The requirements analysis for a shallow water pipelay barge

Abstract

From shallow to deep water, from the well to the ship, from the offshore field to shore or between countries, many kilometres of offshore pipelines for transportation of hydrocarbons (oil and gas) have been installed over the last decades. Allseas’ Tog Mor is a shallow water pipelay barge capable of laying pipes up to a water depth of 25 meters. Currently, Tog Mor is more than forty years old and is due for replacement. Therefore, Allseas has considered to replace Tog Mor with a new shallow water pipelay barge. However, it is unclear whether the current barge is optimised for pipelaying since it is designed for crane supported purposes. The aim of this research is to give insight into vessel properties which determine the pipelaying performances and to define a set of design requirements for the new shallow water pipelay barge, Tog Mor 2.0. To define the requirements of Tog Mor 2.0, the principles of Systems Engineering are followed. Based on the operational analysis, the performances of Tog Mor can be improved by a reduction of the theoretical peak cycle time, optimisations of the pipelaying process and a reduction of downtime. The output of the operational analysis in combination with the principles of pipelaying results in potential bottlenecks. Those performances and potential bottlenecks were the input of the functional definition of Systems Engineering. The aim of this functional definition is to allocate functions to systems and to prioritise the design focus point in terms of functions. Those functions were translated into hardware components and integrated in the total design. Due to the inversely proportional relationship of the number of production days and corresponding production costs, an increased number of stations and lay speed do not have an advantage in terms of costs. In order to reduce the theoretical peak cycle time, the specifications of the functional equipment are improved and the need of buoyancy modules is reduced. Less need of buoyancy modules is achieved by the increased capacity of the tensioner (two tensioners of 100 ton each) and an inner stinger. The pipelaying process is optimised by free deck space, a higher level of redundancy of the functional equipment and new technologies of equipment. The level of redundancy in combination with the improved workability results in a reduction of downtime. Those improvements results in a final concept design which proposes a barge with an off-centre firing-line of five stations (three welding stations, one testing & grit blasting station, one coating station). As a result of an improvement of each individual function and the interactions among those functions, an improvement of 30% in terms of theoretical minimum peak cycle is achieved by an optimisation of the pipelaying process and potential bottlenecks. A reduction of the peak cycle time, an optimised pipelaying process and a reduction of downtime percentages results in an improved overall performance of Tog Mor 2.0 compared to the current Tog Mor.