At 0652 on 8th August 2016 the semi-submersible drilling rig, Transocean Winner grounded on the Scottish island of Lewis. The rig was unmannded, there was no loss of life and a small diesel spill which did not cause in any pollution. The accident was as a result of the tow wire from the tug ALP Forward parting in adverse weather. The accident was investigated by the MAIB and their report published in September 2017.

 

THE VESSELS INVOLVED.

The ALP Forward.

The ALP Forward had been built as Ursus in 2008, owned and operated by Harms Bergung. The fleet had been purchased by ALP, in somewhat mysterious circumstances, apparently under protest from the managers. This could have been as a result of ships and oil rigs often being owned individually by a single ship company or a group of investors and therefore, in some circumstances, vulnerable to take-over.

The ship was a moderately sized anchor-handler 65 metres long, with 19,000 bhp available from four engines driving two screws and giving it a bollard pull of a little over 200 tonnes. It was provided with a conventional towing winch, a single tow drum which had on it 1600 metres of 76mm wire, a workdrum with a similar capacity, and in all probability a spare tow wire held on a storage drum on the deck below the pilot house. The breaking strain of the tow wire had been 485 tonnes when new.The tug was provided with a tension meter with a readout in the pilot house. The tug had a crew of 17, including master who had considerable towing experience as well as three other deck officers.

This is an excellent view of the deck of the ship which was to become the ALP Forward taken by Jeff DeJean. What is known in the report as the "gog-chain" can be seen in the middle of the deck. I don't think there are any rules for this item, but we always used to have the shackle connected to the work wire, which would allow us to extend it as required. It made turning easier.  

The ETV (Emergency Towing Vessel) Herakles.

Within the report there is a section on the history of the ETV activities round the UK coast and how it had ended up with a single vessel based in the Orkneys. The Herakles, unless contracted to do so prior to the event, could never have got anywhere near the casualty and the vessel on the location – the ALP Forward - was in any case best placed to recover the drifting rig could it have been done. So it seems likely that the MAIB’s interest was in promoting the principle of the ETV rather than the application to this event, particularly because those who don’t want to spend the money have used this example as a means of showing that the ETV concept is a step too far.

The Transocean Winner.

The semi-submersible Transocean Winner had been constructed in 1983, and would be described as a mid-water drilling rig probably capable of operating in water depths of about 700 metres. It operational capabilities are not material to the event, since it was unmanned and was on its way to Malta, for sale or breaking up. However the required bollard pull to maintain it stationary in different weather conditions is important and this information was contained in the rig’s operating manual. Like all semi-submersibles its upper structure was supported on columns, themselves positioned on fore and aft pontoons. The Winner had two columns on the port side and two to starboard with transverse bracing provided between the forward columns and the after columns. When operating the rig would be sunk up to a point where the pontoons were well under the sea, minimising the effect of wind and wave action on the structure as a whole. The Transocean spec suggest that its transit draught would be 65ft, and operating draught 79ft. In its operating mode the rig would be held in position by eight 15 ton Stevpris anchors, assisted if necessary by two 2400kw thrusters.

PREAMBLE

The Transocean Winner was laid up (cold stacked?) in Stavanger, and in 2015 the rig was added to the list of those which were to be disposed of. However the decision had to be taken whether to carry out the tow manned or unmanned, and it was decided to carry it out in the latter condition. In any case, since the rig had been cold stacked the work required to bring it into an operational state would have been, one assumes, excessive. It would also be considerably cheaper to do the job unmanned. The anchors had been removed, both to decrease the draught and possibly to recover them for further use.  

The rig had been fitted with a new towing bridle for the voyage which consisted of a couple of lengths of chain connected to the Smit brackets on the pontoons and then two lengths of 76mm wire to a 180t SWL monkey’s face and single 76mm wire from that. All the connections being 120t SWL shackles.

For the tow the rig was provided with an emergency towing arrangement which consisted of a 60m of 76mm wire, apparently secured to the port aft anchor chain and tied to the hand rails at deck level. To the end of the wire 150m of 80mm buoyant rope was attached followed by a 250m messenger at the end of which was a marker buoy.

Aqualis, the warranty surveyor had a look at the tug, the tow and the route initially considering passage through the English Channel. They revieved the bollard pull which was likely to be required and came up with a minimum of 150 tonnes. ALP put forward the ALP Forward with a bollard pull of 218 tonnes which was accepted. ALP also suggested the alternative route to the West of Ireland, avoiding the congested area of the UK south coast. Aqualis approved the alternative route, their recommendations including the following:

4. The Semi-Submersible “Transocean Winner” is to remain in the same ballast condition throughout the voyage.

And

6. All emergency towing arrangements and column mooring arrangements are to remain in place and available for use at all times.

The various managements collectively agreed that weather condition for departure should be suitable for three days these being: maximum wind 15 knots, Max Sig Wave height 2.0 metres.

