Session: 16-10-01 Marine Hydrodynamics and Subsea Technology

Submission Number: 182317

A Novel Ballast System Concept for a Deep Seawater Intake Riser 

Along the course of the innovative SWIR project, the need of a new ballast system concept for the long polymeric Sea Water Intake Riser emerged, substituting heavy (larger than 200t) solid and metallic ballast that had been originally conceived. Primarily, the main issue with the original concept regarded the nearly unfeasible installation operation to be done in Brazilian waters, mainly due to the specialized vessels needed, which should be equipped with very heavy lifting devices. Many cons were raised, not only from the technical and economic points of view, but primarily considering matters of installation risks, which would affect safety during that operation. It is worth mentioning that thorough previous dynamic analyses of the SWIR system, considering the original ballast concept, were carried out in the first phases of the project. It was then found that one major issue was the possibility of occurrence of dynamic compression along the SWIR polymeric pipe, due to resonant behavior of the whole system in the first axial mode of vibration, caused by the oscillatory motion imposed by the FPSO driven by free surface waves. This issue was faced and a proper ballast mass selected, avoiding dynamic compression. As a matter of fact, even a previous flooded ballast concept had been assessed and discarded, since the entrapped water in the designed structure substantially increased the axial inertia, causing the natural period of the first axial mode to fall within the wave induced motion frequency range, leading to severe resonances. Therefore, one of the main premises for the new ballast concept was stated as ‘to avoid any dynamic compression’ all along the SWIR polymeric pipe, which might lead to buckling followed by large curvatures and sudden tensioning.

The feasibility of a new ballast concept, composed of a sequence of modules, linking heavy cylindrical weights to heavy chains, was then investigated. It is worth noting that, at that stage, other possible concepts were discussed and discarded, as a simpler one, made of a single (or multiple) long chain(s) hung from the mouthpiece of the SWIR pipe. Nevertheless, as could be anticipated, such a simpler solution might induce dynamic slackening of the chains, provoking compressive waves to propagate along the pipe, event that may be followed by high levels of tension caused by the sudden stretching of the chains. In fact, it is well known from theoretical and practical studies found in technical literature that such hanging chain systems are prone to the slackening phenomenon, even inside the water, followed by sudden stretching (whipping effect) and sometimes leading to chaotic dynamics, caused by the subsequent random impacts between the links that might take place. The existence of such an undesirable slackening phenomenon has indeed been confirmed for this simpler concept, through dynamic simulations for the SWIR system, carried out with Orcaflex™.

A conceptual design task of the new ballast system took then place, leading to a system composed by a string of modules linking heavy cylindrical weights and chains. The dynamic behavior of the new system was assessed through selected dynamic simulations, carried on by running Orcaflex™. This conceptual development guided the final design, that is now patented, whose main results and corresponding dynamic analyses are shown in the proposed paper. This is believed to be a major contribution regarding the feasibility of the innovative SWIR concept. The results to be presented refer to the design achieved through a rational procedure which, besides dynamics, hydrodynamics, and structural-strength aspects, considered practical construction ones, balancing a set of premises.

Presenting Author: Celso P. Pesce University of São Paulo

Presenting Author Biography: Professor of Mechanical Sciences, Escola Politécnica, University of São Paulo, since 1998.
Head of the Department of Naval Architecture and Ocean Engineering (2024-26).
Head of the Department of Mechanical Engineering (2007-2011)
Director of the Offshore Mechanics Laboratory, USP, since 1994.
ASME - OOAE Executive Committee Member (2019-25).
Visiting Professor, University of Michigan (1999).
Naval Architect (1978), MSc (1984) and DSc in Ocean Engineering (1988), from University of São Paulo.
Engineering Sciences (1984-85), CALTECH.
Research Engineer Officer, Tech Res Institute, SP, Brazil, 1979-89.

Authors:

Celso P. Pesce University of São Paulo
Clóvis De A. Martins University of São Paulo
Gustavo R. S. Assi University of São Paulo
Fernanda C. M. Takafuji University of São Paulo
Rafael G. Savioli University of São Paulo
Kazuo Nishimoto University of São Paulo
Caio H. K. Bonini SBM Offhore Brasil
Eli E. B. Gomes Shell Brasil Petróleo Ltda