SS: Testing in the Offshore: Comparison of tank testing and numerical analysis for the design of a catamaran for deck installation by the float-over method
Authors: Cholley Jean Marc , Vanneste Anne , Petrie Francois , Rousse Benjamin 
Published: July 2009

Extensive tank test programmes continue to be employed in the offshore oil and gas industry i.e. a physical modelling technique as distinct from the numerical modelling that is also employed in the design of the offshore structures. All of the modelling methods have their limitations and sources of inaccuracy, hence the priority of the engineer using them is to control and refine their methods as the demand for reliability and accuracy increases. This is especially true for new offshore concepts during their Research & Development programme.

To validate the development of a new catamaran vessel for deck installation by the float-over method, an integrated set of tank testing and numerical analysis has been conducted to explore the degree of confidence to which tank test experiments can provide benchmark datasets for the numerical model using the hydro-structure coupling method.

Firstly, an overview of the challenges with hydrodynamic verification of a novel vessel design is given. The role of the model testing in the verification process, as well as the range of relevant values to be measured, is then discussed. In particular, the combination of model test and computer simulation for validation of the coupling approach is presented.

Initial considerations, such as the mathematical methodology (coupling) for the investigation of the catamaran phenomenon, are discussed including definition of mass distribution, structural stiffness, and combined catamaran hydrostatics, hydrodynamics, internal loads and wave load induced motions of the two hulls.

For the specification of the tank test programme, considerations such as scale selection, model engineering and construction, instrumentation and definition of the test matrix are presented along with examination of issues such as stress and internal load measurement.

This paper reviews and discusses the procedures used, presents the cases studied, compares the experimental data and coupled hydro-structure computational simulations, and describes the future challenges to apply these results to the real world design, construction and operation of the vessel.

Introduction

Specialised in float-over installation methodology, Technip has developed a new solution allowing topsides installation onto either floating platforms or conventional jackets at low or high air gap.

The DSIV is a catamaran shaped vessel capable of installing or uninstalling up to 20 000T topsides on a jacket or spar at a 20m final airgap. The DSIV is composed of two pontoons (of 142.5m length) linked by a truss beam and a rear gate. Each pontoon is equipped with two elevating systems. These elevating systems consist of motorized legs with a rack and pinion system and a movable shuttle fixed on the leg, giving a stroke of 16.5m in the basic configuration. The spacing between hulls is adaptable according to the width of jacket, the diameter of the spar and the size of topside to be installed or removed. The inner distance between pontoons varies between 37 and 46 m.