Carry on carbon

John Bradbury

May 1, 2015

Carbon fiber rods are the way forward for providing umbilicals in deepwater applications, according to one leading subsea contractor. John Bradbury explains.

Umbilical load out Moss. Photo from Aker Solutions.

Outlining the benefits of using carbon fiber rods inside umbilicals rather than steel to provide bend stiffening, Lars Mehus, chief engineer for umbilicals at Aker Solutions, explained how the lighter carbon reduces strains within an umbilical installed in an ultra-deep zone.

“We have two power umbilicals that are free suspended from a vessel at 2700m (23,911ft) water depth now in operation,” Mehus said, following completion of a project for an undisclosed client.

Mehus said the achievement was due to the decision to turn to carbon fiber rods to provide stiffening for umbilicals, rather than taking the traditional approach of using steel tube umbilicals.

Detailing the success of the carbon fiber application, Mehus explained Aker Solutions has two umbilicals installed at 2700m water depth. The longest is 31km (19.3mi), with a top tension 985 kilo Newtons, and still has high umbilical axial stiffness, and a “reasonable” strain value of 0.15%, Mehus explained.

Mehus outlined how using steel armoring to improve axial stiffness in an umbilical comes with a big disadvantage, by adding more weight. “In deepwater you not only have stiffness but you also add to the weight of the umbilical, so in a way you eat up the benefit you get from the actual stiffness,” he says.

Mehus was speaking at the Marine Construction Europe Deepwater Development conference in London in March. “By using carbon fiber technology we have an element that has much less weight and more or less the same stiffness as steel,” he explained.

Using carbon fiber can be more effective at greater water depth. He said carbon fiber rod technology is much more efficient at deep water due to its low density compared to its stiffness. (see formula below taken from Mehus’ presentation)

“By putting carbon fiber in the umbilical, we can reduce the strain down to a decent level, below the strain limits we have set as an acceptance criteria for the copper cable, but also leaving capacity for the other elements in dynamic scenarios, and capable of taking bending stresses,” Mehus reasoned.

Conference delegates heard how the complex interplay between forces and strains imposed on umbilicals affects their fatigue life; and that the cable strain and loading can be the same in shallow and deepwater, depending on the axial stiffness of the umbilical.

Due to temperature variations, material creep, and the effects of friction in the bend stiffener area of an umbilical, it is difficult to predict loads imposed on umbilical power cores. And this in turn complicates prediction of fatigue life, Mehus told his audience.

While offering a carbon-fiber umbilical, Aker Solutions is specifically developing a deepwater umbilical stress analysis program – USAP – using a 3D finite element analysis approach, looking at linear and non-linear effects, axial strain, longitudinal helix behavior, with bell mouth and bend stiffener modeling, and a full catenary model. The USAP research is also looking at reeling umbilicals with large center tubes, stress control, and installation and temperature impacts on umbilicals.

Aker Solutions is also looking at the process of installing umbilicals with cable splices in deepwater, where Mehus outlined further issues including strain hardening due to conductor manufacturing, hardening during conductor stranding, and conductor compaction.

Using 3D analysis more fully measures axial strain: “If we use a 2D model we will have under-estimated the strain elements,” Mehus noted.

While cable splice welds impart “proper strength” in a welded zone on a cable, there is a change detected in the mechanical properties of the cable, due to the heat input. Aker has found all the copper cable strain is concentrated in a local point, where there is no ductile behavior and the spliced cable exhibits signs of a brittle fracture during a tensile test. To overcome this, the joint is reinforced with a high strength fiber sleeve outside the splice.

“We are working on improving the technology for power umbilicals, and now we have umbilicals in 2700m water depth in operation,” Mehus summarized, concluding: “We have improved our design in order to have a better service condition for the cable itself, especially in the most exposed, dynamic area, and we have also developed technology to take care of cables including splices, to increase the safety of the installation.”