Surface Monitoring Techniques for a Continuous Flexible Riser Integrity Assessment
Authors: Marinho M.G. , Camerini C.S. , dos Santos J.M. , Pires G.P. 
Published: October 2008

Deep water oil and gas exploitation in Brazil remarkably increased the utilization of flexible pipes in conjunction with floating production systems. In Campos Basin, nowadays, oil and gas production, water and gas injection and well control and monitoring are carried out almost entirely by flexible pipes, including risers, flowlines and umbilicals.

Periodic inspections have detected a considerable incidence of damage on the top section of flexible risers, which may affect their structural integrity and eventually induce different failure mechanisms. These include mostly external sheath damage, corrosion and/or fatigue-induced damage to the tensile armours and torsional instability, which are originated during installation or, more frequently, during operation.

In order to mitigate the progression of these damages, apart from the inspection program, surface monitoring techniques such as percolated gas monitoring, nitrogen injection in the annular space, deformation monitoring and visual inspection through video camera are being implemented, for a continuous flexible riser integrity assessment. Other techniques, like alternative methods for torsion monitoring, tensile armour stress measurement, detection through acoustic emission of tensile arm our wire rupture and external sheath wrinkling monitoring through optical fiber sensors, are under development. This paper describes and evaluates these techniques, as well as reports the results obtained from field experience.

Introduction

The incidence of damage on flexible risers during installation or operation caused considerable costs, not only due to the replacement of damaged lengths but mainly, in certain occasions, to the production losses of oil and gas caused by failure.

The necessity of preserving the structural integrity and maintaining the operational continuity of flexible pipes, since these structures are essential in oil and gas production and well control and monitoring, led to the implementation of an integrity assessment program.

In relation to flexible risers, the program is performed through periodical inspection carried out by absailers, from the riser/platform connection down to water surface, and by submarine qualified inspectors, from the water surface down to 30m water depth. From this depth down to the riser/flow connection, the survey is made through remote operated vehicle (ROV) or saturated divers.

Due to the high incidence of damage along the flexible riser top section, surface monitoring techniques are being implemented for a continuous integrity assessment. These techniques consist of percolated gas measurement, nitrogen injection in the annular space, deformation monitoring and visual inspection through video camera.

Other techniques, like alternative methods for torsion monitoring, tensile armour stress measurement, detection of tensile armour wire rupture through acoustic emission and external sheath wrinkling monitoring through optical fiber sensors, are under development.

Flexible Pipe History in Brazil

Since the beginning of the deep water oil and gas exploitation in Brazil, through the early production systems, flexible pipes were considered the best option for fluid transportation from offshore installations. This kind of pipe is not only easier to handle than rigid pipes, but also to transport, install and operate. Differently from rigid pipes, they can also be easily recovered and reinstalled.