MARIN and Oce‰nica recently joined forces to study the tandem offloading operation of the first FPSO in the Gulf of Mexico for the Petrobras Cascade/Chinook development. MARIN performed the model tests in its deepwater Offshore Basin and Oce‰nica, the numerical analysis in Brazil. Cascade and Chinook are located in the Walker Ridge block, around 300 km (180 miles), south off the Louisiana coast. The FPSO will have a 80,000bpd processing capacity and a 16 MMcfpd natural gas export capacity. Featuring a disconnectable turret buoy, the FPSO can move offsite during severe weather events, such as hurricanes. It is moored by a polyester mooring system, linked to torpedo piles and vertical loaded anchors. Crude oil will be offloaded via a shuttle tanker in tandem arrangement and a tugboat will be available at the stern of the shuttle tanker during the offloads. Tandem offloading requires a thorough understanding of the relative motions of the shuttle tanker behind the FPSO in all weather conditions. Obviously, loads in the hawser should remain within safe limits and the shuttle tanker should remain at a safe distance from the FPSO. The combination of model tests and numerical simulations, as well as the cooperation between the two organisations proved very successful in this project.