Christopher Hardee - For Technip
In a comparison between simulation and physical testing, finite element analysis could simulate accurately complex loading conditions involving multiple component contact and nonlinear material behavior. This validated simulation method can be used to perform optimization and other studies on a virtual prototype of an umbilical. With most umbilicals built for purpose, this provides engineering testing and design validation during front-end engineering without requiring physical tests.
The umbilicals of the oil and gas industry are aptly named. They are the lifelines of deep-sea fields, connecting the well to the mother ship, offshore platform, or onshore terminal. Since the early 1990s, seabed-based production systems have become increasingly important. Offshore wells are now being built as far as 100 km (62 mi) from the base and up to 3 km (nearing 2 mi) deep. As a result, umbilicals—whether steel tube conduit, thermoplastic hose, or a combination of the two—are critical for power, control, communication, and fluid injection to keep deepwater wells healthy and producing around the clock.
Steel tube umbilicals are typical in deepwater sites such as the Gulf of Mexico or off West Africa because of the high pressures at increasing depth. Each umbilical is custom-designed to the field requirement, but a typical umbilical can contain a central large-bore tube 7.62 cm (3 in.), a number of smaller tubes (½ –1¾ in.), and electrical and fiber optic cables, all helically spiraled around the central tube—similar to the structure of rope—for axial strength and flexibility.
Durability is essential whether the umbilical is hanging in the water column (dynamic), resting on the seabed (static), or connecting infrastructure. That is because pressure and temperature extremes, wave and current action, and sour fluids conspire to break or damage the umbilical and its individual parts.
To add robustness for dynamic applications, tubes are coated with a polyethylene sheath to prevent metal-to-metal contact, and that bundle is wrapped with fiber-reinforced binding tape. An additional layer of ½-in. tubes and space fillers can be placed around the core, and a polymeric outer jacket, or sheath, extruded over the bundle for mechanical protection.
Deepwater installation
Building failure-proof umbilicals is difficult enough, but it is made even more so by the challenges of deepwater installation. Wound onto storage reels, then mounted on a specialized installation vessel, the umbilicals need to deploy in a highly controlled manner to reach a precise target on the ocean floor.
To do so, the umbilical is fed through a vertical lay system (VLS) that controls the unspooling by applying a holdback tension to the umbilical as it hangs from the ship. The holdback tension is created typically by four caterpillar tracks with V-shaped pads, which use friction from a radial crush force to control deployment.
As the water depth increases, the tension and the crush load required to hold the weight of the lengthening umbilical also increases. Traditionally, up to 30 tons/m of radial load can be applied to a steel tube umbilical during deepwater installation.
This can cause deformation to the tubes, which have point contact (where tubes in adjacent counter-rotating layers cross) due to the umbilical’s helical construction. Det Norske Veritas, Norwegian risk-management specialists, recommends a 3% residual ovality (permanent tube deformation following crushing) as acceptable; higher levels of deformation can affect the umbilical tubes’ resistance to hydrostatic pressure as depths increase. Residual ovality also can impair fatigue resistance to pressure cycling over time. As a result, understanding umbilical crush behavior in detail is critical to ensure product integrity, to establish load limits, and to design out failure.
As projects get more expensive, risks rise, and customers want more up front engineering.
“Being able to prove that the design fits the purpose is critical,” notes Ian Probyn, senior engineer R&D, Technip Group’s DUCO Ltd. “With realistic simulation, we’re able to see inside the umbilical. That’s something you cannot do with physical testing. FEA provides that level of detail.”
Simulation flexibility
DUCO first chose Abaqus FEA from SIMULIA, the Dassault Systèmes brand for realistic simulation, for umbilical R&D in 2005.
“We did an evaluation,” says Dave Fogg, R&D team leader, “and Abaqus stood out because of its Explicit solver capability for analyzing highly nonlinear, dynamic behavior.”
This capability is important, he adds, given the helical structure of the umbilical, interaction between the components, and bending stiffness due to friction.
