Collapse Due to External Pressure by API RP 1111
 
  Calculation of Collapse Due to External Pressure by API RP 1111 when the Source is at Subsea Wellhead
  API RP 1111 Limit State
              (Enter values only in yellow cells)
  DATA INPUT
Flowline/Riser Diameter Nominal,  DN =
 mm Flowline pipe SMYS, S =   MPa
Flowline/Riser External Diameter,  D =  mm  Riser pipe SMYS, S =   MPa
Jacket Pipe Diameter Nominal,  DN =
 mm  Jacket pipe SMYS, S =   MPa
Jacket Pipe External Diameter,  D =  mm
Young’s Modulus, E =   × 10³ MPa
  Wall thickness flowline, t =   mm Poisson ratio, v  =                     
Wall thickness riser, t =   mm Pipe Ovality, δ  =   
Wall thickness jacket pipe, t =   mm Collapse reduction factor, g(δ)  = 
Type of Flowline/Riser welding = 
Water Depth, H1 =   m Type of Jacket pipe welding =
Water Depth, H2 =   m
Density, ɣ =   Kg /mᶟ
     1 MPa = 1 N/mm²                            
  CALCULATIONS
  For a Single Pipe
    Net External Pressure Loading                       
  Design Cases P₀ Pi (P₀-Pi)                      
  Flowline  N/mm²                    
  Riser  N/mm²                    
    Collapse Pressure                      
  Design Cases Py Pe Pc                      
  Flowline  N/mm² Pc= PyPe          
  Riser  N/mm² (Py²+Pe²) ½        
    External Pressure Collapse Resistance                      
Pressure in newton per square milimeter
  Design Cases (P₀-Pi) f₀Pc Inequality (9) Satisfied? (Yes/No)                       
Flowline
foPc ≥ (Po-Pi)
  Riser                      
    Buckling Limit State Bending Strains Collapse Pressure                      
  Design Cases {g(δ) – (Po – Pi)/(fc × Pc)}  εb ε                      
  Flowline   ε = { g(δ) -  (Po-Pi) } x εb  
  Riser   (fcPc)  
    Combined Bending and External Buckle Resistance                      
  Design Cases Installation Operation Inequality Satisfied? (Yes/No)                       
    ε f₁ε₁ ε f₂ε₂                      
  Flowline   ε ≥ f₁ε₁            
  Riser   ε ≥ f₂ε₂            
                                   
  For a Pipe in Pipe (PIP)
    Net External Pressure Loading                       
  Design Cases P₀ Pi (P₀-Pi)                      
  Flowline  N/mm²                    
  Flowline Jacket  N/mm²                    
  Riser  N/mm²                    
  Riser Jacket  N/mm²                    
    Collapse Pressure                      
  Design Cases Py Pe Pc                      
  Flowline  N/mm² Pc= PyPe          
  Flowline Jacket  N/mm² (Py²+Pe²) ½        
  Riser  N/mm²                    
  Riser Jacket  N/mm²                    
    External Pressure Collapse Resistance                      
Pressure in newton per square milimeter
  Design Cases (P₀-Pi) f₀Pc Inequality (9) Satisfied? (Yes/No)                       
  Flowline                      
  Flowline Jacket   foPc ≥ (Po-Pi)        
  Riser                      
  Riser Jacket                      
    Buckling Limit State Bending Strains Collapse Pressure                      
  Design Cases {g(δ) – (Po – Pi)/(fc × Pc)}  εb ε                      
  Flowline                      
  Flowline Jacket   ε = { g(δ) -  (Po-Pi) } x εb  
  Riser   (fcPc)  
  Riser Jacket                      
    Combined Bending and External Buckle Resistance                      
  Design Cases Installation Operation Inequality Satisfied? (Yes/No)                       
    ε f₁ε₁ ε f₂ε₂                      
  Flowline                      
  Flowline Jacket   ε ≥ f₁ε₁            
  Riser   ε ≥ f₂ε₂            
  Riser Jacket                      
                                   
