Wei Yan

Thesis PDF

Summary The most important characteristics of the outfitting processes in shipbuilding are:

  • The processes involve many interferences between yard and different subcontractors. In recent years, the use of outsourcing and subcontracting has become a widespread strategy of western shipyards. There exists not only the vertical relationship between a yard and a subcontractor but also the horizontal relationships between the subcontractors themselves.
  • They require timely and detailed engineering information. Outfitting performance depends largely on the quality, quantity, and timeliness of technical information supplied by engineering.
  • Much ambiguity and tacit knowledge exists especially in the outfitting processes. Planners quite often use rules of thumb abundantly to make important decisions and workers, who carry out daily outfitting assembly work, very largely rely on many years of experience.

All these make the planning of outfitting processes, which have not been sufficiently considered in practice, a great challenge to both shipyards and subcontractors. The study presented in this thesis aims to develop an automatic sequence generation method which is able to give the yard and subcontractors a realistic and reliable outfitting plan that identifies the relationships among outfitting activities, estimates a realistic mounting time, and displays it by means of animation. First an integral planning system for the outfitting processes is proposed. It has been divided into three interrelated steps: activity generation, sequence determination and schedule determination, trying to answer questions "What has to be accomplished?", "In what order will it be accomplished?", and "When will it be accomplished?" respectively. The first two steps comprise process planning and the last is scheduling. The focus of the research is to determine the assembly sequence of the outfitting components, answering the first two questions. Different assembly sequence determination methods, attempted in the mechanical industry, are described and the possibility of their application in shipbuilding outfitting processes is discussed. These methods include liaison diagrams, assembly sequence diagrams, and/or graphs and the binary matrix method. The interference matrix, part of the binary matrix method, is provisionally selected to detect the relationships between outfitting components because of its practical applicability to the shipbuilding situation. Next, the kind of geometry attributes of outfitting components that are important in the determination of their mounting orders is analyzed. After field observations and thorough consultation with workers, the decisive attributes of components--position, material, weight, size, penetration and minimum work distance-- have been chosen. The way to describe these attributes mathematically with a purpose of making them programmable is introduced. Weighting coefficients are used to quantify the relative impact of the components' geometry attributes on the mounting orders. They are calculated using the Analytical Hierarchy Process (AHP) method, applied to questionnaire results. Subsequently, data collection and preparation are reported. For our own laboratory purpose, the detailed engineering CAD model, generated by TRIBON, was chosen to be the original data source. A data preparation model is presented. It is necessary to reorganize the data in a proper format so that it can be read by the sequence generation model. Apart from this, it is also able to extract the necessary information. The assembly sequence generation model is developed starting from the requirement that the sequence should follow automatically from the CAD model of a particular section or compartment in the ship. All physical attributes of a component and their relative importance in deciding its mounting order were modeled, which resulted in the derivation of finish-start relationships between components. Two types of outputs of the model lead themselves to validation:

  • the numerical results for throughput time and individual mounting times;
  • the detailed assembly sequence visualized by Gantt charts or by 3D animation.

The validation study is described to investigate the correctness of the estimation of the mounting time of assembling each type of components, followed by application of the method for four test cases. The validation of the total mounting time for these four representative sections/compartments indicates, but does not prove, that the estimates for the mounting time are realistic. Also the validation work supports the conclusion that the generated sequence is realistic but not flawless. Discussion of the model's result shows that the methodology does not yet consider all technical and organizational aspects of outfitting processes at the same time. In fact, given all the complexities, it is gratifying that the method yields already useful assembly sequences that provide a basis for a better planning method. Nevertheless, before the developed method can be implemented on the worksite, additional effort will be necessary both in gathering validation data and improving the model (including the integration of steel structure). In conclusion, an automatic assembly sequence generation model for the planning of outfitting processes is developed in the thesis. It already allows generation of interference-free and integral assembly sequences including their throughput time. The behavior of the model will further improve and become even more realistic by implementation of all three dimensions of interference-detection, an improved equipment-mounting algorithm and the integration of steel structure information.