Combined Seakeeping and Maneuvering Analysis of a Two-Ship Lightering Operations

Hydrodynamic interaction effects between two ships going ahead in regular deepwater waves were numerically studied during typical maneuvers for ship-to-ship (STS) operations, such as lightering, replenishment, etc. Such maneuvers are usually classified as potentially hazardous situations, due to the possibility of collision between the two vessels when they are operating in close proximity. Since the collision hazard is usually even greater in bad weather conditions, knowledge of the maneuvering capabilities of two ships in a seaway must be available in order to ensure safe and efficient STS operation. In this study, a combined seakeeping and maneuvering analysis of two ships involved in typical lightering operation was performed using a unified seakeeping and maneuvering theory developed by Skejic and Faltinsen [1, 2]. The unified theory integrates seakeeping and maneuvering analysis by using a two time scale assumption and modular concept. This approach allows the maneuvering behavior of the two ships involved in lightering operation in waves to be successfully described. The seakeeping analysis for both vessels uses Salvesen-Tuck-Faltinsen [3] (STF) strip theory for deep water by assuming that there are no hydrodynamic interaction in waves between the two ships. The regular wave field effects upon the involved vessels are described by the mean second-order wave loads. They can be estimated by using one of the available near/far field theories (Salvesen [4] and Faltinsen et al. [5]) that take the complete wave length range of interest for a considered STS maneuver into account. When the incident wave length is short relative to the ship length, the asymptotic theory by Faltinsen et al. [5] is used. The predicted mean second-order wave loads according to these theories are shown in the case of turning maneuver of a ‘MARINER’ type of a ship in specific wave conditions. The maneuvering module of the unified theory model is based on generalized slender-body theory, while calm-water interaction forces and moments between the two ships are estimated using Newman and Tuck [6] theory. Automatic steering- and speed-control algorithms for both ships (Skejic et al. [7]) are employed to achieve high-precision and collision-free lightering maneuvers in waves. This is illustrated by a numerical simulation involving ‘Aframax’ and ‘KVLCC’ (type 2 – Moeri tanker [16]) types of ships. Finally, from the perspective of marine safety and reliability, the future requirements and recommendations for typical lightering operations in a seaway are discussed.