Stability of the Deepwater Horizon

I appreciate such attention to detail... I stopped counting the number of times I've seen that particular, and other similar, misnomers on engineering docs. I recall a previous post by you (Ad Hoc) in which you require your engineers to manually edit the AQWA input files, for the purpose of eliminating the geometric warnings/errors. I won't have a problem creating geometry, but, I'm not sure if there's enough accurate information available to make an AQWA model and actually test the OP's thesis numerically.

I was thinking about analyzing/comparing the frequency response/RAOs with and without the mooring lines. To do this, I really need the proper weight tables, lines drawings, loading conditions, and mooring line specs.

The next analysis I was thinking about was non-linear/time domain in which I'd actually model the Liebherr cranes and force them to move, and measure the motions, if any, of the rig in calm water.

But, again, this is all predicated on having accurate documentation/specs, which we clearly do not...

 

Do you have RAOs of with and without mooring lines which have been validated?...and at different wave spectra and length and tensions of line?..and with some 'degree' of sensitivity analysis and same displacement and arrangement of the caissons/legs etc?

This is not in my general field of expertise, Rigs, it is far more yours than mine; I think you're the expert here certainly not me. But I use the same methodology when I design swaths and other high speed hulls; RAOs etc that is, not mooring lines.

 

I don't know if anybody truly is an expert on this...semi subs/SWATHs, or really any structure with a low waterplane to volume ratio seems to be the hydrodynamic equivalent of an annoying red-headed step child lost in a grocery store.

Given my lack of information, I was thinking about just analyzing the damn thing with and without mooring lines, and see if there's really any big difference. With 5000 ft of 8 mooring lines, there most certainly *will* be, and if we keep the analysis to the first order wave forces only (for now) then we may have preliminary results to support or deny the OP's thesis questions.

 

SWATHs and other similar types, like semi submersibles, the hydrodynamics is actually not the onerous, once you have the data to hand. No more difficult than that of a multihull. The difficultly lies in arranging the structure, layout and getting the natural frequencies sufficiently far from the excitation without compromising the aforementioned.

Sounds like a fair comment. A basic sensitivity analysis just to establish if the difference is say 2-5% or 50-60%. As an absolute is not necessary for this, just a trend or guide.

 

Some statements which I hope are true and may shed some light...........even if you have to draw your own conclusions out of the mix

1. There are many semi-submersibles which have both DP and moored capability. The effect on stability of the different modes of operation is clearly well known to the operators of these vessels, as they move between modes.

2. Some units have been converted from one mode (e.g. anchored to DP) to another in mid life. Same applies as per above.

3. There are more than a few semi-submersibles which do exhibit heel angles of 1 or 2 degrees when the crane is boomed out. Generally (as you would expect) these suffer from low GM. This is not popular, as you can imagine.

4. This effect of heeling under an applied moment when operating with low GM can happen to a moored semi with low GM as well as to a DP semi with low GM.

5. It may be more common for rig hands to complain about heel angles induced by the crane when they are working on a unit in the normally calm Gulf of Mexico or West Africa, but I would imagine that hands used to working in harsher environments where the rig often moves due to the waves and wind might complain less. No evidence for this statement - just my guess.

6. In windy climates rigs can often take up a steady heel of a few degrees by the wind pressure alone.

7. Production (not drilling) vessels working in the stormy North Sea and elsewhere have volatile hydrocarbons permanently flowing onboard. The levels of these oils and gases in the various separators have to be monitored and controlled safely, despite the motions and varying heel/trim angles of the ship. This is done every day on all these vessels - it is a matter of factoring a time averaging into the measurement of the levels. So the concerns about difficulty in accurately measuring levels in the mud return system with simultaneous crane ops are surprising (to me). However, the evidence you quoted does indicate a level of care by the drilling crew - and concern as to the accuracy of their readings - which is surely commendable.

8. There is no doubt that the presence of mooring wire/chain tension does have an effect on the vessel stability - it has to be calculated in the onboard regular stability calculation. Weight of wire or chain onboard (VCG at the winch or chain locker) depends on the water depth the rig is in - as does the tension in the outboard part of the lines (VCG acting low at the fairleader). The line tension is measured onboard. The angle of the chain or wire is usually not.

9. When designing a DP rig from a moored parent there are many different things to consider in the design. Weight and space in the deck box for the extra diesel generators. Weight on deck to remove due to absence of big winches/windlasses. Space in the pontoons for the thruster motors - meaning perhaps less space in the pontoons for ballast tanks (could be the most important factor). More space needed for fuel - less space for pontoon ballast. All these things affect the weight distribution. So the VCG and GM of the unit in various operating conditions will be different as a result of these differences in design.

10. The stability books of the DW Horizon and the DW Nautilus will show this story.

11. This part of the story is a simple subject. No non linear time domain required. I am sure the VCG and GM of the rig in its DP form was considered in great detail when the design was made.

12. It may be that the minimum GM to avoid "nuisance" heel angles was NOT specified in the design of the DW Horizon. Instead the GM would be dictated by the regulatory minimum. If true, I would bet this was also the case for the DW Nautilus. Some companies may have these "in house" GM requirements (which could be based on response to crane movements) - but others (most) will not.

13. The GM in operation depends on the variable deck load (risers, drillpipe, mud etc high up) carried - as well as the basic stability of the design. Do we know what variable deckload was being carried at the times when the "tender" behaviour (heeling under crane operations) was noted ? Was the rig being operated within or beyond its design limits ?

14. By the above, it is likely that any inadequacy in the DW Horizon stability (if indeed it existed) was not a result of an inadequacy in systems engineering in the DW Horizon project, but a lack of a specified owner requirement for this performance aspect of their vessels generally. In fact, the crew evidence you quoted refers to this problem on a number of other vessels in the fleet. So - clearly not a DWH problem only.

