Riser fatigue a serious concern in GoM

Riser fatigue is a critical concern, particularly in deepwtaer operations.

Paul Stanton of Technip Offshore Inc. told participants at the Deep Offshore Technology conference in New Orleans, "As an industry, we are continually evaluating how to analyze this."

Stanton's work focused on dry tree units, which in the Gulf of Mexico means spars and TLPs, he said. There are 13 spars operating today in the GoM in water depth ranging from 1,900-5,700 ft. For all of these units, riser fatigue is a serious issue.

Stanton's approach uses a tool called SHEAR7, which he referred to as, "a typical tool for analyzing vortex induced vibration (VIV) on top tensioned risers (TTRs)."

The analysis Stanton presented took into account several causes of VIV, including Loop and eddy currents (a particular concern in the GoM), submerged currents, background currents, bottom currents, and vessel motions that transfer stresses to the riser. "Each riser has different parameters," he said.

Stanton proposed multi-mode analysis as a better alternative to single-mode analysis in determining fatigue to the risers. Stanton explained that the number of modes included in an analysis is specific to the type of riser and that single-mode analysis yields extremely conservative results.

Once stresses are identified, a number of approaches can be taken toward mitigating them. One useful solution applied in the GoM has been syntactic foam buoyancy modules, which reduce riser weight and lower stress on the riser.

Solutions can only be implemented after analysis identifies the trouble areas and the extent of fatigue, Stanton said. His multi-mode approach is the first step toward that end.

12/1/04