Ballast for Lifting

Abstract

Amazing Grace will be needed for the removal of offshore platforms that are out of Pioneering Spirit 's reach. The base case design of Amazing Grace is an enlarged version of Pioneering Spirit (Figure 1). Amazing Grace uses mainly Quick drop Ballast tanks, releasing a lot of water at once, for lifting. Pioneering Spirit uses mainly a pneumatic-hydraulic system for fast lifting. The idea is to lift bigger platforms with a less complex system. The behaviour of the base case design of Amazing Grace when lifting the upper part of an offshore platform (topside) from its supporting structure (jacket) is studied in this work and the feasibility of the new Quick drop Ballast system is tested. To perform a feasibility study on lifting topsides with Amazing Grace , a set of design requirements is generated. Before using these design requirements, a one tank model is required to simulate the emptying of a tank. The dimensions of an existing Pioneering Spirit Quick drop Ballast tank are used. The model is validated by comparing its output to existing physical test data of emptying the same tank. To lift by means of heave only, a bigger volume is used for the one tank model. Once the Quick drop Ballast concept shows to be feasible, the next step is to introduce waves. Waves are modelled using the linear superposition method. An estimation of the time series duration is done by computing the standard deviation of the heave amplitude. For a converged standard deviation, the time series' minimum length needs to be 1200 seconds. The maximum heave amplitude, derived from another statistical analysis, is applied to the connection plateau (when Amazing Grace connects and starts lifting the topside without creating clearance). The results show that Amazing Grace 's mass, added mass, damping and stiffness (the coefficients of the equation of motion) need to be included to give a more accurate estimation of the dynamics for connecting Amazing Grace to the topside and the possible rebounce to the jacket. Although this is a conceptual study, the dynamics are included to get a better understanding of Amazing Grace 's response. To include the previously mentioned coefficients of the equation of motion, the Cummins equation is implemented. The excitation forces are the sum of the wave- and the Quick drop Ballast force. The Quick drop Ballast force is derived from the one tank model. A convergence study shows which time step is needed for this model to work accurately considering its application. The implementation of the Cummins' equation is verified by showing agreement between time- and frequency domain. A sensitivity analysis is used to show the effect of changing the model's main variables. The peak period has the biggest influence on Amazing Grace 's heave motion. By varying the coefficients of Cummins' equation in the range of 35.5 - 38 meters draught, the computed vessel motions show that the variation of the draught only has little influence. The Quick drop Ballast system is feasible for both the pretension- and fast lift phase when applying the Quick drop Ballast force at the centre of gravity. The Quick drop Ballast force vector is partly relocated at the bow for applying trim during fast lift. A comparison shows that both the required volume and the number of valves reduce by one third compared to the heave only concept. The maximum allowable trim per length of Amazing Grace is 1.5 meter. This maximum is exceeded by 0.2 meters. The 0.2 meters needs to be included for a future topside lift system (TLS) design. It is recommended to apply trim during fast lift since this simplifies the complexity of the Quick drop Ballast system by reducing the required volume of water and the number of valves.