                                 
  Collapse Due to External Pressure by API RP 1111
 
  Calculation of Collapse Due to External Pressure by API RP 1111 when the Source is at Subsea Wellhead
  API RP 1111 Limit State
                    (Enter values only in yellow cells)
  DATA INPUT
Flowline/Riser Nominal Pipe Size,  NPS =
  in. Flowline pipe SMYS, S =   psi
Flowline/Riser External Diameter,  D =   in.  Riser pipe SMYS, S =   psi
Jacket Pipe Nominal Pipe Size,  NPS =
  in.  Jacket pipe SMYS, S =   psi
Jacket Pipe External Diameter,  D =   in.
        Young’s Modulus, E =  × 10⁶ psi            
  Wall thickness flowline, t =    in. Poisson ratio, v  =                     
Wall thickness riser, t =    in. Pipe Ovality, δ  =   
Wall thickness jacket pipe, t =    in. Collapse reduction factor, g(δ)  = 
Type of Flowline/Riser welding = 
Water Depth, H1 =    ft Type of Jacket pipe welding =
Water Depth, H2 =    ft
Density, ɣ =    lb/ftᶟ
  CALCULATIONS
  For a Single Pipe
    Net External Pressure Loading                       
  Design Cases P₀ Pi (P₀-Pi)                      
  Flowline  psi                    
  Riser  psi                    
    Collapse Pressure                      
  Design Cases Py Pe Pc                      
  Flowline  psi Pc= PyPe          
  Riser  psi (Py²+Pe²) ½        
    External Pressure Collapse Resistance                      
Pressure in pounds per square inch
  Design Cases (P₀-Pi) f₀Pc Inequality (9) Satisfied? (Yes/No)                       
  Flowline   foPc ≥ (Po-Pi)        
  Riser                      
    Buckling Limit State Bending Strains Collapse Pressure                      
  Design Cases {g(δ) – (Po – Pi)/(fc × Pc)}  εb ε                      
  Flowline   ε = { g(δ) -  (Po-Pi) } x εb  
  Riser   (fcPc)  
    Combined Bending and External Buckle Resistance                      
  Design Cases Installation Operation Inequality Satisfied? (Yes/No)                       
    ε f₁ε₁ ε f₂ε₂                      
  Flowline   ε ≥ f₁ε₁          
  Riser   ε ≥ f₂ε₂          
                                   
  For a Pipe in Pipe (PIP)
    Net External Pressure Loading                       
  Design Cases P₀ Pi (P₀-Pi)                      
  Flowline  psi                    
  Flowline Jacket  psi                    
  Riser  psi                    
  Riser Jacket  psi                    
    Collapse Pressure                      
  Design Cases Py Pe Pc                      
  Flowline  psi Pc= PyPe          
  Flowline Jacket  psi (Py²+Pe²) ½        
  Riser  psi                    
  Riser Jacket  psi                    
    External Pressure Collapse Resistance                      
Pressure in pounds per square inch
  Design Cases (P₀-Pi) f₀Pc Inequality (9) Satisfied? (Yes/No)                       
  Flowline                      
  Flowline Jacket   foPc ≥ (Po-Pi)        
  Riser                      
  Riser Jacket                      
    Buckling Limit State Bending Strains Collapse Pressure                      
  Design Cases {g(δ) – (Po – Pi)/(fc × Pc)}  εb ε                      
  Flowline                      
  Flowline Jacket   ε = { g(δ) -  (Po-Pi) } x εb  
  Riser   (fcPc)  
  Riser Jacket                      
    Combined Bending and External Buckle Resistance                      
  Design Cases Installation Operation Inequality Satisfied? (Yes/No)                       
    ε f₁ε₁ ε f₂ε₂                      
  Flowline                      
  Flowline Jacket   ε ≥ f₁ε₁          
  Riser   ε ≥ f₂ε₂          
  Riser Jacket                      
                                   
                                                             
    Discussion and References  
     API RP 1111 Design, Construction, Operation, and Maintenance of Offshore Hydrocarbon Pipelines (Limit State Design)  
                                                             
