Prevalent worldwide, massive salt sections add to well construction challenges.

In our industry, good ideas come up like grass. But it takes determination, drive, and a significant investment to take a good idea and turn it into an accepted practice.

A blue-ribbon committee co-sponsored by IADC and MMS is pushing the development and certification of "Best Practices" for the use of surface blowout preventer (SBOP) operations offshore. They hope to have the document reviewed and released by October of this year.

So what's new about SBOPs? Actually, the idea itself is not new. Shell Brunei and (then) Sedco Forex successfully introduced and tested a high-pressure casing riser and a surface stack on the Sedco 135-A in 250 ft (76 m) of water in the 1960s. At the time, the key pre-requisite was relatively benign metocean conditions, like those found offshore Indonesia. In the mid-90s, the idea was resurrected by Unocal, and over the next few years, several Far East wells were drilled in increasingly deeper waters. According to Transocean, the progression involved the Actinia with five wells, Sedco 602 with 68 wells and Sedco 601 with 71, so the practice got some valuable experience in a relatively short time.

Basically, the technique involves replacing the bulky 22-in. marine riser with its associated kill/choke lines and buoyancy collars with a svelte 13 3/8-in. diameter high pressure casing riser and surface BOP stack that is suspended in the moonpool. At the mudline, is a remotely-controlled safety valve that provides a method to disconnect from the well in the event of emergency. The advantage of such a system is obvious. With a much lighter riser it is possible to use 3-D generation floating rigs to drill in deepwater, as opposed to 5th generation rigs costing up to four times as much.

Now Shell has taken the challenge to a new level. While early efforts involved pre-set mooring spreads, Shell has adapted the SPOB technique for use on a dynamically-positioned (DP) floater, and holds the current deepwater record for SBOPs at 9,474 ft (2,889 m), achieved last year in the Campos Basin offshore Brazil using the DP Stena Tay. Key to success was their suspension of the riser and stack using a flexible connection at the moonpool in the form of a clamshell tensioner ring that allows the vessel to heave, pitch , roll, yaw and weathervane. The arrangement reduced the amount of fatigue stress and wear on the riser. At the same time, a careful study of several years worth of metocean data and finite element analysis allowed Shell to model the prevailing stressfields. As a result, they have been able to eliminate the ball joint commonly sited at the seabed and replace it with two tapered fatigue joints at the top and bottom of the riser string. As an additional safeguard, unlike marine riser, casing riser is used only once, then it is set as casing in the next well and a brand new riser string is picked up - like rotating groceries on the store shelf.

Vortex induced vibration (VIV) has long been a concern in deepwater operations, but the smooth profile high-pressure riser is less susceptible to this problem than marine riser. And if needed, strap-on strakes can be easily installed before the riser it run. In fact, running the riser is far more efficient than running marine riser. In practice, Shell was able to run 14 double-joint stands of casing riser per hour as opposed to 4-5 single joints of marine riser. This amounts to huge savings in 9,000 ft (2,745 m) of water.

Shell's version of the seabed disconnect system (SDS) consists of dual 13 5/8-in. shear rams that can be closed in seconds using pre-charged seabed accumulators. A triple-redundant control system is used to send an emergency shut-in message to the SDS. The primary control is a coded acoustic pulse developed by Nautronix for communication with submerged submarines - proven technology known for its high reliability.

Simple, safe and reliable, the SBOP promises to deliver a step-change in deepwater drilling economics. Hats off to Shell and the IADC committee members. It's an idea whose time has come.




Comments