Towage And Installation Of Concrete Gravity-Based Structures
Authors: Reppe Tormod , HeIsø Erling 
Published: October 2008

ABSTRACT
This paper describes the tow-out and installation of the Draugen and Sleipner A platforms during the Summer 1993.
These two platforms were no. 11 and 12 of a series of Condeep platforms which have been designed, constructed and installed by Norwegian Contractors.
Emphasis in this paper is placed on the ability to manoeuvre and control large mass objects in a predictable manner whilst during towing in restricted waters and during precise emplacement on location.

INTRODUCTION
The Draugen field at Haltenbanken off Mid-Norway is the first field to be developed on the Norwegian continental shelf, outside the North Sea area.
A significant milestone in this development was reached when the integrated drilling, production and quarters platform was towed to location and installed in May 1993.
The structure itself-both concrete base and steel topsides - were quite modest in weight and size compared with other Condeep platforms.
The most significant feature of the platform was the single concrete shaft supporting all topside equipment.
One of the Platform dimensioning parameters was the water depth of the tow route. The principal restriction between the inshore deep water construction site and the offshore location was an offshore sand bank of 237 m depth. Therefore the maximum draft was set to 225 m, giving sufficient allowance for the margins deemed necessary by recognized international standards and industry practice.
As for many Condeep installations the contractual accuracy of platform center should be within a circle of 25 m radius. The Haltenbanken was a virgin area and infrastructure for accurate surface navigation was limited. However, the radius of 25 m was regarded to allow ample margins for both navigational inaccuracy and tolerances in positioning control.
However, the project development deemed it necessary to position the platform within ±7.5 m after allowing for 5 m navigation inaccuracy.
This necessity was identified at a such late stage in the project that a change of installation method would mean a schedule impact. It was therefore decided to employ the method originally planned, using free running tugs only.

DESCRIPTION OF DESIGN AND PLANNING

Positioning Method
The idea was to arrange the tugs into a star formation based on very accurate pre-calculated tow-line direction. It was important that the tow-line force acted through the platform center. This avoids unnecessary rotation when altering the tugs power settings for accurate position control.

Navigation
The primary navigation system for the inshore tow was Micro-fix. DGPS (differential global positioning system) was used as back-up. For the offshore tow and installation, the DGPS was the primary system. During approaching location Syledis was utilised as back up.
On location HPR (Hydro Acoustic Reference System) was used for identification of an obstruction very close to the installation area.

Weather Criteria
For platform approach and emplacement the following maximum criteria were used: (Available in full paper)
As it turned out these relatively stringent criteria became absolutely necessary.
To ensure a reliable and continuous weather update it was decided to have a meteorologist ride the tow.