Mooring System Optimisation for Floating Production Systems
Sebastiano Casole - Università degli Studi di Trieste - [1999]
The development of the offshore industry commenced with the use of fixed structures. As development accelerated with the discovery of oil and gas in deeper waters, the use of floating structures became commonplace. The pioneering nature of the search for hydrocarbons from beneath the ocean has resulted in many innovative engineering solutions. As confidence in the successful application of engineering solutions to the development of offshore oilfields has grown, the industry has required the solution of new problems such as:
·searching for oil in ever increasing water depths,
·producing oil in areas that do not have a pipeline infrastructure for exporting the oil,
·economic oil production from small ''marginal'' fields, which have field lives of only a few years (that do not justify the expense of a permanent platform),
·production very soon after discovery (oil exploration is expensive so oil companies can be more profitable by producing oil quickly in order to improve cash flow).
All these pressures have led to an increase in the number and diversity of floating structures used for oil field development throughout the 1980's and 90's. The primary concepts considered for floating oil production are Floating Production Storage and Offloading units (FPSOs), column stabilised semi-submersibles platforms, Tension Leg Platforms (TLPs) and Spars. All the above concepts, with the exception of TLPs, use catenary line mooring.
The design of mooring systems for these floating structures is a compromise between making the system compliant enough to avoid excessive forces due to wave, wind and current induced motions, and making the system stiff enough to elude damage to the riser system due to large offset. For moderate water depths and line lengths, the compliancy is obtained by deformation of the catenary shape. With increasing water depth the static compliancy of the catenary increases at a higher rate than the water depth. On the other hand, the velocity dependent drag force increases, so the low frequency dynamic motion is being reduced with increasing water depth. This makes it difficult to find a good compromise for deep water mooring systems. The relative importance of wave frequency motion compared to mean and slowly varying motions tends to decrease with increasing water depth. Another aspect in increasing the water depth is that the anchor chain becomes a less attractive mooring element, and the lighter steel wire rope becomes more interesting.
All these considerations make it easy to understand that the optimisation of a mooring system (i.e. the determination of the most reliable and cost effective solution) is cumbersome and time consuming, and, at the same time, a task of paramount importance. Optimisation algorithms and computer programs for finding minima of complex functions have been available for three decades. Applications in different fields related to marine technology have been widely published. Typically, several variables are involved, and the design solution has to satisfy a certain number of constraints in order to ensure the functionality and safety of the structure to be optimised. Both the objective function to be minimised and the constraints to be satisfied are often complicated functions of many design variables, so that a non computerised parameter variation is an huge task. Automated optimisation procedures were not feasible earlier, which is mainly due to the complex analyses involved in evaluating the dynamic responses of both the Floating Production System (FPS) and the mooring lines. The development of fast and efficient methods for frequency-domain mooring analysis (including the most important nonlinearities), together with the availability of more and more powerful computers, is improving the situation.
For this reason, a specific computational procedure has been developed combining efficient mooring analysis methods with a state-of-the-art standard optimisation program. The optimisation process regards the mass or the cost of the material of the mooring system. At present, the cost of the installation is not included. This is due to the complexity of modelling the installation operations. On the other hand, the variations in the line geometrical characteristic (lengths and diameters) should not have a significant impact on the installation costs. As a consequence, the evaluation of such costs can be performed ''a posteriori'' on various solutions coming from the optimisation process. Hence, the objective function to be minimised is the weight or the cost of the mooring system, while the constraints to be satisfied are safety factor requirements, maximum allowable offset, angle of attack on the anchors and several others.
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