Influence of Fatigue Issues on the Design of SCRs for Deepwater Offshore Brazil
Authors: Torres Ana Lucia Fernandes Lima , Mourelle Marcio Martins , Senra Stael Ferreira , Gonzalez Enrique Casaprima , de Gam Lima Jose Mauricio Teixeira 
Published: October 2008

Petrobras has been investigating the Steel Catenary Riser (SCR) alternative since the beginning of the nineties. Since then, fatigue verification has been an important issue, demanding a good representation of the loading conditions occurring during the riser lifetime, and also demanding a precise knowledge of all possible aggressive factors and construction aspects that could decrease or change riser materials resistance. The concern with fatigue aspects has motivated Petrobras to develop research in several areas, such as metocean data acquisition, hulls concept for motions optimization, special TDP joints, confident models for VIV analysis, corrosion-fatigue effect, clad pipes, etc.

Introduction

Petrobras has been investigating the free-hanging SCR alternative since the beginning of the nineties. With Petrobras discoveries at water depths greater than 1000 m, the use of flexible lines concept has faced difficulties due to the action of high hydrostatic pressure. Steel risers overcome this problem by increasing the wall thickness. In addition, they have the advantage of using cheaper material and having a simpler production process.

The installation of the P-18 SCR was a pioneer project of a free-hanging steel catenary riser connected to a semi-submersible [1] and determined the technical feasibility of the concept. It is a 10?? riser of gas export located at a water depth of 910m in the Marlim Field, on Campos Basin, Brazil. In order to implement this riser, Petrobras developed internal research through its Research Center, working together with universities and taking part in JIP's. Although this riser was installed as a prototype, it is still working in the gas transfer from P-18 to P-26. It has been monitored since 1999 and the results are being compared with the design data and with simulations that are being performed with in-house computer programs [2] and other commercial packages that take part of the complete design methodology. Other SCR's were studied such as the 12??oil export in the P-19 semi-submersible in a water depth of 770 m [3] and the 10?? oil and gas export lines for P-36 semi-submersible platform [4], these two installed at a water depth of 1360 m.

The free-hanging SCR configuration is considered, by Petrobras, as an available technology for semi-submersible applications, and the interest in the application of SCR's connected to FPSO's (Floating, Production, Storage and Offloading units), due to the trend of using these units for exploration and production in deep waters, has brought the need to study this concept carefully, given the high offsets and heave motions imposed by the vessel at the top of the riser.

In all the developments listed above, fatigue verification was an important issue, demanding a good evaluation of the loading conditions occurring during the riser lifetime, and also demanding a precise knowledge of all possible aggressive factors and construction aspects that could decrease or change riser materials resistance.

The concern with fatigue aspects has motivated Petrobras to develop research in several areas, such as metocean data acquisition, hulls concept for motions optimization, special touch down point (TDP) joints, confident models for VIV analysis, corrosion-fatigue effect, clad pipes, etc.