An anchor line pretensioning method of an anchor line configuration that includes anchor lines (2, 3, 4) which are not diametrically opposed with respect to the structure that is moored to the seabed via the anchor lines. Two anchor lines (2, 3) on a first side of the installation vessel are mutually connected via a connecting member (13), such as for instance a triangular plate (13). The triangular plate (13) is connected to the vessel (10) in a fixed connection point or may be connected to a tensioning device on the vessel. During pretensioning, the pull force exerted on the tri-plate (13) is oriented in the same direction as the pull force exerted on the tensioning line (4) that is located on the opposite side of the vessel.