Optimization of an integrated deck installation process using a fully coupled hydrostructures time domain analysis
Authors: Berhault C. , Gubrin P. , Foulhoux L. 
Published: October 2008

ABSTRACT
In mid January of 1996, the 9650-tons integrated deck of the COB-PI platform operated by Elf Exploration Angola on the COBO field (West Africa) has been successfully installed using an original float-over method developed by Technip Geoproduction. Offshore operations confirmed to be very sensitive to the metocean conditions, particularly during the critical path of docking. Therefore, a more accurate simulation tool was welcome to develop and analyze installation scenarios and to help on-filed decision making. The paper describes the application to COB-P1 deck installation of a new coupled hydro-structure model devoted to the analysis and optimization of marine operations. The different stages of the deck mating were simulated in time domain, including the dynamic behavior of the barge I deck I jacket system, mooring lines, hawsers and cables, fenders and shock absorbers, ballasting and jacking procedures, external loads coming from wind, waves and current. As the most critical stage is the mass transfer of the deck from the barge to the jacket, particular attention has been paid to the modeling of the contact loads and the hydrostatic restoring loads.

Introduction
The 9650-tons integrated deck of the COB-PI platform has been installed in January 1996 using an original float-over method developed by Technip Geoproduction. The barge GIANT 4 was equipped with a jacking system supporting the deck and was moored on to the jacket piles (Fig. 1). The deck load was transferred on the jacket by a combined jacking I ballast technique (Fig. 2). An important stage of the detailed engineering was to define the limiting environmental conditions and to define adequate monitoring of the motions of the barge and of the piles. A critical stage is when the docking piles engaged into the jacket cones (Fig. 3). A particularity of the West Africa coast is an existing permanent swell. Then accurate limited seastate conditions had to be defined. Classical numerical models are widely used for that: linear diffraction analysis in frequency domain, slow drift analysis in time domain. However it appears that these models have some critical limitations. The computations must be performed stage by stage, neglecting the transient effects induced by the jacking and ballast operations. During the deck mass transfer, the contact loads are not included. Built on the COB-PI experience, it was decided to develop a more realistic numerical model for in depth analysis of this type of marine operations. As in-situ measurements of metocean conditions and barge motions were made available from the COB-PI deck installation, the new model presented hereafter was run on this case for validation before to be used for new design scenario.

Description of the deck installation principles
At the time of installation, in 1996, COB-PI was the heaviest deck installed by a float-over method in open sea off West Africa. Barge I deck I jacket configuration. The main dimensions of the deck and on the barge Giant are given in table 1. At the beginning of the mating process the deck is fully supported on the barge by 8 hydraulic jacks. Initially, the jacks are fully extended.