Improved calculation method for the workability during pipe laying: Composition of a standard format

Improved calculation method for the workability during pipe laying: Composition of a standard format

Kooy, M.

Ligteringen, H. (mentor)
Meek, J. (mentor)
Pinkster, J.A. (mentor)
Steenhuis, A.L.J. (mentor)

Civil Engineering and Geosciences

2004-06-24

To support cost calculations for tendering a pipe lay project, the influence of weather conditions has to be incorporated by applying workability predictions for the pipe lay vessel. When assuming constant good weather conditions it is possible to accurately determine the duration of an offshore pipe lay project. In practice however, changing weather influences have to be incorporated to determine the actual total project duration. The objective of this study is to find a new calculation method that is more accurate and has a short calculation time. First the existing method to predict the workability during pipe laying has been analysed and shortcomings have been identified. Historical project data for the dynamically positioned pipe lay vessel Solitaire has been used. Four areas for improvements of the existing method have been identified: - Wave Prediction: Extra deviation factor - Criterion Limits: Incorporation wave period - Wave Heading: Chosen on solid base - Abandonment and Recovery (A&R): Dependent on project location The first aspect of interest is the wave prediction. Currently used wave measurements are satellite data combined with buoy measurements. A scatter diagram represents the occurring sea states for a certain location, as combinations of the significant wave height related to the zero crossing period. This is at the moment the best available representation used for wave predictions. In this study a program has been written to determine the standard deviation of the workability prediction caused by variation in the weather conditions. As input for the composed program the limiting sea states for the vessel, project duration and scatter diagram are used. Logically, the project duration has a direct relation to the deviation. Five criteria, (i) pipe strain, (ii) stinger tip clearance, (iii) stinger forces, (iv) tensioner dynamics and (v) dynamic positioning (DP) system limit the sea states for the workability. The vertical stinger tip motion is a good parameter to determine the workability for the criteria concerning the pipeline (i to iv). When the critical value for one of the criteria is reached, the accompanying vessel motion is determined. The vessel motions are known for the whole scatter diagram and compared with the critical motion to determine the workable limit. For the criterion (v) DP system, which can compensate displacements in the horizontal plane, the same program as for the existing method will be used. For the other criteria the new method incorporates the influence of the wave period, while for the existing method only the wave height is regarded. The third area is the wave heading. As mentioned before, the vertical stinger tip motion is a good parameter to give an indication of the influence of waves. For different wave angles the stinger tip motion is plotted. Two angles are chosen as representative values for the whole compass rose. The existing method treats the average and worst wave angle for the vessel. By using the new method more accurate results can be achieved based on solid assumptions. A&R is the lay down process of the pipeline because of bad weather conditions. Currently a fixed percentage of the production time is used to predict the duration of A&R. Not only the duration of bad weather but also the number of times a critical sea state is exceeded determines the duration for A&R. By using old project data a relation between the duration for A&R and the workability percentage has been found. For North Sea projects a relation with the probability of exceedence of the significant wave height is found.

workability
pipe laying
wave prediction
wave heading
criterion limits

http://resolver.tudelft.nl/uuid:9577409f-47d4-44e3-8e33-c851c35b4e1b

TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering

Student theses

master thesis

(c) 2004 M. Kooy