This paper presents the technical feasibility study performed to develop an alternative for the sale flexible risers of the Campos Basin deep water fields production systems.
The target was to reach diameters larger than 12" for 500 to 1000m water depths. This represents a present flexible pipe industry limitation in terms of manufacturing and installation, and brings a potential source of cost reduction.
The study is based on the conclusions of a Joint Industry Project (JIP) about the matter. In addition to the steps covered by the JIP, it has been investigated the fatigue life, embracing the first and second order rig motion as well as the vortex shedding loads during its operation.
In order to develop deep water fields with FPS'S, Petrobrás faced technical and economical limitations on large diameter flexible risers (larger than 12") for exporting systems.
The conclusions of the JIP about Deep water pipeline, flowline and riser installation, encouraged Petrobrás to consider SCR as a feasible alternative.
A specific study of a 16 oil export SCR for Petrobrás XXV (a future FPS of Albacora field, to be anchored at 575 meter water depth) started in mid 1993, in cooperation with the University of São Paulo (USP), This case was chosen among several others because it was the most critical one.
A preliminary study recommended the following basic characteristics for the SCR:
OD: 16;
ID: 14.75";
steel: API 5LX60.
The first step in SCR design is the determination of the static configuration. The following steps are the verification of the SCR behavior when subject to the critical environmental loads (waves and current) and the fatigue analysis.
The static configuration was established considering the maximum of 35% of the steel yield stress. The main criteria established for the design were:
the maximum SCR Von Mises stress reached during a critical environmental condition should be limited to 72% of the steel yield stress of the material;
the maximum accumulated damage (Miner's rule) should be limited to 10% of the specified service life.
As the service life specified for the production system is 20 years, the SCR should be designed for a 200-years fatigue life to be coherent with the established criteria.
To determine the maximum dynamic response of the SCR it was used the critical loads, obtained from the combination of a 100-year wave and a 10-year Current, or vice-versa, which one is more severe. For this case, the maximum stress obtained was 62.2% of yield stress.
The service life was calculated considering the cumulative effects of the first order motions, the slow drift motions and the oscillations induced by vortex shedding. The service life obtained from the analysis was 102 years, being lower than the established criteria of 200 years.
Other remarkable aspects extracted from the study are:
The main contribution for the fatigue damage is given by the slow drift. The vortex shedding gives the smaller contribution among the three considered effects.
