This paper addresses quality of service aspects of web service orchestrations created using WS-BPEL from the standpoint of a web service integrator. A mathematical model based on operations research techniques and formal semantics of WS-BPEL is proposed to estimate and forecast the influence of the execution of orchestrated processes on utilization and throughput of individual involved nodes and of the whole system. This model is applied to the optimization of service levels agreement process between the involved parties.