Internal Pressure (Burst) Design and Wall Thickness Calculation by API RP 1111  
   
  Calculation of Internal Pressure (Burst) Design and Wall Thickness by API RP 1111 when the Source is at Subsea Wellhead  
  API RP 1111 Limit State  
                     
                (Enter values only in yellow cells)  
  INPUT DATA  
  Flowline/Riser Diameter Nominal,  DN =
 mm   Pi at subsea wellhead, Ps =   bar    
Flowline/Riser External Diameter,  D =  mm
 N/mm²
  Water depth at subsea well, H₁ =    m   Flowline pipe SMYS, S =   MPa    
Water depth at platform, H₂ =    m
Flowline pipe ultimate strength, U =  MPa
Density, ɣ =   Kg /mᶟ
 Riser pipe SMYS, S =   MPa
GS =
Riser pipe ultimate strength, U =  MPa
  Internal Corrosion Allowance =  mm   Burst design factor for pipelines, fd =       
          Burst design factor for pipeline riser, fd =       
     10 bar = 1 N/mm²       Weld joint factor, fe =       
     1 MPa = 1 N/mm²       Temperature derating factor, ft =       
                     
  CALCULATIONS  
  For a Single Pipe  
                     
  Description Flowline at Subsea Well  Bottom of Raiser  Top of Raiser             
  Pi, shut-in pressure -  N/mm² Note: For the single-pipe case, from the      
  Po, external pressure -  N/mm² top of riser at the waterline to wellhead,       
  (Pi – Po), shut-in pressure difference -  N/mm² external hydrostatic pressure will cancel      
  Pt, test pressure at raiser top - -  N/mm² out internal hydrostatic load, making      
  resulting pressure during hydrostatic test -  N/mm² resulting pressure during hydrostatic      
  maximum pressure for calculating D/t ratio  N/mm² test constant.      
  D/t ratio for hydrostatic test pressure by Eq 4          
  Verify D/t (< 15 use Eq 4, ≥ 15 use Eq 5)          
D/t ratio for hydrostatic test pressure by Eq 5
  tdesign, wall thickness  mm        
                     
  Specified minimum burst pressure, Pb ≤  N/mm²          
  Hydrostatic test pressure, Pt ≤  N/mm²          
  Incidental over pressure, Pa  ≤  N/mm²          
  Pipeline design pressure, Pd  ≤  N/mm²          
                     
  Select tnominal, wall thickness  mm          
  SCH            
  From ASME B36.10M                  
  Specified minimum burst pressure, Pb ≤  N/mm²          
  Hydrostatic test pressure, Pt ≤  N/mm²          
  Incidental over pressure, Pa  ≤  N/mm²          
  Pipeline design pressure, Pd  ≤  N/mm²          
                     
                     
  For a Pipe in Pipe (PIP)  
                     
  Description Flowline at Subsea Well  Bottom of Raiser  Top of Raiser             
  Pi, shut-in pressure  N/mm²          
  Po, external pressure  N/mm²          
  (Pi – Po), shut-in pressure difference -  N/mm²          
  Pt, test pressure at raiser top - -  N/mm²          
  resulting pressure during hydrostatic test -  N/mm²          
  maximum pressure for calculating D/t ratio  N/mm²          
  D/t ratio for hydrostatic test pressure by Eq 4            
  Verify D/t (< 15 use Eq 4, ≥ 15 use Eq 5)            
  D/t ratio for hydrostatic test pressure by Eq 5            
  tdesign, wall thickness  mm          
                     
  Specified minimum burst pressure, Pb ≤  N/mm²          
  Hydrostatic test pressure, Pt ≤  N/mm²          
  Incidental over pressure, Pa  ≤  N/mm²          
  Pipeline design pressure, Pd  ≤  N/mm²          
                     
  Select tnominal, wall thickness  mm          
  SCH            
  From ASME B36.10M                  
  Specified minimum burst pressure, Pb ≤  N/mm²          
  Hydrostatic test pressure, Pt ≤  N/mm²          
  Incidental over pressure, Pa  ≤  N/mm²          
  Pipeline design pressure, Pd  ≤  N/mm²          
                     
                     
                   
   Internal Pressure (Burst) Design and Wall Thickness Calculation by API RP 1111  
   
  Calculation of Internal Pressure (Burst) Design and Wall Thickness by API RP 1111 when the Source is at Subsea Wellhead  
  API RP 1111 Limit State  
                     
