Design and operational evaluation of offloading operations for deep water FPSOs

Offshore offloading operations take place in many locations around the world. Presently a great number of new deepwater developments are under consideration, not only for the offloading of oil but also offloading of LNG from LNG FPSOs. Scenarios normally considered include:
a) Single-point-mooring (SPM) offloading from a CALM buoy or similar facility. The offloading tanker can freely weathervane and assume a heading of ‘least’ resistance relative to the prevailing weather. Apart from the existing SPM systems in shallow water (typical for terminals), they are nowadays planned as offloading facility in the neighbourhood of FPSOs for deep water fields.
b) Tandem offloading from a turret-moored floating production storage and offloading system (FPSO). The offloading tanker can still weathervane, but its behaviour is influenced by -and coupled to- that of the FPSO. Depending on their relative loading conditions and possible support by stern thrust or tug(s), they will assume headings with respect to the environment. Beside the passive mooring with a bow hawser, DP tandem offloading is used at a number of locations. Moreover, tandem offloading is seen as an option for LNG offloading from future LNG FPSOs.
c) Tandem offloading from a spread-moored FPSO. The export tanker is typically a vessel of convenience and assisted by one or more tugs. Such operations take place for instance offshore West Africa, where the prevailing environmental conditions are relatively benign compared to the North Sea, but often characterised by non-collinear swell, wind, local waves and current conditions. Due to the ‘fixed’ heading of the spread moored FPSO, the vessels do not both have the ability to assume freely a heading with respect to the environment. The behaviour of the offloading tanker is also heavily influenced by the presence of the FPSO (shielding, etc).
d) Side-by-side offloading from a turret-moored floating production storage and offloading system (FPSO). These operations in general take place at locations with a moderate wave climate. The mooring system, e.g.: lines and fenders, and the relative motions of both vessels become the critical factor. Advantages are that the combined system can rotate to a favourable heading and that existing loading arms can be used in the case of LNG offloading.

Offloading operations have a large impact on the design and operation of FPSOs, because:
• Possible weather downtime of the offloading operation effects the overall economic performance of the FPSO
• The choice of the offloading system effects the capital cost as well as the operational cost of the FPSO significantly
• Safe day-to-day operation requires clear procedures and trained personnel
• Finally offloading operations have a large safety impact, because by definition they involve the operation of two or more structures in close proximity.
The safety of personnel and the offshore structure, plus the possible environmental impact (pollution) have been of concern of regulating authorities as well as the industry itself. The subject of offloading for instance plays an important role in the investigations of the MMS related to FPSOs in the Gulf of Mexico.

Irrespective of the type of system selected, two stages can be distinguished in the offloading process:
• the approach and connection of the export tanker to the FPSO and the departure from the FPSO
• the offloading phase.
During the design process the evaluation of each phase requires its specific tools and methods. Both phases will be discussed in the present paper.