There were a number of formal documents intended to provide guidance to rig owners and towing companies as to the safest means of carrying out an ocean tow. Transocean had provided the Transocean Winner with an Operations Manual, there was industry guidance provided by DNV-GL and ALP themselves had available a “Towage and Anchor-Handling Manual” which referred to the DNV and the MSC guidance. The investigation report quoted some of these document and a selection of extracts are reproduced here.

From the ALP Towing Manual we find the following:. 

Length of tow line and Tow line – shortening

In areas of shallow water, water depths less than 100 meters, the Tug master will determine well in advance to shorten towline and adjust speed accordingly. Crossing over water depths less than 200 mtr must be avoided where possible during the sea passage, in view of the catenary of the towing line and the maximum draft of the tow. Proper calculations of the catenary are being carried out every watch accordingly. 

The tow line length will be determined during the voyage by the Tug master, according to weather and sea conditions and available water depth. During the voyage the towing line length will be adjusted every 24 hours to avoid chafing of the wire at the stern roller. 

When the convoy meets deteriorating weather conditions, which have not been predicted in the daily weather forecasts, the Tug Master will inform ALP Maritime immediately. 

ALP Maritime Services will contact and discuss with the Companies designated Meteorological Agency and request for immediate weather updates for the vessel. The weather updates must include the expected weather conditions and extra weather outlook services. 

Upon receipt of the weather updates and weather outlook the master will decide if course/speed alterations are required, all in line with the Motion Assessment (Meaning unknown)of the Transocean Winner. In consultation with the OIM \Master and Tug Master, at any time during the passage whenever the sea/swell/ waves increase sufficiently to cause heavy slamming on the lower horizontal bracings, the unit should be ballasted down to survival draught until such time as conditions improve to allow the passage to continue. The unit is to be stopped in the water during ballast and de-ballasting operations. [sic].

The manual went on to say:

Consideration should be given to water depth and catenary. The goal is to pay out as much wire as possible in relation to the anticipated water depth, as this gives maximum protection against shock loads.

A passage plan was drawn up in accordance with the attached copy of the appropriate chart giving a closest approach to the coast of 12 nm.

A close lookout this chart shows that the intended route did not ensure that as far as possible the water depth was more than 200 metres (As required by the ALP manual). Perhaps they thought it didn't matter.

THE SEQUENCE OF EVENTS

July 26th 2016.

 The ALP Forward arrives in Stavanger.

August 1st 2016.

Aqualis Offshore Marine Services approve the tow and the various managers agree that a three day clear weather forecast is required before departure. 

August 3rd 2016.

Transocean and the master of the ALP Forward agree that conditions are suitable for the departure.

1300. Tug and tow depart Stavanger. Tow wire set at 760m. Anticipated arrival at Malta 21st August.

August 4th 2016.

Afternoon. The ALP Operations assistant and the master of the tug discuss the weather. A deepening low out in the Atlantic is discussed. The assistant suggests that the master go west – more sea room. The master says he will maintain his current plan for the moment.

August 5th 2016.

0300. Waypoint 11 is reached and the tug and tow alters course to pass north of the Orkneys.

August 6th 2016.

0656. Tug and tow pass abeam of the Flannan Isles, heading SW.

1200. The low in the Atlantic has deepened and wind is now gusting to 35 knots. Speed over the ground is reduced to 5.7 knots.

1400. The tow begins to take control and the tug’s course becomes erratic. The master tells the watch officers to maintain a maximum tension of 120 tonnnes.

1500. The tug and tow are 12.85 miles northwest of St Kilda and course is altered to 194 deg. The length of the tow wire is extended to 909m. Water depth 156m.

2115. The tug is being dragged backwards by the wind force on the Transocean Winner.

2259. The tug submits the daily report to the company. It does not tell anyone that the tug and tow are now moving backwards.

August 7th 2016.

0035. The wind veers southwest and the tug and tow, still moving backwards begin to track northeast parallel to the Scottish coast.

0900. Some progress ahead is made.

0941. The wind force is increasing and the tug and tow are now being dragged backwards towards the Flannan Isles.

1048. Master calls the DPA and tells him that the tug and tow are being dragged towards the Flannan Isles, but he says that he expects the weather to abate.

1200. The wind is seen to abate a bit and the master puts the wind on the beam and tried to steam away from the coast. 

1420. In shallower water the tow line is shortened to 568m and the master attempts to turn the tug and tow to the north. What is described as the “gog-chain” breaks and the tow wire sweeps across to the starboard rail knocking off the “stag horn” the towing stop. The wire then sweeps back across the deck and hits the port stop. This has resulted in the master’s ability to control the situation is reduced.

1634. The tug and tow pass 3.7nm north west of the Flannan Isles. The wind speed increases and the tug and tow are being pushed backwards at a speed over the ground of over 4 knots.

1805. The master telephones the coastguard on Stornaway appraising them of the sitution.

1844. The coastguard determine that the nearest towing vessel is the ETV Herakles which has to steam from the Orkney Islands. Its ETA is 0615 on 8th August.

August 8th 2016.

