Simulating the voyage of the TPI Mega Caravan with the focus on vibrations due to Hogging, Sagging and Slamming

Abstract

In the summer of 2016 heavy lift transport company TPI will transport a topside from Busan, South Korea to the Gina Krog field in Norway. For this voyage TPI selected their second largest vessel, the TPI Mega Caravan as heavy lift transporter. Since 2010 the Dutch based engineering company Hydrographic and Marine Consultants (HMC) were hired by TPI to simulate the voyages of the TPI fleet. The objective of this thesis is to: Simulate the voyage of the Mega Caravan from South Korea to Norway with a focus on the structural responses of the vessel during the voyage. The voyage is simulated in Safetrans, a simulation software program which can be used to derive both the hydrographic data of a voyage and the motion responses of the vessel. In this software the vessel is modeled as a rigid body. To determine the structural responses during the voyage, a finite element method model of the vessel is created in FEMAP. In this model the bending moments of the vessel due to vibrations of the hull are analyzed. The analyzed structural responses are caused by wave bending moments and slamming. Wave bending moments When a ship is located on top of a wave, or between two waves, with a wavelength corresponding to the length of the vessel this can result in large tensions. As ships are flexible, they will deflect to the shape of the wave. When the crest of the wave is amidships, the midsection of the vessel is pushed upward. This causes large tension in the deck and compression in the keel of the vessel. This phenomenon is called Hogging. When the bow and stern of the vessel are pushed upward the vessel deflects the other way around. There will be a large tension in the keel and compression in the deck. This phenomenon is called Sagging. Slamming Wave slamming is an impulsive load caused by sudden impact of waves onto the vessel. This phenomenon occurs when the bow of the vessel is emerged and “crashes down” with an impact against the next wave. The vessel can start to vibrate due to those slam impacts, which is called Whipping. The input data to simulate hogging, sagging and slamming in the FEMAP model is gained from Safetrans. The result of the FEMAP analyzes showed concerning bending moments due to hogging and sagging. Safetrans is able to calculate the maximum slamming pressure during the voyage. However, the program gives no information about the time duration of the impact and the exposed area. This is why Safetrans is not the optimal software to simulate slamming. With some assumptions the slamming pressure is applied to the FEMAP model. The analyze showed that the influence of slamming on the bending moment is small compared to the other loads. However, it is recommended to research the combination of slamming and wave bending moments.