Submerged Towing Performed by a Monohull Vessel

During a submerged towing operation of a subsea module, the vessel motions combined with the hydrodynamic properties of the considered module will induce drag forces and dynamic loads on the towing configuration. These loads are important when evaluating different submerged towing concepts and will have implications for structural design with respect to the relevant limit states. However, since it is difficult to quantify the hydrodynamic properties of the towed module, model tests are useful for verifying coefficients which are further used in dynamic analyses of the towing configuration. The objective of this paper is to present an analysis methodology for determination of dynamic forces and hydrodynamic properties of a marine structure during a submerged towing operation. Emphasis is made on a simple analytical model of the dynamic forces based on DNV-RP-H103, and a detailed multibody time integration by means of the MARINTEK computer program SIMO (Simulation of complex marine operations). In order to model the submerged towing operation in SIMO, hydrodynamic properties of the considered towed structure are determined based on analytical formulas presented in DNV-RP-H103. The results from the SIMO model are in addition compared with measured response obtained from an experimental investigation. In general it is found that there is good agreement between experimental and calculated values when the hydrodynamic properties of the components have been identified.