Carbon Footprint Minimization for Deepwater Pipelay Construction

Abstract

With recent oil and gas discoveries in deepwater offshore, these regions have become the hotspots for oil and gas exploration. It is for this reason that major pipelay contractors are developing more advanced construction vessels with high lay tension capacity, payload and high specification dynamic positioning (DP) systems to operate at even deeper water depths. It is shown that at water depths of greater than 1000 m, one of the major construction costs is fuel consumption, which is directly related to the level of thrust and hold back tension the laybarge is required to maintain during pipelay operations. Furthermore, the fuel consumption and the resulting carbon footprint, is shown to increase disproportionally as the laybarge thrust increases at deeper water depths. For example, a deepwater laybarge (DP3 class) with a typical operating power of 40MWe can consume 130 metric tonnes of diesel fuel per day (1.5 kg/s) with carbon dioxide equivalent emissions (CO2e) of 3,200 kg per tonne of fuel. This is a substantial measure of emissions, typical of a pipelay vessel during pipe lay operations. It is for this reason that American and European air pollutant emission inventory guidelines expect environmental impact documents for all marine activities, including construction, to be calculated and submitted to relevant environmental protection agencies. By comparison, a typical car will produce around 4,600 kg of CO2e per year. Currently, deepwater pipeline engineering and design is based on relevant offshore design codes and standards, e.g. DNV-GL and API. Within the framework of those codes and standards, a design approach is presented within this paper that shows that, by properly combining pipe strength and stiffness characteristics with pipelay construction loads, a unique bending strain limit can be defined that would lead to the most economical solution that minimizes the vessel thrust and thereby radically reduce fuel consumption and associated CO2e emissions during pipelay activities. This unique design approach would be of interest to operators, pipe manufacturers as well as the pipelay contractors. Because of the construction economy and the minimizing of the carbon footprint, this approach is an attractive design method to all concerned parties, including environmental protection agencies. Since the design approach promotes higher steel grades, it would be very much in the interest of pipe mills to further develop and elevate the use of higher steel grades higher than the present widely used API 5L, X-65. Pipelay contractors will benefit by installing pipe with lower levels of thruster power, resulting in safer and a more reliable station keeping and, most significantly, a lower fuel consumption.