonlinear Problems for Sub-Sea Applications Smitha G, Mahesh Bhat GE Oil & Gas, Bangalore Abstract: In deep-sea oil fields, metal seals play an important role to facilitate a smooth operation during drilling and production by withstanding high internal pressure and temperature. The metal seal is one of the most important components of a wellhead system. A wellhead is that part of an oil well which terminates the well, whether on land or offshore, where petroleum or gas hydrocarbons can be withdrawn. Sealing mechanism in a wellhead is a must to prevent any leakage of the formation fluids during production and withstand the higher pressure that may be encountered during the drilling and production process, hence safeguarding the environment. The metal seal is installed into the wellhead using a running tool, while the installation process, in itself, is a complex phenomenon involving high plastic strains. Finite Element Analysis (FEA) proves to be an indispensable method in understanding the quasi-static nature of seal installation process. Simulating seal installation with conventional implicit approach gives rise to high element distortions with the development of high plastic strains. Thus, a need arises to overcome this high element distortion in order to achieve the final assembly configuration. Explicit FEA has the capability to optimize the mesh to address such high element distortion. Arbitrary Lagrangian Eulerian Adaptive meshing technique available in Abaqus/Explicit is used in simulating this seal installation. The dynamic mesh smoothening in this technique helps in controlling the mesh distortion, in the quasi-static domain. The results such as high plastic deformations, strains and final assembly configuration are captured and are well validated with test data after extensive FE model calibration. The paper describes the technique by which the seal installation is simulated through FEA, addressing the challenge of high mesh distortion. Keywords: Explicit FEA, Wellhead, Metal Seal, Subsea, Element distortion, Quasi-static, Adaptive meshing, Formation fluids, Plastic strain SIMULIA India Regional Users Meeting 11 Page 1 of 10

2 1. Introduction Energy resource is a vital ingredient in the growth of any economy. Fossil fuels constitute a big share when meeting the world energy demand. Oil & Gas contributes to more than half of this demand. Petroleum products and natural gas plays a significant role all around us. Automobiles, power generation, industries, household applications etc depend on these sources. Figure 1: World consumption of energy resources [1] Maintaining the seesaw of supply and demand is quite an arduous task. Figure 1 illustrates the world consumption of various energy resources. The demand is growing at a steady pace over the past few decades. Not all oil is accessible on land or in shallow waters. One can find some oil deposits buried deep under the ocean floor with a water depth over 10,000 ft. World oil consumption is projected to increase annually by 1.8% over the next two decades. To meet this goal would be a mammoth task. Unless oil production can also continue to rise, it is only a matter of time before oil production can satisfy demand. Hence, Oil exploration and production (E&P) companies are drilling further out into the sea and deeper under the ocean floor, (at depths greater than 10,000 feet) to tap into one of the last remaining pockets of oil and natural gas in the world. SIMULIA India Regional Users Meeting 11 Page 2 of 10

3 As the offshore drilling progresses over great depths, the pressure and temperature increase drastically. Hence deep-water exploration of oil requires the subsea drilling equipments to be designed for such harsh conditions (high pressure & high temperature - HPHT, sour fluid, etc.). Moreover, containment of such high pressures becomes vital to avoid any catastrophe. A wellhead is one such system that contains these high pressures and operates at harsh environmental conditions. SIMULIA India Regional Users Meeting 11 Page 3 of 10

4 2. Subsea Wellhead System A wellhead is that part of an oil well which terminates at the surface, whether on land or offshore, where petroleum or gas hydrocarbons can be withdrawn. Its permanent equipment used to secure and seal the casing strings and production tubing and to provide a mounting place for the Christmas trees. The primary role of the wellhead is pressure containment and to hold the casings and the production tubing. The casing hanger lands on a shoulder in the bore of the wellhead housing. The casing is cemented in place. Then a seal is used to contain the pressure (sealing) between an exterior wall of the casing hanger and the bore of the wellhead housing. The seal positioning in the wellhead is indicated in figure 2 below. Figure 2: Metal seals in wellhead system SIMULIA India Regional Users Meeting 11 Page 4 of 10

