发表在我在Nature Blogs上的博客（英文）
Rheology is indeed involved in oil drilling engineering, including enhanced recovery method and spill control. Therefore I thought I could tell quite a lot about the disater in the Gulf of Mexico, until I found that the problem is mainly mechanics — the spill occurs because of the unexpected failure of blow out preventer (BOP). From asking “what is a BOP” I have to extend my knowledge to the whole mechanic aspect of oil drilling engineering. The following texts are the result of my books and webpages ad hoc reading and are not meant to be expert opinions.
Semi-submersed platform
Ocean has the amount of energy and resources that probably define how long in the future can human race last on this planet, energy and resources on the land approaching to the end. However the courage to built positioned bases in the ocean — a huge brave trial — is finally granted by high profits behind the abundance of oil under the seabed (among many other e.g. wind, tidal, etc.).
US was the first of the world to design and practice offshore drilling rigs. As the drilling depth increases, different designs appeared in the history. Deepwater Horizon is a semi-submersed rig, a design known to support the deepest oil well offshore ever known. Allegedly DH has the ability to drill the worlds’s deepest oil well, without using a newer design. No design, however, can be a final one and support an infinite drilling depth. DH may be a sign that the old design, not only the rig but the whole drilling design, after more than a decade of application if not misinformation, has now reached its limit. Or put it in another word, it was the unexplored depth that causes the failure. To answer against my ill-disposed, arbitrary conclusion one should know clearly what cause this event actually and is it unpreventable under the same rig design? or is it unrelated to the rig design?
Dynamic positioning
We all know that first the rig caught fire and then sank. Although up till now I don’t know how the fire started but it could hardly be related to the design of the rig of anything to do with the hardware. because the same design is used elsewhere. However the fire was really a big one. It causes the rig to sink and, catastrophically, offset from its original position. As you can see from the structure of a offshore drilling system, considering the length of the oil pipe submersed in the sea water, the rig must be precisely positioned above the oil well holding the pipe perpendicular to the sea horizon, otherwise the oil pipe must be broken. Before the accident DH was drilling a new main well. That means once broken the oil well has well extra pressure to push the oil out by itself, causing an oil spill accident. Historically offshore rigs were positioned by anchors. With the development of powerful propellers, GPS, and signal processing technology, dynamic positioning — positioning without anchors — played its role in newer drilling rigs including the DH. Under dynamic positioning the real time position of the rig is continuously analysed and fed back to the propeller system with proper commands of new strengths and directions. Because this feedback cycle is delicately maintained by AI rather than physically by anchors, once the signal/control system fail (e.g. in a fire) the rig will be suddenly turned back to a simple unchained flowing boat on the sea. The design of dynamic positioning, although hi-tech, adds risks of oil spilling accident to the system.
The blow out preventer (BOP)
The Wall Street Journal did an excellent explanation about the BOP. The blow out preventer is located at the oil well on the sea bed. As the oil pipe broke at somewhere, it leaves to the blow out preventer to stop the oil from the exit of the well (triggering it perhaps by a dropping signal of flow pressure). According exclusively to BP, however, the BOP ceased to work as expected by unknown reason. News reporters also found from the safety evaluation of DH 10 years ago evidence that BP planned not to implement emergent manual “stop” button for the BOP, i.e. a remote acoustic trigger. It seems that the producer of the BOP was very confident with their product at that time, and successfully convinced its customer BP not to implement additional remote trigger for DH due to economic reasons. All these information represent an atmosphere of over-confidence around the construction of this platform which is to drill the deepest seabed ever.
Historically when the offshore drilling task did not involved very deep sea, people use BOPs with the same design as those used on dry land oil wells. As we drill from deeper and deeper the seabed this simple copying started to cause problems. Under increased hydrodynamic pressure the hydraulic transmission system used for the BOP become more inefficient, limited by the diameter of hydraulic tubes. Thus the deeper the BOP goes down the sea, the slower for the hydraulic transmission system to response, and in the worst condition, the system just fails. Although this has been history and now these problems must have been overcome, the experience tells us that drilling from new depth poses new unexpected factors and challenges. Confidence on the BOP component base on previous success in shallower constructions is faint.
The rescue
Underestimation extended to the rescue phase of this event. As I wrote this post news that the containment domes plan has failed come to my eyes. The idea of using domes is not the first idea, but after the earlier failure of remotely operated underwater vehicles (ROVs). However, the speed of spilling was already warned of by many source of information before these failures, while BP kept assuring us of the effectiveness of the ROVs, then the containment domes by previous success of these methods, despite public questions whether these would also work under the depth we never have explore.
The ocean is no more accessible than the moon
Offshore projects, although started more than 50 years ago in USA and later in other countries (including China), are still kind of adventures. The most important obstacles — ocean currents, winds and environmental impacts, are currently still out of mankind’s prediction and control. We are proud to say that a small step deeper of the drilling rig is a large step of the mankind, just like what we call about the moon landing project. However, as BP described earlier, although it took full response of the accident but it is not BP’s own fault (from Chinese news sources). In other words, any small fault in this adventure may also be a big one of the mankind, just like what we have in the space shuttle Columbia disaster.
Does the US government treat their offshore exploration projects equally well as their moon or Mars exploration projects?
