SUBROUTINE Utube
    
    !! This subroutine calculates flow rate when pump is off, pump is disconnected from drill pipe
    !! and both annulus and drill pipe are exposed to atmosphere pressure and thus a U-tube situation is occurs
    
    !!      Record of revisions
    !!          Date            Programmer          Discription of change
    !!         ------          ------------        -----------------------
    !!        1396/07/29        Sheikh              Original code
    !!        1396/08/09        Sheikh              Two-side U-tube
    !! 
    
    USE FricPressDropVarsModule
    USE MudSystemVARIABLES
    use UTUBEVARSModule
    USE Fluid_Flow_Startup_Vars
    
    IMPLICIT NONE
    
    
    
    INTEGER :: i ,j, ibit , ij , ijk
    REAL :: AreaBeforeBit       !!! Area of element before bit in U-Tube condition [in^2]
    
    UTUBEVARS%QUTubeInput = 1.0
    UTUBEVARS%QUTubeOutput = 1.0
    FricPressDropVars%TotFricPressLoss = 0
    BitPressLoss = 0
    UTUBEVARS%PressureDp = 0
    UTUBEVARS%PressureAnn = 0
!!!!!!!!!!!!!!!!!!!!!! Bit
!!!!!!!!!!!! Calculating Pressure at the bottom of drill string from mud columns in drill pipes and annulus space

    UTUBEVARS%PressureDp = SUM(FlowEl(FricPressDropVars%StringFirstEl : FricPressDropVars%StringLastEl)%StaticPressDiff)
    
    UTUBEVARS%PressureAnn = SUM(FlowEl(FricPressDropVars%AnnulusFirstEl : FricPressDropVars%AnnulusLastEl)%StaticPressDiff)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!  U tube:: flow from string to annulus
    IF ((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) > UTubePressTolerance) THEN
        DO ijk = 1 , 10
            FlowEl(FricPressDropVars%StringFirstEl : FricPressDropVars%AnnulusLastEl)%Flowrate = UTUBEVARS%QUTubeInput
            !WRITE (*,*) 'QUTubeInput', FlowEl(AnnulusLastEl)%Flowrate
            DO ij = FricPressDropVars%StringFirstEl , FricPressDropVars%AnnulusLastEl                       !!!!! Updating values of flowrates
                CALL FricPressDrop(ij)
                CALL PartialDerivativeFricToFlowRate(ij)
                !WRITE (*,*) 'FricPressDrop, PartialDerivative', FlowEl(ij)%FricPressLoss, FlowEl(ij)%FricToQPartialDiff, FlowEl(ij)%Length
            END DO
        
        !!!!!!!!!!!!!! Bit pressure drop calculation
            IF (BitTrue) THEN
                i = FricPressDropVars%NoHorizontalEl + FricPressDropVars%NoStringEl
                AreaBeforeBit = FlowEl(i)%Area * Convfttoinch**2
                BitPressLoss = FlowEl(i)%density * Convft3toUSgal * (FlowEl(i)%vel**2 * ((AreaBeforeBit/BitTotNozzArea)**2 - 1.)) / 2. / Convlbftolbm / Convfttoinch**2
            END IF
            
        !!!!!!!!!!!!!!!!!!!!!!!!!!!
            i = FricPressDropVars%NoHorizontalEl + 1
            j = FricPressDropVars%NoHorizontalEl + FricPressDropVars%NoStringEl + FricPressDropVars%NoAnnulusEl
            
            FricPressDropVars%TotFricPressLoss = SUM(FlowEl(i : j)%FricPressLoss) + BitPressLoss
            IF (ABS((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) - FricPressDropVars%TotFricPressLoss) <= UTubePressTolerance .OR. UTUBEVARS%QUTubeInput < 1.0) EXIT         ! tolerance set to 1.0 psi
            IF ((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) > FricPressDropVars%TotFricPressLoss) THEN 
                UTUBEVARS%QUTubeInput = UTUBEVARS%QUTubeInput + ((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) - FricPressDropVars%TotFricPressLoss) / SUM(FlowEl(i : j)%FricToQPartialDiff)
            ELSE IF ((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) < FricPressDropVars%TotFricPressLoss) THEN
                UTUBEVARS%QUTubeInput = UTUBEVARS%QUTubeInput + ((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) - FricPressDropVars%TotFricPressLoss) / SUM(FlowEl(i : j)%FricToQPartialDiff)
            END IF
            !WRITE (*,*) 'QUTubeInput, TotFricPressLoss', QUTubeInput, TotFricPressLoss
            !WRITE (*,*) '1) PressureDp, PressureAnn', PressureDp, PressureAnn, TotFricPressLoss, QUTubeInput
            
