Reeling of Pipes With Thick Insulation, A Simple New calculation Procedure
Authors: Chaudhury Gautam 
Published: October 2008

Abstract
Basic pipe reeling check is simple, keep ratio of radii of drum to pipe high and limit the ratio of pipe radius to pipe wall to control local buckling and ovalization. In general, this has worked well, with some occasional problems.
Recently, reeling of pipes with thick insulation coating encountered more than fare share of local buckling. Sophisticated non-linear analysis has confirmed this and further revealed that any mismatch of bending stiffness, or material yield, or combination of both increase the chance of local buckling, no more simple calculation for reeling.
The paper depicts a simple new procedure for pipe reeling calculations, which incorporates all the non-linearity associated with pipe geometry, pipe material property, mismatch of material property, and insulation coatings. A nonlinear property relationship has been developed for insulation material. The paper outlines techniques to control local buckling of pipes and how the new procedure and techniques can improve the reel-ability of very thick insulated flow lines for deep waters.

Introduction:
Traditionally, reeling calculation procedure was simple. Check the ratio of drum radius to pipe radius such that strain is within the allowable limit. Depending on the level of allowable strain, a certain ratio of pipe radius to pipe wall is required. This is to prevent the chance of local buckling and control ovality of the pipe within acceptable limit to avoid collapse. Local buckling and ovality check calculations are done using empirical relations available in the codes (1), recommended practices (2), and in open literature (3). These empirical relations are based on test results of bending pipes with constant stiffness and strength, and in some cases welded pipes with equal stiffness and strength.
As we advance into deeper water and longer distance wells, we need thicker insulation to ensure flow assurance requirement. Insulation coating is applied to each pipe stalk keeping small gap at each end of the pipe to facilitate welding. After welding two pipes the gap is filled with polyurethane field joint coating. Reeling of pipe with insulation means a step change in bending stiffness due to the presence of field joints. Unless properly calculated local strain concentration is missed leading to possible local buckling (4). Even though the perception within the industry is that thick insulation is the primary reason for unexpected local buckling, this is not the only reason for changes in stiffness or strength. There are other reasons, which can lead to substantial change in stiffness and or strength to cause local buckling due to local strain concentration.
Needless to mention that a buckled pipe cannot be accepted to install as is. This means expensive repair process and possible delay in schedule. A safe and reliable reeling procedure, therefore, should identify all possible causes of local strain concentrations in advance, if possible take appropriate action to mitigate them, and implement them into reeling calculation as required. Such detailed calculation generally requires non-linear procedure, using sophisticated software program.