This thesis investigated the possibility of alternative ways to loading and offloading flexlay vessels and provides a concept design. This was investigated because currently, the vessels designed by IHC, have to sail back to port to restock on pipes. During this time the expensive pipelayer is not laying pipes and thus not making money for its operator. Some operators have already moved to designs which use cranes to load pipes at sea. However, lifting heavy objects results in limited operability and wide vessels. The problem owner (IHC) wants to avoid this. The goal was to find a way to transship pipes offshore, which improves on the current performance of the reference vessel without resorting to cranes to transfer the pipes.

The research was mainly focused on the modelling of the logistical process of getting pipes from the port to the location. The results of this were used for the conceptual design of a flexlay vessel capable of being supplied at sea. The reference vessel built by the problem owner turned out to be a suitable candidate for adaptation for this purpose. To reach this conclusion, a logistical model was made, which was used as a basis for parametric studies. As input to that studies, a seastate timeseries is generated together with different transshipment methods. Two sets of simulations were used to evaluate the effect of the transshipment method in relation to a range of operational limits, and the effect of the distance on the operation.

The resulting pipelayer is fitted with two mailbox type docks in the side, this elongates the vessel and decreases its initial draught. These docks are closed and drained afterwards in order to secure the barge. The main conclusion is that it is possible to redesign the original pipelayer to accommodate the transshipment of pipes offshore.