          (Enter values only in yellow cells)  
  INPUT DATA  
Flowline/Riser Nominal Pipe Size,  NPS =
 in.
  Flowline/Riser External Diameter,  D =  in.       Pi at subsea wellhead, Ps =  psi    
Water depth at subsea well, H₁ =   ft
Flowline pipe SMYS, S =   psi
  Water depth at platform, H₂ =   ft   Flowline pipe ultimate strength, U =  psi    
Density, ɣ =   lb/ftᶟ
 Riser pipe SMYS, S =   psi
GS =
Riser pipe ultimate strength, U =  psi
Internal Corrosion Allowance =  in.
Burst design factor for pipelines, fd = 
Burst design factor for pipeline riser, fd = 
          Weld joint factor, fe =       
          Temperature derating factor, ft =       
                     
                     
  CALCULATIONS  
For a Single Pipe
                     
  Description Flowline at Subsea Well  Bottom of Raiser  Top of Raiser             
  Pi, shut-in pressure -  psi Note: For the single-pipe case, from the      
  Po, external pressure -  psi top of riser at the waterline to wellhead,       
  (Pi – Po), shut-in pressure difference -  psi external hydrostatic pressure will cancel      
  Pt, test pressure at raiser top - -  psi out internal hydrostatic load, making      
  resulting pressure during hydrostatic test -  psi resulting pressure during hydrostatic      
  maximum pressure for calculating D/t ratio  psi test constant.      
  D/t ratio for hydrostatic test pressure by Eq 4          
  Verify D/t (< 15 use Eq 4, ≥ 15 use Eq 5)          
  D/t ratio for hydrostatic test pressure by Eq 5          
  tdesign, wall thickness  in.        
                     
  Specified minimum burst pressure, Pb ≤  psi          
  Hydrostatic test pressure, Pt ≤  psi          
  Incidental over pressure, Pa  ≤  psi          
  Pipeline design pressure, Pd  ≤  psi          
                     
  Select tnominal, wall thickness  in.          
  SCH            
  From ASME B36.10M                  
  Specified minimum burst pressure, Pb ≤  psi          
  Hydrostatic test pressure, Pt ≤  psi          
  Incidental over pressure, Pa  ≤  psi          
  Pipeline design pressure, Pd  ≤  psi          
                     
                     
  For a Pipe in Pipe (PIP)  
                     
  Description Flowline at Subsea Well  Bottom of Raiser  Top of Raiser             
  Pi, shut-in pressure  psi          
  Po, external pressure  psi          
  (Pi – Po), shut-in pressure difference -  psi          
  Pt, test pressure at raiser top - -  psi          
  resulting pressure during hydrostatic test -  psi          
  maximum pressure for calculating D/t ratio  psi          
  D/t ratio for hydrostatic test pressure by Eq 4            
  Verify D/t (< 15 use Eq 4, ≥ 15 use Eq 5)            
  D/t ratio for hydrostatic test pressure by Eq 5            
  tdesign, wall thickness  in.          
                     
  Specified minimum burst pressure, Pb ≤  psi          
  Hydrostatic test pressure, Pt ≤  psi          
  Incidental over pressure, Pa  ≤  psi          
  Pipeline design pressure, Pd  ≤  psi          
                     
  Select tnominal, wall thickness  in.          
  SCH            
  From ASME B36.10M                  
  Specified minimum burst pressure, Pb ≤  psi          
  Hydrostatic test pressure, Pt ≤  psi          
  Incidental over pressure, Pa  ≤  psi          
  Pipeline design pressure, Pd  ≤  psi          
                     
