Hydrate-Induced Vibration in an Offshore Pipeline
The objective of this study is the numerical simulation of hydrate-flow-induced vibration and stress analysis of an M-shaped jumper of a subsea oil and gas pipeline. These objective is divided into the following tasks:
Developing a steady-state and transient model of hydrate flow using multiphase-modeling techniques to capture the realistic phenomena and validating the simulation results with experimental results available in the literature
Conducting sensitivity analysis of flow-condition parameters, such as hydrate volume fraction and flow velocity, in order to minimize flow-assurance challenges
Conducting stress analysis of the pipeline using a fluid/structure interaction static structural model
The CFD simulation is coupled with a finite-element-analysis (FEA) -based program, and sensitivity analysis is performed using different pipeline construction materials.
In this study, a straight pipeline was used first for validation. Later, an M-shaped jumper is simulated as an offshore pipeline. The computational grid of the M-shaped jumper consisted of 180,720 hexahedral nodes.
The M-shaped jumper used for the simulation has a diameter of 25 cm with a suspended span of 30 m and six sharp elbows. The jumper is assumed to be fixed at the two ends under the water, and gravity works in the downward direction. The numerical simulations were performed with commercially available CFD software. The software uses an element-based finite-volume method to discretize the computational domain with fine meshing. The mesh creates finite volumes, which are used to solve the mass, momentum, and energy equations. Discretization helps linearize a large system of nonlinear algebraic conservation and transport equations. The hydrate slurry flow inside the jumpers is governed by 3D incompressible Navier-Stokes equations. The density and viscosity of the hydrate-carrier phase used in this study were 1000 kg/m3 and 0.001 Pa·s, respectively. The hydrate phase density was 1138 kg/m3. The hydrate was assumed to be formed already, and the mean particle size of the hydrate was 70 μm. A Eulerian-Eulerian approach was used for the multiphase modeling. The flow was considered turbulent and isothermal, with aggregation and breakage of the hydrate particles in the turbulent pipeline flow and no interphase mass transfer. These physical processes are modeled as a set of partial differential equations with boundary conditions. The theoretical framework of the CFD simulation is based on the solution of conservation equations—namely, mass, energy, and momentum conservations.
The simulation results were compared with experimental and numerical data in the literature, and the results were a good fit. The pressure gradient was observed to increase as the carrier-phase velocity increased. If the hydrate volume fraction increased, the pressure gradient also increased. The proposed CFD codes then further extended to predict the hydrate flow for the M-shaped rigid jumper.
A refined tetrahedral unstructured mesh was used in the simulation. The turbulent eddies near the wall are very small, and a special consideration was required to obtain good results in the CFD simulation. It was recommended that the mesh near the wall be refined enough to resolve the small turbulent eddies. A dimensionless quantity was used to check the near-wall mesh treatment. For a standard wall function, the value of the dimensionless quantity for the first cell should be within the range of 30 to 150. In this study, the near-wall treatment was performed to ensure that it is in the acceptable region.
After the CFD analysis of the flow phenomenon of hydrate transport in a turbulent flow through subsea rigid jumpers was studied, the internal-flow data of the pipeline was exported to the FEA program and a fluid/structure interaction study was conducted.
The FEA model of the pipeline was developed by use of a fluid/structure interaction static structural model. The flow information from the CFD results were imported to the static structural model, and the stress and total deformation of the pipelines from the flow-induced vibration was analyzed.
Results and Discussions
Jumper-Stress Analysis. The equivalent stress (von Mises) was calculated with changing hydrate volume fractions, as presented in Figs. 1 and 2. Maximum stress developed by a hydrate volume fraction of 37.8% is higher (28 MPa) compared with the maximum stress developed by a hydrate volume fraction of 14% (18 MPa). If a higher hydrate volume fraction passes through the jumper, the impact made by hydrate particles is greater, causing greater stress. The stress development is not equal in all locations of the jumper, however. With 14% hydrate volume fraction flow, the minimum stress is 0.05 MPa and the maximum stress is 18 MPa. With 37.8% hydrate volume fraction flow, however, the minimum stress is 0.1 MPa and the maximum stress is 28 MPa. The maximum stress developed in the jumper is critical to its structural integrity. The jumper’s deformation will be greater at these locations, and the jumper can fail eventually because of critical deformation in those locations.
Effect of Hydrate Volume Fraction on Stress Development. If the hydrate volume fraction increases from 15 to 37%, stress formation in the jumper increases from 18 to 28 MPa. Higher hydrate volume fractions have a greater effect on the jumper structure, creating stress in the jumper.
Effect of Hydrate Volume Fraction on Jumper Deformation. Without hydrate flow in the jumper, no stress is developed and no deformation occurs because no impact is made by hydrate particles. When hydrate flow exists in the jumper, stress is created and deformation occurs because of the impact of the hydrate particles.
If hydrate volume fraction increases from 15 to 37%, deformation in the jumper increases from 0.022 to 0.35 m. The deformation may cause a rapture or structural failure in the flow line and may lead to a catastrophic event. Jumper failure occurs because of pressure buildup at the pipe walls caused by high flow rates and high hydrate volume fractions.
Sensitivity analysis of flow parameters such as hydrate volume fraction and flow velocity provides more insight into hydrate flow through the complex geometry of the jumper. Results show that the total deformation on the jumper increases with increasing hydrate volume fraction. They also show that the minimum deformation of the pipeline occurs at a very low fluid velocity. Identifying the regions of maximum stress and deformation for various flow conditions will help operators take preventive action against structural failure.
Analysis of the effect of different structural materials on deformation revealed that deformation increases with increasing Reynolds number because flow inertia increases with Reynolds number. Analysis showed that aluminum experiences maximum deformation, cast iron experiences intermediate deformation, and steel experiences minimum deformation. Choosing the right structural material for jumper construction can help reduce the risk of structural failure.
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01 August 2018
30 July 2018