Innovative Design Method for Deepwater Surface Casings

Abstract

Abstract This paper presents a method to calculate the load bearing capacity of surface casings. The method clearly describes how to calculate the immediate capacity of the conductor and how to estimate the soil set-up with time. The recommended equations for set-up calculations are based on the results of three proof-load tests performed on partially set-up conductors.The paper also lists a series of key parameters that need to be controlled for a successful jetting operation. The overall method has been calibrated against 20 successful installation records. Introduction When BP and Amoco merged, considerable differences existed between the two companies with respect to deepwater drilling practices. In particular, surface casings (also known as jetted conductors, or surface conductors, or jetted pipes) were designed and installed with drastically different methods, leading to considerable debate and disagreement in the newly integrated drilling community over which method was most appropriate for future wells. Both companies' previous methods had been successful in preventing loss of wellhead equipment. The work described in this paper consisted of harmonizing past practices by developing and implementing an innovative design and installation method for deepwater surface casings. The work included:Clearly documenting and understanding BP and Amoco's pre-merger practices.Analyzing each practice and comparing it with actual field installation records.Replacing the two practices by a new innovative method that was fully calibrated against pre-merger successful installations.Implementing the new method into the design and installation of deepwater surface casings. The work also defined specific installation procedures for the surface casings and gives clear guidance on how a set of key parameters should be controlled during installation. The shortcomings of past installation processes were also highlighted throughout the project. Should the recommend work process not be followed during offshore installation, the new method also allows the drilling personnel to properly account for the deviation in installation procedure and to calculate the "penalty" to be paid when landing subsequent strings. Pre-merger Methods: The pre-merger method used by Amoco to calculate the capacity of jetted conductors was developed and documented by Beck et al (1991). In summary, the method:was developed from jetted conductor installation observations made from 1984 to 1986.was calibrated at 6 Amoco sites.used the undisturbed soil shear strength, MV, and the residual shear strength, RMV, to calculate a lower bound and an upper bound immediate capacity. Both shear strength measurements were done using a minivane. Some of the limitations of the method were as follows. The method:was intended for conductors with squnch joints, not threaded flush connectors.assumed that soil data was available for the site.This was usually obtained by means of soil sampling with the drilling vessel immediately prior to jetting the conductor.Gave only the immediate capacity and did not provide set-up curves.