Bonded type hose can accept considerable torsion without structural damage or loss of shape. The inclusion of a swivel would add an unnecessary point of potential leakage and increase maintenance aspects.
The helixing of the riser is achieved partly through its natural tendency (as a result of the counter helix carcass structure), and because the bottom collar is always oriented in a fixed direction to the seabed, and the top collar in the same direction as the vessel ‐ all the intermediate collars taking up appropriate intermediate angles.
This collar orientation is achieved through either:
“fixed” collars, ie rigid with individual links of chain and hence use the chain torsion for the helixing (specific chain torsion tests were undertaken for various chain sizes and tensions, to enable formulae to be developed for the correct spacing of collars);
“free” collars, when the chafe chain is free to rotate inside the collars, and their orientation is achieved solely by the hose itself; the lower collar orientation being maintained due to the lazy wave down to the seabed. Which collar type to use can be decided from factors such as design weather criteria, water depth, riser size and weight etc.
If the collar design is not correct this could occur. The interior surface and the bell‐mouthing are important. Typical bonded hose has a chloroprene outer protective skin which does not form part of the structural integrity and thus provides a margin in case debris enters the collars.
Firstly, the way the TCMS works, the whole chafe chain assembly, chain, collars and riser, are all moving together as a unit, since the ship motions are taken up in the catenary sections of the mooring. Thus there is no relative movement of the hose to the chain. Certainly in waves these can cause the hose to slam against the chain, however by correct choice of collar spacing the impact pressure is so far within tolerances for bonded hose it is not an issue.
Either can be, although it is normally prudent to lay the mooring before the hose to avoid the risk of chain, or worse still something heavier and sharper, falling on the hose. A dummy gantline is drove through the collars during mooring deployment, and the flow-line installation vessel can pull the riser through the collars with this. “Ears” are then bolted on the end riser flange to stop it falling back through the collars.
The TCMS is typically a less stiff system than other tanker moorings. This means that the tanker has a range of offsets from the zero position. These offsets are controlled by the design of the system damping the natural vessel fore and aft motion. Not only do the mooring catenaries contribute to this damping, but there is a sort of pendulum effect of the chafe chain. The design of the load/offset curve for the TCMS is critical.
For example, a 90,000mt tanker in 100m water depth in a 7m significant wave height sea, would be surging between 50 and 60m from the zero point. The movement of the mooring node is considerably less than this, the rest of the movement taking place because the chafe chain takes up a larger angle from the vertical.
A tanker of this size will typically have a second order surge period of between 6 and 9 minutes on the TCMS, and so this movement is far from violent.
The distance from the node to the tanker bow is a fixed length (typically about 40m for a 90,000 dwt Aframax tanker). What affects the riser catenary shape is the movement of the mooring node, and this is considerably less than the tanker movement. Horizontally the node will move perhaps only 10m, and vertically perhaps 25m. These sort of motions are easily taken up with the lazy wave of the riser to the seabed. In shallower waters, it is even feasible to rely on the inherent stretch in bonded hoses to accept some of the node offsets (bonded hose can have elastic stretch to as much as 40% of its original length).
In the case of a TCMS where the collar orientation is achieved by connection to the chafe chain links, this could actually be detrimental to the working of the system, since the vessel on connection would never know what pre‐twist is in the collars. In either fixed or free collars the swivel also provides a potential source of weakness and unnecessary complication.
No tug is necessary. A normal single fixed pitched propeller tanker can normally adequately connect without any support vessel at all. The weather criteria in which this is achievable is dependent on the tanker size, but it has been performed numerous times with a 8,500mt dwt tanker in 3.5m significant seas, and in 4m or above with a 90,000mt dwt tanker. From the time the pick‐up rope is grappled (by hand or air rocket) to being in a safe moored condition, is typically ten minutes, for a lot of which the vessel has steerage.
A DP offtake tanker requires some additional protection in case it has a propulsion failure or complete black‐out, to stop it drifting on to any other facility. A TCMS moored tanker already has two means of protection against such a hazard. Firstly, if it does black out, the vessel remains moored; if the mooring should fail, the vessel still has its propulsion system. Whether a further level of protection is prudent can be determined by a risk assessment, taking into account proximity, weather direction frequency, ability of facility to move, if any, etc.
Designs of TCMS without any single points of failure have been made, but they are considerably more complicated, hence expensive in both capex and opex (although still competitive with turret moorings etc).
Instead, an alternative approach to the issue is made, in line with Class rules. An alternative to the single line failure has been approved in the past by both LRS and ABS for systems that incorporate a/ an adequate factor of safety, b/ a proper means of disconnect of both riser and mooring, and c/ maintenance of the tanker as able to manoeuvre. DnV have recently introduced new criteria specifically for non‐redundant systems such as the TCMS.
In short term exports, either short term projects or where the tanker will be proceeding to a discharge terminal, this alternative approach is particularly applicable, although it has also been approved for long duration and high pressure riser systems.
The limit is typically caused by the acceptable torsion of the chain rather than the bonded riser; the normally accepted standard for chain is 9° per link, which in a typical Aframax tanker system results in 3 full turns. Recent tests indicate this may be a conservative limit. Operationally we have never found the need to weather‐vane past 1.5 turns, although we have allowed a vessel to go further for the experience. Our Operations Manuals state that a normal limit of two turns is permissible.
Turns occur either through the result of tidal currents (up to one turn per 12 hours), or as a result of a depression passing across the region (typically max 3⁄4 of a turn per 24 hours). Above calculated wind speeds, the tidal currents do not turn the vessel across the wind typically 12 knots in ballast, and 19 knots loaded, for a Aframax tanker in the North Sea. Tidal currents will not always turn the vessel round and round, but may take it backwards and forwards across a semi‐circle, or the vessel can be kicked in that direction. But even if weather‐vaning does occur, the tanker can use its own engines to take a turn or more back out. This can either be done near slack water if the tidal currents are predominant, or after a period of bad weather. This operation has been performed a number of times, the data carefully recorded and checked against dynamic modelling programs such as Ariane, so that limits for future scenarios can be estimated in advance.
This could only occur in relatively calm weather, as a bad weather regime will not continuously turn a vessel; coupled with the risk of power failure, the chances of this happening are very small. But even so it is not an ultimate problem to the TCMS, as in the periods of slack water the chafe chain connection can be disconnected from the chain stopper forward and put on an alternative wire or chain, thus taking out the twists. With a fixed collar system it will also be necessary to disconnect the riser and pass this around the chafe chain to take out the helixing. Typically a chafe chain is made up of a lower and upper section, the connection between them being just outside the fairlead. The principal reason is to permit change out for close inspection of the upper chafe chain ‐ the section that might be seeing abrasion on the fairlead or stopper. The standard inspection program for the TCMS recommends this at 6 monthly intervals. Past experience has shown that this change‐out operation can be achieved in about 6 hours.