motion control during offshore lift operations

Abstract

Offshore lifting is present in many offshore operations. Offshore lifting operations is very sensitive to environmental loads which reduces the weather window for the entire operation. The weather criteria for offshore heavy lifting according to DNV recommended practice is set to 2.5 m significant wave height. To increase the weather window of the offshore lifting operations, it is necessary to control the motion of the lifted object and understand the basis of behavior. The most common way of controlling the motion of the lifted object is by use of tugger lines. In this project, a SIMO model of a general heavy lifting operation was provided by Marintek and modified according to this project. Several simulations are carried out for three different phases representing different initial positions of the lifted object or module. The use of tugger lines proved to reduce the module displacement relative to the ship drastically according to the simulations. And a deeper understanding of how tugger lines affect the behavior of the system was retrieved. The relevant theory for the SIMO software is provided in the report. The simulations are not verified, but seems to give fundamentally correct physical behavior with the exception of the relative module displacement at low peak periods which should be studied further.

Further work can roughly be summarized in the following points: Verify the time domain analysis carried out in this project and improve the quality of the simulations. Simulate the offshore heavy lifting operation with a continuous crane module position from where it is lifted off deck to where it is hoisted down in the water. Study the system behavior of reaching the maximum velocity of the tugger winch. Study different damping models of the tugger lines.