Float-Overs Offshore West Africa
Authors: Tribout C. , Emery D. , Weber P. , Kaper R. 
Published: October 2008

Major oil and gas field developments in remote areas on the continental shelf with less benign environmental conditions require an efficient method of delivery and installation of the production topsides. West Africa is one of those areas with its severe swell conditions and location far away from the major fabrication yards.

Technip has developed a special technology, called the UNIDECK??concept, which enables topsides float-over installation in these swell conditions, even during the normally restricted season for installation from October to April each year.

This paper gives an overview of the technical aspects involved in combining the UNIDECK??system with a transport vessel for the transportation and subsequent installation of two recent float-over deck installation operations offshore Nigeria.

Introduction: The UNIDECK??concept

Due to the long swell period conditions in West Africa, conventional float-over by ballasting is too slow and thus, not advisable because it causes excessive impacts between the topside and the jacket. Technip has therefore developed a special technology, called the UNIDECK??concept, which enables a very short installation time in these swell conditions (typically Hs=1.5 m in period of 10 seconds or Hs=1.2 m in period of 14 seconds), thereby ensuring a safe installation operation and limiting the risk of weather downtime. The method is applicable to three main categories:in shallow water where there is not sufficient draft for derrick barge to enter the site,when heavy lift derrick barges are not readily available resulting in prohibitive mobilization cost,when an integrated deck would be too heavy for the available derrick barge.

The technology combines ballasting and jacking to improve the stability of the heavy transport vessel during the transportation phase and uses jacking to provide a quick, and therefore safe, transfer of the integrated deck weight onto the pre-installed jacket in order to avoid high dynamic impact loads.

As the topsides are fully completed and tested onshore, this concept also enables significant cost savings compared to a multi-module lifted topsides in transportation and installation by minimizing the installation time, offshore hook-up and by eliminating the need for heavy lift derrick barge. The method also integrates the operations of weighing, load out and if necessary, topside supports during the construction phase.

This technology, for which Technip acts as an installation contractor, has been implemented in West Africa with the COB-P1 production platform (9,500 tonne), the Amenam Kpono AMP1 (11,000 tonne) and AMP2 (9,600 tonne) platforms for TOTAL and the East Area Project GN compression platform (18,000 tonne) for EXXONMOBIL. In each case, in order to minimize the cost of the operation, the design of the platform arrangement and the installation method have been adapted to the particular project in terms of layout, topside weight, environmental conditions and final elevation of the deck.

Detailed Engineering

The activities normally included at the engineering stage are:structural engineering,detailed engineering of the operation,vessel preparations engineering,specification of the installation equipment,transportation engineering,procedures for the installation operation,supervision/assistance during the operation.