After exploring hydrocarbon deposits onshore and offshore for about 20 years, Soekor in 1984 embarked on a feasibility study to determine whether offshore gas deposits could be viably processed to produce fuel products. Two contractors were appointed, one to study the offshore development and the other the onshore development. At the outset there was concern about the offshore development, as this was a new venture that had never been undertaken in South Africa before. The establishment of a new industry in a country far from traditional areas of offshore developments and industries and with mandatory sanctions imposed by the United Nations made it a real challenge for the project managers and the client. Coping with these adverse conditions, South Africa had the courage to embark on this project, rallying the industry to set up a grass roots capability. This paper descriptionbes the three stages of the project from the feasibility study to final execution and handover to the client.
The detail design of the jacket for the Mossgas FA platform was simply based on good engineering. It encompassed many disciplines of engineering and naval architecture and was checked by independent audits by the certifying authorities. This paper descriptionbes the jacket and outlines the design procedures that were necessary to ensure its safe installation and the requirement lor a 30-year operational life.
The design of the Mossgas FA jacket for fatigue due to the aggressive wave conditions found off the Massel Bay coast proved to be one of the most complex of all the structural analyses. This paper descriptionbes the election of the fatigue wave loadings and the manner in which they were applied to the structure in order to assess the resulting stress ranges throughout the jacket. The effect of dynamic behaviour of the structure as well as local stress concentrations are considered and the method of assessing the fatigue life at any position on the structure is descriptionbed. The paper concludes that the use of the most up to date technology for this project has resulted in a fatigue-efficient structure that will be safe for the 30-year life of the platform.
The Mossgas FA gas and condensate platform is the first permanent offshore structure to be installed off the South African coast. Although the technology used for this platform was tested in such places as the North Sea, the lack of any field experience in South African waters posed particular problems for the offshore engineers involved. One such problem Was that of installing the tubular piles, which are the longest vertical piles ever installed on an offshore platform. This vertical piled concept, which was chosen over an angled or battered system for cost reasons, required the development of a special installation procedure. This paper descriptionbes the development and successful implementation of this innovative procedure, with very significant cost savings, for the client Mossgas.
The topsides structures for an offshore platform, although serving essentially the same purpose as in an onshore process facility, have to be modularized to facilitate onshore fabrication and subsequent movement of the relatively large packages for installation offshore in their final location. The movement of the substructures, the size and weight of each package and the availability of transportation vessels and equipment place demands and constraints on the design that make it different from conventional onshore process plant design.
With the trend in recent years towards unpropped cantilevered flare booms with high natural frequencies, it has become increasingly important to investigate the effect of short duration gust-induced vibrations on the fatigue life of the structure. In addition, the slender individual members are prone to vortex shedding and a trend towards smaller heat shields necessitates careful investigation of thermal effects on the structure.
The two export pipelines transporting the products from the offshore platform to the onshore refinery some 94 km away are required to operate in a variety of environmental conditions, ranging from severe offshore to moderate overland conditions. This paper highlights some of the design and installation methods employed to resolve the potential problem areas.
The construction of the Mossgas FA jacket heralds a new era in the technological development of the steel fabrication industry in this country. Never before has a structure of this magnitude and complexity been fabricated and transported on South African soil, and the success of this project bears testimony to the co-operation of all the parties involved. This paper provides an insight into some of the unique challenges tackled during the fabrication of the jacket, from component fabrication, through to erection and final load-out of the 14500 t structure onto the barge at Saldanha Bay.