TenneT TSO B.V. (Client), as the Transmission System Operator (TSO) in the Netherlands, is responsible for constructing, operating, and maintaining the offshore grid infrastructure that connects Offshore Wind Farms to the national transmission network. This infrastructure includes a combination of Offshore Substations (OSS) and the associated onshore connection points (land stations), which together ensure the safe and reliable transport of electricity generated at sea to the onshore high‑voltage grid. In the Netherlands Client operates 7 independent 700MW offshore Grid Connection Systems (GCS) by means of a standardised Offshore High Voltage Sub Station (OHVSS). Future developments include 7 independent 2 GW GCS’s, each with a standardized DC OHVSS. These DC systems are located further offshore and for now are not included in this service scope The OHVSS 's are remote operated, normally unmanned, with planned maintenance scheduled during relative short periods, i.e. several sequential days per OHVSS. Around these campaigns, Client must be able to reach a OHVSS when need arises. This means the teams of maintenance engineers including equipment and spares require a fast and reliable means of transport, with extensive vessel capabilities in relation to the year-round prevailing weather conditions. The TenneT TSO B.V. (TenneT) business unit Offshore Operations and Maintenance is the organizational unit managing the operations of the Dutch Offshore High Voltage Substations (OHVS) including associated export cables from sea to land, also called a Grid Connection System (GCS).  For this tender we are looking for: Lot 1: year round CTV : one party that can provide the year yound (365 days - annually) Crew Transfer Vessel services to the following grid connections/platforms: Lot 2: seasonal CTV     : one party that can provide seasonal (minimum  90 days coverage between may and october) Crew Transfer Vessel services to the following grid connections/platforms: -    Borssele Alpha -    Borssele Beta -    Hollandse Kust Zuid Alpha -    Hollandse Kust Zuid Beta -    Hollandse Kust Noord Alpha -    Hollandse Kust West Alpha -    Hollandse Kust West Beta One candidate may be awarded for one or two lots from (asap) Q4 2025 to 31 december 2028 with the option to extend the contract with 2 times one year.