15. There are technical papers in the public domain describing the design of the DW Horizon. Check out OTC, MTS of Houston, DP conferences around the time of the build.

16. There are other designers (GVA, Friede & Goldman, Moss Maritime, Gusto) who market the same basic rig design in a moored configuration, DP configuration, DP & moored configuration. They could show you the effect of these modes on the GM.

17. It would seem obvious that the chain catenaries provide a resistance against "nuisance" low angle heeling that is not available to a pure DP unit.

18. Finally, if the stability of the DW Horizon was inadequate (against whatever contractual standard the original owner specified), it could have been enhanced by adding blisters to the columns. This could have been done in the design stage - or even before it left the yard if the problem was discovered after the inclining experiment prior to delivery. Sadly, the evidence of this failure of prudent design or good weight control (or both) is visible on many semisubs.

 

Thank you for the very informative reply. WRT to your point 13, at the time the telemetry data was taken the rig was carrying a full complement of drill pipe and had an 8000 foot string of drillpipe attached to the top block. The mud pits had been emptied to an adjacent support vessel.

Cheers,

Earl

 

JRMcGr

Thanks for those insights. These vessels are not my bailiwick, different field of deisgn from mine, but I am interested in them, certainly from a hydrodynamic perspective. The list of your comments/thoughts are as I suspected, but too, this data should be available 'somewhere' too!

 

Extracts from the Deepwater Horizon bridge manual:

Keeping the drill pipe centered in the hole depends on trim.

Trim can be adjusted without changing draft -- important when cementing or testing BOP.

Do not move ballast without permission from drill floor during well control situations.

Most changes of trim are do to crane movement. Moving of mud will also change trim.

Stability is to be calculated each night, using Ocean Motions software.

Deck surveys should be done at least once per week, or if large changes in deck load.

Variable Deck Load (VDL) must be calculated each night. VDL limit is 8202 metric tons at drilling draft and 7839 at transit draft. Loading large volumes of mud can exceed limit, especially in transit when riser is on deck.

FWIW.

Cheers,

Earl

 

Wasnt it a combination of factors? Including an inadequate cement job?

 

I have never worked on a DP rig. I can imagine there are differences in the sense that you mention, but have never considered them or heard of them in the past.

In terms of the monitoring of the pits on surface for well control purposes, offshore on floating rigs, in bad weather, always adds a "fogginess" to the accuracy. Whether they be DP or anchor stabilised, those differences have no effect on rig heave. or do they? Crane movement effect would be independent of rig heave effect on the pit levels.

Where the differences might be are in pitch and roll of the rig and the reaction of the DP - these factors might lead to a difference in stability that might effect the pit room levels.

 

The term "crane operations" is a bit confusing because of the number of cranes on the rig. There were two large arm cranes on either side used to lift material on and off the service vessel. There was another arm crane on the foredeck used to lift drill pipe onto a conveyor that moved up onto the drill floor.

The crane that appears to have had the most effect was a gantry crane on the aft deck that moved on elevated tracks going from one side to another. This was used to move casing sections (heavier than drill pipe) and place them on a different, aft conveyor to the drill floor when the crew was running casing.

The pits were also arranged athwartships, which magnified the effect of roll induced by the gantry crane moving a load of casing from side to side, but I suppose would tend to minimize the effect of heave, which might be why the designers did it that way.

The pit volume sensors were acoustic rangefinders, one per pit, that measured the distance from the top of the pit to the surface of the liquid. The variation caused by crane-induced roll could be significant and is clearly visible in places on the pit volume telemetry that was sent to shore. It was a common practice for drillers to call the bridge and ask that crane operations be suspended when they needed accurate pit volumes.

All in all, not a very sophisticated design. It does not seem that it would take much to incorporate motion compensation into the sensors and eliminate the possibility of confusion during well control analysis.

Cheers,

Earl

 

One per pit.

Normally, one sensor per pit is not enough, in rough weather. Two per pit in differing areas of the pit and the results averaged out, helps cancel out some of the effect. But yes, crane movement with or without load can often affect the rig pits levels as seen by the sensors. Very often in fact.

I am surprised there was only one sensor per pit. Then maybe not as corner cutting in key areas isn't such a strange thing in the industry at times.

 

Actually, Earl, the reason why I registered here was because I was hoping to find out a bit more about semisubmersible stability. I know both the Nautilus and Horizon have diagonally spread columns, unlike most semisubmersibles and my conclusion is that they bring stability in terms of port / starboard movement but maybe not forward aft? So if the rig pits were longer from port to aft than forward to starboard then that may have been a reason for 1 sensor in each pit. However, I just cannot help but think that it wasnt as well planned as that....

 

Confusion

The demise of the Deepwater Horizon was as a consequence of a combination of issues, as described by the Swiss Cheese Model. There is documentation about in the public doman that describes them. There was no one single cause, but a number of them. Operational corner cutting accounts for a percentage of the causes. The difference between the mooring systems will not have had a bearing.

http://www.bp.com/content/dam/bp/pd...nt_Investigation_Report_Executive_summary.pdf

It will be out as a movie soon.

 

Semis usually operate in two modes. In the transit condition, the rig is de-ballasted until the lower pontoon hulls break the surface. This gives the rig quite a bit of longitudinal and transverse stability.

In the drilling condition, the rig is ballasted down so that the vertical columns provide the necessary waterplane for stability purposes, and the ballast also serves to lower the c.g. of the rig in this condition.

If you look at pictures of the Horizon before the accident, you'll notice that the columns of the rig appear to be built bigger at approximately the operating draft of the rig when in drilling mode.