On the rank of the fibres of rational elliptic surfaces

We consider an elliptic surface $\pi :\mathcal{E}\to {\mathbb{P}}^1$ defined over a number field $k$ and study the problem of comparing the rank of the special fibres over $k$ with that of the generic fibre over $k({\mathbb{P}}^1)$. We prove, for a large class of rational elliptic surfaces, the existence of infinitely many fibres with rank at least equal to the generic rank plus two.