Page 569 - Marine Structural Design

Chapter 30 Risk Assessment.Applied to Offshore Structures 545

30.6 Case Study - Risk Assessment of Floating Production Systems

30.6.1 General
A risk assessment may be conducted as part of the offshore field development and includes the
following,
All critical elements are appropriately selected and the corresponding performance
standards are adequately defined for the life cycle of the FPS in terms of its functionality,
availability, structural integrity, survivability, dependency and influence on the other
critical elements. It should e demonstrated that the critical elements fit for purpose and
meet the performance standards.
Risk acceptance criteria are defined prior to the execution of risk assessment, and to
provide a level of safety that is equivalent to that defined in the prescriptive rules and
codes.
All hazards with a potential to cause a major incident have been identified, their risks are
evaluated and measures have been taken (or will be taken) to reduce the risk to the level
that complies with the risk acceptance criteria.
Type of risks for FPS depends on the type of vessel used and the geographical region it is sited.
FPSOs used in the North Sea are mainly new vessels with turret system. The offloading
tankers come to empty the storage tanks at frequency (approximately) once per week. The
offloading tankers may represent a collision hazard to the FPSO with medium frequency and
potentially high consequence. So far FPSOs in the west Afirica offshore are mainly based on
spread mooring system and a single point mooring for oil export. FPSOs used in other
geographical regions are mainly based on converted tankers.
In the following, an FPSO Floating Production Storage and Offloading) for the Gulf of
Mexico is chosen as an example to illustrate methods of risk assessment. The methods
illustrated in this section may also be applied to other types of floating production systems
such as TLPs, Spars and semi-submersibles.
A risk assessment of FPSO may include evaluation of the following systems:
Process Systems
The process systems include, e.g.:

e Process plant with three-stage separation, gas compression for export and gas turbine-
driven power generation on deck
piping, pressure vessels in production and storage facilities
cargo tanks and crude pumping systems, offloading systems and its operation
Process risk is mainly initiated by loss of hydrocarbons containment that might escalate to
explosion and fire accidents. The risk assessment of process systems may be conducted using
a conventional offshore QRA approach (Wolford, 2001),
Development of isolatable sections
Summarize the loss of containment frequency by using a parts count approach
Identifylng spatial interactions that could lead to escalation