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Umbilical designs for deepwater are complicated and present a number of challenges that must be considered during early design stages. If not correctly taken into account, they can cause problems and limitations during installation and in service. The trend of incorporating more functional components into the design leads to heavier umbilicals and, coupled with greater water depths, results in much higher topside hang-off loads. This drives umbilical designers to further understand the behavior and limits of their products.
The need for lightweight components and stronger materials increases as umbilicals continue to go deeper and topside weights increase. To help reduce the topside tension, Technip Umbilicals developed aluminum power cables that reduce the weight of umbilicals, resulting in greater water depth capability. The cables provide higher strength and fatigue life compared to traditional copper cables, improving service life capabilities of deepwater umbilicals. To complement the technology, high-strength strain members also have been developed. These strength members reduce strain on the product at the topside hang-off by increasing axial stiffness and further reduce risk for the product.
Umbilicals are typically installed via a caterpillar tensioner system that exerts a radial squeeze load to the outer sheath of the umbilical. A friction interface between the installation equipment and umbilical generates a holdback tension that reacts to the topside tension of the umbilical hanging below the installation vessel. Internal friction between the individual components of the umbilical ensures that the holdback tension is transferred from the outer sheath to the vital load-bearing members, typically steel tubes or high-strength members. It is critical that the holdback tension is sufficiently greater than the hang-off load and that the load is efficiently shared across components to ensure safe and risk-free installation. The key factors that govern successful installation are crush capacity of the umbilical structure and frictional load transfer.
Umbilicals are a composite mixture of components helically laid in counter rotating layers with multiple interactions between components. If the structural behavior is not fully understood, issues can occur during installation. If the crush resistance of the umbilical is conservatively underestimated, this could lead to a lower-than-required crush force being specified, resulting in added costs for larger installation equipment and vessels. Conversely, if the allowable crush load is increased without fully understanding the limits, it can result in damage to the functional components such as ovalization or squashing of tubes. If all issues have not been taken into account, the umbilical may have complications when in service that affect its fatigue, utilization and service life.
When designing the umbilical, it is important to calculate the crush capacity of the cross section. Current industry methods include physical testing of prototype samples, mathematical modeling and 2-D analysis. However, these methods are overly conservative and can introduce risks due to assumptions and unknowns that generally underestimate crush capacity. The company has developed a finite element analysis (FEA) method using 3-D modeling that accurately captures the structural behavior of umbilicals during installation. The models have been validated against component level and full-scale umbilical tests to prove the correlation between model and reality.
A proprietary software tool, FEMUS, uses a technical cross-section design database to automatically build the FEA model using proven techniques. This process provides a fast and accurate 3-D model ensuring that the crush analysis pinpoints any weak areas of the design and provides a knowledgeable view of the needed installation process. Allowing interrogation and optimization of the umbilical design while ensuring the crush capacity is tailored for specific installation gives the installer increased confidence, reduced risk and cost savings by identifying potential issues early in the design stage.
Overall, using the 3-D software ensures that the company gains a greater insight into the behavior of umbilicals, creating an advantage for its design and manufacturing processes by producing designs that are robust and ease installation. Other 3-D modeling methods are currently used within the industry; however, FEMUS is unique in its ability to model the radial crush behavior.
The company has also applied its 3-D modeling tool from umbilical analysis to installation equipment analysis. Using the same approach, the method directly replicates the installation equipment by building a model based on the design and interrogates the installation equipment setup. The installer is advised of the best installation scenario, creating greater confidence in the installation process, reducing potential risks and minimizing cost by eliminating physical testing and late changes to installation equipment.
In addition to analysis of installation equipment, understanding the frictional transfer of load through the umbilical is needed for a safe installation. Some umbilical designs contain large-bore center tubes, which can provide up to 70% of the tensile strength. During installation, it is imperative that this key component is securely gripped by the frictional load generated by the surrounding components.
From full-scale testing, the weakest friction interface was found to be between the outer circumference of the steel tube bundle and the inner surface of the outer sheath. If slip occurred at this interface, the outer sheath would remain gripped by the installation equipment, but the bundle of components would pass through, resulting in a tear of the sheath at the exit of the caterpillar.
The company has developed a patented high-friction tape called Compressi-Grip that increases by up to double the installation friction between the bundle of components and the outer sheath. Compressi-Grip locks together the bundle and outer sheath, and due to the increased friction, the factor of safety during installation can be increased, or a smaller installation vessel can potentially be used, resulting in reduced cost. Friction levels return to normal levels once the umbilical is released, maintaining fatigue and bending performance.
To reduce the strain experienced by functional components in a deepwater umbilical, the stiffness needs to be increased or weight-reduced. The company has qualified use of high-strength steel members that can be incorporated into umbilical structures. However, if the steel strength member is used along the entire length of the umbilical, it can add strength and mass where not required. To extract the maximum benefit, the company has developed a special joint that allows the steel strength member to transition to a lightweight, cost-effective polymer filler as the umbilical extends down the water column. This enables the dynamic configuration to be optimized with added strength at the topside hang-off where it is needed, then transition to a lighter structure with polymer fillers as the umbilical goes deeper, reducing weight.
Once the umbilical has been installed and is in service, these new analysis techniques and products enable greater water depths to be reached, reduce risk and unknowns during the installation process and optimize in-service loads and life of the structure. Overall, these technologies improve deepwater umbilicals by evolving the products to adapt to future industry needs. The developed engineering analysis tools provide structural behavior, creating greater confidence and knowledge in the overall product design and service and helping Technip Umbilicals take umbilicals deeper.
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