Not knowing the exact quantity of water in the ballast system is dancing with fate which can have catastrophic results.
Is this as potentially dangerous as you make it out to be?
As a company involved with the Oil and Gas upstream exploration segment for the past 20 years, we can confidently state that the failure to accurately record ballast tank volume manually via sounding tubes and accurately knowing the quantity in that tank can indeed result in a drilling unit listing within minutes. This is serious stuff!
What are Sounding Tubes?
These are tubes running vertically from a deck level down to the bottom of a ballast tank. Marine personnel on board use a 'sounding tape' via the sounding tube to measure the level of water within the ballast tank. The level indicated on the tape is converted to actual weight for each individual and uniquely different ballast tank.
What is a ballast tank?
The drilling unit, in a Semi-Submersible rig, has several tanks in the pontoons below the water. They are filled with seawater as means of ballast. There may be 50 or more of these tanks strategically located to enable the Ballast Control Operator (BCO) trimming abilities in keeping the drilling unit on an even keel, being level. This is no easy task as daily operations influence the fine balance continuously, for example; pumping SBM, heavy loads, taking on fuel, cementing, heavy weather and deck load all upset the balance. As these tasks are taking place, the BCO continuously adjust the ballast tank content to counter-balance a listing/heeling effect.
What if I have an electronic tank volume measurement system?
These devices can be extremely accurate when calibrated properly, however, not only do they fail, but they may send through inaccurate information. The only sure way of knowing the volumetric content in a ballast tank is via physical sounding on a regular (weekly) basis. Relying on electronics alone is a dangerous gamble, hence the compulsory sounding tubes for manual soundings.
What is the danger if I don't know the volume of water in a Ballast Tank?
There are two fatal dangers here; if you think you have the counter-weight to balance the rig when an upsetting force is applied, and you don't, the unit will list heavily towards the upsetting weight (e.g., transfer of mud), or, if you thought you had less weight on the counter-balance side, but in reality, you have excess weight, then there will be over compensation, resulting in a list. The second danger is called the Free Surface Water effect. When a tank is not completely full, the remaining water 'sloshes' around and has a detrimental effect on stability when suddenly pulled to one side of the tank due to rig movement. It pulls/pushes the water force to one side, much like a half full tub of water that is disturbed. Any listing is abnormal and requires immediate intervention to rectify.
What is the cause of blocked Sounding Tubes?
Rust is the main cause. The flaking of the inner pipe wall dislodges and causes an obstruction which then acts as a barrier with more than rust scalings piling onto the original, eventually causing blockages up to 6 metres long. Other causes are foreign items (rags, billets, pipes, tools) being dropped down the tube. Poor design also causes blockages with tank slurry becoming trapped, which hardens to a solid clay like blockage.
How can I confirm that none or any of our Sounding Tubes are blocked?
The Barge Engineer/Supervisor will keep a record of periodic manual soundings (this is compulsory). These manual soundings should be within a three-month periodic period; however, vigilant operators will have a monthly, or even weekly sounding schedule recorded. Any discrepancies will be clearly visible by looking at the difference between sounding depth and sounding obtained.
How can I have the Sounding Tubes 'unblocked'?
Offshore Support Services (OSS) Pty Ltd operates globally. Contact Anita on +61 438138526 or via email firstname.lastname@example.org. Anita will arrange for a survey to be conducted which includes full video and still digital footage of the problem with a report of the extent thereof and the proposed solution.
Can this be done whilst at sea in full operational drilling mode?
Absolutely. The techniques developed allows OSS to unblock a tube whether the tank is in use or not – dry or wet. No tank access is required as all work is done from deck level in close consultation with the Barge Engineer and BCO.
Once unblocked, what can be done to prevent them from blocking again?
As a short-term solution OSS can coat the internal walls of the tube to prevent any further scalings from becoming dislodged. However, the ideal solution is to have the tube walls lined.
How many personnel do you bring on board?
A typical technical marine crew involve three personnel. The equipment varies according to the specifics of the sounding tubes and their dynamics.
How long does the process take to clear and refurbish Sounding Tubes?
Each blockage presents itself with a unique set of challenges, but on average a tube will take 24 hours from start to finish (unblock, refurbish, coat).
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