Bey Edwards
Oceaneering Production Systems
Houston
A multipoint conventional buoy mooring, often called a Mediterranean moor, provides a low-cost solution for loading tankers from the new onshore Isla de Bioko terminal in Equatorial Guinea, West Africa.
This mooring method is a basic no frills terminal system, suitable for protected areas with moderate sea conditions.
The system will be used for transporting condensate produced in the offshore Alba field. Walter International, a Houston-based independent producer, together with its partners, is developing this gas condensate field in about 250 ft of water, some IS miles off the coast.
The high gravity, 47-48 API, Alba field gas condensate is piped to a shore-based tank farm for processing and storage.
Current production from the field is about 2,200 b/d and may double when an additional well is drilled in 1992. The tank battery has a 300,000 bbl capacity.
LOADING FACILITY
In April 1991, Walter International commissioned Oceaneering Production Systems to design and construct the tanker-loading facility.
The mooring system consists of a spread of four large, 8 x 15 ft, mooring buoys that are anchored to the ocean floor with 3-in. chain and 30,000-lb Moorfast anchors (Figs. 1 and 2).
Construction was basically completed by late 1991. Final loading hose connections were made in January.
MOORING PROCEDURE
The mooring system is located in 125 ft of water, about 2,500 ft from the coastline.
The system will accommodate vessels up to about 45,000 dwt.
For mooring, tankers drop their bow anchors that dig into the ocean floor. Then, a line-handling vessel attaches four stern anchor lines to quick disconnect pelican hooks on the mooring buoys.
Oil is loaded through twin 16-in. diameter hoses connecting the pipeline end manifold (PLEM) on the ocean floor to the tanker rail.
Navigation buoys mark the location of the 10-in. gas/condensate pipeline that brings the raw product to the tank farm and the 24-in. finished-product loading pipeline and end manifold on the seafloor.
DESIGN AND CONSTRUCTION
Oceaneering retained Aker Omega Marine of Houston as design consultants for the project. Drawings and specifications were prepared in April and May. The complete mooring and loading facility was designed and built for less than $2 million.
All of the components for the facility were fabricated in shops in Texas and Louisiana.
The pipeline end manifold was fabricated in May by Victoria Machine in Victoria, Tex. The fabrication involved a round-the-clock welding effort.
Anchor buoys and navigation buoys were built at the same time by Universal Fabricators in New Iberia, La.
Anchors and chains, meeting American Bureau of Shipping specifications, were supplied from stocks in Morgan City, La.
The buoys were fabricated in about 1 month and in June all of the equipment was shipped by ocean freight to West Africa.
Installation of the spread mooring system went smoothly. Anchors, chain, and mooring buoys were assembled on the dock at Malabo, a port city on Bioko island, and loaded onto a large anchor-handling tug.
Satellite positioning systems on the tug accurately located each anchor-chain-buoy package before the package was lowered over the stern rollers of the tug. Anchor chains were pulled to allow deep embedment of the 30,000 lb Moorfast anchors into the seafloor. The whole operation took about 1 week.
SYSTEM LIFE
Mediterranean mooring is easy to install and easy to relocate after a field is depleted. This is particularly, attractive for producing isolated, marginal, or short-life fields. In the case of the Alba field, the reserve life is estimated to be over 20 years. The field has sizable estimated reserves of over 500 bcf of gas and 20 million bbl of condensate.
Currently, the gas is being flared, but either a methanol or urea plant may be built in the future. The mooring system is expected to handle possible future methanol shipments.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.
