Results Comparison of Computer Simulation, Model Test and Offshore Installation for Wandoo Integrated Deck Float Over Installation
Authors: Chu N. , Cochrane M. , Mobbs K. , Mitchell D. 
Published: October 2008

Computer simulation and model tests were used to develop and evaluate the Wandoo B integrated deck float over installation design. The design included moorings, fenders, ballast system, mating point and supporting point mechanisms. The mooring line dynamic loads, impacts between barge fenders and concrete gravity sub-structure (CGS) shafts, mating point impacts and support point impacts were simulated in the time domain. Model tests were used to check the design parameters and computer simulation results. Decay tests were used to estimate system damping for the final computer simulation. Mooring line tensions, barge motions and environmental conditions were monitored during the integrated deck float over installation. A comparison of the computer simulation, model test and field installation results are given in this paper. Possible improvements to the computer simulation, model tests and the integrated deck float over installation design are highlighted.

Introduction

The 6,500t (dry weight) integrated deck was successfully installed onto the pre-installed concrete gravity substructure at the Wandoo location, North West Shelf of Australia in January 1997. The transportation barge Ocean Orc carrying the integrated deck was docked with the assistance of two anchor lines, four mooring lines attached to the CGS shafts and two ocean going tugs. The barge was positioned by eight mooring lines and four hard fenders during the deck mating. The nominal gap between the hard fenders on the barge and the CGS shaft surface was 100 mm. A rapid ballast system was activated to lower down the integrated deck onto the preinstalled CGS during a falling tide. Environmental conditions, barge motion and mooring line tensions were monitored during the installation. Safely and economically installing an integrated deck onto a pre-installed CGS requires detailed engineering and optimized design. Accurately predicting the possible loads on the structures and the installation equipment along with the development of practical procedures for the float over installation can eliminate incidents, minimize required installation time and reduce the cost of the installation. Detailed engineering of the integrated deck installation included three phases:preliminary engineering design and computer simulation to develop a feasible positioning system for the float over installation;model test to check computer simulation results and evaluate the preliminary installation design;final computer simulation to consider the changes made in the final installation design.

This paper briefly covers the procedures used for the engineering of the Wandoo B integrated deck float over installation.

Computer Simulation

The float over installation were simulated for the following stages:DockingPrematingFirst contact between mating points50% of integrated deck weight transferred.Last contact between support pointsPost mating

Hydrodynamic Analysis.

Hydrodynamic forces RAO's, added mass and damping of the floating barge were generated in the hydrodynamic analysis Submerged parts of the barge and the Wandoo B CGS were modeled by using small quadrilateral and triangular elements. Figure 1 shows the numerical model used for the hydrodynamic analysis. Wave diffraction radiation theory was used to calculate hydrodynamic forces (pressure) on each element of the floating barge.