Siem Offshore has embarked on a project with two KPIs for anchor handlers: efficient rpm and eco-friendly transits
Offshore vessels featuring hybrid and electric propulsion systems deploy multiple engines for various loads, including dynamic positioning (DP) operations. Each of the engines has an optimum load bandwidth in which the engine performs best in terms of fuel efficiency and emissions. Quite often, optimum engine performance may conflict with the operational requirements of offshore vessels. Redundancy and backup can have more importance than efficiency.
Siem Offshore has embarked on a project to achieve improvements in two key performance indicators in its fleet: engine load optimisation and vessel speed. Through a monitoring system on the vessel and data collection and interpretation software, the project seeks to optimise vessel operation for safety requirements as well as engine load.
Siem Offshore serves the global oil and gas industry and its vessels are on duty across the world. The fleet consists of 35 vessels including large anchor-handling tug supply (AHTS) vessels, platform supply vessels, multipurpose field and ROV support vessels as well as offshore subsea construction vessels. Recently, the company added two well intervention vessels to its fleet.
The project started with the company’s 10 AHTS vessels, which handle anchors for rigs, tow them and secure the rigs with the anchors. Winch capacity of 500 tonnes and bollard pull of 275 tonnes are the norm in these vessels and engine loading supports these functions.
A typical AHTS from the fleet has the following propulsion configuration: two four-stroke main engines – Wärtsilä W16V32 can deliver 8,000 kW at 750 rpm each; two four-stroke auxiliary engines – Caterpillar CAT3516C are rated at 2,100 kW and 1,800 rpm. The main engines can operate on PTO mode and supply power through the shaft generators at 3,400 kW. In PTI mode, the electrical motors can deliver 1,500 kW to run each of the two controllable pitch propellers (CPP). The CPPs are rated at 9,215 kW at 144rpm for normal and 84 rpm PTI.
Auxiliary engines are mainly for electric power and electric motors on shafts. Other propulsive loads include two bow thrusters of 1,000 kW each, two aft thrusters of 880 kW each, as well as a forward azimuth thruster of 830 kW.
Two years ago, as part of the drive to manage engine loads efficiently, a vessel performance monitoring solution was installed on selected vessels in the fleet. The integrated system is a collaboration between Høglund Automation and Yxney Maritime.
Siem Offshore started this project with Høglund in 2016. Due to installation costs, only a couple of vessels were initially selected. With the support of the Norwegian government’s Enova project – which aims to cut greenhouse gas emissions – more vessels received the integrated system.
For the selected vessels, Høglund Marine Automation’s Ship Performance Monitor (SPM) – which collects and stores fuel consumption data for an individual vessel or entire fleet – is integrated with Yxney’s cloud-based toolkit for energy efficiency, Maress.
Jon August Houge, operations manager at Siem Offshore, describes it as a simple system that measures fuel consumption per hour or per nautical mile through tank soundings and power output. The values are calculated without the use of expensive coriolis flowmeters measuring the fuel flow.
In addition to general fuel oil measurements in different operation modes, two company-selected KPIs are the focus areas for improvement. The first is engine load in percentage of MCR: “The goal is to have it in the 70% to 85% range where specific fuel oil consumption (SFOC) is optimum,” says Mr Houge. “At loads like 20% or when the DP is operating, fuel consumption is high relative to the power and there is a lot of sooting,” he adds. For baseline data, the Maress software had calculated in a three-month rolling average for the engine load in percentage terms.
The second KPI is the number of transits made in a pre-defined eco-friendly speed range. “If it is 15 knots, we are using too much fuel,” says Mr Houge. “Typically 8 to 11 knots is the ideal speed range for transits, depending on hull shape and draft. We then monitor how many times the vessel makes transits in these speed ranges,” he explains. “If the client had requested high speeds then there was no choice; but that is rare,” he adds.
Traditionally, the crew onboard would select the diesel mechanical mode with both or all main engines running. This gives comfort and redundancy. Today they run either one main engine or two auxiliary engines, generating electrical power for driving the slower RPM PTI-motors, i.e. PTI-mode. This improves engine load as well as propeller efficiency.
In the past, at standby outside the 500 m zone, vessels pushed the DP-button engaging all engines and thrusters; today they do this by running only one engine, combined with the autopilot controlling only one thruster, typically the azimuth thruster. During inshore transits in calm weather, during which two or more main engines were previously selected as the operating mode for safety reasons, now two auxiliary engines are often considered.
There is currently an ongoing internal competition among the Siem Offshore vessels using the Høglund-Maress system to determine which is performing better in the KPIs. The main goal is to make the captain conscious about these choices. “Once the captain is on his toes, given the many engine set-ups possible in hybrid propulsion, he will select the speed and load on the engine that is optimum,” says Mr Houge. “Making him conscious of what he is doing has a consequence as far as fuel consumption is concerned.”
Typically, a vessel’s efficiency is improved through initiatives by the shipowner. “Cleaning of hull and propeller, trim, weather routing and other SEEMP plan measures are the typical ways, but what we are doing is entirely user-driven, since the speed is rarely demanded by the client,” adds Mr Houge.
An AHTS vessel of the type in Siem’s fleet burns around 15-20 m3 MGO every day, depending on the type of operations. A typical improvement of 3-5% can contribute to substantial savings. “Three to five percent is easily achieved by a captain following the recommendations and benchmarking by the system,” says Mr Houge.
These operating measures are controlled by ship staff and the monitoring is done real time on a live screen displaying parameters such as sea and wind condition, along with the relevant maps. “If the vessel is inshore or near the platform, then redundancy requirements kick in. Safety takes precedence. But in a drift-off situation when the weather is calm, the captain could possibly go down to fewer engines running,” says Mr Houge.
Company information: project uses an integrated monitoring system supplied by Høglund Automation and Yxney Maritime