The rigid arm or yoke type Single Buoy Mooring (SBM) system is replacing the conventional SBM system as the oilfield storage mooring. Since 1973, fourteen yoke type SBM systems have been ordered. The popularity of this type of-tanker mooring is accelerating. In 1977 alone, five new systems were ordered. To date, four types of yoke systems have been developed to the building stage.
Yoke moored tankers installed vary in size from 54,000.DWT to 167,000 DWT. Total tonnage for all systems combined in use and on order exceeds the 1.3 million DWT mark. Rigid arm moorings have been installed in water depths ranging from 140 feet to 400 feet, and maximum wave heights from moderate to as high as 52 feet. A design has been completed for the yoke mooring of a VLCC in the North Sea.
This paper describes and catalogues the various types of yoke SBM systems installed or under fabrication. The application, characteristics, and component buildup will be discussed. Operational properties and fields of application of yoke moored tankers will be reviewed. Some general design aspects will be mentioned.
The usage of SBM systems to provide the mooring of storage tankers at offshore oilfields started in the middle sixties. Basically two types of operational procedures have been used.
The permanent storage, where the storage vessel remains moored at all times. In this type of operation, the export function is performed by a separate unit (tanker/barge).
The filling station concept, where the storage and export function are combined in the same unit. Here, crude is produced into the storage vessel at the production rate of the field.
The true filling station concept does not apply in those cases where storage or a storage buffer is provided by other field facilities. In these cases, the export tanker has no actual storage function and loading is usually performed at rates far exceeding the field production capacity.
Tables (1) and (2) give an overview of the permanent storage and filling station concept installations. The authors have given as complete an overview as possible in these tables. Due to the scarcity of literature on the specific subject, an incidental installation might have been overlooked.
Initially, existing technology developed for transportation terminals was used to provide moorings for storage tankers. Soon it became clear that the storage function demanded characteristics quite different from those provided by the classical transportation terminal. In the next chapter, a synopsis of these differences will be given. The yoke mooring was developed to improve on the shortcomings of transportation terminals used as permanent storages at the oilfield. To date, four types of yoke moorings have been developed.