Dry transporting large heavy floating structures has been an important development in the innovative history of Heavy Marine Transport. This is done by carrying these type of cargoes on the deck of Heavy Transport Vessels. Loading and discharge is preformed by submerging the transport vessel, and positioning the cargo above the deck using tugger winches. Traditionally, these operations take place in sheltered locations, like harbours, where virtually no waves occur. From the beginning Heavy Marine Transport has served the oil and gas industry in the exploration, development and production phase of offshore oil and gas fields. In search of optimizing profitability of remote offshore fields, optimizing fabrication and installation of facilities is essential. Delivering the facilities directly at the remote fields is recognized as a potential cost saving. Also, Inspection, Maintenance and Repair by offshore dry-docking is a potential cost optimisation. Both imply that loading and discharge operations need to be carried out in exposed areas, where wave conditions do occur. It is evident that, to ensure acceptable workability and safe operation, it is important to accurately prediction the dynamic motions and to assess and design the mooring and handling equipment. Experience and model testing has shown that prediction of vertical relative motions using industry standard software was inaccurate; generally, the relative vertical motions were significantly over-predicted. Also, it has been shown that horizontal relative motions are too large to safely place the cargo on cribbing support, while using the standard handling equipment. These issues have resulted in the development of an accurate method to predict relative vertical motions and the development of cargo handling equipment for accurate positioning of the cargo. @en

In Heavy Marine Transport it is common practice to drytransport large and heavy floating offshore structures. In general, loading and discharge of these floating cargoes on- and from heavy transport vessels is done at sheltered locations like harbors where sea-state and swell conditions are insignificant. Often these locations are at large distance from operating fields of the offshore structures, which means that the structures need to be towed from- or to these fields. To save time and costs, it is beneficial to perform the loading and discharge operations in the field. This necessitates a reconsideration of the maximum allowable wave condition such as to perform the loading- and discharge operations within specified time frame whilst ensuring safety of crew, cargo and heavy transport vessel. Since precise positioning of the cargo on the HTV cribbing beams is of importance to support the cargo on its structural strong points, the allowed relative horizontal motion during loading or discharge operations is limited to a fraction of the width of these cribbing beams. When increasing the maximum allowable wave conditions, relative horizontal motions between heavy transport vessel and cargo easily exceed these limits if only the standard handling equipment is used. Also, the loads in the handling equipment may exceed safe limits. This paper presents two methods including complementary equipment to reduce- and limit the relative horizontal motions. The first method is based on increasing the stiffness of the con-nection between cargo and heavy transport vessel. This means that there is a transition from a soft (standard handling) system with a low natural frequency to a stiff (clamping) system with high natural frequency. During this transition the system natural frequency will coincide with the wave frequent excitation force. Resonant behavior during the transition is avoided as the complementary equipment also employs a damping force. The second method is based on a closed-loop controller applied to the desired relative horizontal position. The resulting desired load to control the relative horizontal motion is then allocated to several line tension actuators. Contradictory to well-known Dynamic Positioning systems which control low frequent motions, motion control during offshore loading and discharge is performed on wave frequent behavior. This implies that the line tension actuators also need to deliver loads within a wave frequent time-frame. In fact, the peak tension needs to be obtained within a quarter of a wave period. System design and simulation results are presented. Depending on the cargo type, different solutions and operational aspects are discussed. Simulations are done for a typical cargo where both methods to reduce the relative horizontal motions are utilized.@en

In Heavy Marine Transport, it is common practice to drytransport large and heavy floating offshore structures. In general, loading and discharge of these floating cargoes on- and from heavy transport vessels is done at sheltered locations, like harbors, where sea-state and swell conditions are insignificant. Often these locations are at large distance from operating fields of the offshore structures, which means that the structures need to be towed from- or to these fields. To save time and costs, it is beneficial to perform the loading and discharge operations in the field. This necessitates a reconsideration of the maximum allowable wave conditions such as to perform the loading- and discharge operations within specified time frame whilst ensuring safety of crew, cargo and heavy transport vessel. Therefore accurate prediction tools to determine the relative motions between heavy transport vessel and cargo are required. In the past, studies have shown that standard prediction tools over-estimate relative vertical motions compared to model tests and practical experience. This paper discusses the prediction of relative vertical motion, which is dominated by the phenomenon squeeze flow. From model tests and CFD calculations non-linear hydrodynamic loads related to squeeze flow are recognized. Standard linearized solutions do not cover the non-linear loads and therefore result in a lack of accuracy in predicting the relative vertical motions. Linearized solutions assume small motion amplitudes with respect to characteristic dimensions of the flow problem; in the case of squeeze flow this assumption is not valid as the motion amplitude may be in the same order as the gap between cargo bottom and the deck of the heavy transport vessel. With linearized potential solvers it is found that the added mass is strongly dependent on the gap height, which verifies the analytical work of Molin et al. [1]. Following the work of Molin, the change in added mass due to changing gap height gives a large contribution to the non-linear hydrodynamic load. Additionally, a second important contribution is related to the relative vertical velocity, recognized as a drag component. As such, a non-linear formulation is found which can be used in a time-domain approach. This formulation requires gap-height dependent added mass as found using the linearized potential solver. As potential solvers are known to have difficulty dealing with the small gap, different methods have been investigated. Results of model tests and CFD calculations are shown, which are used to tune the non-linear formulation. Tuning is done by adapting the drag component. Furthermore, results of a multi-body problem based on standard linear hydrodynamics and the non-linear formulation are compared.@en