Offshore Application of a Novel Approach to the Inspection of Flexible Risers

Abstract

Abstract The inspection of unbonded flexible risers poses a number of challenges several of which are linked to the complex nature of the pipe's construction. This precludes the application of most conventional inspection techniques. Further difficulties arise because in many cases the critical region of the riser is at the top end; from the splash zone to the termination. This area is particularly congested where the presence of the bend stiffener and I-tube structures means there is no access for inspection equipment. This paper describes a unique inspection approach which overcomes these challenges and its implementation on an FPSO operating in the UKCS by Shell. The inspection technique concerned employs electromagnetic stress measurement technology that when mounted onto the external surface of the riser can sense into the riser construction and measure the stress in the tensile and pressure armour wire layers, the primary load bearing elements within the riser design. From this information the integrity of these layers such as the presence of wire breaks can be determined. A key feature of the approach is the concept of ‘reach’. As the layers within the pipe wall are unbonded the effect of a wire break can be seen many metres away from the location of the break itself: so an inspection at a single location can determine the state of the riser armour wires over the complete length of the critical top section. The application on the FPSO formed part of Shell's riser integrity management and life extension programme. A simple scanning device was deployed allowing a stress measurement probe to sense the state of all of the armour wires in the riser from a single point in the turret region. The results of the scan demonstrated that the armour wires of the riser were in a sound condition for continued service. This paper will provide the background to the underlying technology and its embodiment within an inspection system, together with the results from the inspection campaign.