Anchored Vessel Ship-to-Ship Operations: Environmental Limits Considering Mooring Equipments and Ship Maneuver

Abstract

Ship-to-ship (STS) operations have been widely applied worldwide for the last years from military operations to oil/LNG cargo transfer, in order to improve operational efficiency. In the last years, the lack of available berths/terminals in Brazil has increased transfers in anchored Ship-to-Ship operations, avoiding pier occupation. This operation is already performed in Brazil and abroad for sheltered locations. However, most of the sheltered locations available have also ecological restrictions, motivating the accomplishment of this kind of operation in non-sheltered regions, i.e. close to the coast and under wave action. These last cases of STS have been investigated in the last years, for instance, FSRU and FLNG operations. This study is focused in the evaluation of the maximum operational condition for oil transfer operation, using conventional VLCC and Suezmax tankers for a given location, close to the Brazilian coast. The first stage of the study computes the minimum required water depth and anchor capacity to provide a safe operation in terms of vertical motions, since the wave height reduces with the UKC. The methodology to define the required UKC followed PIANC Report n° 121 – 2014 using the vessel motions computed in frequency domain using the Response Amplitude Operator (RAOs) in finite water depth. The anchor hold capacity is evaluated for 720 environmental conditions (12h interval of simultaneous wind, current, tide and wave condition), following OCIMF recommendations for a single vessel and side-by-side configuration. For this study it was considered current, wind and second-order wave forces, in order to compute the equilibrium heading, similar to a Turret moored platform. The second stage of the study was the mooring integrity analysis following OCIMF STS recommendations, selecting thus the required pneumatic fenders and cables properties/arrangement/pre-traction for the mooring design. The static loads due to current and wind were computed using CFD coefficients to evaluate the shadow/interaction effect between vessels, in order to achieve more reliable results compared to the conventional single vessel coefficients. The static second order forces are computed in frequency domain using potential theory considering the side-by-side configuration with some additional numerical treatments/discussion to deal with the resonance of the gap between the vessels. The mooring arrangement (cables+fenders) were modeled as an equivalent linear stiffness matrix applied to the vessel motions computation, also providing a Response Amplitude Operator for the cables/fenders elongation, providing an expedite methodology to compute the cables tension in frequency domain. The incident wave heading applied in the computation of the vessel motions/cables elongation is defined for each condition, based on the STS equilibrium heading defined in the previous stage. The operation up-time is computed based on the cables MBL and compared to a simplified criteria defined by means of the the wave height/period/heading. The third stage of the study was the maneuvering analysis carried out in TPN-USP Simulation Center. Since the operation is executed in non-sheltered regions, the wave action on the tugboats must be carefully evaluated, so three integrated simulators were used. The approaching vessel was in the full-mission simulator, commanded by the maritime pilot. A tug-master commanded a part-task simulator, with the main ASD tugboat in each maneuver. The moored vessel was in the third simulator, emulating the point of view of a second maritime pilot in charge of the maneuver. The other tugboats were commanded as vector forces, and an experienced tug-master defined the actuation rules depending on the wave height. The three stages of the operation were tested: approaching maneuver, heading control of the moored vessels and disconnection. Important parameters of the operation were monitored, including motions, velocities, cables and towlines tension, tugboat effective forces, etc. Different environmental conditions were tested. As a final conclusion of the study, the limiting wave height and period could be defined for both mooring integrity and maneuvering. Therefore, these results could be used to evaluate the availability to perform STS operation in each region, based on the metocean conditions.