5 A seal assembly is a mechanism which provides pressure isolation between each casing hanger and the wellhead housing and acts as a pressure controlling equipment [2]. The annulus between the casing and the wellhead must be sealed off to contain the higher pressure that may be encountered as the drilling operation progresses at greater depths, while the casing string installation takes place. The pressure encountered may be above the burst pressure of the previous casing string set, so the seal isolates the string. In addition, the seal must also be able to seal from below (inside the annulus) should the cement allow leakage from the formation to pressurize that annular area. If there were a seal failure, it would create a potential leak point during drilling, completion and work-over operations and the consequences would be catastrophic. Hence it is critical to maintain the integrity of the seal throughout the life of the well. It is quite difficult to understand and visualize the various operations happening at 10,000 ft below the rig floor. Also, it is quite challenging to predict potential problems that one could encounter at such depths in subsea. FEA proves to be an indispensable method in understanding these processes. An FE simulation would give us the flexibility to work with various combinations of materials, geometric profiles/features and extreme conditions of operation. A few test correlations with FE results would validate the FE simulation, which in turn could be used to design the components with lesser risk and in a cost effective manner. SIMULIA India Regional Users Meeting 11 Page 5 of 10

6 3. Simulation of metal seal installation The process of installation of seal into the wellhead system is a complex phenomenon. Installation of seal is a process of preloading the seal, where the preload helps the metal seal, and bites radially on to the hangers of wellhead. This would give rise to high plastic strains and result in severe element distortion, as indicated in Figure 3. These severe distortions cause solution unconvergence, when the conventional implicit approach is adopted. Also, implicit solvers, using the Newton-Raphson method would create certain contact difficulties, when the contact status changes from open to close and vice versa. In such cases lot of effort is needed to achieve the solution convergence with sufficient accuracy. Figure 3: Excessive element distortion with implicit approach Thus, a need arises to overcome this high element distortion in order to achieve the final assembly configuration. Explicit FEA has the capability to optimize the mesh in order to address such element distortions. The main advantage of Abaqus/Explicit for quasi static problems is that, due to the explicit time integration scheme, no iteration of nonlinear systems required and hence no convergence controls need to be addressed. For the current analysis, the quasi-static method proved to be advantageous in terms of computational cost, as it gave us the liberty of having a comparatively lower mesh density. It was, however, required to choose judiciously, very small steps in the analysis to obtain the results in close proximity with the test. 3 and 4-noded, reduced integration, axi-symmetric elements (CAX3 SIMULIA India Regional Users Meeting 11 Page 6 of 10

7 and CAX4R) were used in the analysis. Amplitude based displacement control approach was used to simulate seal installation. For explicit dynamic analysis, it needs to be ensured that, throughout the progression of the analysis, the inertial forces stabilize the system motion. Therefore, a constant check was performed throughout the analysis and made sure the kinetic energy is less than 10% of the total energy. Arbitrary Lagrangian Eulerian Adaptive meshing technique available in Abaqus/Explicit was adopted for simulating this seal installation. The dynamic mesh smoothening in this technique helped in controlling the mesh distortion as shown in Figure 4, in the quasi-static domain. Figure 4: Mesh integrity with adaptive meshing (explicit approach) SIMULIA India Regional Users Meeting 11 Page 7 of 10

8 The results such as high plastic deformations, strains and final assembly configuration were captured and well validated with test data after extensive FE model calibration. A comparison plot of test data versus FE simulation for the applied preload displacement versus force generated due to sealing is shown in the Figure 5. Figure 5: Comparison of test data vs. FEA SIMULIA India Regional Users Meeting 11 Page 8 of 10

9 4. Conclusion Although conventional implicit method has a lot of advanced non-linear features, solution methods and convergence tools, a quasi-static Abaqus/Explicit analysis can prove to be an advantageous alternative in case of systems containing multiple highly non-linear effects. The primary objective of overcoming high element distortion is successfully addressed through quasi-static approach using Abaqus/Explicit. Following the successful validation of FE results with test data, Abaqus/Explicit is employed to understand the behavior of various other sub-sea components. SIMULIA India Regional Users Meeting 11 Page 9 of 10

10 5. References [1]. BP Statistical Review of World Energy: bp.com/statistical review. [2]. API Spec 17D, section a, page 82. SIMULIA India Regional Users Meeting 11 Page 10 of 10