Criteria for Analysis of Offloading Operation
Authors: Morandini Cedric , Legerstee Franck , Mombaerts Johan 
Published: October 2008

As the operators want more and more to optimize the offloading operations in terms of withstanding rougher and/or crossed weather conditions in order to minimize downtime, the determination of specific criteria associated with numerical tools and proper model test, may help them in extending the operating limits for such operations.

The purpose of the paper is to help the designer by listing and presenting the specific criteria to be used when designing offloading operations and the tentative ways to assess such criteria using state-of-the-art time domain mooring analysis software tools.

Typical systems to be analyzed include:CALM Buoy + FSO + Shuttle tankerTurret system FPSO + Shuttle tankerSpread-Moored FPSO + Shuttle tankerCompliant-Moored FPSO + Shuttle tanker

The different criteria specific to assess offloading operations, presented in this paper, are as follow: Relative headings & fish tailing Hawser azimuth relatively to vessel heading. Hawser tension and their occurrence.

Introduction

Offloading operations are an important part of the whole FPSO concept and are the key element of the success encountered by FPSO, as offloading tankers are almost available anywhere.

Operators want more and more to optimise such operations, not only from a point of view of withstanding rougher weather conditions, but as well limiting the use of external elements (such as tugs, etc) and/or increasing the availability for such operations.

Meanwhile, guidelines to help the engineer in designing offloading operations are less available than the ones for designing permanent mooring system; As a matter of fact., these latter generally do not fully embrace the offloading operation analysis. Furthermore, in some cases the offloading operations are the designing cases for the mooring system of the main unit.

The settings of offloading operations can involve from 2 floaters to 4 floaters in the following configurations :FPSO + offload tankerFPSO + offload tanker + tug(s)CALM Buoy + FSO + offload tankerCALM Buoy + FSO + offload tanker + tug(s)

The mooring system connecting the F(P)SO to the seabed can be either in spread or single point or compliant configurations. Generally the floaters are connected to each other by a series of hawsers, working either independently or by couple. The offloading operation can take place either in a tandem mode (connection between stern and bow of the floaters) or in a couple mode (offtake tanker alongside with F(P)SO).

This paper is a tentative way to propose guidelines and criteria to design properly the offloading operations by taking into account their specificities.

Specificities of multiple-bodies system analysis

A methodology for time domain analysis of mooring systems, the "quasi-dynamic analysis", has been developed, and presented in [2]& [4]. Since then, this methodology has been expanded to address the analysis of offloading operation [3], that were initially not covered.

In addition to the methodology presented and proposed in [1]&[2], specific requirements are to be met for the description of offloading operation.