Service discovery and service composition are two crucial issues in the emerging area of Service-oriented Computing (SoC). The introduction of semantic annotations in Web service descriptions can support the discovery process of services and guide their composition into workflows. In this paper, we propose a Structured Relationship Algebra that extends existing frameworks for semantic service descriptions by a set of algebraic operators and allows to define in a uniform way formal semantic annotations regardless the framework adopted to describe service semantics. The proposed algebra is comprised by a set of three algebraic operators and allows to define new objet properties at the ontology level called derived properties. Derived properties are used to describe the data transformation processed by a service by associating constraints on both properties and instances of the concepts and their related ontological relationships.