Transport and Installation of Protective Barrier Ekofisk 2/4 Tank
Authors: Donoclift P. , Gijzel T.G. , Hjelde H.G , Veldkamp J.R. 
Published: October 2008

ABSTRACT
After an engineering and construction period of sixteen months in June 1989 both halves of the Protective Barrier for the Ekofisk 2/4 Tank were ready for tow-out and installation. To enable installation of both halves immediately after each other both structures left the inshore construction site with approx. 1 day in between. Consequently two towing fleets had to be mobilized totalling 18 towing vessels with 2215 t bollard pull, the largest towing fleet ever assembled.
At the Ekofisk Field during the approach to the 2/4 Tank the structures had to pass neighbouring installations with clearances down to 10 m. Arriving close to the 2/4 Tank clearances during manoeuvering reduced to 2 m, whereas the acceptable final positioning inaccuracy was lessthan 0.20 m, over the full height (84m)of the joints between the two halves.
To enable this complex and critical operation in a fully operational Ekofisk Field (as a shut down was not acceptable), several installation systems and facilities were used, together with detailed preparation of the activities and training procedures of the responsible personnel.
The installation was completed successfully and the required installation accuracies were met. The achieved position accuracy at the seabed was within 0.05 m of the target.
This paper outlines several operational aspects of above described towing and installation and shows the feasibility of positioning large gravity structures with very high accuracies.

1 INTRODUCTION
In 1984 the seabed around Ekofisk was found to be subsiding at a rate of about 0.5m per year and the total subsidence expected to reach 6.0 m. This subsidence made it necessary to protect the existing installations against severe wave action due to the fact that an increased part of the structures were submerged with consequently a reduced airgap.
The field lay-out of the Ekofisk Complex is presented in Fig. 1.
The topsides of all the jacket structures and all the bridges were raised 6 m in a jacking operation in summer 1987.
However the design and lay-out of the concrete 2/4 Tank (see Fig. 2) including the two main decks at +90m and +100m above seabed level with all the modules and processing equipment did not allow for a jacking operation. As subsidence of this magnitude was not accomodated within the original design Phillips Petroleum Company Norway(PPCoN) and their partners investigated alternative solutions to protect the 2/4 Tank topsides and finally selected a Protective Barrier structure forming a complete ring around the existing 2/4 Tank. This is shown in Fig. 3. In February 1988 Peconor Ekofisk A/F was awarded the contract for the detailed design, procurement construction and installation of the Protective Barrier. Peconor Ekofisk A/F is a joint-venture of three norwegian and two dutch companies. In Refs(l) and (2) a review is given of detailed design aspects of the structure and its foundation, whereas in Ref(3) information on hydrodynamic analysis and model testing programmes being part of the detailed design work is presented. A general review of the marine design work is outlined in Ref(4).