On the exploration side of deepwater drilling, progress toward breaking the direct relationship between water depth and operation costs has been slow. This relationship has resulted from contractors taking a ‘conventional’ approach to rig design where larger and more complex pieces of equipment such as BOPs and risers for deepwater drilling have increased rig size, and thereby cost.

Kiriman dari : A_Dharmawan

Surface BOPs free modest semis for immodest depths

from: Offshore Engineer

by: Marshall DeLuca

Thursday, March 01, 2001

On the exploration side of deepwater drilling, progress toward breaking the direct relationship between water depth and operations costs has been slow.This relationship has resulted from contractors taking a ‘conventional’ approach to rig design where larger and more complex pieces of equipment such as BOPs and risers for deepwater drilling have increased rig size, and thereby cost.

Work on this front has been aiming at developing technology to reduce rig costs by allowing smaller and cheaper moderate depth rigs into the not-so-moderate water depths of the latest newbuilds.

To date, several joint industry projects are underway, such as dual gradient and composite materials, that aim to meet this goal. But commercialization is still some way off, with testing only now getting underway.

One way of beating the curve has been developed, however, by re-visiting a relatively old offshore industry technology – the use of surface BOPs or surface stack on floating rigs. This concept, while so far used in a relatively exclusive environment, has begun to catch on and operator Unocal has employed the use of surface BOP applications extensively while other operators are also exploring the concept.

‘We have utilized and designed several configurations for the surface BOP system including the use of the existing 18 ¾ in subsea BOP in surface stack mode, but the current rig, the Sedco 601, utilizes a 13 5/8 in land BOP suspended on a hanger system in the moonpool of a moored semisubmersible,’ says Glen Olivera, deepwater superintendent at Unocal Indonesia Company (UIC). We also have several different designs for varying severity environments.’

The current hanger system is a custom-built ring receiver plat lowered and stabilized on side rails fitted with sheaves on the top.

These sheaves are connected to the standard riser tensioners on the rig and support the entire weight of the stack. The stack then connects to 13 3/8 in casing which is run into the seafloor and acts as the riser, providing a much lighter, high pressure riser configuration. The pipe which is used as the riser section is run in the ground as casing in a later well.

With this standard casing/land BOP configuration alleviating the need for riser and riser handing capacity and subsea BOP requirements, a rig with less than maximum payload/top tension and space capacity can be used in deeper water. This drastically reduces rig costs and moves the industry a long way towards the goal of extending the operating envelope of smaller rigs into deepwater.

While this decreases the major charge of rig time, the key driver for it, according to Unocal, is time savings. Without the need to run the BOP to the seabed and install the riser, Unocal has proven it can make the whole operation faster and thereby lower well costs.

But the concept of using surface BOPs on floating rigs is not new. In fact it was the only method of operation before development of subsea BOPs. The first drilling operation of this kind took place in the 1960s in shallow water off California with the Cuss-1 drill barge, and for Shell offshore Brunei using a land BOP with Sedco 135-A. The subsea BOP was consequently developed out of a need to be able to disconnect the rig from the well in cases such as station-keeping failure.

Saturation exploration

Some 35 years later, in 1995, Unocal revisited the idea as part of a new exploration program in Indonesia. The company saw that after decades of drilling in the region, its development costs were decreasing an average of 20 to 30% per year, but exploration costs remained high.

‘We asked why can’t we do this with exploration?’ says Kevin O’Donnell, presently drilling manager, deepwater Gulf of Mexico with Unocal Deepwater USA, and previously stationed in Indonesia. ‘We wanted to approach our exploration business in the manner we had approached development. We felt our costs were high and our exploration success was lower because everything was spread out and the programs were intermittent so we couldn’t learn as fast.’

Analysis of the two programs led to the development of what the company called ‘saturation exploration’ or ‘SX drilling’. The SX concept aims at drilling a large number of holes by using inexpensive techniques such as slimmer holes, running fewer strings of pipe and eliminating much of the drill stem testing to expand the knowledge base and lower overall costs.

O’Donnell refers to the program as risk management. ‘Basically we said we would not take any additional risk at the surface with personnel or equipment, but we would take additional risk in that we might lose some well bores,’ he says. ‘We felt we could cut our exploration time by an estimated 70% by doing this, and that we might lose one in ten wells.’

Additionally the company used what it calls geoscience for drilling. ‘The old paradigm was: we need all the data we can get because we might not get anymore. That extends well time and increases cost so you don’t get as many shots.

‘So it gets to ‘would you rather perform a drill stem test and work this to 99%, or would you rather have another penetration over here with logs in it?’ And when you really start looking at it, mot would want another penetration.

‘It doesn’t mea you are indiscriminately limiting data collection. What you are doing is making sure you collect only the data you need, you are justifying those needs, and that you collect the data in the sequence that you need it,’ O’Donnell says.

The company originally started in the shallow water and in the first year drilled 30 wells as compared to previous year averages of between five and six wells. In addition well costs were cut from $5.5 million to $1.3 million with this technique on shelf exploratory operations and around 30 million barrels of reserves were discovered.

‘The whole concept was tat you create an envelope, you do your design work, you push to the edge of that envelope and you measure everything you can,’ O’Donnell says. ‘We measured wind, wave, weather, and currents.

‘We put strain gauges on everything to measure what was going on at the surface and the riser. And when you hit that envelope you analyze, you re-design, and you create another envelope and just start pushing yourself out step-by-step.’

To get deep, the company undertook a rig upgrade program to increase a 600ft water depth rig to 2500ft using the surface BOP technique in combination with pre-set moorings – an industry first at the time.

The company then assessed the drilling requirements and performed extensive detailed analysis, risk management assessments (hazops), and engineering to develop system upgrades that allowed it to move capabilities into 5000ft and currently beyond 6500ft water depths.

Selengkapnya :