The motions and mooring loads of storage/production tankers are dominated by large amplitude. low frequency components related to wave drift forces. In the design stage use is often made of time domain simulation and/or model tests to determine those quantities. In view of the relatively long periods associated with low frequency motions and forces long simulation or model test times are required in order to reduce statistical variance of the results.

In this paper a theoretical expression is given for the statistical variance of these quantities for the case of a linearly moored tanker in irregular head seas. This expression is compared with results of model tests and time domain simulations. For a non-linearly moored vessel model tests and time domain simulations are used to quantify the statistical variance. The results indicate that the main elements governing the statistical variance of the data are the natural period of the moored vessel. the system damping and the simulation/model test duration.


The dramatic fall in the oil prices and the resultant need to reduce oil production costs has resulted in an increase in the interest for the application of floaters as low cost floating production/ storage systems. Such units, both tanker based and semi-submersible based have been in operation for several years, see ref. [1]. [2] and [3]. The experience gained through the operation of these units although not always positive in all respects (see ref. [4]) has strengthened the conviction that taking into account in a proper manner all those aspects which have a significant bearing on the system performance at the design stage reliable, cost effective systems can be designed and built.

One of the major components in the design of floating production systems is the mooring system by means of which the unit is kept on station.

The mooring system can be a critical factor in the design in those locations where environmental conditions lead to high moooring loads. Such conditions are met, for instance, in the northern North Sea and east of Canada.

In general, mooring systems are designed based on two major load cases. i. e. extreme loads associated with extreme environmental conditions and fatigue loads. In all cases attention is mainly focussed on the dynamic parts of the mooring loads.

For most offshore locations, wave loads are the major source of the dynamic loads in the mooring system. In particular, the large low frequency horizontal motions and mooring loads associated with low frequency second order wave drift forces form a dominant part of the system behaviour and mooring loads under extreme wave condition. The origins and characteristics of second order wave drift forces in irregular waves have been the subject of study for some time, see for instance ref. [5], [6] and [7].

Methods to compute wave drift forces have been developed which, for many practical cases can be used for an assessment of environmental loads in the preliminary design stage. The motion response of a moored vessel to low frequency drift forces has also been the subject of research.

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