A techno-economic model of ship operation with special reference to hull and propeller maintenance in the face of uncertainty
A description is given of a new computer based techno-economic model designed with particular reference to investments in improved hull and propeller maintenance. The model combines the principles of accountancy with technical and operational variables so as to facilitate an operational simulation of most ship types in a selected economic environment. The technical and engineering economic basis for the proposed new model is discussed with particular emphasis on the relationship between hull surface roughness and ship resistance, the effects of hull roughness and fouling upon propulsion efficiency and quantitative measurements of hull roughness and fouling experienced on ships in service. Results from a set of full scale experiments on two sisterships are also presented in support of a proposed modification to an existing approximate relationship between roughness and ship resistance. The new techno-economic model is sub divided into three principal parts, based respectively upon deterministic analysis, dynamic programming and probabilistic cash flow simulation. Each part serves a different function in the decision making process between alternative hull and propeller maintenance strategies. A new technique is presented for obtaining probability distribution functions of individual variables associated with uncertainty when only a limited amount of subjective information is available. This new method serves as a basis for the proposed probabilistic cash flow simulation model, having the primary function of providing quantitative assessments of uncertainty in investment calculation. The initial requirement for considering the hull maintenance problem within the complete commercial context of ship operation has been confirmed in a series of case studies for different ship types where principal variables and recommended maintenance strategies have also been identified. Quantitative assessments of uncertainty are provided, indicating a potential high degree of uncertainty associated with this type of investment. A separate case study on the hydrodynamic and economic penalties of propeller roughness has established the relative difference between hull roughness and propeller roughness in economic terms. Finally, the deterministic case study evaluations have resulted in the introduction of two simplified methods of calculation from which approximate solutions to alternative hull maintenance strategies may be obtained.