7.2.1
Signs of riser deterioration identified during visual examination should be addressed as follows.
a) Wear or corrosion on riser pipe—Conduct thickness inspection of identified location. If any measurements of thickness are less than minimum wall thickness, lay joint aside until a detailed examination can be conducted.
b) Obvious signs of damage—Lay the joint aside for more detailed examination.
c) Loose attachments—Secure or replace.
d) Damaged buoyancy modules—Remove and replace or repair.
Periodic Detailed Inspection 7.2.2
7.2.2.1 General
The likely sources of deterioration in integrity and the associated requirements and limitations of the applied inspection techniques are described in 7.4. In addition, the possible implications of noncompliance, such as
requirements for repair or derating of the tension capacity or pressure (depth) resistance of joints, are described.
7.2.2.2 Wall-thickness Reduction
Prior to fabrication of a drilling riser joint, quality control checks are conducted to ensure that the pipe wall complies with specified tolerances. Reduction in wall thickness occurs in service due to drill string wear and corrosion. Thickness reduction can be evenly distributed around the riser wall due to corrosion in service or localized, due to storage, drill string wear, or pitting corrosion. Wear on the external pipe surface from running can also result in corrosion and is likely to occur near critical welds. The procedures used for inspecting wall thickness shall be capable of identifying both global and localized thickness reductions. Inspection acceptance criteria should account for a wide range of global and local forms of wall thickness reduction. The implications of deterioration in condition are as follows.
— Localized loss—Local reduction in wall thickness might not significantly affect global bending or tensile strength, but pressure resistance can be impaired. Consequently, the pressure resistance of a riser pipe should be reassessed where localized wall thickness is less than the minimum expected wall thickness.
— Longitudinal loss—Longitudinal loss of wall thickness over a small circumferential length can be caused by keyseating from drill string rotation. Localized longitudinal wear might not have much effect on bending and tensile strength of the riser pipe, but internal and external pressure resistance and collapse resistance are reduced. Where thicker-walled joints are used to compensate for wear, it might not be necessary to assess fitness-for-purpose on the basis of nominal thickness less manufacturing tolerance.
Alternatively, less stringent dimensional criteria may be devised in conjunction with the equipment manufacturer.
— Circumferential loss—Circumferential loss of wall thickness can result in reduction in bending, tensile, and pressure resistance. In addition, curvature due to bending can become localized, possibly decreasing the fatigue life of the riser joint. Hence, the circumferential wall thickness should not be less than minimum acceptable wall.
Where dimensional measurements fall below minimum acceptable wall thickness, it is probable that a joint can be derated.
Figure 4 illustrates a typical riser system stack-up and the scope of equipment detailed in this section.
7.2.2.3 Surface and Through Wall Cracks
Methods of crack detection can be broadly classified into two categories: surface-flaw detection methods, such as dye-penetrant, magnetic-particle, and eddy current, and volumetric methods, which detect through-thickness flaws, such as ultrasonic and radiographic testing.
Surface cracks are critical and grow fastest in the region of couplings with the presence of high stress concentrations and welds. In coupling load shoulders, where no welding is present, cracks grow fastest on the surface at points of highest stress concentration. In such locations, surface crack inspection methods are sufficient to confidently detect the presence of cracks.
Drilling-riser coupling welds may be single-sided or double-sided. In single-sided welds, the most likely location of imperfections is at the weld root from incomplete penetration or inclusions. Surface-crack detection methods applied to the outside of the pipe are incapable of detecting flaws on the inside. Consequently, either internal inspection or a suitable volumetric method applied from the outside that can reliably detect root imperfections is required.
Key 1 casing
2 conductor (structural pipe) 3 lower stack
4 LMRP 5 lower flex joint
6 slick and buoyant joints 7 telescopic joint
8 tension ring 9 upper flex joint 10 drill floor
11 extent of integrity guidelines
Figure 4—Schematic Illustration of Riser System 7.2.2.4 Ovality and Collapse
Pipe ovality can impair the passage of tools inside the riser and reduce collapse resistance. In terms of the drift requirements, any denting damage sustained through mishaps during running should be measured and the limitations assessed. Though dented, a joint can prove serviceable in shallow-water applications where collapse resistance is not a dominating design requirement. In deeper water, collapse can become the criterion that determines the serviceability of a riser joint. As collapse resistance is affected by both wear and ovality, it is necessary to pay greater attention to the global shape of the riser pipe. The limitations for ovality
vary from application to application, but it is necessary that the criteria for each application be developed based on the following:
a) functional performance—passage of tools inside the riser limited by ovality;
b) ovality/wall thickness—combined limits on wall thickness and ovality or denting that produce unaccepta- bly low resistance to withstand external hydrostatic pressure.