A different anchor thread ….

[dunedin]

How can an oil rig lose 4 out of 8 anchors? And assuming these are evenly distributed around the rig, if these were the windward ones in the storm, how didn’t it break free - or worse tip over if only anchored at the leeward side? North Sea platform 'secure' after losing anchors

 

[robmcg]

They were probably using CQR's with undersized chain and didn't reckon on the catenary. That's my guess .


I'll get my coat.......

 

[RunAgroundHard]

Quite easily. This is the second event I am aware of in the last few years. The anchors are set and tensions quite hard to hold the rig in place while it drills over the wellhead, on the seabed. When a storm approaches, the drill string is recovered and the rig disconnected from the wellhead. The marine manual will have anchor tensions, also part of the mooring plan, that need to be in place for storm conditions. These tensions are much less than the moored tensions for drilling.

What should happen is that the rig calculates the T time to storm based on recovering drill string, disconnecting the riser between rig and wellhead, ballasting the vessel for survival storm conditions and releasing tension on anchors to the storm setting.

If the anchors tension is not reduced, the lines cannot sustain the dynamic loads and will fail. If the T time is miscalculated, the anchor tension cannot be reduced because the rig needs to remain over the well while drill string et cetera is recovered.

In recent cases, it has been found that competence and understanding was less than required and tension was not released. The recovered lines were tested and found to meet the required strength. In this case, I have no idea. However, the industry is seeing a sharp decline in experienced personnel. Usually anchor lines are supplied by 3rd party and the vessel just hooks up its own lines and winds in tension with the aid of the anchor handler vessel.

 

[Neeves]

If the anchors tension is not reduced, the lines cannot sustain the dynamic loads and will fail.

for fail, also read drag.

Should have read threads in PBO on snubbers.

But more seriously - thanks for the background

Jonathan

 

[boomerangben]

Like Neeves, I suspect the anchors dragged rather than fractured, but I don’t know. We did take some of those evacuated off the rig to other platforms by SAR home on Sunday and would love to have found out more. I suspect they were glad to be on dry land.

Yesterday there was no wind but a horrible looking swell pattern. It’s amazing how quickly the North Sea changes its character

 

[RunAgroundHard]

The anchors may or may not have dragged but all 4 have "detached" themselves from the Stena Spey. Dragging is a red herring and not relevant. Also anchor fracturing is not a realistic failure mode. The anchors will dig deeper in north sea seabed sediments.

They tend to fail at the fairleads where the hawser turns sharply but there are a number of areas that failure can happen as the rode assembly will consist of a few parts: anchor, shackle, bridle, possible an acoustic release device with associated connectors, another bridle and connections to the semi subs cables via the fairleads and onto the anchor windlasses. The anchors have what we would call tripping lines, attached to large marker buoys. This line is used to both preset and remove the anchors by the anchor handling vessels.

This summer one of the AHV pulled the rigs line apart on a semi sub hired by my employer. The line was picked up from the semis sub, and the skipper of the AHV applied too much throttle and the AHV shot forward, powered up and sheared the line. Other issues are that fairleads on the hull can seize up and stop articulating, or the sheave stops rotating.

It will be subject to an investigation.

 

[noelex]

Thanks for the insights into the industry, RunArroundHard.

The shallow waters of the North sea make for a tough sailing area. I cannot imagine anchoring anything offshore away from land protection.

Oil rigs manage this with seemingly less problems than we leisure boaters do when anchoring in far more protected locations. I am sure there are some lessons that we can learn despite the obvious size and operational differences.

 

[Kelpie]

Thanks for the insights into the industry, RunArroundHard.

The shallow waters of the North sea make for a tough sailing area. I cannot imagine anchoring anything offshore away from land protection.

Oil rigs manage this with seemingly less problems than we leisure boaters do when anchoring in far more protected locations. I am sure there are some lessons that we can learn despite the obvious size and operational differences.

It sounds a bit like a grown-up version of what fish farms use- lots of very heavy anchors laid out in a grid surrounding the moored object, with permanently installed trip/marker buoys.
Many newer farms are located in 'high energy' locations where you wouldn't dream of anchoring a boat.
Everything is on a totally different scale to what we use on yachts. But of course we have to be able to lift our great and carry it with us!

 

[Kola]

Everything is on a totally different scale to what we use on yachts.

6 Ton Bruce.

 

[noelex]

6 Ton Bruce.

If they read YBW anchor forums then they would know a 10kg anchor has more holding power .

 

[Neeves]

to what we use on yachts.

And their vessels don't look like yachts either, they can have more windage



Jonathan

 

[boomerangben]

And their vessels don't look like yachts either, they can have more windage

View attachment 166729

Jonathan

I think the anchors for one of those will be a different scale again - We banged 10 36m long 5’ diameter piles into the seabed for an FPSO mooring.

 

[Neeves]

I think the anchors for one of those will be a different scale again - We banged 10 36m long 5’ diameter piles into the seabed for an FPSO mooring.

I suspect Penguin might have used anchors - the waters are/were deep 150 - 300m. Too deep for piles?

Too deep for any rode I have carried

Jonathan

 

[boomerangben]

I suspect Penguin might have used anchors - the waters are/were deep 150 - 300m. Too deep for piles?

Too deep for any rode I have carried

Jonathan

We certainly used hydraulic hammers to pile subsea structures to the sea bed at Heidrun in 350m of water so piles would certainly be an option. Your post has me intrigued so I did some reading this morning - I left the industry nearly 25 years ago so out of touch. It seems that suction piles are common in this application and indeed the Penguins field being developed in the Brent field east of Shetland (is it the same Penguin as your photo? - look like it but not sure) is being secured with piles although it is not clear whether they are suction or friction piles.

