In the design of underwater cables and umbilicals for the offshore oil and gas and renewable industries, engineers must account for deepwater pressure, wind and wave forces, and the rough ocean bed over extended design lives of up to 25 years. Known as umbilical systems, these cables house links for power, hydraulic control, electric signals, fiber optics, and chemical injection. The cables must be extremely durable because they undergo severe coupled bending as well as high torsional and axial loads in service and during installation.
Installation can happen as many as a 150 times in an umbilical’s lifespan, so it must be able to withstand high tension and tight bending radiuses. Armored umbilicals are generally big and difficult to handle and, as a result, this makes physical testing cumbersome and expensive. It is also very difficult to fully represent the in-service conditions accurately, leading to very conservative fatigue testing regimes.
To gauge fatigue properties and performance, a typical fatigue test for an umbilical includes undergoing 100,000 usage cycles around a sheave wheel on a large fatigue rig. At approximately 6,000 cycles/day, plus all the other required testing and taking into account setup and downtime, it takes at least a month to complete the testing. These tests cost about $30,000 to $50,000 and can often become the critical path of a project as the testing on the prototype length may need to be completed prior to the manufacture of the main product.
As explained by Senior Design Engineer Tim Poole of JDR in the U.K., “Typical umbilical analyses incorporate multiple layers of armor wire with helical geometries and multiple contact points. Other umbilicals, such as intervention work-over control systems (IWOCS) contain an aramid (Kevlar) braid strength member. This synthetic material is difficult to analyze because of its braided construction and fatigue properties.
JDR worked with Comsol Certified Consultant, Continuum Blue, in the U.K., on these complex analyses.
Director of Continuum Blue Mark Yeoman explains the method of the analysis. “Our starting point was to create a 2D cross section of a cable, including material specifications,” he says. “Of concern was that the cable cross section had a double- armor layered structure with 50 to 60 armor wires in each layer. Each layer is twisted along the length in the opposite direction to the other layers. We built the model to reflect bend and axial-load conditions with contact for the internal structures and added in the contact for the counterrotating armor wires. This resulted in well over 3,000 localized regions of highcontact pressure along a unit length of cable, creating high stresses at every point of contact.”
Continuum Blue built a custom program so JDR could generate the 3D cable structure through Comsol’s Livelink for Matlab and then build the Comsol cable model.
The Matlab code added advanced material properties and relations from Continuum Blue’s materials database. The code used these properties to help define the custom contact expressions and parameters necessary to solve the contact analysis. Everything was then automatically imported into Comsol Multiphysics so it could be solved with one click of a button. Engineers modeled wires as contact pairs moving between a sliding surface and analyzed the cables in pure bending, pure tension, and a combination of both.
It now takes two days to build a full 3D cable model ready to solve from an initial design and 2D drawing,” says Yeoman. “From there, Comsol solves the load conditions. We can conduct seven or eight different types of analyses. For example, we test different axialload conditions and various bend radiuses.”
The analysis has helped JDR assess many design scenarios where comprehensive stress, strain, and contact-analysis plots can be studied. The results help ensure cables survive during installation and service. This boosts the life of the cable and cuts the costs involved in testing and manufacture. “The first time we adopted this approach it worked really well,” says Poole. “The models were clear and the local stress analysis was reliable.”
JDR can now analyze subsea cable structures with multiple internal counterrotating structures and up to six protective armor layers. From 10 weeks on the original project, turnaround time is now down to two weeks, all while producing five times more data. “Not only are we able to analyze the umbilical-fatigue characteristics using Comsol, we can also analyze their thermal characteristics,” says Poole. This is good news because analysis can cost one-half that of physical testing.