Comparisons Between Api And Npd Drilling
Authors: Ghalayini S.A. 
Published: October 2008

Design and fabrication of standard drilling structures in North Sea are becoming more common with the advent of extended reach drilling technology. Most of the structures are built according to API and NPD requirements which dictate a specified set of regulations, design codes and standards. The design authority shall ensure consistency within the framework of these specifications and codes and standards utilized for the performance of the design work. It is not the intention that the most stringent requirement be picked from each and combined but that the most conservative set or coherent group of requirements be applied. In many cases either API or NPD requirements are applied. The complexity of the design and the safety level of the drilling structure are directly affected by the specified code.

The present paper describes the differences in the design requirements specified by API and NPD. Design aspects and factor of safety recommended by both for various structural components are presented and differences are identified. Effect of these aspects in the computation of critical member stresses is discussed. The study performed indicated that NPD design requirements prescribe procedures which result in an increased level of safety as compared with API specified codes and standards.

In addition a case study of an existing driling module is presented. The module consists of a drilling mast, storage buildings, substructure boxes and skid beams. The module is analyzed using a three dimensional finite element computer package. The analysis is performed using both API and NPD specified codes. Hook, wind, waves and earthquake loading are combined according to each code requirements and applied to the drilling module. The resulting critical unity checks of the members are presented. Critical unity checks as computed by NPD specified code are found to be higher than those computed by API specified code, which indicates that the factor of safety used by NPD design requirements are higher than those used by API.

INTRODUCTION

Due to the recent increase in drilling activity generally in the North Sea area, and in particular in the Norwegian Sector, more standard offshore drilling structures are designed and fabricated for installation in this area. Design regulations and specifications have been developed in European countries for offshore steel structures based on the presently available design principles and the offshore conditions that structures are likely to be experienced. Offshore drilling structures built in North Sea has to comply with the requirements of the national codes and statutory instruments, along with additional requirements of the operating oil company.

Most of these codes and regulations for the design of the drilling structures are largely derivative from publications of the American Petroleum Institute. But the regulations recommended by the Norwegian Petroleum Directorate have more meticulous restrictions when designing for the environmental loads due to the severe climatic conditions of the North Sea. This paper identifies and discusses the differences in design between the NPD and API and their effect on the structure.