Radial Instability of Flexible Pipes With Defects in the High Resistance Bandage and External Sheath
Authors: Favaro Borges Marcelo , Talgatti Otaviano Luis , Mosquen Amauri 
Published: October 2017

Exploration and production of oil and gas in ultra-deep waters will face several technical challenges. One major concern of the static failure of flexible pipes is the occurrence of damage on external sheath and high resistance bandages that in some cases can generate radial instability of those structures. The new ultra-deep water fields will require a better understanding of those failure mechanisms and relationship between compressive loads and defect sizes to guarantee a safe operation, otherwise the expected service lifetime will not be achieved. Besides that, historical data of in-service failures of this type of equipment shows that several flexible pipes have to be early replaced due to missing of proper data to evaluate damaged structures. Therefore, even worse results are expected on ultra-deep water field application.

Flexible risers comprise multiple structural layers, which combine leads to characteristics of resistance, tightness and desired flexibility, both for its installation and operation. Regarding the mechanical strength, flexible riser structure must withstand several load modes acting together or isolated. Within this context, axial compression acting individually or combined is the responsible for radial instability of flexible pipes.

Radial instability occurs mainly when the flexible pipe suffers damage on the outer layers, which are responsible for radial strain resistance. This damage on the external sheath and bandages occurs due to the launching procedure, project or material failures, wearing, excessive loading, abandonment procedures or possible falling. Damages on the external bandage layer, together with axial compressive load may lead to catastrophic failures due to radial strains, as well known as birdcaging or lateral bucking, thereby leading to a complex local analysis in the search for solutions capable to predict riser behavior.

Therefore, this study intends to build a relationship between the size of the defects and compressive loads for flexible risers that leads to birdcage formation, which consequently reduces the pipe expected life. The measurements were performed in full scale mechanical tests of two sizes of flexible risers. After that, finite element method models calibrated and validated with mechanical tests data, were used to extrapolate the results for other possible defect scenarios. The case studies for an analysis of the relationship between compressive loads and sizes of defects which lead to radial instability and consequently to pipe stiffness decreasing, were two flexible pipes of different sizes, widely used on offshore applications, with produced defects. Besides that, thirty-two conditions were analyzed through the model developed with variations in the size of defects, according to riser geometric limits in length and width.

The results indicated that the radial instability in flexible pipes with defect on high resistance bandages does not reach the failure criterion for axial stiffness if compressive loads are limited to a threshold. Also, the defect size on bandage of flexible pipes subjected to compressive loads influences the radial instability, reducing the stiffness up to five times according to obtained results, especially depending on its length and without significant dependence of the width. One of the simulated conditions presented a change on the deformation distribution located near the manufactured defect, indicating another type of instability known as lateral buckling.