Fraser Bennie, Alpha Process Controls, UK, examines the role of the fall arrestor in LNG ship-to-ship transfers.
“The dark clouds squatting on the horizon had been glowering at the Master of the LNG carrier for some time now, taunting her, threatening to come in rapidly and ensure dramatic waves form and lift her vessel in spite of the previous weather forecasts.
It may happen a couple of times per year at a location like this, except that this was also an unplanned cargo transfer of an immobilised vessel out in the gulf area.
This was becoming a potential situation that may require her to interrupt operations abruptly.
The Master was well aware of this so, keeping her neighbouring Master informed and her head office well updated, she was working with the local weather office while the Chief Engineer was working closely with the STS crew brought on board the day before.
She had brought her LNG carrier alongside yesterday and the Chief had managed the transfer of the LNG transfer equipment very efficiently.
This operation was being done without much fuss. The team was very experienced and a very reliable, simple set of equipment had been sent. No external power required.
Just as well, this type of job required no equipment failures, less complexity, just simple, safe and reliable.
The start of the LNG transfer was going well, and the weather, well, as soon as they are done, they will be pulling away and get going for the terminal.
An hour later, that dark weather had closed in, a front of frothing waves was coming in like teams of galloping horses.
The STS operation had to be cut short. There was no time at present for normal isolation, purging and dismantling. The short, sharp method was to pull the vessel away, and fast.”
With ship-to-ship (STS) LNG transfers becoming more commonplace in the marine industry, this scenario is increasingly likely with operations being undertaken in sea states of varying complexities.
LNG transfers are made STS and also at times ship-to-jetty, using flexible hose emergency release systems. A rapid change in meteorological conditions is just one factor which may need to be dealt with that may require the emergency release collar (ERC) coupling release system to be activated either by the operator, or by the Master.
The large majority of LNG transfers go smoothly and are accomplished within predictable conditions and the primary safety function of the ERC is not employed.
But when needed, even if rarely, as with any safety device, they do need to perform on the day. The safe closure of the two valves within the ERC and the subsequent coupling separation needs to be achieved consistently, with no mistakes and no failures.
In the case of Alpha Process Controls, this has been achieved through the creation of a mechanical ERC system, which removes the requirement for expensive hydraulic power units, which are reliant on the availability of power – a resource that may not be available in an emergency ship to ship transfer situation (ESTS).
Utilising the advanced in-house design and manufacturing knowledge it has developed as part of the GT Group and with the backing of its parent company, the rail and commercial vehicle braking systems leader, Knorr Bremse, Alpha Process Controls is moving rapidly into supply of innovative, yet simple, LNG marine STS and ESTS equipment for all LNG transfer requirements.
Its marine transfer equipment is designed to bring back simplicity, operational time savings and limited service requirements in shipping LNG transfers. Based on proven designs used in other marine and on shore cryogenic transfers where Alpha Process Controls is a well-known name, its design principals are founded on keeping it simple and safe while retaining a high level of reliability.
Alpha Process Controls supplies a complete package for STS and ship-to-jetty transfers featuring ERC couplings, manifold reducers, hose saddles and flexible hose and arrestor systems.
The company’s ERC system features a two-stage operation that enables the controlled shutdown of internal valves (ESD1) and the coupling separation (ESD2). Through controlled shutdown of the ERC coupling, even at high flowrates, pressure surges are maintained to a level without compromising pipework or pumps in the event of a vessel drift-away. It also includes an innovative inbuilt damping system to reduce the effects of pressure spikes should the coupling be called upon to separate during full flow transfer. Servicing is done economically by a local operator trained by Alpha Process Controls to overhaul and correctly-maintain the equipment on-site.
Figure 1. Alpha Process Controls’ Fall Arrestor during testing at its facility in County Durham, UK.
Manufactured in sizes from 4 in. to 10 in. in stainless steel, and offering a wide range of seal trims available to suit specific applications, the couplings meet the guidelines set out by the Society for Gas as a Marine Fuel (SGMF), and the Society of International Gas Tanker and Terminal Operators (SIGTTO).
In addition, the ERC couplings are class approved to BS EN 1474-1 (2008) for installation and equipment for LNG.
Its systems undergo intensive testing programmes that utilise Alpha Process Controls’ in-house, state-of-the-art cryogenic facilities at its base in North East England, which features a purpose-built viewing area to allow engineers to watch cryogenic separation tests. The ERC couplings are subject to tests in the facilities, which hold 2000 litres of liquid nitrogen, to temperatures of -196°C, below the standard temperature of LNG.
However, it is not just the ERC coupling that needs to be simple, safe and reliable. Another part of this system is also brought into use when an ERC release occurs. This is the fall arrest system (FAS).
The FAS is a braking cable system attached to the ERC itself, on the outboard half, which is the coupling half that goes overboard in an ERC separation. Alpha Process Controls’ recently-launched solution is a braked, cable-guided FAS, which has been fully-tested in the company’s dry dock facility in North East England.
The system, designed to handle variable sea conditions, protects against loss of mooring and an operator-controlled release is also incorporated.
The primary function of the FAS is to control the descent of the split ERC and flexible hose assembly as it is guided by the hose saddles over the side of the vessel and into the sea.
The flexible hose contains LNG (or LNG vapour depending on the line) and the vessels are still relatively close at the moment of separation. The FAS must allow the released ERC and the flexible hose to be guided, through a controlled descent, within a controlled envelope of movement, down the vessel side, into the water.
Figure 2. Animation of Alpha Process Controls’ ERC system during separation during STS transfer.
At the end of the descent the FAS cable is released into the water to be recuperated with the hose and half-ERC.
The system is installed in the saddle area and the FAS cable rolls out as soon as the ERC splits. The cable release is slowed by a closed loop oil brake within the FAS body.
The oil is, of course, selected for the ambient temperatures of the region, which ensures that no oil heating system is required.
This means that the complete package of Alpha Process Controls LNG STS equipment can be used in any climate of the world, from Arctic to tropical conditions, with all components designed for the arduous conditions of the marine environment.
The separated ERC is guided over the ship’s side over single or double saddles. The saddles, designed and manufactured by Alpha Process Controls in Peterlee, County Durham, UK, may be fixed height, adjustable or with specific extension equipment depending on the vessel involved.
Marker buoys installed on the system will appear in the water as the FAS cable unreels, to show the crew where to locate the pick-up point for the ERC flexible line. The system will be recovered by the vessel’s craneage for reinstallation. Alpha Process Controls’ FAS include an in-built winding system to reinstate the cable onto the arrestor system drum for rapid reset for future transfer operations.
Not only are the number of STS transfers of LNG increasing, but the marine and land-based infrastructure to transfer LNG for use as a fuel is also evolving very rapidly.
What is clear, having spent considerable time in Europe and Asia meeting well-established or new LNG operators, is that the requirements for user-friendly transfer and emergency shutdown systems is increasing rapidly.
It is central to some of the discussions on subjects, in addition to STS operations, which range greatly from supply to FSRU newbuilds, conversion projects, retrofit and jetty back-up solutions.
The flexibility of new or converted FSRUs is bringing LNG to many regions of the world. However, there is also an impact on charter rates for these vessels due to the low gas prices.
Safety solutions must ensure not only effective and secure LNG transfer, but also prevent product loss, damage or injury while remaining as simple and economic to use and maintain as they can.
Read the article online at: https://www.lngindustry.com/liquid-natural-gas/19102017/under-arrest/