Experimental and Numerical Evaluation of the Installation of Sub-Sea Equipments for Risers Support

ABSTRACT The installations of sub-sea equipments are very complex operations, requiring previous analysis in order to define the correct procedure and the environmental "window" for a safe operation. This paper addresses the installations of a Mid Water Arch (MWA) that consists of a structure to provide risers support. Connecting the risers to the MWA largely eliminates the dynamic forces that would otherwise cause friction and fatigue. Such structure is composed by the riser guides and several buoyancy tanks. It is kept in the water by means of tethers connected to an anchor. The MWA is to be installed 42m from the seabed. The installation procedure has several steps evolving the launching of each component of the MWA (anchor, main structure and the tethers). A tug boat with an Aframe is used during the whole launching, and an assisting vessel is required to keep the buoy away from the tether and the launching cable. The presence of exciting waves induces oscillatory motions in the whole system, and may cause large dynamic forces in the cables and tethers. Due to the complexity of the multi-body system, a comprehensive numerical and experimental analysis was then carried out in order to dimensioning the launching cables and to define the limit environmental condition. The numerical analysis was carried out in the Numerical Offshore Tank (TPN), a multi-processor offshore system simulator that considers the 6 DOF of each body and all environmental forces acting in them. The lines are modeled by finite-element method. Furthermore, a full set of small-scale experiments were carried out at the State of São Paulo Technological Research Institute (IPT) towing tank, considering the system excited by a sinusoidal motion at the top, emulating the wave excitation. Comparisons between numerical and experimental results were performed, with good adherence between them. The validated numerical simulator was then used to make predictions of the behavior of the systems during the installation, considering several environmental conditions and configurations. INTRODUCTION The present paper addresses a methodology used to analyze complex offshore operations, evolving sub-sea installation and several support vessels. In such cases, the execution of complete experiments, evolving all the vessels and the components of the real operation may be extremely complex and expensive. Furthermore, depending on the necessity of the offshore industry, the time required to prepare and execute such experiments may be another drawback. Numerical simulator is another tool that engineers may use for performing such analysis. However, due to the enormous complexity of some operations, the engineers still do not absolutely rely on the numerical results to make important decisions. The present paper presents an example of a hybrid methodology for the analysis of a complex installation of subsea equipment. Simplified experiments were used to validate a numerical simulator that was then used to make simulations of the complete operation under real environmental conditions. This paper addresses the installations of a Mid Water Arch (MWA) that consists of a structure to provide risers support. The MWA consists of a buoyancy tank assembly (BTA), an anchor and two tethers connecting them. During the installation, other cables and two tug boats are used

OMAE2009-79380 MITIGATION OF VORTEX-INDUCED MOTIONS OF A MONOCOLUMN PLATFORM

ABSTRACT A great deal of works has been developed on the Spar VIM issue. There are, however, very few published works concerning VIM of monocolumn platforms, partly due to the fact that the concept is fairly recent and the first unit was only installed last year. In this context, the present paper presents a meticulous study on VIM for this type of platform concept. Model test experiments were performed to check the influence of many factors on VIM, such as different headings, wave/current coexistence, different drafts, suppression elements, and the presence of risers. The results of the experiments presented here are inline and cross-flow motion amplitudes, ratios of actual oscillation and natural periods, and motions in the XY plane. This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction