Smart?Leg Computations Validated During the Ekpe Gas Compression Project Float-over Deck Installation
Authors: Legras J.L. , Bouyssy V. , Berek E. , Gu G. 
Published: October 2008

The Smart?Leg system developed by ETPM ensures the shockless float-over installation of heavy fully-commissioned integrated decks on offshore jacket structures. ETPM successfully installed the Ekpe Gas Compression deck offshore Nigeria in June 1997, using, for the first time, the Smart?Leg float-over system. Two years before, ETPM undertook a program of numerical simulations and model test work as part of the development of the system. The numerical program was validated against the basin model test results. The design of the Ekpe deck, the cargo barge, and the Smart-Leg equipment was based on extensive numerical simulations performed with the Smart?Leg software. A detailed weather analysis and forecast were conducted during the Ekpe deck installation. Furthermore measurements of environment, barge motion and pressure and stroke of Smart?, Leg jacks were carried out to monitor the operation. A later study verified correlation between measured and predicted motions and forces. This paper presents the model basin test results, the calibration and computer work, and the field results from the Ekpe deck float-over installation. The results of the correlation between the predicted motions/forces and the as-measured values obtained during the float-over deck installation are presented, too. The main advantages of the method have been confirmed: smooth suppression of deck and jacket relative motion, absence of shock either horizontally or vertically, and great positioning accuracy.

Introduction

In June 1997, the 4,100 tonne Ekpe Gas Compression deck was successfully mated on the OY jacket using the Smart?Leg float-over system, as described in the companion paper reference [1]. The Smart?Leg float-over method is fully described in reference [2]. The main steps of the mating operation are listed below as well as the parameters that need to be evaluated in order to design structures and equipment and to define limit environmental conditions for the operation. Environmental conditions to be considered for offshore Nigeria operations include:–wind and current, although corresponding loads are much smaller than those due to waves–maximum tide of 2 meters–long swells with maximum incidence of ±20° relative to the jacket longitudinal axis–wind generated waves from any direction

Analytical Work and Design Procedure

Several computer programs have been used to check the feasibility of the mating operation. The main software is a new time domain simulation program developed by BMT [2] to describe the dynamic behavior in regular and irregular waves of three bodies: the barge, the deck and the jacket linked by the "Smart" components - i.e. the Smart?Fins, Smart?Fenders, Smart?Legs and Smart?Shoes - with their complex mechanical and structural characteristics. Other programs are MOSES commonly used for the simulation of marine operations and structural verifications, and FASTRUDL, a general purpose structural program. Vertical clearance between jacket and deck Legs. It is essential to be sure that there is no risk of shock between jacket and deck until the deck legs are immobilized. This was ensured by first designing the Smart?Leg system to leave enough clearance between the deck bottom and the jacket top, based on the numerical simulation results, and then by measuring, during the installation at the site, the heave motions prior to the beginning of the mating operation.