Waste heat recovery technologies for offshore platforms

Highlights

• Optimal waste heat recovery technology for offshore platforms.

• Organic Rankine cycle, air bottoming cycle and steam Rankine cycle as waste heat recovery units.

• Multi-objective design-point optimization approach to compare the three alternatives.

• Three objective functions considered: yearly CO₂ emissions, weight and net present value.

• Case study: Draugen offshore oil and gas platform in the Norwegian Sea.

Abstract

This article aims at finding the most suitable waste heat recovery technology for existing and future offshore facilities. The technologies considered in this work are the steam Rankine cycle, the air bottoming cycle and the organic Rankine cycle.

A multi-objective optimization approach is employed to attain optimal designs for each bottoming unit by selecting specific functions tailored to the oil and gas sector, i.e. yearly CO₂ emissions, weight and economic revenue. The test case is the gas turbine-based power system serving an offshore platform in the North Sea.

Results indicate that the organic Rankine cycle technology presents larger performances compared to steam Rankine cycle units, whereas the implementation of air bottoming cycle modules is not attractive from an economic and environmental perspective compared to the other two technologies.

Despite the relatively high cost of the expander and of the primary heat exchanger, organic Rankine cycle turbogenerators appear thus to be the preferred solution to abate CO₂ emissions and pollutants on oil and gas facilities.

As a practical consequence, this paper provides guidelines for the design of high-efficiency, cost-competitive and low-weight power systems for offshore installations.

Introduction

Owing to environmental concerns there is an urgent need to reduce greenhouse gas emissions and pollutants in the industrial, civil and transport sector. As reported by Nguyen et al. [1], the North Sea oil and gas platforms were responsible for about 25% of the total CO₂ emissions of Norway in 2011. On offshore facilities the major contributor to the overall emissions is the power system which typically releases a large amount of heat to the environment [1]. Since 1991 Norway levies carbon tax on hydrocarbon fuels and the Norwegian government has recently increased the taxation by 200 NOK (32 $) per ton of CO₂ in 2013 [2]. Thus, increasing the performance of power systems in offshore applications has become a focus area from an environmental and economic perspective. On oil and gas facilities one or more redundant gas turbines supply the electric power demand. As an example, a standard operational strategy is to share the load between two engines, while a third is on stand-by or on maintenance. The two gas turbines typically run at fairly low loads (around 50%) in order to decrease the risk of failure of the system, which would cause a high economic loss to the platform operator. On the other hand, this operational strategy reduces significantly the system performance, which in turns results in a large amount of waste heat contained in the exhaust gases exiting the engines.

A viable solution to enhance the efficiency is to implement a waste heat recovery unit at the bottom of the gas turbines. Major design criteria are compactness, low weight and high reliability. A mature technology accomplishing these duties is the steam Rankine cycle (SRC). Kloster [3] described the existing SRC units in the Oseberg, Eldfisk and Snorre B offshore installations. Aiming at minimizing the weight of the heat transfer equipment, Nord and Bolland [4], suggested the use of SRC turbogenerators equipped with single-pressure once-through boilers (OTBs), instead of the heavier drum-type heat recovery steam generators. Air bottoming cycle (ABC) systems constitute a valid alternative to SRC units as they employ a non-toxic and inflammable working fluid. Moreover, ABC modules do not require a condenser as they operate as open-cycles, thus potentially leading to high compactness and low weight. Bolland et al. [5] performed a feasibility study on the implementation of ABCs offshore. Results proved that, despite the low gain in performance, low weight and short pay-back time are attained. Pierobon et al. [6] proposed instead the use of organic Rankine cycle (ORC) turbogenerators by tailoring their design to an exemplary oil and gas platform. The authors employed a multi-objective optimization approach to minimize the volume requirement, and, simultaneously, maximize the thermal efficiency and the economic revenue. As surveyed by Walnum et al. [7], supercritical CO₂ power units may also be utilized to decrease the greenhouse emissions and pollutants as they exhibit high performances both at design- and part-load conditions. Mazzetti et al. [8] showed that combined cycle performances using a dual-stage supercritical CO₂ bottoming unit are comparable with the values obtained with a SRC module. Furthermore, due to the high working pressure, the power module is expected to be extremely compact and light.

Notwithstanding the above-mentioned works, to the knowledge of the authors there is a lack in the literature of a comprehensive comparison among waste heat recovery technologies for offshore applications. In this context, this paper aims at finding the most suitable waste heat recovery unit to be implemented on existing and future oil and gas platforms. The multi-objective optimization procedure coupled with the genetic algorithm is utilized to search for the optimal system designs (i.e. Pareto fronts) of each technology. The objective functions are the economic revenue, the weight of the bottoming cycle unit and the daily CO₂ emissions. The detailed design of the heat transfer equipment and the material selection enable to include geometric quantities (e.g. tube length and tube diameter) among the optimization variables, thus allowing the estimation of the weight. Moreover, the implementation of part-load models within the optimization routine consents to evaluate the CO₂ emissions over the entire year, and to estimate the profitability of the alternative investments. The case study is the power plant installed on an offshore oil and gas platform in the North Sea. The three power units considered in this work are the organic Rankine cycle, the steam Rankine cycle and the air bottoming cycle. Supercritical CO₂ cycle configurations are not analysed here since such systems are still in the development phase.

This paper is structured as follows: Section 2 introduces the case study, while Section 3 describes the methodology utilized to perform the multi-objective design optimization. The results are then reported and discussed in Section 4. Concluding remarks are given in Section 5.