Matt Dalton Transocean Offshore Deepwater Drilling Inc. (Gulf Coast Region)
Division Engineer 1311 Broadfield Blvd. Suite 400
Houston, TX 77084.
To : Eddy Redd
Cc : Logan Puckett, Thomley, Donnie
From : Matt Dalton
Date : 26th March 2001
Subject : Report - Deepseas Riser Tensioner Rod Repair (C2 and P1)
Reference : DDS-8597
Introduction
On 4th – 5th March 2001, during rough weather, fluid was seen to be leaking from the centre aft tensioner cylinder (C2). The tensioner and it’s opposite were de-selected and pressured down. No visible rod damage was seen until the next weather front on the 12th March 2001. A photograph of the initial discovery is shown below:
Similar damage had been seen on both the Discoverer Enterprise and Discoverer Spirit, however, the Deepseas operating time of the system was approximately only 32 days.
During the recent rig move, we mobilized to the vessel with National Thermospray and Hydralift. The former had recently performed shop and field repairs to the Enterprise and Spirit tensioner rods. The intention was to field "repair" the rod damage on C2 and change out the rod seal, which was believed to be damaged also.
Onboard
With the tensioner cylinders attached to the SDC ring, all cylinders were stroked out to there maximum 50’ stroke. The rods were cleaned and inspected. The damage on C2 rod was inspected and marked. No other damage was seen on any of the aft rods.
The Photograph below shows the damage area of rod C2:
As can bee seen from the photographs above, the area of the coating damage was substantial. It covered approximately 50% of the circumference of the rod at a stroke of approximately 22’. The height of the damage area was approximately 8".
It would be necessary to blast away approximately 2" of coating around the damaged area to give a good bond of the new coating with the original.
The forward tensioner rods were inspected and a damaged area was discovered on the port forward rod P1. The following photograph shows the damaged area.
The damaged area was approximately 4" wide by 8" high. Again, an additional 2" would be blasted away around this area for the repair. It should be noted that no signs of leaking had been seen on this tensioner during operation and it had been online as one of the four remaining cylinders for the duration of the well.
In addition to this area, visible indications of coating thickness difference could be seen throughout the length of the rods. The indication was that of a silver sheen where the coating appears thinner than other areas.
After cleaning and inspection, the forward and aft cylinders were stroked up to zero feet. The aft centre cylinder C2 was pressured up gradually to test the integrity of the rod seal. The final test pressure was 2000 Psi for 2.5 hours with no visible signs of leaking fluid or pressure drop. This gave confidence that the seal was satisfactory on a "good" section of rod, however further testing was planned for after the repair.
The same test was performed on cylinder P1, even though no leaking was witness during operation, with the same results.
We went on to blast the damaged areas on C2 and P1 and spray on the new coating. The coating was sprayed to give approximately 8 thousands of an inch above the measured diameter of each rod. The repair areas were hand finished to as near as practicable the tolerance stated by Hydralift that is, 2 thousands over, 5 thousands under. The base line figure taken (as all rods resulted in different diameters – see Figs 1 and 2 for micrometer readings) was done two feet below the repair areas.
On completion of the hand finishing, the areas were sealed and the rods cleaned.
The repair patches were measured and are shown in figures 1 and 2. These readings will be used in the near future to see if these areas are "wear" areas. New readings will be taken on the next rig move and a comparison made.
Figure 1 – Micrometer readings taken on Tensioner rod P1 Patch.
P1 rod diameter – 9.070"
(C1 rod diameter – 9.065", S1 rod diameter – 9.063")
Figure 2 – Micrometer readings taken on Tensioner rod C2 Patch.
C2 rod diameter – 9.055"
(P2 rod diameter – 9.050", S2 rod diameter – 9.051")
After completion of the repairs, the aft C2 cylinder was stroked up and down over the repair area (approximately 22’ stroke of rod). The port P2 and starboard S2 cylinders were pressured down, all 6" valves were closed and all 3" valve closed 50%. The SDC ring was stroked up and down on the C2 rod only, requiring 300 – 350 PSI to stroke up. The cylinder C2 was visually inspected during the stroking (six times) for leaks. Non were seen. The SDC ring was stroked up to zero stroke and the cylinder C2 was again pressure tested to 2000 PSI. The pressure was maintained overnight and so the decision was made not to change the seals.
Cylinder P1 was also tested in this way and held 2000 PSI overnight with no pressure drop.
Conclusions
As reported previously by the Enterprise and Spirit, we must work with Hydralift for a solution to this rod coating problem. For the most part, the damage appears very similar throughout the vessels and occurs in the same area of the rods, i.e. approximately 22’.
In the short term, we should plan for similar patch work to be required on the remaining cylinders and so regularly inspect the rods, especially during inclement weather. During the next rig move, measurements of the rod diameter at the repair areas should be taken and compared with the readings in Figures 1 and 2. This would give an indication of wear in that area.
Any sign of fluid leak should be managed as it was after the discovery of the C2 leak, that is, de-pressurize the affected cylinder, as the prompt action of the Subsea Department more than likely saved us a seal change.
In the long term, I believe we have to look closer at the coating in use, it’s application and the method by which it is applied, and decide whether a different coating would be more suited to this application. Should the position system rod machining be a factor in the damage, we should change the rod design all together and seek an alternative method for position indication.
As we have no "good" spare cylinder rods or complete cylinders we should try to formulate a plan as soon as possible, otherwise we may find ourselves in a situation where we cannot field repair and will have our equipment down. Had the damage on P1 caused a cylinder leak, we would have had only two of the six cylinders in service – the last chance before major downtime and/or equipment damage.
NATIONAL THERMOSPRAY, INC.
5120 Oak Ave.
Pasadena, Texas 77503
Contact Ken Norris
OFFICE: 281-373-0609 or 281-991-1133
Cell 713-898-6844
Fax: 281-991-9911