Numerical Analysis of Anchor Handling and Fish Trawling Operations in a Safety Perspective

Abstract

A vessel, with slender structures (like mooring line or wire) hanging from the sides or at the stern, handles or transfer objects from one place to another. This is a typical scenario in marine operations. Anchor handling operation (AHO) and _sh trawling operation (FTO) are two typical examples of such a scenario. In this scenario, the interactions between the vessel, the slender structure, the object being handled or transferred and other subsea infrastructure are important. Such interactions could be hazardous and lead to accidents. How to reduce the associated risks is a demanding issue. However, marine operations rely usually on experience. Only limited literature is found for the two aforementioned operations. Therefore, it is a relatively new and challenging area for scientific research. The main purpose of this thesis is to study the typical hazards revealed by previous accidents in AHO and FTO by performing numerical analysis, and to provide more insights into these operations in a safety perspective. This thesis consists of two parts. The first part deals with the safety of anchor handling vessel (AHV) during AHO, where the emphasis is on the interaction between vessel and mooring line. The second part deals with a special hazard associated with trawl board-pipeline interaction, the hooking event in FTO, with a focus on the interaction between trawl board and pipeline. Anchor handling operation is considered to be one of the most potentially hazardous and demanding marine operations in the offshore industry. The heavy mooring line hanging at the stern of an AHV represents a risk factor for the vessel because the line introduces extra loads and acts as a constraint. In this thesis, the excessive drift from the planned anchor track observed in the accident of the Bourbon Dolphin AHV was considered to be an initiating event, suggesting that the positioning capability of the vessel was insufficient. Therefore, the thrust utilisation plot concept was proposed to illustrate the positioning capability of an AHV during AHO, in which the mooring line load was considered in addition to the environ- mental loads. The method provides a clear and straightforward criterion for selecting vessels and establishing operational limits. To study the behaviour of an AHV during AHO under different environmental conditions, a dynamic model was also developed. The model inherently includes the effect of the mooring line on the vessel, i.e., the initial heel and trim angles. These angles are directly related to the safety of the vessel and are therefore important to consider. By using the proposed model, more realistic vessel behaviour can be captured. The model provides a good basis for developing the operational criteria to keep the risk at an acceptable level for AHVs. In fish trawling operation, trawl board hooking at pipeline is a hazardous event. The trawl board hooking load is an important design factor of pipelines. However, there is no existing method to quantify the hooking probability in bottom-trawling operations. In this thesis, an approach was proposed to quantify the trawl board hooking probability using numerical simulation tools and statistical data. An important aspect of this approach is the numerical model capable of simulating trawl board hydrodynamics and trawl-pipe interaction. The model was first established to study the pull-over and hooking events and validated against model tests. Moreover, based on the observations from previous model tests, simplified hooking criteria were proposed. The criteria link the pipeline data to the fishing activities data, enabling the quantification of hooking probability. Sensitivity studies were carried out to identify the critical parameters in hooking events, thus providing more insights into the governing physical effects and means to mitigate the associated risk.