Integrated layout and routing optimization for offshore oil and gas decommissioning yards with hazardous-material flow-path segregation

End-of-life offshore oil and gas structures are dismantled and processed at specialized onshore yards, where facility locations, access points, transport aisles, hazardous-flow segregation, and land use are closely linked. Most existing layout models address safety mainly through separation distances between facilities; route-level segregation and the joint treatment of layout and routing receive less attention. This paper formulates the yard layout as six coupled mixed-integer modules: block placement, boundary entrance/exit placement, facility I/O assignment, rectilinear flow paths and aisle envelopes, hazardous/non-hazardous path segregation, and functional clustering with safety distances. Four objectives are handled lexicographically: inter-zone flow distance and Type 1 cluster compactness, frequency-weighted active-segment measure, intra-zone facility-to-I/O distance, and yard-dimension compactness. After spatial discretization, the model is solved with Google OR-Tools CP-SAT, so optimality claims apply to the chosen discrete feasible domain. For the Yard X case, comprising 36 facilities and three yard-level I/O interfaces, the distance-priority sequence cuts the complete 39-flow Stage-Z1 inter-zone distance by 62.6% and the derived yard area by 41.4%; the HAZMAT distance falls by 77.6%. Placing area first produces a smaller footprint but longer HAZMAT routes. Removing the flow hierarchy leaves nonzero solver optimality gaps within the time limit.