Drop the anchor! Sounds simple, right? On a mobile ship or platform, the moorings aren’t simple but they are less complex than permanently moored assets. Every time a mobile vessel changes locations they pull in the mooring lines and can more easily inspect or change out components. A permanently moored Floating Production Unit has a complex mooring system with many constraints. A wide variety of factors must be considered to try to predict the future and account for degradation of the system.
A mooring system’s main function is to maintain positioning thereby protecting the risers and umbilicals. No matter the sea state, the system must keep the installation close enough to its equilibrium so that the strain on the risers does not exceed its design. In shallower water the risers have less slack which means a smaller excursion radius is required.
Weather and Environmental Loading
Where is the wind coming from? How about the waves and the current? Are they consistent or will they shift regularly? Perhaps consider spread moored vs. weathervaning to reduce loading. A mooring system must hold up to not just the operational sea state but also a 10-, 100-, or even 1000-year storm depending on the life of the field. These are when the loadings are greatest. But it is a balance: having high stiffness in the system may lead to operational fatigue where low stiffness may lead to higher tensions in storms or snap loading of leeward lines
Ultimate, Accidental, and Fatigue Limit States
These industry standard recommended practices are the simulation tests of a system’s design. Ultimate Limit State is a test that simulates a 100- or 1,000-year storm with all lines intact and a safety factor of at least 1.67. Accidental Limit State is a similar test, but includes the loss of 1 line or thrusters. Fatigue limit State simulates a range of loadings and considers the most probable real-world scenarios as well as incorporating safety factors. The amount of scenarios considered will impact cost of the system.
It has been widely accepted that having uninspectable areas in a mooring system is not desirable. Keeping components easily accessible for inspection means that changeouts may not have to occur at the end of design life. If you cannot inspect a component it must be assumed to have no residual capacity at the end of its intended design life, and you won’t know if it is degrading faster than designed. This can lead to serious issues including mooring failure and loss of production. Furthermore, if the field life is extended, replacement of uninspectable components is an unavoidable cost.
This is an obvious consideration that impacts more than just the length of the lines. The deeper the system the heavier the risers and mooring lines will be. Can the vessel support the weight of these systems? Perhaps consider fibre moorings which are close to neutrally buoyant and do not contribute to the restoring force of the system.
What type of risers will be used? Flexible or steel catenary risers will have a big impact on the allowable excursions and therefore the design constraints. Asset motions will impact fatigue of the risers and must be incorporated into the design.
Newbuild vs. Conversion
If converting a single-hull tanker to an FPSO, it is quite costly to install an internal turret whereas going with an external turret or spread moored option will typically be much cheaper.
Life of Field
The life of the field is a major consideration. Sea state and degradation are easier to account for over a 7-year period vs. a 30-year period. It is possible that a return period of less than 100 years can be used, or even that a reduced corrosion allowance or fatigue life is warranted. This will bring the cost of the system down; however, possible tie backs or enhanced production might lengthen the required design life, which may require costly life extensions or component changeouts in the future.
Generally the budget will set what is possible in terms of safety factor and flexibility of design. The design process is a difficult balance between meeting all the above considerations while still being within budget.
The design process is not linear. Assumptions must be made initially and then simulated, altered, and so on. Each iteration will bring the design closer to completion.
Recommended practices are updated by drawing new lessons from real world failures. Regulations and design theory is typically guided by what has happened in the field. This means industry is continually getting better at mooring system design, but it is a slow process. Sharing lessons learned through JIP’s and research groups will help accelerate the industry’s learning and improve design sooner rather than later.
The considerations detailed above are not exhaustive, so please share you experience with mooring design below and stay tuned for the next post in this series as we delve deeper into the different types of mooring systems.