ABSTRACT Field and laboratory measurements of forces for pipelines in wave plus current flow conditions demonstrate that peak forces may not be proportional to the total ambient velocity squared. Horizontal and vertical peak forces occurring in the half cycle when the wave and current velocities oppose (smaller total velocity) may be nearly as large as;. or under some circumstances even larger than peak forces occurring when the wave and current velocities combine (larger total velocity). This force feature has important implications for pipeline response; it generally leads to a much smaller net pipe displacement than that predicted with Morison's equation, and may even cause a pipeline to move into the current. The phenomenon is explained in terms of the wake concept, and ranges of parameters which could potentially cause pipe movement into the current are presented. INTRODUCTION Recent test data of hydrodynamic forces for pipelines have demonstrated force features which are not predicted by the conventional Morison's equation based on the ambient. flow. These data have brought about the development of the wake force model (Lambrakos et al., 1987; Verley et al., 1987), and the Fourier expansion method (Fyfe et al,. 1987; Jacobsen et al., 1988) for predicting hydrodynamic forces for pipelines. Pipeline response (movement) calculations show that unexpected features observed in the forces significantly affect the response, and can therefore impact pipeline design for on-bottom stability (Wolfram et al., 1987). The force data discussed in this paper were collected from one field test and two laboratory tests. All data are for full or near full scale, and for wave plus current flow conditions. These tests are Exxon's Pipeline Field Measurement Program (PFMP), and a Joint Industry Program (JIP) and the Large Scale Test Program (LST) conducted by Marintek (Norway). The PFMP was conducted in the Strait of Juan de Fuca offshore Washington state in 60 ft (18.3 m) of water with a 2-ft (0.6 m) diameter pipe. The test program details and selected data have been presented by Lambrakos et al. (1987).. The JIP was a laboratory test program to measure hydrodynamic forces on a pipe section by oscillating it in a towing tank. Two pipe diameters were tested: 0.5 m and 1 m. The test program has been discussed by Fyfe et al. (1987). The LST was an extension of the JIP'and test details have been presented by Verley et al. (1987). A number of basic findings that are not accounted for by the conventional Morison's force model have been discussed by Lambrakos et al. (1987). The present paper will expand on one important finding from the measurements: that peak forces when wave and current velocities oppose may be larger than when they combine. The phenomenon will be interpreted in terms of the wake concept presented in Lambrakos et al. (1987) and Verley et al. (1987).