The First Docking Operation of a Large North Sea Platform Over a Subsea Template
Authors: Baur Michael P. , Clark Howard H. , Lee Ming Shang 
Published: October 2008

This paper describes the method selected and used to install a deep water North Sea jacket structure over a subsea template containing previously drilled oil wells. The engineering studies, the subsea template design and the systems employed to facilitate placement of the structure are examined. A reveal of the actual installation summarizing operation times and activities is given. Improvements on this and other such operations are suggested.

Introduction

In September, 1981, Amoco U.K. Exploration company successfully installed a large, 8 legged, 40 well jacket over a pre-drilled-rutty-well subsea template. The installation was accomplished through controlled docking of the jacket to the subsea template in 144.3 meters of water in the NW Hutton field. The NW Hutton field is located, as shown on figure 1, approximately 160 kilometers east north east of the Shetland .Islands.

The subsea drilling template was installed in its seafloor location in early October, 1979. In the period until June, 1981, while the jacket structure was under fabrication, seven future production wells were drilled to an approximate average vertical depth of 3,660 meters. Upon fabrication completion, the 152.5 meter tall platform jacket, with an installed weight of 16,500 tones, was accurately placed over the subsea template. This unique jacket installation was the first of its kind in the hostile Northern North Sea.

This method of field development significantly reduces the lead time required and allows increased oil production early in the project life. Enhanced panics resulting from accelerated oil production should promote development of fields with reserves previously classified as marginal if developed using conventional procedures.

Engineering

Engineering began with preliminary studies directed toward establishing feasible concepts which could be used to erect a large platform jacket over a subsea template. Small jackets had previously been placed over single and multiple subsea wells under a crane hook control. Various methods of controlling the placement of a massive steel structure were analyzed. Prime considerations in selecting an installation method were:Maneuverability and controlled placementWeather sensitivityOperational safety and reliabilityCost An alignment/placement system had to be developed which would give maximum assurance that the final position of the jacket would be within acceptable tolerances. The dynamic motions of the bottom of the jacket structure must be held within specific limits to avoid damage to the template and the jacket.

Four placement methods were inside red in the preliminary engineering. These were:Method 1A vertical jacket descent over the template with mooring barges controlling jacket positioning.Method 2A vertical jacket descent over the template with a mooring system integrated into the jacket.Method 3A minimum seafloor clearance horizontal jacket approach to the template with mooring barges controlling jacket positioning.Method 4A minimum seafloor clearance horizontal jacket approach to the template with a mooring system integrated into the jacket.