Channels of data

Andrew Stafford, Technical Director at Trelleborg’s marine and infrastructure operation, UK, details the use of mooring tension data to ensure safety and longevity of equipment on vessels.

Mooring a vessel is not a trivial matter. As mooring loads securing the vessel to a berth are held within the mooring ropes, failure of a mooring line can result in a significant amount of tensile energy being released in a very short period of time, potentially leading to safety issues. This article will explore how mooring equipment guidelines highlight the importance of gaining access to accurate data on the condition of mooring equipment to ensure the equipment’s safety and longevity on vessels. It will also discuss the incompatibility of legacy infrastructure and the alternate solutions available to keep the mooring interface operational within a fleet.

Updated mooring guidelines

Following several incidents, the Oil Companies Marine Forum (OCIMF) updated its flagship mooring guidelines in 2018 to Issue 4 (MEG4) to now include actions around mooring safety. One of these actions was replacing steel-wired mooring ropes on larger vessels throughout the shipping industry, with safer, synthetic ropes of equivalent strength, such as those constructed from high modulus polyethylene (HMPE). Whilst these newer designs are inherently safer under snapback conditions, knowing the status of a mooring line, its operational history,

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

Figure 1. Quick release hooks mooring a vessel at an LNG facility. 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

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

equivalent converters into a format capable of being transmitted over longer lengths of cable that RS232 allows.

MLM repeater

The vessel repeater used by Trelleborg was originally developed to read data at Japanese terminals; a requirement in the LNG carrier build specifications issued by Korean shipyards. The development involved sending engineers on board trading LNG carriers to all terminals in Japan in order to ‘sniff’ the data of existing systems. The communication protocol was then reverse engineered to produce a computer application capable of fully communicating at the Japanese terminals without generating any connection alarms. As Trelleborg also produces QRH mooring systems, it was simple to add the various jetty data profiles into a single application for installation on LNG carriers.

As a result of continuous development of this software, as well as displaying data real-time, functionality was added to record each piece of data received to allow offline review of the mooring at a later date. This repeater coupled with the full terminal database is one of the most complete MLM repeater solutions in the world.

New berthing arrangements

Recently there has been a noticeable shift to alternate trading practices within the LNG sector, which have led to some novel mooring solutions involving multiple assets with active equipment installed.

The introduction of floating storage-based applications has changed the understood concepts of both SSL and QRH from traditional LNG carrier type applications. The addition of more complex process emergency shutdown systems (ESDs), telecom and data requirements, along with the new mooring arrangements of double banked vessels undergoing cargo operations, has led to the application of new and novel solutions. The situation is still fluid with new functionality requested on newer projects.

FSRU to LNG carrier MLM interface

The MLM repeater requirement for the FSRU to LNG carrier interface is exactly the same as for a standard terminal to LNG carrier. The LNG carrier is the lowest common denominator in these applications and needs treating as a vessel equipped with industry standard interfaces only; an LNG carrier should not have to install any additional equipment if it is trading with an FSRU.

If the FSRU is equipped with QRH and has a requirement to transmit MLM data to the LNG carrier via SSL, it must have the standard shore interface of a V.23 modem and an RS232 data source.

FSRU to jetty MLM interface

FSRU to jetty interfaces have a subset of issues that can be catered for depending on the application: � FSRU displays jetty hook data, exactly as an LNG carrier would.

� FSRU QRH controller communicates directly with the jetty QRH controllers. � FSRU QRH controller communicates and controls the jetty QRH controllers.

To expand on the SSL interfaces available in an FSRU to jetty connection, typically the principle follows: If there is an LNG product transfer, the interface has to be ISO28460 analogue fibre optic and 37 W electric links on defined pins. If it is a gas transfer, then the interface is via digital (SONET) fibre optic and an alternate 37 W electric pin configuration.

The digital link adds an ethernet channel, which is capable of bulk data transfer from multiple sources. In the 37 W electric mode, there is also an ethernet option available, but this is via ethernet extenders.

There are situations where an FSRU would read the data from the jetty QRH system and add this to the FSRU-LNG carrier QRH arrangement to create an overall view of the mooring. By using the ethernet channels on the SSL, this can be undertaken either with direct ethernet connection or via serial/modbus interfaces. This means the jetty data is received by the FSRU. The FSRU can then create the holistic overview and resend that data back to the jetty over the ethernet link and/or send the MLM data to the LNG carrier via the standard interface.

Figure 2. A vessel moored at an FSRU facility.

Mooring tension data

To conclude, as the mooring tension data can be received by the LNG carrier using standard equipment, it opens up the possibility to log and use the data as part of a rope management system. These systems can simplify the current methods of rope management, and would even allow for accurate, operational data to be used as part of the required recording programmes.

As non-standard and different POLL strings in an open communication standard is not helping the industry, there is certainly scope for the mooring load monitor repeater system to be made easier in terms of jetty compatibility. However, just having this data channel available could allow for additional functionality to be applied.