Engineering the Procedure for Installing the North Sea Forties Field Platforms
Authors: Hruska Stanley J. , Koehler Albert M. , Shaw Larry K. , Bourne M.J. 
Published: October 2008

The hostile environmental conditions inherent to the North Sea, the water depth and the seabed soil conditions existing at the site of the Forties Field were the principal factors which made the installation of these platforms unique. In conjunction with these factors, the functional and operational requirements dictated a platform of enormous size and weight. This installation of the Forties Field steel platforms presented problems of a magnitude which had not been previously encountered.

This paper presents the problems associated with the setting and securing of the Forties Field platforms to the seabed. The paper includes discussion of the engineering study which ultimately led to the development of an installation procedure designed to minimize risk during the installation period. Reference is also made to the input from naval architecture which was performed as an integrated activity in the development of the installation concept.

Installing the platform to within the specified tolerances of verticality and orientation and ensuring that the platform has an adequate foundation to withstand wind, wave and current forces during the installation period is extremely important. Maximum allowable wave heights were determined for each stage of the platform installation. This information, combined with an assessment of the length of time required to perform each specific offshore operation, was used to determine the most expeditious and safe sequence for installing the platform. Alternate programs were developed in the event that it became necessary to modify the installation program in the field, as could result from unforeseen installation problems or weather conditions. The installation program and weather conditions on site were therefore continually monitored. Based on experience, the time required to perform each scheduled field operation was assessed, and installation procedures were altered in the field if the changes would expedite the program or improve the in place condition of the platform with respect toits vulnerability to environmental conditions.

INTRODUCTION

Since the construction of the first offshore platform, the offshore industry has continued to expand into deeper and more hostile waters. These early platforms in very shallow water were erected in place using derrick barges and floating cranes normally used in harbor construction. As the size and weight of the offshore platforms continued to grow, the lifting capacities of the derrick barges and cranes were exceeded to the point that the platforms could no longer be erected on site. At this stage it became necessary to develop a technique for installing the platform that would be less dependent on the lifting capacity of the derrick barge. As a result, the launch technique was introduced. This method involves transporting the platform jacket to the offshore location on a barge and winching the jacket off the barge and into the water. Once such a typical jacket is in the water, it generally exhibits positive buoyancy, i.e., the weight of the water displaced by the jacket is greater than its dead weight. In some cases it is necessary to add small removable buoyancy tanks at the top of the jacket to make it adopt the proper attitude in the water.