Structural Integrity Guarantee of Semi-Submersibles Converted to Production
Authors: Henriques C.C.D. 
Published: October 2008

In the last fifteen years PETROBRAS has converted many old drilling semi-submersibles into floating production units. The low cost of existing units compared to high naval construction cost has favored this type of system.

On the other hand, the increase in the plants size has become conversion projects more and more difficult, The continuous increase in the water depth has been yet another challenge. This work covers PETROBRAS experience in conversion projects of different sizes and levels of complexity. Emphasis is focused on structural and naval aspects that are the most critical to ensure a good conversion project. The specific design criteria that we developed arc discussed here. We stress the special care that should be taken in the choice of the unit to be converted and in the conversion work. The guarantee that an old unit (sometimes more than 20 years old) might operate with increased deck load for 20 more years is a key factor that is highlighted here, also.

INTRODUCTION

Current oil prices, around US$ 18, press the oil industry to a continuous search for cost reduction, specially deep water and marginal oil fields. Converting old semi-submersibles to floating production systems is a practical and inexpensive alternative for exploiting deep water oil fields.

Small or medium size fields will probably prevail in the future deep water developments, since the giants or even big fields are becoming more and more rare. According to Ref. 15, 40% of the new oil prospects in 1994 were located over 300m waters. These medium size, deep water fields seem to be a perfect scenario for using converted semi-submersibles. Besides, the increase of exploration in new mild environment areas, like the Extreme Orient and West Africa Coast enlarges the potential of use of this alternative. Offshore production from floating units increased worldwide from 2% in 1984 to 4.5% m 1994 (Ref. 12).

The size and complexity of the plants that PETROBRAS installed over semi-submersibles has been increasing, since we began to use this system, in the Enchova Field, in 1977. Tables 1 and 2 list the Floating Production Systems we have installed, since then and Fig. 1 shows the location of FPS'S that are operating at our main oil province, the Campos Basin.

CONVERSION X NEW BUILDING

There has been a lot of discussion in the offshore industry about building new semi-submersibles for production or converting old units. Ref. 11 shows that in the last 5 years the number of converted units (including FPSO's) overcomes the number of new ones by a small margin, but this margin is clearly increasing, Table 3 shows the installed and designed floating production systems around the world. We can absence that the number of converted semis, like the GB 388 project in the Gulf of Mexico, is much greater than newbuilts. For bigger fields like the Njord field (Nonvegian Sea), however, semi-submersibles specifically designed for production seem to be more suitable.

The low cost of existing units compared to high naval construction costs has favored this type of system. The main advantage of converting old units to floating production systems is the anticipation of the 011 production, since the conversion work is quicker than the construction of new hulls (purpose-built production semis or TLP's) Typically, converting an existing unit takes 12 months less than building a new one. In our major projects this may represent an anticipation of 18 MMboe, increasing the Net Present Value of the project by more than 140 MMUS$. However, the complexity of the conversion work may result in delays that could overcome this advantage.

Today more than 50% of PETROBRAS offshore oil product