Modeling of risers using hybrid fea

Editor's Notes

• #2: I’d like to share with you some of my former experiences on marine riser FEA.
• #3: First this is an overview of the presentation.
• #4: I’d like to mention several things as background information of this presentation. As a former automobile engineer and a naval architecture and ocean engineering major in college, and having been working in the oil industry for the past 9 years, I have the privilege to involve in FEA activities of these two gigantic yet distinctive industries, and know something about both worlds. The differences in practices and design philosophy are large, for example, the auto design has been using very detailed FE modeling for local as well as global simulations, and for the oil industry, since uncertainties of environmental conditions and the severe nature of the consequences of any failure in an open and deep sea, a safety factor of 10 is usually used in design, which is unheard of for its auto counterpart. I think a sharing of these experiences would help, and one such example is for the offshore engineering, detailed FEA in a global setting, i.e. using hybrid modeling, or shell elements locally and beam globally for riser simulation. Buckling is a failure mode due to elastic instability, characterized by a sudden failure of a structural member subjected to high compressive stresses, where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. It is proved that linear FEA buckling load calculation in combination with material yielding property consideration will give a more accurate, convenient and consistent way of buckling check. Beam elements are used in global FEA, and shell elements in detailed FEA. They can be used jointly in a hybrid FEA, where shell elements are used to model an area of concern with local features, such as ovality, local defects and buckling, and beam elements to simulate riser global behaviors in various boundary and environmental conditions.
• #5: For an accurate simulation of a structure, a precise material stress-strain curve should be used. Plastic strain is a useful concept in structure failure mode simulation. Here is a definition of plastic strain, and an illustration of the engineering stress-strain curve, which is obtained from material sample test, and the true stress-strain curve. Usually an engineering stress-strain curve without modification is used in FEA, and this gives a conservative simulation. For compressive buckling of a cylindrical pipe, here are what the usual design codes say. An ‘08 paper published by researchers from JFE, one of the largest Japanese steel producing companies, showed test results of 30” pipes, that criteria of plastic strain for pure compression is t/2D, and for pure bending t/D. As for risers, especially in TDA of SCR, its loading can be characterized as compression + bending. Detailed FEA will show buckling mode and associated plastic strain. For a specific event, a fracture would not occur unless local strain passes the material elongation, which means the plastic strain could reach 10~15% or even more.
• #6: Some details of FEA using hybrid modeling.
• #7: The first example presents the export SCR (Steel Catenary Riser) on Independence Hub in GoM. Characteristics of the SCR and material stress-strain curve used are displayed here. This has been published in an OMAE ’07 paper.
• #8: About 500 beam elements and 2400 shell elements were used. The shell elements were for a 300ft portion of SCR.
• #9: Two meshes, with 2400 and 9600 shell elements and different aspect ratio ranges, were used to see sensitivity of mesh size. This was not a complete investigation.
• #10: Two simple test cases with theoretical solutions were used. It can be seen with finer mesh, the results are converging. The coarse mesh, Mesh 1, over-predicted both displacement and stress, while the fine mesh, Mesh 2, over-predicted displacement and under-predicted stress. Mesh 1’s results are more conservative, and will be used.
• #11: An example of compression, where tension at TDA and 100-yr Hurricane heave motion time-traces are displayed. It can be seen the compression is about 200 Kips.
• #12: Simulation results for 100-yr Hurricane at near condition, of both Mesh 1 and Mesh 2, are displayed in tabulated form with those of beam model. Pictures of stress and plastic strain distributions for Mesh 1 are also shown. More details can be found in the published paper.
• #13: An example of animation of stress distribution of about 50s duration. It should be noted this is not for the original problem as the original video clip has been lost.
• #14: An example of animation of plastic distribution. This is for the the same problem as in the previous slide.
• #15: The second example is for a production TTR(Top Tensioned Riser) on a TLP (Tension Leg Platform), during an extreme metocean event where compression is present at top of the stress joint. Water depth is 3800ft. TTR is 14” in OD and 0.65” in wall thickness for outer casing, and 7” in OD and 0.362” in wall thickness for inner tubing . Details of the hybrid model is shown. 6550 shell elements are used to model the stress joint, transition joint, and several standard joints of 780ft length. Details can be found in the paper published in an engineering magazine.
• #16: Von Mises stress distributions at a moment on TTR outer casing with and without current.
• #17: Plastic strain on TTR inner tubing.
• #18: Tension variation on different tensioner cylinders (5) with one cylinder damaged, in comparison of heave motion.
• #19: Tension and moment variations at stress joint top with one tensioner cylinder damaged. It can be seen for the case w/o current there is a compression of about 40 kips on TTR outer casing, and tension of about 175 kips on inner tubing.
• #20: Time-trace variations of forces on top 2 and bottom 2 centralizers.
• #21: Animation of stress distribution on outer casing w/o current of 200s duration.
• #22: Animation of plastic strain distribution on outer casing w/o current of 200s duration.
• #23: If there is any question, please contact me at yhliu_vt@hotmail.com.