Float-over of Arthit PP Deck
Authors: Kocaman Alp , Kim Daejun , Seto Jian 
Published: October 2008

In December 2007, the 17,500-metric ton, Arthit PP deck was installed over the substructure in a single piece by a using J. Ray McDermott's (J. Ray) transportation and installation barge Intermac-650 (I-650), specially designed for float-over installation. The Arthit Field is located in the Gulf of Thailand in 80 meters of water.

A number of technical challenges were overcome to accomplish the successful and safe float-over.

A single-piece deck installation using the float-over technique provides significant advantages over other methods of deck installation for heavy topsides, especially in areas of the world where access to heavy construction equipment, trained labor and supplies are not readily available or reliable. Time spent on offshore hook up and commissioning is also minimized by utilizing a single-piece installation.

J. Ray engineers developed a simulation model to predict the I-650 barge motions and the loads between the deck and the barge, as well as the tensions in the mooring lines. Model tests were completed and the simulation method was verified. The analysis tools developed yielded reliable and repeatable results in selecting and designing components critical to the success of the float-over. Such components include the mooring system, the shock absorbing cells, sliding surfaces and vessel ballasting systems, and with careful analysis, risks can be minimized and the installation method could be guaranteed as successful.

In this paper, computer analysis methods are explained; comparisons to the model tests and the results achieved in the real world are compared. The main conclusion is that the float-over method is an important, cost-effective, state-of-the-art tool in installing large sized decks in single piece.

Introduction

Installing production decks (topsides) offshore has always posed technical challenges. The main offshore construction equipment is the floating crane (derrick barge). For relatively light topsides, less than 3,000 metric tons, a number of lift vessels are available in most offshore areas of the world. For lifts of structures heavier than 3,000 metric tons, the number of vessels drops dramatically. To make matters worse, the vessels which have the capability to handle heavier topsides weights are committed to work in certain parts of the world and are very costly to mobilize for a project in a distant location.

Apart from the lift capacity, several other factors that put severely limit the operation of even the high capacity derrick lift-barges. Although the " name plate?? capacity exceeding 14,000 metric tons is being advertised in the market; the geometry, hook reach and water depth restrictions limit the ultimate lifting capacity to a single piece of about 10,000 metric tons.

In addition, floating cranes are sensitive to the prevailing weather conditions at the installation site. For a swell dominated offshore site, it is not unusual for a floating crane to be de-rated 40-50% from its rated lift capacity.

If a topsides heavier than 10,000 metric tons is to be installed using floating cranes, it will have to be divided into smaller modules that can be lifted individually. In such a scenario, offshore hook-up and commissioning will need to be extended at a cost premium.