Too small for the job

Dear Sirs,


Instructions received
We are instructed by XXXXXX to provide an opinion on anchor handling offshore India in deep water in the Bay of Bengal with water depths down to 1,200 metres and with strong currents at the location with up to 6 knots being encountered. We understand that ONGC is asking for a vessel with DP1, a maximum of 120 tonnes bollard pull and with 300 tonnes lifting capacity on the anchor handling winch. The scope of work for anchor handling is to work down to 1,200 metres water depth with a combination of 76 mm stud-link chain and wire, 15 tonne Bruce anchors, in an 8-point mooring. Rolf Berg Drive AS have asked for our opinion on the required vessel capacities.

The make-up of the mooring legs
In 1,200 metres water depth, each leg of the 8-point mooring will be made up from the following components:
· 2,100 metres of 76 mm diameter wire, unit weight 25 kg/m
· 1,200 metres of 76 mm diameter chain, unit weight 130 kg/m
· 15 tonnes Bruce anchor.

Maximum power requirement
The arrangement has been modelled using in the software package OrcaFlex 9.0e. This is a finite element package capable of modelling flexible lines in the dynamic time variable environment. OrcaFlex can consider the effects of a complete environmental loading on a vessel in six degrees of freedom based on user-defined loads for the wind, wave and current environment.

In the first instance, we have investigated a worse case scenario when the rig has maximum wire out, and with all of the chain deployed with the anchor handler holding the anchor on the end of pennants prior to touch down on the seabed. This is, in effect, when maximum power is being applied by the anchor handler.

The model was produced, and run, in the static environment only, with no effects from the environment considered. Having defined an indicative stand-off position for the vessel from the rig, the wire ropes and chain were connected to an individually modelled anchor. The lines types were produced using the software standard defaults for wire rope and chain, inputting the known data where possible. The anchor itself was modelled as a 3 degrees of freedom buoy, giving greater control over the model.

Having defined the basic model, a sensitivity analysis was run to find the most indicative starting position for the vessel based on an assumed starting point on the seabed for the anchor. Once this stand-off position for the AHT was found at 4,000 metres from the rig, the AHT was stepped forward an indicative 350 metres in 10 metre intervals to represent the lifting of the anchor off the seabed.

Once the analysis of the full system was completed, two further analyses were run to look at 50% of the chain length deployed from the rig, then 100% chain length plus 50% of the rig wire length. The depth of the anchor in the water column, and consequently the length of working wire deployed, was determined by inspection.

In summary, the following runs were completed:
1) Full mooring system +1600m AHT working wire, Stand-off = -4000m
2) 50% Chain + Anchor +500m AHT working wire, Stand-off = -800m
3) Full chain + 50% Rig wire + 1200m AHT working wire, Stand-off = -2500m

The results, which are for the static case only, i.e. no wind, wave, or current forces, clearly show that, in all three runs, the in-line tension at the stern of the vessel exceeds 110 tonnes in every step with the mooring clear of the seabed.
1) Full mooring system, tension at the stern of AHT 187 tonnes
2) 50% chain, tension at the stern of AHT 120 tonnes
3) 50% wire, tension at the stern of AHT 131 tonnes

Practical considerations
Experience has shown with a 12,240 bhp AHTS with a bollard pull of 150 tonnes and anchor handling drums of 430 tonne brake holding capacity, the practical limiting water depth is of the order 1,500 metres when running all-wire moorings. This was on locations which did not have very strong cross currents.

If a vessel has to work across strong currents and there are no stern thrusters, she will have to hold the stern against the current reducing the already limited pulling capacity. For dealing with large cross currents the vessel would ideally be equipped with an azimuth thruster forward in addition to tunnel thruster(s). It should be noted that if a vessel has to lay across a current (as opposed to in line with it, where the main propellers can control the vessel’s position) then a substantial proportion of the vessel’s available power is lost to using transverse thrusters and rudder input to maintain position, this is in addition to power that may be required for tensioning or recovering anchor lines etc, etc. Therefore an operator should be guided by the old adage ‘the more power available, the better’.

If the vessel is operating in 1,200 metres water depth, then a work wire with a minimum length of about 1,500 to 1,600 metres length and diameter of about 76 mm to 81 mm would be required. The winch would ideally have two large capacity wire length work drums and be rated at 350 tonnes lifting capacity as a minimum. As you are aware, if a winch is specified as having a 350 tonne lifting capacity, this indicates its maximum pull would be 350 tonnes and that would apply only to the first wrap of wire on the winch drum. So, the winch capacity of 350 tonnes only applies to the first 200 metres or so of wire (hence a working water depth of less than 200 metres). As the second and subsequent wraps of wire (up to perhaps 9 wraps) build on the drum, so the lifting capacity falls proportionately, perhaps down to as low as 70 tonnes or less. Thus the longer the wire the vessel would have to operate with, the less lifting capacity is available.

The introduction of chain into the system increases the bollard pull and winch handling requirements. In our experience, for 1,200 metres water depth and strong currents a vessel with a bollard pull of more than 200 tonnes is required. In our view the OrcaFlex static analysis confirms this requirement. The vessel should be equipped with large chain lockers, capable of holding at least the water depth of chain if chain is to be handled as part of the anchoring process.

Crew experience/ability at these water depths is also very important. In particular, if for operational reasons another vessel has to be brought in to share the load with a J hook, poor skill levels can give rise to a major incident, in the worst case even lead to the loss of one or both vessels.

Our final practical comment is with regard to redundancy. Even the best maintained of vessel can suffer mechanical failure. If it is proposed to have only one ‘large’ tug in the field, the chances of the entire operation grinding to a halt because the one and only vessel in the area capable of doing the work is operationally deficient, increases.

Yours faithfully,

About allatsea

Sixty year old master-mariner. Absolutely gorgeous. Well wedged.....when compared to a Nairobi street urchin. Sorted, in that I haven't been in court recently. Hopelessly optimistic, terminally disappointed. Good with cats and other fluffy things. No musical talent. Generous to a fault provided it's someone else's round. Political centreist with far right and left viewpoints. A green activist from the hydrocarbon position with nuclear leanings. Averse to avarice but always happy to receive lottery wins, gifts, windfalls, legacies, prizes and wet sloppy kisses.
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