In the last years Petrobras has optimized and reduced the cost of the mooring systems by the use of torpedo anchors. It consists of a tubular pile with heavy weight ballast, installed by a process that uses the energy generated by the free fall of the pile from a supply vessel. It's a low cost anchoring concept, with simple fabrication and installation. More than two hundred torpedo anchors were installed offshore Brazil in the last four years.
Based on the success of the torpedo anchors for mooring purposes Petrobras Drilling and Exploration department decided to employ the same idea for installing the conductor casing. The benefit was that this installation could be accomplished from an anchor handling vessel saving important rig time. Another advantage from this installation method is the better foundation provided since no soil is removed during this process.
The paper will give an overview of Petrobras Torpedo Base experience in the Albacora East field as well as a few other applications of this technology for the production side.
In late 1984 Petrobras started its deepwater campaign in Campos Basin in the Marlim field using dynamic positioned (DP) drillships. Among the many problems related to deepwater drilling such as high wellhead stresses, wellhead equipment inadequacy, lack of experience with DP rigs, the loss of wells during the spud in caused considerable prejudice. Many actions were taken with the goal of reducing this problem. New dedicated equipment were designed, an extensive soil sampling campaign was carried out, adequate operational procedures were written, etc.
After the implementation of those actions the number of well losses has dropped considerably but some wells were still lost per year.
Thus a group of engineers aiming to improve the well performance decided to use Petrobras mooring torpedo experience to be fitted for well spud in. The idea is to install the 36" or 30" conductor casing previously to the DP arrival to the location. This would be carried out through the use of a DP tug with ROV support while delimity the well location with buoys. This would save significant money since the drillship daily rate cannot be compared to the tug's rate. Besides the well drilling time is reduced of about 1 to 1.5 days since the conductor casing is already installed.
The standard deepwater casing well design at Campos Basin is 36"/30" × 13 3/8" × 9 5/8" × 7" (optional). Normally the 36" or 30" conductor casing with 36m long can be installed by drilling and cementing or by jetting depending on either soil conditions of the location.
The jetting procedure in clay soils is reliable in most of the cases and adopted all over the world but from time to time a very poor spud in happens causing in some occasions the loss of the well. To understand this occurence it is important to know that the conductor casing is held in place by soil friction with the outside of the casing.