Composite tubes increase umbilical fatigue resistance

As oil production moves into deeper waters, problems of hydrate formation in subsea wells and flowlines become greater.

As oil production moves into deeper waters, problems of hydrate formation in subsea wells and flowlines become greater. Methanol injection is one method of combating hydrate formation, but in deeper waters a greater volume of methanol must be injected, requiring the use of large center tube umbilicals.

Although steel tube umbilicals are suitable in dynamic umbilical applications in conventional water depths, they suffer limitations which make them unsuitable for deepwater, notably limited fatigue properties and limited bending capacity. Both limitations imply restrictions on tube diameter.

Spoolable composite tubes for use in dynamic, large-center tube umbilicals could be the solution. In Norway, NAT Compipe is spearheading an R&D program known as DUCT - dynamic umbilicals with composite tube.

Pilot installation

Having established the feasibility and potential of composite tubes for this application and developed a design, the company will next produce and qualify a prototype umbilical and prepare for pilot installation. Funding for this phase, which is planned to take place as a joint industry project, has been arranged with the Research Council of Norway but oil company and contractor support is still being sought, says Rolf Jemne, NAT Compipe's Director of Business Development.

Strain tests carried out in the first phase of the DUCT program showed that composite tube has a strain capacity nine times higher than super duplex steel tube when looking at strain width at one million cycles. This increased fatigue capacity means the tube diameter can be increased from 1.2 in to 3.0 in. while keeping the cycles to failure at the same level.

Smaller bending radius

Further benefits are that the smaller bending radius of composite tube makes installation easier and requires a smaller bend stiffener. In overall terms, composite tube provides greater freedom of umbilical design for deep waters.

Composite tube is also much lighter than steel tube, which has beneficial implications for the topside tension on the floating platform from which the umbilical is deployed. This benefit becomes greater with increasing water depth.

The tube designed for the DUCT program is a hybrid laminate made up of aramid and carbon fiber. The matrix material is a ductile epoxy resin with a cycloalphatic polyamine hardener, and the liner material cross-bound polyethylene (PEX).

Meanwhile, with backing from a leading European contractor, NAT Compipe is working on a steel/composite hybrid riser for use in deep water which takes advantage of composite's weight-reducing potential. Plans are now being drawn up to build and test a prototype, Jemne says.

With their beneficial fatigue properties and weight, composite materials also have the potential to be developed for deepwater flowline service, in which application both steel and flexible pipe again suffer limitations. NAT Compipe recently delivered the first spoolable composite injection pipe to Statoil's Åsgard Field, and is currently qualifying a composite flowline for use in conventional water depths.

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