0236. The wind has dropped sufficiently for the tug to begin to make some headway, and the tow line is extended to 740m.

0421. The towline between the ALP Forward and the Transocean Winner fails close to the rig. Thereafter the tug makes attempts to find the buoy which indicates the end of the emergency tow line. The messenger is seen streaming between the pontoons and so is inaccessible. 

0652. The rig grounds on the north coast of Lewis.

THE INVESTIGATION

To assist with the analysis of the accident the MAIB commisioned three studies. The first was a weather hindcast by the UK Met Office which, as we would expect, determiend that the seas had been rough, or very rough with winds gusting to 59 knots during the afternoon of 7th August. They also had the remnants of the tow wire tested, and it was found to be internally degraded, the strength of the wire possibly being reduced by 21.3% . In addition they had a study carried out as to the capability of the tug to hold the rig against adverse weather in a variety of conditions. 

The investigators concluded that the voyage planning had not considered sufficiently the effect of high winds on the structure of the rig, and that by the time the master realised he was in trouble he did not have the sea room or the ability to control the rig in the worsening weather.

They then assessed the loading that caused the failure of the tow wire, which had already been found to have had a reduced strength, and of course as the rig was pulling the tug back towards the shore it was necessary for the tow line to be shortened in order to keep the catenary off the seabed. On 7th August the tow line hand been reduced to 568 metres which, it is thought, had caused some considerable weakening of the wire, and the analysis considered that even if the tow wire had been brand new it still might have failed.

The investigators went into some detail about the catenary of the tow wire, and for those not involved in the towing business, tug masters have to hit a median between the length of the tow wire, the longer the better, to reduce the shock loading, and the lepth of the caternary, the sag of the cable towards the seabed. And the manuals provided cautioned the master of the  ALP Forward against having too much cable deployed in case it dragged on the seabed. As a result the correct length of tow wire, which was at leasty 800 metres was seldom deployed due to the anxiety the master felt about the wire touching the seabed. And it is obvious that as the tug and tow neared the shore, the water depth was less, and therefore the tow was shortened. Actually with a bollard pull of 120 tonnes used a much longer tow wire could have been deployed for much of the time.

The route taken by the tug was assessed and the fact that all the interested parties had accepted it mentioned. However it was determined that a possible alternative route might have been through the Minch, where there would have been more shelter, and anyway maybe 12 nm from the nearest land might not have offered sufficient sea room.

The ALP Towing manual was considered to be less than adequate for the operation of towing an unmanned semi, particularly the Transocean Winner.  In addition to a lack of some information and guidance there was an explicit statement that the rig should be ballasted down to survival draught if the seas were slamming on the cross members. The lack of information about the effect of wind on the rig and hence the tow it was felt had resulted in the master not taking the appropriate action to gain sea room early in the passage.

There were recommendations for an improvement in the process of approving unmanned rig tows, and Transocean decided tha tthey would send a “Rig Move Supervisor” on unmanned rig tows.

COMMENT.

Oh dear, we would say. Surely they could have avoided this disaster? Reading through the report I get a sense that the tug master was not used to towing semi-submersibles. Of course I could be wrong. But it seems that the constant problems with the catenary relate more to tows of ship shapes, which in general slide through the waves and it is quite difficult in fact to apply high levels of bollard pull, and if you can’t you’ll get a lot of catenary. Towing an oil rig is like towiing a brick.

And it is difficult to believe that they did not give themselves sufficient sea room, even after discussion with the assistant back in the office. It is also suggested that the captain did not realise how much the rig would be affected by windage, but anyone who has ever towed a semi would surely be aware; they are just like sailing ships and can make five or six knots downwind and in adverse weather you ballast down and wait for things to get better. The report says that if the rig had been ballasted down it would not have made any difference. Well, had the Winner been manned the whole situation might have been different. The crew could have operated the thrusters, adding several thousand BHP to the operation, without increasing the weight on the tow wire, and finally had there been any anchors they could have deployed them, even in quite deep water. But then we would have had to consider the safety of the crew, so who knows.

As early as 4th August there was a discussion about gaining sea room, but maybe the captain was offended by being offered advice by an office wallah. If you have a familiarity with the progress of depressions out of the Atlantic towards the UK you recognise that very strong SW winds can accompany them, although we accept that this is less likely in August.

There are also sprinklings of disinformation where no-one, particularly the warranty surveyor has realised that some of the instructions relate to manned semi-submersibles rather than to one up on the pontoons with no-one on board. The analysis of the final hours suggest that even if the tow wire had not failed the rig would still have gone aground.

But the manuals lacked the guidance that might have helped the master in his decision making, particularly the need to gain sea room when threatened by adverse weather. And incidentally had he chosen to take a course further out into the Atlantic, more tow wire could have been deployed even using his extremely conservative approach. The intended course was available to Transocean, Aqualis and ALP so why did no-one just tell him? Only too often managers rely on others to know more about what’s going on than they do, and its easy, you might say, to be wise after the event, but it’s not difficult to be wise before the event either.

 
Copyright © 2019 Ships and Oil. All Right Reserved.