Another benefit of Abaqus is its ability to customize scripting tools. This is important because each application has unique umbilical requirements. Since each product is essentially one-of-a-kind, the FEA tool and simulation process need to be flexible enough to accommodate this design variability.
To help accommodate the need for customization, DUCO, in collaboration with French-based IFP, a public-sector research and training center, developed a proprietary, validated engineering software tool—FEMUS or finite element model of an umbilical structure. This tool interrogates a database that includes all of the information required to build a model of the umbilical. It then automates 3D model building by gathering all the data into a Python script (programmable language file used by Abaqus), which it then executes within Abaqus/CAE to create the FEA model.
Once the data is loaded in Abaqus, the script does the rest: It builds the umbilical-specific geometry; constructs the assembly; applies the section properties, element types and materials; and creates the load steps, contacts and request for history; and supplies field output data. Developed for Abaqus and for non-FEA experts, the interface aims at rapid model building using proven techniques. In approximately 10 minutes, the team can have a run-ready base model inside Abaqus.
Looking deeper
During the VLS installation, the umbilical is subject to tension, bending, and the crushing load from the caterpillar pads. For the crush load portion of the analysis, the 3D model in Abaqus/Explicit captures all the interactions in the helically oriented structure. The 3D analysis gives the relationship between the crush load and the resulting ovality of the tube while under that load.
When the umbilical leaves the caterpillar, the crush load is relieved and the tubes elastically relax, resulting in a reduction of tube ovality. For the recovery of the tube, a simpler 2D analysis in Abaqus/Standard is efficient. In the 2D environment, the team conducted a number of analyses for each tube and built the relationship between the maximum ovality under load and the residual ovality of the tube following elastic recovery. The results from the 2D and 3D analyses in combination determine the overall residual ovality of a tube for a given caterpillar crush load.
FEA model of the umbilical (one pitch in length) showing internal umbilical components and the external caterpillar pads, which apply the crush force.
For further efficiency, all analyses were run on models constructed from a single pitch of the umbilical—the length at which the helical pattern starts to repeat—which in this case was several meters. The team used shell elements for the tubes and solid elements for the polymer sheath, outer sheath, and fillers. For the crush pads, they used rigid elements and dimensions that matched the umbilical pitch length.
Even streamlined, an umbilical installation simulation can be computation-intensive. A recent DUCO analysis had approximately half a million nodes and a similar number of elements.
“It looks like a simple structure,” says Probyn, “but it’s a reasonably big model and quite a solver challenge.” To handle this, the team used a cluster of CPUs with significant capacity. “The goal was to deliver an analysis in a reasonable time,” adds Probyn, “and we’ve succeeded.”
Diagnosing umbilical health
In the umbilical installation analysis, DUCO considered the key variables: tube wall thickness, VLS crush load, internal tube pressure, and caterpillar pad geometry. To gain confidence in the results, four simulations ran that match conditions of four full-scale physical tests for a combination of internal tube pressure and caterpillar pad angle.
Comparing results, the team found good agreement between the FEA predictions and the physical tests. For most loads, the differences between the FEA and test results were within the measurement tolerances. The FEA predictions also showed the same trend as the test data in predicting reduced residual ovality as the geometry of the caterpillar pad V-angle was altered from a high to low angle. In addition, the model and tests agreed when an internal pressure was present in the steel tubes during application of the crush load. All results indicated that residual ovality was below the recommended 3% limit for all loads — within the nominal crush loads. This gave the team confidence that, in specific cases, crush loads beyond the typical values could be applied.
Overall, the analyses demonstrated to DUCO that FEA could simulate accurately complex loading conditions involving multiple component contact and nonlinear material behavior. “Now that we have fully validated the FEA, simulation can be employed as a virtual prototype to perform additional analyses, such as optimization and reliability studies,” says R&D team leader Fogg.
Offshore Articles Archives
iew Oil and Gas Articles on PennEnergy.com
