A Discussion of Technical Challenges and Operational Limits for Towing Operations

Abstract

This thesis is written on the subject of marine towing operations. The purpose of the thesis is to identify and discuss technical challenges related to towing operations with respect to safety and operability. In addition, a simplified model used to estimate towline tension is created to establish operational limits when towing a semi-submersible drilling rig in the North Sea.

Towing operations are the most common marine operation today. They range from transportation of self-floating objects and large structures to subsurface tows of long pipes and bundles. The frequency and diversity of these operations brings several challenges as no operation is alike i.e. they are non-routine. As such, towing operations are accident prone. Statistics show that the transportation phase of marine operationsmake up for 26% of the registered accidents.

Almost all marine operations, towing operations included, are at the mercy of the weather. Uncertainties in weather forecast are accounted for via the alpha-factor which is dependent on location, planned operation time and operational limit. A method to estimate operability has been described and hindcast weather data is used to determine the operability at Heidrun in the North Sea. The results show that time needed to wait on weather is considerably lower for the summer months compared to the winter months.

Findings indicate that dynamics are important for the towline tension with large incoming waves, and should be well understood to avoid overload and failure. A base case has been developed and the operational limit, given by significant wave height, determined to be equal to 5,7m. Results from operability calculations give a probability for weather window of 47% for the base case. Variation of operational parameters have shown potential in improving the operability. The most promising way of improving operability seems to be a combination of increasing the operational limit and decreasing the operational period. This has the potential of leading to massive cost savings for the marine operation as waiting on weather can be expensive.