        END DO
        UTUBEVARS%QUTubeOutput = 0.0
!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!  U tube:: flow from annulus to string
    ELSE IF ((UTUBEVARS%PressureAnn - UTUBEVARS%PressureDp) > UTubePressTolerance) THEN
        DO ijk = 1 , 10
            DO ij = FricPressDropVars%NoHorizontalEl + 1 , FricPressDropVars%NoHorizontalEl + FricPressDropVars%NoStringEl + FricPressDropVars%NoAnnulusEl              !!!!! Updating values of flowrates
                FlowEl(ij)%Flowrate = UTUBEVARS%QUTubeOutput
                CALL FricPressDrop(ij)
                CALL PartialDerivativeFricToFlowRate(ij)
            END DO

        !!!!!!!!!!!!!! Bit pressure drop calculation
            IF (BitTrue) THEN
                        i = FricPressDropVars%NoHorizontalEl + FricPressDropVars%NoStringEl + 1
                        AreaBeforeBit = FlowEl(i)%Area * Convfttoinch**2
                        BitPressLoss = FlowEl(i)%density * Convft3toUSgal * (FlowEl(i)%vel**2 * ((AreaBeforeBit/BitTotNozzArea)**2 - 1.)) / 2. / Convlbftolbm / Convfttoinch**2
            END IF
        !!!!!!!!!!!!!!!!!!!!!!!!!!!
            i = FricPressDropVars%NoHorizontalEl + 1
            j = FricPressDropVars%NoHorizontalEl + FricPressDropVars%NoStringEl + FricPressDropVars%NoAnnulusEl
            FricPressDropVars%TotFricPressLoss = SUM(FlowEl(i : j)%FricPressLoss) + BitPressLoss
            
            IF (ABS((UTUBEVARS%PressureDp - UTUBEVARS%PressureAnn) - FricPressDropVars%TotFricPressLoss) <= UTubePressTolerance) EXIT         ! tolerance set to 1.0 psi
            IF ((UTUBEVARS%PressureAnn - UTUBEVARS%PressureDp) > FricPressDropVars%TotFricPressLoss) THEN 
                UTUBEVARS%QUTubeOutput = UTUBEVARS%QUTubeOutput - (((UTUBEVARS%PressureAnn - UTUBEVARS%PressureDp) - FricPressDropVars%TotFricPressLoss) / SUM(FlowEl(i : j)%FricToQPartialDiff))
            ELSE IF ((UTUBEVARS%PressureAnn - UTUBEVARS%PressureDp) < FricPressDropVars%TotFricPressLoss) THEN
                UTUBEVARS%QUTubeOutput = UTUBEVARS%QUTubeOutput + (((UTUBEVARS%PressureAnn - UTUBEVARS%PressureDp) - FricPressDropVars%TotFricPressLoss) / SUM(FlowEl(i : j)%FricToQPartialDiff))
            END IF
            !WRITE (*,*) 'QUTubeOutput, TotFricPressLoss', QUTubeOutput, TotFricPressLoss
            
            WRITE (*,*) '2) PressureDp, PressureAnn', UTUBEVARS%PressureDp, UTUBEVARS%PressureAnn, FricPressDropVars%TotFricPressLoss
        END DO
        UTUBEVARS%QUTubeInput = 0.0
!!!!!!!!!!!!  No U-Tube
    ELSE
        UTUBEVARS%QUTubeInput = 0.0
        UTUBEVARS%QUTubeOutput = 0.0
    END IF
    
END SUBROUTINE