and safety inspection records are key to ensuring the safe continued use of the mooring rope. Compared to other vessels, LNG carriers have a unique feature. A dedicated hardwired communication cable between the jetty and vessel allows for the implementation of a data transfer channel to send accurate mooring load data for each mooring rope to the vessel. This can be used not only to manage safe mooring and cargo operations, but has the potential to record the duty data for mooring ropes, keeping accurate records of the ropes’ usage.
Mooring load monitoring
The usual causes of disruption to mooring at a berth include tidal change, strong berth currents, significant wave events, wind, ballast vs loaded draft, and winch slippage. Disruption issues are not only limited to the risk of ropes breaking. A vessel moving along or away from an intended jetty position, can affect the loading systems. This is especially relevant for marine loading arms, which have a limited operation window that can easily be exceeded if a vessel moves excessively from its required position. Pre-tensioning of mooring lines is important as it ensures that ropes have a minimum load applied. When set to an optimum tension, all the slack and material stretch are removed from the mooring rope. Quick release mooring hooks (QRH) can accurately measure the tensions of mooring lines by utilising a simple strain gauge in the hinge pin of the hook assembly. This allows terminal operators to have a full overview and control of a vessel’s mooring condition. However, one major issue is that the winches that control line tension are located and controlled onboard the vessel while the data is gathered onshore. If this data were available to the crew on the vessel, it could manage the mooring actively.
Communication channels
Unfortunately, there is a misconception that mooring load monitoring (MLM) data availability onboard LNG carriers comes as standard or that implementation is easy. There
are many ways that data can transfer between the jetty and vessel, and these multiple alternatives can cause incompatibility.
Ship-to-shore link options
The primary systems used to transfer MLM data are via the ship-shore link (SSL), which is capable of transmitting data directly over the link in either fibre optic or 37 W electric modes. This data transfer is usually displayed on a dedicated computer in the cargo control room and requires an application to be installed that can read the data transmitted by the terminal. There is no single MLM repeater application that includes all terminal data layouts, as each terminal is unique in terms of its mooring layout, metocean availability, and its data transmission regarding the data points it sends. There is however, no reason why the computer in the cargo control room cannot have multiple MLM repeater software programmes installed on it. The vessel is expected to send a correct POLL data string within a maximum time frame. If the POLL matches the terminal data requirements, the terminal will respond with a data transfer of the terminal specific live information. Typically this is repeated on a 30 sec. cycle. On the vessel, the data received can include status conditions that can be used as a status alarm. As well as YEWMAC format, other solutions include bespoke messaging arrangements that can contain alternative software communication handshaking and encryption to enhance data security. These terminals will have a specific decryption key and software installation available for LNG carriers visiting the terminal to install on their onboard hardware. The terminal will send data at pre-configured intervals, typically faster than the 30 sec. YEWMAC style, and a repeater application with a matching security key will be able to read and display the data. To utilise the SSL and existing infrastructure, the transmission of data between a terminal and an LNG carrier is a simple 1200baud RS232 serial data link. However, to accommodate the specific link interfaces, certain modems and converters are required and will be detailed below for the specific link types.
SSL – fibre optic and electric channel
Figure 1. Quick release hooks mooring a vessel at an LNG facility.
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In order to take the RS232 serial data and send it via a fibre link, the data first needs to be converted to a suitable telecom signal via a modem. For the purposes of existing terminal and fleet compatibility, these modems have a specific protocol matching the original NEC DATAX V.23 modem protocol. The original modem is no longer manufactured, however compatible modem solutions are available. The modulated data signal is sent over the link channel, where it is received and demodulated by a similar modem, and presented out as the data in the original RS232 1200baud format. So long as compatible modems are used, and the terminal QRH system and the vessel repeater application are capable of interfacing to the modems, then the data channel is transparent in terms of the SSL. Similar to the fibre solution, the electric solution simply converts the RS232 signal via MTL3058
