PETROBRAS has developed a new mooring system for production and storage tankers moored in offshore Brazil. The system is denoted DICAS (Differentiated Compliance Anchoring System), and is basically a spread mooring system with different stiffness at the bow and stem of the ship. This difference in stiffness allows the ship to weather vane partially under environmental conditions. Due to the characteristics of this system, the design has to take into account the varying direction of the weather and the best layout of the production risers and mooring lines. Since DICAS is a partial weather vaning system, it makes the production swivels and turrets unnecessary. Therefore, the cost for production systems based on existing tankers will be drastically reduced, so it shall be very useful for low production and marginal fields in mild environments. This paper presents some results from experiments and simulations, and discussions on the design criteria for such systems. Important factors to be included in the design are the damping coefficients, namely the wave drift damping, the mooring line damping and the linear and quadratic viscous damping. The effect of interaction between current and waves is also important.
Neste trabalho é apresentada uma metodologia para o planejamento da operação de instalação de âncoras de equipamentos offshore, navios e plataformas, visando a otimização dos recursos necessários, embarcações de apoio e cabos de trabalho, por meio da minimização de uma função objetivo com base em múltiplos critérios. Como ganho adicional, inerente à metodologia proposta, consegue-se a automação do processo de planejamento de instalação, que nos moldes tradicionais é feito na base da tentativa e erro, onde o planejador, utilizando algum aplicativo para o cálculo de ancoragem, decide quanto de recursos deve ser aplicado ao problema tentando fazer com que a âncora atinja o alvo pré-definido no projeto do sistema de ancoragem.
In this work is explored a planning methodology for deep water anchor lines deployment in offshore platforms and floating production systems aiming operational resources optimization, by minimizing a multi criteria objective function. As an additional advantage, inherited from the proposed methodology, the planning automation is achieved. The planning automation overcomes the traditional way to do in a trial error basis, where an engineer, using a anchoring software, decides how much of work wire and anchoring line must be paid out from both the floating system and the supply vessel and additionally which horizontal force must be applied to the line trying settle the anchor on a previously defined target in the ocean floor
For mooring chains of offshore floating production units, API (American Petroleum Institute) recommends the use of its TxN fatigue curve considering the MBL (Minimum Breaking Load) of an ORQ (Oil Rig Quality) chain even if the chain has a higher grade. This curve has been used in mooring system design of offshore floating production units since the draft edition of API Recommended Practice for Design, Analysis and Maintenance of Catenary Mooring for Floating Production Systems in May 89 and several fatigue tests have been done by petroleum industries, chain manufacturers and research centers. Those fatigue tests show that the use of the MBL of an ORQ chain for higher grades is a conservative assumption. This paper will present an overview of the fatigue curves of materials for mooring lines: stud and studless chains, steel wire ropes and polyester fiber ropes. This overview is based on recent tests, rules and published papers.
This paper describes the experience of Petrobras in the design, installation and operation of deepwater mooring systems incorporating high efficiency polyester fiber ropes. These ropes have been used now for more than 12 years in catenary and taut configurations to moor drilling and production units as well as storage and offloading facilities. The paper provides a history of the development of this technology, highlighting the key issues involved in introducing in mooring systems a component that has non-linear visco-elastic behavior. Differences to all steel systems in terms of: behavior, failure modes and design criteria and procedures are discussed. The paper presents the life cycle management strategy adopted by Petrobras in the early years of the use of this technology and its evolution that have guaranteed that the only failure mode observed over all these years was mechanical damage. It is expected that this paper shows that the use of polyester (polyethylene terephthalate) ropes in deepwater moorings is a mature technology.
Model test verification of floater systems in ultra-deep water meets limitations when it comes to available laboratory sizes. Systems in depths beyond 1000–1500 m cannot be tested at reasonable scales without the truncation of the mooring and riser system. The development of methods and procedures to overcome this problem has been addressed through extensive research programs at MARINTEK (VERIDEEP, VERIDEEP Extension, NDP, DEMO2000). This led to a hybrid verification procedure which combines reasonable truncation principles, model tests of the truncated system, and numerical simulations, to estimate the system’s response at full depth. There is, however, still a need to address the actual influence from the truncation procedure, and from the integration with simulations, on the final extrapolated full depth results. This paper presents a case study for the validation of the procedure, that compares full depth model test results of a semisubmersible in water depth 1250m against the extrapolated full depth results obtained from a truncated system of 500m. Results are presented for line tension and vessel responses in 3 seastates. In general the extrapolated full depth results were found to be in good agreement with the full depth model tests. However, the results confirmed expectation that the low frequency response has the greater uncertainties and presents the greatest challenge for the procedure.
Definition of air gap is an extremely important issue in the design of floating offshore systems such as semi-submersible or TLP platforms. For these systems, any unnecessary increase in the static value of air gap generally demands the payload to be decreased or leads to a larger buoyant hull, which, in any case, has a negative effect on the project economics. Designers face a difficult challenge since there is no well-established methodology for predicting the air gap demand in the early stages of the design. This is a consequence of the inherent complexity involved in the problem of predicting the free-surface elevation around large structures in steep-waves, such as the largest wave expected during a design storm-sea spectrum. Non-linear diffraction models are usually called for a more consistent evaluation of the wave field under the deck and the wave run-up upon the columns, but even second-order analysis is not free of uncertainties. Therefore, air gap evaluation still relies heavily on experimental analysis. This paper presents some towing-tank results performed for the evaluation of the dynamic air gap of a large-volume semi-submersible platform. Regular wave tests were performed for the small-scale model in both restrained and moored configurations and results were confronted with numerical predictions. Air gap response at different locations of the hull was evaluated under three different sea states and results were compared to some semi-analytical models proposed in literature for preliminary air gap estimation. The role of dynamic coupling provided by a taut-leg mooring system on the air gap results is also discussed based on the experimental results.
With the introduction of the polyester ropes as mooring lines of large systems such as semi-submersibles, the need to simulate these lines in model tests became a necessity. Although the non-linear behavior is clear, depending on the type of cycling, the polyester rope responds in ways that may be considered linear as a steel wire rope. Because of that, the early model tests have been performed using a linear restoring capability, with different restoring coefficients. The use of equivalent springs seemed the proper way. However, with the help of fundamental investigation on the similarity laws, the present work shows that the use of very thin polyester lines in model scaling is feasible and will indeed allow a closer physical representation. By avoid using springs, but using the same material as in full scale, the same non-linear behavior is present during the tests and even the response to random excitation due to random waves is better represented. The paper closely describes the application of these ideas in a model test of a FPSO (Floating Production Storage and Offloading) comparing both the linear springs and new approach with the model scale equivalent polyester line.
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