A dual firing line drilling installation has a drilling tower main structure with four frameworks that are each over a major portion of their height U-shaped in horizontal cross-section. A four-sided central (11) space is present, and along each side of the central space one framework is arranged with its opened vertical side facing the central space. First and second storage devices (25,27) are provided for vertically storing drilling tubulars. The first storage device is housed in a first framework, the second storage device in a second framework. A first hoisting device is supported by a third framework of the drilling tower main structure, to manipulate a string of drilling tubulars in a first firing line (21). A second hoisting device is supported by a fourth framework to manipulate a string of drilling tubulars in a second firing line (23). A first and second pipe racker (29,31) are housed within the drilling tower main structure and adapted to move drilling tubulars while supported in vertical orientation between a storage devices and a firing line.