Rigid, or flexible? Both STS technologies have their drawbacks, but by bringing together the positives, one company hopes to have found the ideal solution
The development of ship-ship (STS) transfer technology has essentially followed two strands: flexible and rigid. Flexible systems use a cryogenic hose setup that is relatively inexpensive and requires no modifications to the vessel, but which suffer from pressure losses during transfer operations and are time-consuming to put in place, take down and drain for each STS operation.
Rigid systems, on the other hand, are expensive. They require structural modifications to the vessel and are not adapted for use when sailing. Their rigidity does, however, mean faster transfer rates, no need for set up or take down and easier draining.
Seeking a solution that combined the best of both worlds, TechnipFMC turned to its considerable experience to devise its Articulated Rigid Catenary Offloading System (ARCOS).
Business development director Renaud Le Dévéhat explains: “What we are coming up with today is really the result of the work we have done in integrating those two technologies – flexible and rigid. We went through all the possible combinations: fully rigid, fully articulated, hybrid solutions from the top, from the bottom and we came up with one solution that we believe checked all the boxes.”
The system is an arrangement of three 16-inch lines – standard steel – placed in a catenary arrangement, using swivel joints on the loading arms, which ensured six degrees of freedom between two vessels, says Mr Le Dévéhat.
With the swivel joints as a focal point, he says the development team was trying to clarify misperceptions about the system’s flexibility and its longitudinal range of movement.
“Sometimes, there is the perception that a rigid system has limitations in terms of degrees of freedom. Basically, all the six degrees of freedom are absorbed by swivel joints, so there is no limitation in terms of rotation. In terms of the overall operating length of the element, we have based today’s design on the same thing we do with the hoses. You have a normal operating envelope and then you have additional space for ESD 1s, stop transfer and close the valves, and additional space for ESD 2. The system must be able to follow all the movement in that space. Beyond that, you ensure the emergency disconnection,” he explains.
“The development team was trying to clarify misperceptions about the system’s flexibility and its longitudinal range of movement”
The development team also considered the minimum distance necessary between two vessels and the use of floating fenders, typically between 3.5-4 m in diameter.
“We checked the safety gap when the bottom element arrives in the horizontal position; we keep a safety gap in maximum compressed fenders between the two hulls,” he says.
The ARCOS system is built using components already in common use in the industry, and the whole system is compliant with OCIMF and SIGTTO standards as well as European (CSN EN1474-3) and international standards (ISO 16904).
The system is also undergoing qualification by class society DNV GL. According to Mr Le Dévéhat, it has successfully passed most of the classification levels and is working towards the final stage of qualification.
Mr Le Dévéhat says ARCOS offers similar reductions in pressure loss that come with rigid arrangements, equating to some 2.5 times less pressure than is needed to achieve the same transfer flow on flexible arrangements.
This, Mr Le Dévéhat says, results in cost-saving opportunities from faster transfers, or the better management of boil-off gas.
“With all the energy of the pumps pushing this liquid through the system, the friction will create boil-off gas, so boil-off gas has to be managed, and quite often it’s a limiting factor in the transfer operation,” he says.
“By dividing boil-off gas by a factor of 2.5, we can increase, significantly, the flow rate, or we can reduce the equipment required for boil-off gas management and therefore save a lot on the capex of the system.”
By using metallic pipes as opposed to hoses, the cooling down process also occurs approximately 30% more quickly, according to TechnipFMC estimates.
Keeping the arrangement to three large-bore lines improves thermal performance, too: “Those lines are not insulated, but overall the thermal performance is better, so we also reduce boil-off gas from heating waste into the system.”
The number of lifting operations is reduced versus flexible arrangements, saving time and costs and adding revenue from faster transfers.
“As we use only three lines, it’s only three ERSs, three emergency release systems, to be tested before each transfer, instead of six to eight,” explains Mr Le Dévéhat. “So we save time and improve reliability here, [by] minimising the number of manual bolting operations, eliminating up to 130 bolting operations per transfer.”
The system also means the draining process is expedited, with a patent-pending function that over-pressurises the lowest line with gas from the topmost line to force the liquid through. On average, according to TechnipFMC figures, the process saves 25% over typical drainage setups.
And, Mr Le Dévéhat is quick to point out, the company believes that the system is “totally compatible” with today’s fleet.
With one hybridised system nearing completion, TechnipFMC says there is room for more.
“For bunkering applications, we have other ideas, other systems. Basically, for bunker operations we know we must go fast, ensure several operations per day and minimise connections. We have other ideas based on hybrid systems and using flexible technology for bunkering,” Mr Le Dévéhat says.
But the innovation must be feasible, a point emphasised by referencing a system that uses the hydraulic quick connect/disconnect (QCDC) feature. “The next stage – to keep it simple – is to use manual QCDC. So, we greatly reduce the number of manual operations. There is no [defined] limit; we could propose a hydraulic QCDC. It will work, it will save time, but we are getting further from the simple and lean system,” he points out. “So, yes, it is possible, but I don’t think it is in the spirit of what we are trying to do.”