LNG carriers may deal with one homogeneous cargo but transporting large quantities of this cryogenic liquid at its boiling point makes their cargo-handling operations more complex and riskier than those on other tankships, writes Mike Corkhill.
Adding to the challenges that confront officers responsible for cargo-handling and machinery operations on LNGCs are utilisation of cargo boil-off gas (BOG) as propulsion fuel, these ships’ high cargo-transfer rates and the introduction of new cargo containment and propulsion systems, as well as new ship types such as floating storage and regasification units and bunker tankers.
LNGC cargo and machinery operations need to be controlled and monitored in an integrated manner to ensure safe, efficient ship operations. Marine technology firms such as Kongsberg, Honeywell, Emerson and Yokogowa have helped LNGC owners to deal with increasingly complex ship operations by developing integrated automation systems (IASs).
The basic functions of an IAS are sensing, monitoring, alarm and control and its basic components are engineering work stations, the control network, distributed and scalable controllers and the human/machine interface in the form of supervisory computers with mimic diagram screens in the cargo controlroom (CCR).
An independent emergency shutdown (ESD) system is integrated with the cargo-control system (CCS) through the hardware and software of both.
The IAS enables simultaneous oversight of boiler and power management, cargo loading/unloading operations, custody-transfer measurement, cargo containment-system control, cargo ESD, BOG control, ballasting operations and alarm management. It also incorporates open interfaces to allow the supervisory computers to access trend data, messages and process data.
LNG carriers use among the most sophisticated IASs in commercial shipping. A typical vessel hosts upwards of 500 data collection points that gather readings from gas detectors and pressure, temperature and tank level sensors. An IAS with 3,000 digital input and output modules is usual for a modern ship.
The system operates myriad hydraulically controlled valves and regulates the flows of LNG and gas vapour throughout the ship.
IAS suppliers aim to make use of complex algorithms and computations and present crew with simple, understandable recommendations that require straightforward, unambiguous responses.
The Society of International Gas Tanker and Terminal Operators (SIGTTO) provides guidance on industry best practice to a worldwide membership of gas carrier and terminal operators. It established a new human element committee last year, to consider competency and training, design and ergonomics and the human element in incident investigation.
Shipboard experience provides valuable lessons for the industry. SIGTTO is quick to study the details of incidents and near misses on the ships and at its members’ terminals to understand what went wrong and to improve the industry’s rigorous safety regime.
In recent years, investigations into LNG carrier ship/shore interface mishaps have revealed difficulties in interpreting CCR screens and alarms. Misinterpretations can prompt incorrect responses and, sometimes, a reluctance to act for fear of the consequences of being over-zealous.
SIGTTO therefore decided that its new committee should focus on gas ship CCR design and ergonomics. It established a working group to consider the topic. At the committee’s third meeting in August, the CCR ergonomics working group identified alarm management as its initial focus.
The group is prioritising cargo alarms as per the IMO Code on Alerts and Indicators 2009, reviewing the ship-shore checklist and highlighting the importance of safety-critical systems and the timely activation of ESDs. Once it has completed the alarm-management task, the CCR working group will consider the layout of workstations and positioning of mimic screens in the CCR, along with the content and layout of the screens.
The IMO Code on Alerts and Indicators 2009 identifies four categories of alert: emergency alarm, alarm, warning and caution. Emergency alarms and alarms require immediate action to be taken, whereas warnings and cautions alert seafarers to changes in conditions to which they may need to respond, to prevent a hazardous situation.
The code is a generic maritime instrument, covering all types of ships and with the emphasis on alarms relating to propulsion machinery and navigational devices rather than those for cargo-handling equipment. Although it lists the types of cargo alarms utilised on gas carriers, the code does not specify the alarm categories for these devices or name the ship operational situations in which alarm prioritisation may be relevant.
One example of where the lack of alarm prioritisation can be confusing on gas carriers is the “cargo pump low amp” alert. This can be activated when the pump is inactive or active.
In the former case, this alert makes perfect sense and has the status of a caution. However, if the pump is operational and cargo handling is underway, this alert constitutes an alarm that requires immediate action to find and respond to the cause of activation.
SIGTTO’s working group is aiming to complete draft guidance on gas-ship alarm prioritisation by early 2018 and, once the society’s human element committee has approved the guidelines, to consider other CCR matters.
Other ergonomic challenges
The lack of co-ordination between shipyards, naval architects and IAS suppliers can also hinder a gas carrier’s cargo monitoring and control equipment from achieving its optimal performance. This is especially true when a series of ships is ordered and stems from the characteristics of today’s newbuilding contracts.
Yards contract IAS suppliers to provide the same equipment, set up in the same way, for each ship in a series of gas carriers. However, it is inevitable that the IAS arrangement in the lead vessel will be tweaked as the ship is commissioned and after feedback from early cargo-handling duties.
There is no formal route to feed the experience of the lead ship back into the IAS setups of later ships during their fitting-out phases at the yard.
IAS suppliers also need to accommodate into their systems the software of manufacturers of critical LNGC components such as cargo pumps, compressors, reliquefaction plants, gas-combustion units and fuel-gas supply systems.
Bearing in mind the range of equipment available to the market, this requirement adds to the challenge of providing the shipowner with a user-friendly, integrated monitoring and control system.
Although it is impossible to eradicate human error from IAS packages, the gas-shipping industry acknowledges that there is still room for improvement.
Artificial intelligence has a role to play in the drive for more consistency in the CCR and still greater levels of ship safety. The gathering, transmission and storage of real-time ship operational data will no doubt facilitate machine learning in future.
And those at the coalface – ships’ officers – are ideally positioned to say what works best in gas-carrier cargo and machinery monitoring and control.