Reducing fuel consumption of Dynamic Positioning operations from Offshore Vessels

Abstract

The demand for offshore marine contractor services is expected to increase in the coming years, whilst contractors, such as Boskalis, receive growing pressure to reduce greenhouse gas emissions from their offshore operations. Dynamic positioning (DP) is a crucial aspect of most offshore operations, but the high redundancy levels and frequent load variations present several problems and negatively impact fuel efficiency.

This MSc Thesis proposes improved diesel generator (DG) load allocation strategies as a method for reducing fuel consumption from DP operations of offshore vessels, considering existing ship designs and industry practices whilst maintaining high safety standards. The main principle of this solution is to improve the overall fuel consumption by allowing prime movers to operate at different loads. The main research objective of this report is to determine the fuel savings achieved through improved DG load allocation strategies for a selected case study, considering realistic operational constraints, standard industry practices, and a dynamic load profile which approximates real conditions.

A diving support vessel is selected as case study vessel based on its power system characteristics, operational profile, and expected load profile during DP operations. Realistic vessel limits and practices are gathered from operational vessel documents, and constraints are established for different load allocation strategies. Instead of using evolutionary algorithms to create DG sequences that are optimised for fuel efficiency, DG sequences are manually created and correspond to ten different load allocation strategies in order to maintain a clear structure which operators are able to utilise in real life.
The best DG sequences of each load allocation strategy are selected and further analysed in a preliminary analysis, providing the hourly fuel consumption of each load allocation strategy, between 0% and 110% MCR w.r.t. the benchmark strategy.
In addition, a dynamic model is developed to simulate a 24-hour DP operation of the selected case study vessel, providing information on the resulting fuel consumption and potential savings of different improved DG loading strategies for a dynamic load profile. This load profile changes over time to approximate real-life conditions, including most frequent power levels and occasional load transients.

Results indicated that all improved DG load allocation strategies provide fuel savings for both load cases w.r.t. the benchmark strategy. Savings are maximally 5.5% for open bus-tie configurations and 11.3% for closed bus-tie configurations.
Additionally, the outcomes of the dynamic model are compared to those provided by the preliminary analysis, which represents current state-of-the-art methods. The frequently low power levels and occasional load transients are not well represented by an operational average load, whilst these significantly influence the prospected fuel savings of the proposed solutions. Therefore, the results emphasise the importance of considering realistic industry practices and determining fuel consumption for a dynamic load profile which approximates real DP operations.

Although similar fuel savings are expected for other conventional diesel-electric DP vessels, the extent greatly depends on aspects such as the operational profile in DP mode or the fuel consumption characteristics of the power system. Actual fuel savings are likely lower as the effects of load transients are not widely considered within this research. Nevertheless, significant fuel savings are expected, as methods could be devised to limit this.
Ultimately, fuel savings from improved diesel generator load allocation strategies have been provided for realistic operational constraints, presenting an alternative out-of-the-box solution for reducing fuel consumption of Dynamic Positioning operations from Offshore Vessels.