In the mid 1990s our client realized that their recent offshore find would yield a viable Liquefied Petroleum Gas content. With the proposed field located over 20 Nautical Miles offshore from an underdeveloped area, the option of exploitation on site was identified. Oil Floating Storage/Offloading units, usually adaptations of existing oil tankers, were at that time already an established alternative to piping product ashore. Therefore tenders were invited for the provision of an LPG Floating Storage/Offloading unit. Any party tendering for this task would be covering new ground as this installation would be among the first of its kind. In particular, the physical properties of LPG demand different handling systems and techniques when compared to oil.
In tendering we drew from the diverse marine enterprises within our group. We also engaged a partner specialized in offshore solutions. We had access to existing LPG tonnage of suitable capacity and our own yard for conversion work. We also had a competent technical support structure and potential within our personnel base for operational requirements. As with any new project, once tender requirements were met and accepted, evolution occurred. This evolution is of course not limited to the period leading up to commissioning but has continued through to the present.
In the course of describing the project the following prominent items will be covered:
Performance specifications and environmental factors.
Conversion, installation and commissioning process.
Upgrades and maintenance challenges.
Specialized aspects of mooring and cargo handling systems.
Classification and personnel.
Cargo export and operational record.
Management of safety.
It is often the case that delivery of a specialized service holds a narrow spectrum of interest. However it is our hope that this project, although relatively specialized, may also have value for any concern engaged in offshore storage and offloading. By this time we have operated the unit for 13 years and can share some valuable experience.
The field's position in the tropical portion of the West African Continental shelf promised a favorable climate. The main environmental concern during cargo transfer to export vessels was therefore exposure to open ocean swell conditions. The common export method for oil FSO installations at exposed locations is by tandem transfer. In this familiar arrangement the export vessel attaches astern of the FSO and picks up a floating hose to connect to her cargo manifold. This avoids any contact damage between the FSO and export vessel, which individually ride to the prevailing environmental conditions.