I am not aware of FPSOs being secured with drag anchors but as I say my experience is limited to the North Sea and out of date. I guess it is all down to the environmental conditions, seabed structure, vessel parameters and so on. But mooring design criteria are obviously more onerous for multi year installations rather than multi week installations for FPSOs and drilling rigs respectively so drag anchors are less likely to be used for long term applications.

 

[Neeves]

My picture is of a, or the, Penguin for the Penguin field. The picture is taken in Norway.

Interesting reply Ben,

What was interesting is that they are intent on squeezing every last drop of energy out of the field - and have invested significant sums to do so. Sitting in Australia we get a very blinkered view of global warming. In the North Sea much of the business continues as usual, with tweaks like the Penguin and the Penguin field investment, with much money still being spent. You might have better sources but Google comes up with a description of the developments of Penguin. Penguins Oil and Gas Redevelopment Project offshore UK

We have big arguments here over every new cent being invested in coal and no-one wants a marine wind farm on their coast.

Its not entirely clear here where our children are going to derive their energy and who, or what, is going to replace the Governmental revenue of coal. Maybe they are developing methods to derive income and energy from all the hot air from the various protagonists.

Jonathan

 

[Stemar]

Maybe they are developing methods to derive income and energy from all the hot air from the various protagonists.

Global warming solved! Give that man a Nobel Prize.

 

[RunAgroundHard]

... In the North Sea much of the business continues as usual, ... Its not entirely clear here where our children are going to derive their energy ..-

The perception may be business as usual but it is anything that that. For example, tax revenue from North Sea O&G extraction is very, very low, it mostly comes from sales of fuel, not profit from selling hydrocarbons into the market. The UK government gives significant tax breaks to the north sea, unlike the past where Thatcher's economy modernisation projects was significantly funded from from NS revenue.. Also, many of the large O&G reservoirs are now abandoned with peak production around 2000; also true for some other big country O&G players.

We also do more work now with less energy as efficiencies have improved remarkably over the decades, so the need is less. Alternative energy resources are abundant and capable of being utilised with existing technology, hence O&G replacement is well addressed if not currently clear. Governments will find ways of raising revenue, they are not just going to stop because coal is being replaced with something else.

The problem with older people, is that they become myopic. Don't worry, the young will address your issues.

(-;

 

[Neeves]

The problem with older people, is that they become myopic. Don't worry, the young will address your issues.

(-;

Rubbish you do not need to be old to be myopic

Jonathan

 

[boomerangben]

I do wonder about why, with a back ground of increasing environmental concerns amongst the electorate, governments are still allowing further exploitation of fossil reserves. To my mind politicians have either have personal vested interests, real concern over energy security, real concerns over the short to medium demand for energy or concerns on the reality of transition to alternative energy. I suspect it’s a mixture of all four. We see back tracking on both sides of the house on environmental agendas, specifically with oil field licensing which to me says that the reality of fossil fuel demands is very different to aspirations for alternatives.

With regard to Penguins, it is an established reserve which is already being exploited through a facility which long past its original design life (Brent Charlie). Shell used to operate at least 9 platforms in the east Shetland basin. They sold 4, have decommissioned another four and in the process of decommissioning the Brent Charlie. Of the four they sold, 2 are in the early stages of decommissioning. Also within the east Shetland basin, a further 7 platforms have already or are in the process of being decommissioned. I have no idea how many subsea fields have been decommissioned. Business as usual in the ESB is decommissioning with only a small amount of development and as Runagroubdhard says much reduced production. The Penguins field reserves are clearly viable for Shell and strategically important for the UK government. The reason why it (and other recently licences fields) are strategically important is, I imagine, unpalatable to the green direction western governments like to be seen to be taking.

But whatever the politics and economics, they still use blooming big anchors, chains, wires and shackles, one of which would provide more than ample mass for a mooring for me!!

 

[RunAgroundHard]

I do wonder about why, with a back ground of increasing environmental concerns amongst the electorate, governments are still allowing further exploitation of fossil reserves. To my mind politicians have either have personal vested interests, real concern over energy security, real concerns over the short to medium demand for energy or concerns on the reality of transition to alternative energy. I suspect it’s a mixture of all four. We see back tracking on both sides of the house on environmental agendas, specifically with oil field licensing which to me says that the reality of fossil fuel demands is very different to aspirations for alternatives.

With regard to Penguins, it is an established reserve which is already being exploited through a facility which long past its original design life (Brent Charlie). Shell used to operate at least 9 platforms in the east Shetland basin. They sold 4, have decommissioned another four and in the process of decommissioning the Brent Charlie. Of the four they sold, 2 are in the early stages of decommissioning. Also within the east Shetland basin, a further 7 platforms have already or are in the process of being decommissioned. I have no idea how many subsea fields have been decommissioned. Business as usual in the ESB is decommissioning with only a small amount of development and as Runagroubdhard says much reduced production. The Penguins field reserves are clearly viable for Shell and strategically important for the UK government. The reason why it (and other recently licences fields) are strategically important is, I imagine, unpalatable to the green direction western governments like to be seen to be taking.

But whatever the politics and economics, they still use blooming big anchors, chains, wires and shackles, one of which would provide more than ample mass for a mooring for me!!

Boomerangben is Ben van Beurden.