    Tables and Standards  
  -  Table D-1 Specified Minimum Yield Strength for Steel Pipe Commonly Used in Piping Systems  
  -  Table A842.2.2-1 Design Factors for Offshore Pipelines, Platform Piping, and Pipeline Risers  
  -  Table 841.1.8-1 Temperature Derating Factor, T, for Steel Pipe  
-  ASME B36.10M-Welded and Seamless Wrought Steel Pipe
  -  ASME B36.19M-Stainless Steel Pipe  
Collapse Due to External Pressure:
The collapse pressure of the pipe shall exceed the net external pressure everywhere along the pipeline as follows:
Where:
                              fo  is the collapse factor;  
    foPc ≥ (Po-Pi)             (9)       = 0.7 for seamless or electric resistance welded (ERW) pipe;  
      = 0.6 for cold expanded pipe, such as double submerged arc welded (DSAW) pipe;
                              Pc  is the collapse pressure of the pipe, in N/mm² (psi).  
    The following equations can be used to approximate collapse pressure:  
      Pc = PyPe               (10) Where:                              
      (Py²+Pe²) ½             E is the modulus of elasticity, in N/mm² (psi);  
                              v is the Poisson ratio;   
      Py = 2S ( t )             (11) Pe is the elastic collapse pressure of the pipe, in N/mm² (psi);  
      D             Py is the yield pressure at collapse, in N/mm² (psi).  
The collapse pressure predicted by these or other equations should be compared to the hydrostatic
pressure due  to  water  depth  to  ensure  adequate  wall  thickness  is  chosen  for  the  range  of
          ( t ) 3           (12) water  depths  to  be encountered.  
      Pe = 2E D                                            
      (1-v²)                                            
                                                             
    Buckling Due to Combined Bending and External Pressure                              
    Combined bending strain and external pressure load should satisfy the following:  
      ε + (Po-Pi) ≤ g(δ)     (13) Where:                              
      εb fcPc      fc  is the collapse factor for use with combined pressure and bending loads;                               
                              recommended value for  fc =   fo                    
      ε = { g(δ) -  (Po-Pi) } x εb       g(δ)                    
      (fcPc)     For installation conditions, consideration can be given to higher collapse factors up to 1.0.  
                              Regardless of the selection of the value for fc, the conditions for collapse in Equation (9) need to be  
                              satisfied.   
                              g(δ)  collapse reduction factor =                            (1+20δ)ˉ¹            
                              δ  Ovality =     Dmax - Dmin                        
                                Dmax + Dmin                        
                              Dmax is the maximum diameter at any given cross section, in mm (in.);       
                              Dmin is the minimum diameter at any given cross section, in mm (in.).       
                              εb buckling strain under pure bending =      t            
                                  2D            
                              ε is the allowable bending strain in the pipe [in the presence of external pressure];   
                              NOTE:   Equation (13) is acceptable for a maximum D/t = 50.       
    To avoid buckling, bending strains should be limited as follows:                               
    ε ≥ f₁ε₁                 (14) ε1 is the maximum installation bending strain;     
                              ε2 is the maximum in-place bending strain;    
    ε ≥ f₂ε₂                 (15) f1 is the bending safety factor for installation bending plus external pressure;     
                              f2 is the bending safety factor for in-place bending plus external pressure;       
                                                             
    f₁, bending safety factor for installation bending plus external pressure:  
    The safety factor of 3.33 for installation allows for a large increase in the bending strain before the critical buckling bending strain is reached.  
    This safety factor should be selected based on positional stability of the lay vessel during dynamic positioned pipelay and subjective degree of  
    risk to be tolerated. Lower safety factors may be justified for exceptional conditions; for instance pipelay equipment limits, economic constraints,  
    or other factors. (f1 = 3.33)  
    ε₁, maximum installation bending strain;  
    The maximum installation bending strain is typically determined by installation analyses, contractor equipment limitations, and pipeline owner  
    specifications. The selected value of 0.15 % has been used on numerous pipeline projects. (ε1 = 0.0015)  
    f₂, bending safety factor for in-place bending plus external pressure:  
    The safety factor of 2.0 for operation allows for a significant increase in the bending strain before the critical buckling bending strain is reached.  
    This safety factor is reduced compared to the installation safety factor since the maximum expected bending strains can be defined with higher  
    precision due to the known boundary conditions. In many cases it can be demonstrated that operational or in-place bending strains are self-limiting  
    due to the support geometry.  (f2 = 2.0)  
    ε₂, maximum in-place bending strain;   
    In-place structural pipeline analyses and pipeline owner specifications typically determine the maximum operational bending strain. The selected  
    value of 0.15 % is typical for pipeline projects. (ε2 = 0.0015)