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Increased Subsea BOP weights and heights to comply with API S53 BOP configuration
23 July 2020
Hi to you all,

I have a couple of questions here relating to mid-water (moored) semi-subs BOPs and how the changes to reconfigure from the previous class 5 1A 4R or class 6 2A 4R to the current 1A or 2A 5R  increases BOP handling height and weight.
  1. Do you recall what was the typical weight and height of 2A 4R subsea BOP?
  2. What is now a typical weight and height of a 2A 5R BOP?
  3. Is there any particular make/type of a 15k WP 2A 5R BOP that is the most compact and perhaps lighter in weight?
  4. Is there a condition that points towards deploying Subsea BOP Tethering systems. (Albeit based on the Wellhead system design or the BOP weight).
  5. What experience does the forum have in deploying BOP Tethering systems and what are the actual times to deploy online v offline where ever possible. (Not the marketing sale pitch times quoted by suppliers).
  6. Did the Tethering system dramatically reduce wellhead fatigue as originally expected or was the system not as efficient as marketed?
Thanks

Ron
3 answer(s)
gomdriller
Regional Deepwater Drilling Manager EMED
Noble Energy, Inc.
Total Posts: 18
Join Date: 24/06/14
Ron;
Just a note on tethering systems, they do work, and do exactly what it says on the packet in terms of protecting the wellhead/conductor, however in doing so it moves the point of inflection up to the SSBOP stress joint, so that needs to be looked at very closely. 

Cheers

Myles
Augusto
Consultant [retired Shell staff]
SPREAD Associates
Total Posts: 277
Join Date: 02/09/05

A few thoughts from an Auld hand

I have never used a tethered subsea BOP. 

As an engineer I would have a look at the entire system, starting from the guide bases, gimbal and 36”x 30” foundation including the 36” or 30” housing extension positively centralized into the 36” or 30”, even 48”. That is often neglected or underdesigned, as the fit must be snug!

There were studies by Exxon (testing 15K BOPs), Koomey (developing new BOPs) and SINTEF (testing HP BOPs), which addresses the issue. Paul Koomey thoughts were my preferred ones.

There are solutions to reduce weight/height often compounded by the hang-off pipe ram position i.e. the landing pipe ram below the Shear Blind Rams. This forces the use of one higher dual BOP or having the SBR  at the top of the bottom dual preventer, taking advantage of the connection between the two ram preventers to create room for the Hang-Off tool. (or on the combination trebble BOP + Single Bop to have the hang off rams on te second position from above).

Obviously, we should refrain from using kill/choke in dedicated spools. We must use the inlets/outlets provided on the preventers.

The type of connection between preventers can be selected /engineered to reduce the height and consequent bending moment. There are techniques such as blind bolts, and my preferred BOP tie down rods (see ssBOP on “Discoverer Seven Seas”).

The same applies for the hydraulic connectors where we recommend larger swallow rather than hub style to resist bending moments.

There is a preference for forged pieces, in general heavier. The casting pieces may be lower in weight, but the inspection of castings to ensure good ones, may be more elaborated (see Stress Coating techniques, used, for instances, by SEDCo).


The riser top tension also plays an important role, as a not too small part of it, helps in keeping the BOP vertical

(I will try to remember – and revert – with more technical details related to this subject, which I dealt with some 30 years ago…)

DBASM
Principal Engineer
AS Mosley
Total Posts: 1
Join Date: 08/07/20
Hi Ron,

The answers to your questions are very subjective depending on the circumstances, but based on my experience:

1. Around 200-235 Te
2. 250-300 Te
3. Don't have this info to hand sorry.
4. 30" conductors, non rigid-lock wellheads, wells with high historical fatigue damage and/or large BOP stacks all point towards a requirement to mitigate loads on the well which can be done through BOP tethering.
5. They can be deployed in 12-24 hours, sometimes less, this depends on the vendors experience and the site-specific conditions. If the soils are very soft, then the tethers may need to be anchored with piles, though the aim is to use gravity base anchors as standard.
6. Performance varies between tether systems/vendors. For a properly setup system, fatigue damage can be reduced by around a factor of 200 or more. It is a game-changer for operations which have fatigue issues. The extreme loading on the subsea well can also be reduced significantly with a tethered BOP system. Extreme loading can generally be reduced by around a factor of 4 to 8. This can increase the operating limits very significantly and allow the well to sustain vessel offsets from a loss of position (i.e. DP blackout or a single mooring line failure).

There have been a fair amount of tethered BOP operations carried out now. We have been involved with ones where accelerometers/inclinometers have been used to confirm the effectiveness of the tether system and we always see a large change in system response between the untethered and tethered BOP configuration on a well.

See the following link for a case study on a tethered BOP operation to P&A some old weak wells. https://www.asmosley.com/clients/projects/bop_tethering/

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