Exact Installation of Concrete Platforms Over Predrilled Wells
Authors: Gabrielsen P. , Bjordal B.N. 
Published: October 2008

ABSTRACT
This paper describes positioning of large concrete platforms over a subsea template having predrilled wells. Emphasis is put on operational procedure and accuracy of installation.
The operational procedure has evolved from experience obtained from installing eight Condeep platforms in the North Sea. The installation of the Gullfaks A platform, the last of such operations, is described in some detail to illustrate this experience. It is shown that the basic principles are the same for positioning similar platforms over predrilled wells, although the installation equipment would be different.
Platform motion, the main item for discussion on accuracy of positioning, is verified through full-scale platform motion measurements.

INTRODUCTION
The very nature of the offshore oil industry makes it imperative to adapt a conservative attitude towards new development. A harsh and alien environment, complexity of equipment and potential risk of loss or damage to lives, the environment and material value demand cautious planning, construction and operations. As the tasks become ever more demanding, new methods and equipment have to be developed, but it is essential that each step in the process is taken carefully in order to avoid mistakes. Valid proof of any new development can only be obtained by successful practical implementation.
This paper covers one such development, theinstallation of concrete gravity base platforms over predrilled wells. This task has not been carried out as yet, in spite of the economic potential inherent in achieving production start six months to one year earlier than with a conventional installation.
A seemingly large step in the development process is shown to be modest when previous practical experience, theoretical knowledge and full-scale measurements are integrated in the design process.
In the past, tugs have been used to position platforms. To obtain sufficient control during the final close positioning, dowels which protude underneath the platform are brought into contact with the sea bed to provide resistance.
Because greater accuracy of positioning is required when installing platforms over a template, the platform will be positioned by means of an anchoring system and winches rather than by tugs and dowels.
It will be shown that while the physical systems required for controlling and measuring position are further refined, the basic priciples for platform installation remain the same when installing over predrilled wells. These priciples may be summarized as:–Operation is reversible at all times before platform position is secured.–Redundancy in all important systems.–Simple, step-by-step procedures. Clearly identified milestones and criteria.–Few important parameters to supervise in order to minimize human strain during the operation.

GULLFAKS A INSTALLATION
The Gullfaks A platform was installed on the Gullfaks field in 135 metres of water in May 1986 as the eighth Condeep platform in the North Sea. The platform carried a topside load of about 43,000 tonnes during tow to field and installation, see Figure 1. The towing draft was 125 metres, displacement 773,000 tonnes and the offshore tow was carried out by a fleet of six tugs. Figure 2 shows the platform during the cutloose from the inshore mooring.