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Abstract Deep-water steel lazy wave risers and steel catenary risers are subject to simultaneous loading from different sources. Environmental loading comes from waves and vortex induced vibrations (VIV), and in addition internal slug flow may also excite dynamic stresses. Design codes account for this either by applying conservative damage summation formulas, such as the method from DNV-RP-C203, or by recommending adding stresses in time domain. Actual design practice is more inconsistent, with different levels of scientific foundations, like, e.g., linear summation of damage contributions. A study has been performed to assess methods for combining fatigue damage from two independent simultaneous processes. This has been studied for time series simulated directly from spectra, and on simulated response time series of a deep-water steel lazy wave riser. The case study considers two loading combinations: Two years of measured environment (waves, current profile), divided into a series of short-term conditions of waves and VIV. This gives a consistent description of the long-term fatigue loading from waves and current, taking the correlation into account.Representative slugging conditions for the lifetime of a field, combined with wave scatter. Global riser analyses have been run in time domain for waves and slugging, and in frequency domain for VIV. For best-estimate results, combined analyses have been run for waves and slugging, and time series of stresses have been combined for waves and VIV. Linear combination of damages has been applied on the separate results to compare with the best-estimate approach. The results show, as expected, that the industry approach of linear summation of fatigue damage from two processes is unconservative. The amplification of fatigue damage due to an additional process is considerable, even for comparably low stress ranges. Although the study is focused on steel lazy-wave risers, the problem itself is generic and applies to other structures exposed to combined fatigue loading, such as flexible risers, dynamic umbilicals and cables, subsea spools, etc.