Floatover Feasibility in Brazilian Sea Water
Authors: Zhang Wenbin , Jeong Chan , Spreeken Ardaan van 
Published: June 2014

This paper describes the feasibility of topside float-over installation in Brazilian sea water. Float-over installation, which is the method to install a topside onto a substructure, such as jacket or floating platforms, using a transportation barge or vessel, is becoming more common as crane vessel lifting capacity is limited and not available in certain areas.

The main benefit of float-over compared to multi-module lifting installation is relatively short offshore hook-up and commissioning time, reducing offshore exposure hours and cost. The float-over installation method is increasingly becoming the installation method of choice in both mild and harsh environments as topside weight grows. However, the float-over method has never been used in Brazilian sea water mainly due to the unique weather characteristics typically represented by swell.

In this study, weather characteristics in several project sites worldwide are analyzed and several workability analyses are performed based on estimated weather limits that are calculated from extensive float-over analyses. The risks and costs related to a float-over installation operation will be a part of an important factor determining topside fabrication philosophy.

From this study, the float-over method in Brazilian sea water is found to be feasible when using specific supporting structure design such as surge/sway fenders, Leg Mating Unit/Deck Support Units (LMU/DSU) and mooring system. Although workability in the Brazilian sea water is relatively lower than other regions due to swell conditions, there is plenty of room to improve the workability by introducing float-over systems designed specifically for swell dominant area.

Introduction

The float-over deck installation is becoming a favorable and reliable method in recent years not only because the float-over method is a cost-effective and state-of-the-art tool to install large integrated production decks, but also because the latest technical advancement makes float-overs feasible in remote and harsh environments in the world. Furthermore, the float-over substructures can be almost all kinds of both fixed and floating systems such as Jackets, GBSs, TLPs, Semis, and Spars. Seij et al. (2007) introduced general information and technical trends of the float-over method, and the typical procedures in the float-over deck installation can be divided into six stages: load-out, transportation, stand-by preparations, docking operation, mating operation, and undocking operation.

The history of the float-over deck installation can be traced back to the late of 1970s when deck mating operations were performed initially in sheltered areas. As technology advanced, the float-over installation moved offshore with the help of barges and recently self-propelled HTVs (Heavy Transport Vessels). Recently, more major oil and gas fields are developed in remote and harsh areas with less favorable environmental conditions. Larger and heavier production decks are also required and constructed. Both harsh environment and heavy production decks substantially limit the installation operation by floating cranes. However, many special techniques have been developed to extend float-over deck installations into remote and harsh environments. These techniques include: Hi-Deck, smart-leg, strand-jack lifting, Unideck, Ampelmann System, twin-barge, versa-truss system, TML system, catamaran, T-shape barge, and DP vessel.