Subsea pipelines are critical for oil and gas production and transport, more than 100.000kmof subsea pipelines have been laid. These pipelines are laid by specially designed ships which can lay these pipelines at great depth and a high speed. However, designing the supply chain to deliver joints (pipe pieces) to these pipe lay vessels can be very complex; there aremany possible supply vessel combinations andweather influences that can impact the supply chain. This makes trying to find a good solution a very difficult and time-consuming process currently with a lot of manual iterations. This is called the Allseas problem. To solve this supply chain problema mathematical model is developed, thismodel can solve the supply chain problem while taking into account the many constraints. The research question that is discussed in this thesis is: Can the optimal usage of supply vessels, required to solve the pipelay vessel supply problem, be determined using a scenario based mathematical optimisation model? This model can eliminate the need for the many manual iterations and deliver an optimal solution that the Logistics and chartering department can use.
To supply joints to the pipelay vessels several different types of supply vessels can be used. These supply vessels have different characteristics such as; hold capacity, sailing speed, capital- and operational expenditure, and workability (ability to work in bad weather). The supply vessels must supply joints to the pipelay vessel which can have one or two pipe transfer cranes. The number of pipe transfer cranes that can be used significantly influences the speed at which the supply vessels can be unloaded. A final important influence on the supply chain problemis the weather; specifically the wave height and wind speed: they determine if a supply vessel can unload it joints, or if the pipelay vessel can lay pipe. The goal of the problem is to design a fleet of supply vessels that can deliver the cheapest possible set of routes that will supply all the required joints to the pipelay vessel on time.
A scenario based mathematical optimisation model is developed to find the solution to the Allseas problem. Some important parameters for this model that are thoroughly researched are: the desired stock level of the pipelay vessel, i.e. how full should the holds be? The demand window, i.e. how close to the demanded number of joints should the model deliver. The final parameter of interest is the node duration, all joints must be delivered during a certain time window the parameter node duration changes the size of the time window. The influence of these parameters is also investigated when weather is simulated.
The results of the developed program show that it is possible to make a mathematical model that determines the optimal pipe supply vessel usage. Some other interesting parameters settings have also been found by the model. Allseas always tries to keep the pipelay vessels stocked as full as possible, however, the mathematical model has shown that it ismore advantageous to try to keep the vessel at 80%capacity sine the supply vessels can unload faster that way ensuring shorter routes. Furthermore, the mathematical model has shown that it can deliver cheaper solutions than are currently designed. This has been proven by comparing the results of the mathematical model with a project that Allseas has completed. The mathematical model presented a solution that was ¼ 10% cheaper than the solution Allseas implemented.