Fresh volatility in the oil markets and the approach of a new era in ship fuel use are combining to demand more accountable vessel efficiency modelling, says Janne Niittymäki, head of hydrodynamics, Foreship
Fuel buyers have been watching oil price volatility closely over recent months, as a moderate recovery morphed into a three-year high on the threat of shortages from Iran and Venezuela before Saudi Arabian and Russian offers to turn up the taps prompted a 7.5% decline.
These circumstances are beyond the buyer’s control and encourage hedging, but they also flag up a challenge for those ship owners believing they can simply pass on fuel surcharges to cargo owners and charterers after the IMO’s 2020 global cap on 0.5 per cent sulphur content comes into force. While not so restrictive as the 0.1 per cent sulphur content limit allowed in Emissions Control Areas from 2016, the worldwide cap has implications for availability, while LSHFO quality may vary from different sources: both are drivers of price.
In fact, what those ultimately paying the fuel bills will be demanding will be transparency – also the cornerstone of new EU requirements for owners to monitor and report fuel consumption so that CO2 emissions are measurable. Already, shipper attitudes towards surcharging are hardening. After a three-year taste of lower fuel bills, the Global Shippers Forum pushed back hard in May 2018 against ‘emergency’ $120-$140 charges per 40ft container levied to cover rising oil prices.
At Foreship, we are convinced that pressure is mounting to demonstrate that every effort has been made to optimise efficiency. After all, while cleaner fuels may be more expensive, they are also more ‘energy efficient’; unlike HFO, they do not need heating up to be useable, while lower sulphur contents are not so corrosive, making more heat recoverable.
Speed and change
As practices developed in the container segment over the last decade have shown, operating speed is a key determinant for fuel consumption. At 18 knots, the average daily fuel consumption of a 13,000TEU capacity container ship is around 90 tonnes, according to National University of Singapore1 analysis. At June 2018 bunker fuel prices, that would cost $40,680. At 16 knots, the same ship would consume 70 tonnes a day, costing $31,640.
However, what the scatter analysis developed by the NUS also demonstrates is that, between 16 knots and 18 knots, fuel consumed can range between 60 tonnes and 110 tonnes a day. This emphasises that the efficiency of the hull form depends on displacement, stability and seakeeping requirements, as well as the speed-power balance. Weather/sea conditions and displacement trim are also significant for fuel consumption, which can increase dramatically when a ship faces strong bow waves, for example.
The point here is that maximising efficiency is about more than slow steaming. Optimal efficiency would also see engines running on optimum load, ballasting done correctly, excessive speeds to ‘guarantee’ on time arrival avoided, and optimised trim. As the naval architecture and design engineering company with the largest reference list in the cruise industry, we believe that cargo ship owners facing uncertain fuel pricing would be advised to consider efficiency measures whose ROI is proven in a segment known for its technical innovation.
To prove it, Foreship has developed a new software-driven methodology within which to consider ship design attributes. Christened ‘operability analysis’, it draws on our database of projects to analyze and model the performance of different designs and design modifications.
Foreship has already demonstrated that gains can be made in efficiency through designing ships for the conditions they face at sea using CFD (Computational Fluid Dynamics). Its ‘in-wave’ analysis factors real sea states into hull form optimisation and has supported a case for the superior performance of vertical stems over bulbous bows at lower wave heights than was previously acknowledged.
CFD-software has also contributed strongly to propulsion efficiency in new ships: a design from today could expect to use 10% less propulsion power to achieve the same speeds as one considered hydrodynamically excellent a decade ago.
However, CFD methods are only one part of the picture. Foreship’s operability analysis relies on established ship design and construction software that has been augmented in-house to introduce data such as voyage route planning at an earlier stage than is customary. It calculates fuel consumption and motions on board based on hull form, propellers, rudders, general arrangement and main engines, but also the operating conditions themselves.
Voyage simulations are port to port, and are scheduled as starting on a given day at a given time and having a target arrival date and time. Realism is added to the model by using hind-cast environmental data of the wind, waves and currents on the actual route.
Operability analysis of this type offers a platform to assess the impact of design changes on fuel consumption and comfort using a single, consistent approach based on the same model and realistic weather conditions and a realistic route than to deal with these issues separately.
One reason for considering this approach now is that it will allow cargo ship owners to evaluate, accurately and easily, whether innovations first seen in the cruise ship market can make a true impact on optimising cargo efficiency.
Foreship, for example, has developed pre-swirl stator fins to minimise turbulence behind ship propellers and enhancing thrust, with fin shapes custom-developed for each hull form. Again, owners could evaluate the optimised ‘water shield’, where Foreship simulations have modelled the benefits of chamfered hydrodynamic covers fitted to a vessel’s side tunnel thrusters. Conventionally, thrusters generate homogeneous water flow out of the tunnel; when thrusters are positioned one behind the other along the hull, flow disturbances lead to suboptimal performance. Adding a water shield at the forward section of the thruster reduces the disturbance, but Foreship has also optimised the shield’s profile, with simulations pointing to about 3% overall resistance reduction
The operability analysis can also be used to offer guidance on more radical proposals to enhance fuel efficiency, such as Foreship ALS – the first under-hull air lubrication system in the market to secure a truly commercial application originally designed for cruise ships. Running a stream of air bubbles under a ship’s hull means propulsive power can be reduced while ships continue to proceed at their cruising speeds.
Foreship began developing the ALS from June 2011, initiating full scale CFD simulations to apply the solution to both newbuildings and existing ships. Detailed air distribution box geometry was optimised by CFD simulations, and then verified in a cavitation tunnel. Development also included testing the concept onboard a first cruise ship, with commercial installations now working across several vessels.
For owners, the projected net saving on fuel is inferred from the percentage decrease in the braked power used for propulsion. In the cruise ship applications, net fuel savings equivalent to 7-8% have been achieved. Overall net average fuel savings within a defined speed range and conditions have subsequently been confirmed at around 5%.
Discussions in the ferry market have also progressed, with a detailed feasibility study undertaken for a large RoPax ferry. Simulations indicate that the Foreship ALS is especially suitable for ferries operating at <20kn featuring high propulsion power.
Most recently, Foreship conducted a full feasibility study for installation of the ALS to optimise fuel consumption on a container ship which looks very promising. Clearly, the hull shape is different from that used in the case of a cruise ship, but our simulations indicate that 5% savings in fuel consumption are achievable at operating speeds from 12 knots upwards.
The same operability analysis can, of course, be applied to more established cruise ship solutions, including the podded propulsors whose inherent fuel efficiency can be enhanced through the greater hull design freedom they confer, or the ‘ducktail’ that can improve stern performance. Again, if hull lengthening were being considered, the effects on fuel consumption, speed and engine profile could be simulated and the different possibilities for engine configurations investigated in a single process.
Furthermore, operability analysis can be easily updated throughout the ship design process as the design changes and new performance data become available. The model can be tuned further as data from onboard systems is compiled from the operation of the vessel. This feedback from operations will help in the decision-making process during future newbuilding and conversion projects.
On another level, such an approach makes ship owners aware of what is and is not viable, putting them in a position to negotiate the vessel’s performance specification – or indeed any bonus scheme for surpassing expectations – from an informed position.
- BUDGETING THE FUEL CONSUMPTION OF A CONTAINER SHIP OVER A 1 ROUND VOYAGE VIA ROBUST OPTIMIZATION – Revised paper submitted to the 94th Annual Meeting of Transportation Research Board, 1 August, 2014
Janne Niittymäki, head of hydrodynamics, Foreship, contributing author