Ship collision frequency during pipeline decommissioning process on surabaya west access channel (SWAC)

Surabaya West Access Channel (SWAC), which located in Madura Strait, is one of the busiest shipping channels in Indonesia with the depth about - 9m LWS and width 100 m. Due to the shallow and narrow waterways, the large vessels with deeper draft cannot pass the channel. Therefore, Indonesia Port Corporation plans to revitalize the channel to accommodate the larger vessels for passing the channel. Thus, the cost of logistics is reduced and the number of cargo increases. The shipping channel will be deepened to -13 m LWS and widened to 150 m. If dredging is done, the pipeline will be exposed to the seabed. Hence, the revitalization project cannot be conducted until subsea pipelines removal in the shipping channel is completed. Moreover, according to Minister of Energy and Mineral Resources Indonesia, if the off-shore installation included pipeline is not used anymore then they must be decommissioned. Pipeline decommissioning may cause unavoidable hazards such as ships collision between pipelay vessel and passing vessel. Hence, this study has two main objectives of selecting the best method of pipeline removal and determining the frequency of ships collision. The two alternative methods of pipeline removal offered are reverse S-lay and cut and lift. The selection was carried out using the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) then verified by performing Analytic Hierarchy Process (AHP) method, while the collision frequency was calculated using CRASH Model and compared by using SAMSON Model. TOPSIS selected Reverse S-Lay as a better method for pipeline decommissioning. The result frequency assessment showed that the number is less than unity (1) for both collision scenarios in those two models, therefore, it can be concluded that the risks due to ship collisions are acceptable.