                                               
      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  
Equations for Wall thickness calculation of straight pipes on Offshore pipelines:
The hydrostatic test pressure, the pipeline design pressure, and the incidental overpressure, including both internal and
external pressures acting on the pipelines, shall not exceed that determined by the equations 
Where:
                      fd  is the internal pressure (burst) design factor, applicable to all pipelines;              
      Pt ≤ fd fe ft Pb (1)         0.90 for pipelines;                         
                      0.75 for pipeline risers;
      Pa ≤ 0.90 Pt (2)         fe  is the weld joint factor, longitudinal or spiral seam welds. See ASME B31.4 or ASME B31.8.           
                      Only materials with a factor of 1.0 are acceptable;                  
      Pd ≤ 0.80 Pt (3)         ft  is the temperature derating factor, as specified in ASME B31.8 [1.0 for temperatures less than 250 ⁰F].
                      Pa  is the incidental over pressure (internal minus external pressure), in N/mm² (psi);            
                      Pb  is the specified minimum burst pressure of pipe, in N/mm² (psi);                
Pt is the hydrostatic test pressure (internal minus external pressure), in N/mm² (psi). 
The specified minimum burst pressure (Pb) is determined by one of the following equations:
      Pb = 0.45(S+U)ln D   (4)   D/t <15     Where:                    
      Di     D  is the outside diameter of pipe, mm (in.);  
                          Di is D-2t = inside diameter of pipe, mm (in.);   
      Pb = 0.90(S+U) t   (5)   D/t >15     S  is the Specified Minimum Yield Strength (SMYS) of pipe, in N/mm² (psi)  
      D-t         (see API 5L, ASME B31.4, or ASME B31.8 as appropriate);  
                           t  is the nominal wall thickness of pipe, in.;   
                          U  is the specified minimum ultimate tensile strength of pipe, in N/mm² (psi);   
                          In is the natural log.   
                                               
      NOTE 1  The two equations, Equation (4) and Equation (5), for the burst pressure are equivalent for D/t >15. For low D/t pipe (D/t<15), Equation (4) is recommended.   
      NOTE 2  Determination of specified minimum burst pressure for unlisted materials is in accordance with Annex A.                 
      NOTE 3  Improved control of mechanical properties and dimensions can produce pipe with improved burst performance. The specified minimum burst pressure may be    
                      increased in accordance with Annex B.                                         
      NOTE 4  See Annex C for example calculations of internal pressure (burst) and Annex D for example calculations of external pressure (collapse).          
                                               
      When a corrosion allowance is required, the design process should consider the following adjustment to the wall thickness used in the design equations:     
      1)  the hydrostatic test pressure prior to first placing the pipeline in service shall not exceed the code test limit where the wall thickness            
     includes the corrosion allowance; 
2)  the MOP (usually equal to the shut-in pressure for a flowline) shall not exceed the code operating limit where the wall thickness does
           not include the corrosion allowance.                            
                                               
      For a Single Pipe Case             For a Pipe In Pipe (PIP)     Where:
                                     
      Po = ɣ x H               Po = 0         Po is external pressure, N/mm² (psi);
    144                       Ps is pressure source, N/mm² (psi);
                                    Pi is shut-in pressure, N/mm² (psi);
    Pi at subsea level =  Ps             Pi at subsea level =  Ps     H₁ is water depth at subsea well, m (ft);
                                    H₂ is water depth at platform, m (ft);
    Pi at top of the riser =             Pi at top of the riser =     GS is Specific gravity of fluid;
      Pi = Ps - ( ɣ x H₁(GS) )           Pi = Ps - ( ɣ x H₁(GS) )   ɣ is seawater density,  Kg /mᶟ (lb/ftᶟ)
      144           144              
                                               
    Pt at Top of Riser               Pt at Top of Riser                
        Ps - ( ɣ x H₁(GS) )             Ps - ( ɣ x H₁(GS) )              
      Pt = 144           Pt = 144              
      0.8           0.8              
                                               
      Pt at Top of Riser = Pt at Bottom of Riser     Pt at Bottom of Riser                  
                            Ps - ( ɣ x H₁(GS) )              
Pt at Top of Riser = Pt at Flowline at Subsea Wellhead
Pt = 144 + ( ɣ x H₂ )
                          0.8 144      
Note: For the single-pipe case, from the top of riser at the waterline to wellhead, external hydrostatic pressure will cancel out internal hydrostatic load, making resulting pressure during hydrostatic test constant.
        Pt at Flowline at Subsea Wellhead                
        Pt = Ps                  
        0.8                  
      Calculate pipe wall thickness for riser and flowline with:                        
      D =  2     Solving D/t from Equation (4)                        
      t 1   -  1       D =  1  + 0.90 (S+U)     Solving D/t from Equation (5)    
          Pb       t Pb      
e 0.45(S+U)
      D =  2     For Flowline                        
      t 1   -  1       D =  1  + 0.810 (S+U)     For Flowline        
          Pt       t Pt          
e 0.405(S+U)
      D =  2     For Raiser                        
      t 1   -  1       D =  1  + 0.675 (S+U)     For Raiser          
          Pt       t Pt            
e 0.3375(S+U)
    Note: The two equations, Equation (4) and Equation (5), for the burst pressure are equivalent for D/t > 15. For low D/t pipe (D/t < 15), Equation (4) is recommended.