Mobile Bay Project Float-On Deck Installations
Authors: Birdwell J.R. , Jordan J.E. 
Published: October 2008

A float-on deck mating procedure was successfully used instead of a conventional derrick barge lift for installation of the integrated topsides on three production platforms. This procedure involved mooring the deck transportation barge inside a jacket/pile foundation and then ballasting the barge to mate the deck legs with the piles. The three platforms had topside weights ranging from 3700 to 4800 short tons. Two of the platforms were installed outside Mobile Bay in water depths of 40 and 46 feet and the third platform was installed inside the bay in 14 feet of water.

This paper presents aspects of the structural designs and describes the installation procedures, mating hardware, and monitoring systems used to accomplish the float-on deck installations. Factors influencing the selection of this concept are also presented.

INTRODUCTION

The development of three Mobile Bay area fields in Alabama State waters required three large production platforms to handle production from a number of satellite and bridge-connected well templates. An area map outlining this development is shown on Figure 1. Table 1 summarizes some general information on these platforms.

The size and weight of the production platform decks and the shallow water depths presented an installation challenge. A cost effective float-on installation method was selected over a more conventional Iifted installation. Although the float-on concept has been used previously, these decks represent the largest float-on installations to date in the Gulf of Mexico. These successful installations demonstrate the viability of the concept.

SELECTION OF INSTALLATION METHOD

A conventional lifted deck installation method, first considered for this project, had the following shortcomings. Without dredging, the shallow 14 foot waters of the bay restricted derrick barge access to only those with a maximum lift capacity of 800 short tons. The 40 to 46 foot water depths at the offshore locations restricted lifting equipment to the 2,000 to 3,000 short ton capacity barges or ship-shaped derrick vessels. Larger lift capacity semisubmersibles required an operational water depth of 70 feet or more. Use of a one-of-a-kind 5,000 short ton capacity stiff leg would have required dredging in the environmentally sensitive bay and the lift weight of the heaviest deck exceeded its capacity. Therefore, conventional lifted installations would have required setting these large decks in multiple pieces or modules increasing the offshore hookup cost and duration. Finally, some restructuring and down sizing of the offshore construction industry during this project led to concerns about lifting equipment availability.

The float-on deck installation method offered the following benefits over a multi-piece lifted installation:The decks could be outfitted and tested onshore reducing offshore hookup cost and time.The flare towers, ranging in height from 135 to 200 feet, could also be erected onshore as lift rigging interference was not an issue.The float-on method is relatively insensitive to growth in facility weight.The 3700 short ton deck in Mobile Bay could be installed without dredging.Equipment to facilitate the float-on installation was readily available.A derrick barge was not required as personnel could work off a jackup work boat at a fraction of the spread rate of a denick barge.