This paper addresses the main drivers and issues related to offloading buoy systems in deep and ultradeep water. Implicitely the following topics are thus discussed:

  • Maintaining a simple, safe and basic offloading operation for conventional tankers of opportunity;

  • Location of offloading system relative production facility;

  • Mooring system configuration, behaviour and fatigue loading;

  • Operational limits for tanker connected condition;

  • Export riser configuration, behavior and fatigue loading;

  • Pressure drop and booster pump requirements;

  • Inspection and maintenance requirements;

APL is presently designing and fabricating the Offloading Buoy for the Total Dalia Project in 1360 m water depth offshore Angola. This paper will therefore have some basis in the design and verification work conducted for this surface buoy system. Main focus will however be on alternative submerged deepwater offloading systems.


New field developments tend to go into deeper and deeper water, where either the pipeline infrastructure for exporting the product does not exist (ref. West Africa) or the extension of existing infrastructure (ref. Gulf of Mexico) may be overly expensive. The alternative to pipelines will then be to export the product directly into export tankers via an offloading buoy system. Most of these offloading buoy systems are passively moored to the seabed by 6–9 mooring lines with two or more product offloading lines between the production facility and the offloading buoy.

The traditional offloading buoy system is the CALM Buoy System, which is widely used all over the world. However, there are only a handful of these systems presently in operation or in the final design/fabrication/installation stage for deep-water applications, and all of them are offshore West Africa. The experience of remote offloading systems are extensive for shallow and moderate water depths, the experience is very limited for deep and ultra-deep water.

Offshore developments in deep and ultra-deep waters initiate new issues with respect to offloading buoy systems, and the experience from shallow and moderate water depths can therefore not be transferred directly to the deepwater applications. The main issue is that the larger weight from the mooring and riser (offloading lines) system requires larger offloading buoys. This implies larger 1st and 2nd order buoy motions, which give significantly larger loads, especially fatigue loads, on the mooring and riser system. In addition, the increased buoy motions give increased hawser loads between the buoy and the export tanker, which again means that the operational limits needs to be lowered in order to obtain hawser loads that are acceptable to the bow chain stoppers on the export tankers.

This paper addresses some of the design challenges with offloading buoy systems in deep and ultradeep water. It also presents an alternative offloading system where some of the major buoy motion issues have been reduced by utilizing a submerged buoy instead of the traditional surface buoy.

Offloading Systems

Oil offloading systems can be divided into two main categories:

  • Tandem offloading systems

  • Remote offloading systems

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