PAPUSHA A.N., FEDOROV I.V., SHTRASSER V.V.,
Murmansk State Technical Univercity. Russia,FGUP "Arkticmomeftegazrazvedka", Russia

It is known, that equipment for offshore and onshore chilling have to provide safe and profitable working operations, but offshore drilling process is sufficiently more difficult because of specific technical devices and systems, which never been used onshore.

Some of this devices are:

- System of anchoring and dynamic positioning of floating drilling rigs and vessels;
- Underwater control wellhead and blowout preventor equipment;
- System of heaving compensation on floating drilling and others.

It is necessary to say. that equipment, which is applied for operations on marine drilling rigs and vessels, can not be designed without preliminary dynamic and strength calculations of its component parts.

The first time, these elements are drill strings with underwater part, which is hardly loaded due to sea conditions. Feature of offshore technical tools projection is not suitable for automatic applying of onshore projection methods for offshore because of some specific factors which influence on choice of project solution.

It is necessary to allow for:
- sea flow;
- movements of rig (vessel);
- wind and wave stress;
- emergency situations.

These cases can decrease reliability of underwater part of drill string and, consequently, safety of work operations. Problems of offshore well projection depend on geographic position of oil (gas) field. Arctic seas of Russia are characterized as a region with low temperature and ice-covered. These factors complicate the task of technical devices projection. Besides, dynamics and performance of underwater part of drill string influence on dynamics and strength properties of its underground part.

Such problems do not exist for onshore drilling; therefore it is necessary to create a new methodology for sea working operations, which includes the tasks:

? stability, strength and reliability of extended technical devices;
? dynamics and statics of extended-elastic constructions for exploration and exploitation of marine oil and gas fields.
To solve these problems, we applied the modem computer program Mathematic for modeling the drill process.
There are great achievements in projection and construction of offshore wells, but some questions in fundamental problems:
? nonuniplanar bending of drill string (underwater and underground part).
? riser bending,
? its stability and strength in unloaded condition and interaction with: floating unit, system of controlling-system of compensation of vertical wave oscillation, sea flow,
- underwater floating objects (for example, emergency impact of object and riser or drill string)

are not studied now.

A number of these problems include bending strength and stability of drill string and riser during its exploitation in Russian Arctic seas. It is one of the most responsible elements in drilling process. Therefore, success of operations depends on its safety work and how these elements have been constructed.

The next and the same important question is emergency situation with extended-elastic construction. This task have not ever been tried to be solved by Russian scientists. Though, answering this question is important for environmental analysis, for example.

Another part of problem, which have not been studied by Russian researchers is dynamic and static strength of drill string and riser during marine earthquakes. In this report such case will not be studied but it will be done later.

As it is known, the first who set a task about drill string strength was A.E. Sarojan in his monograph [1], where was described the reasonable choice of drill string construction. Sarojans calculations based on classic way of research of compressed by self-weight bar (problem, first was set by Eiler).
Developed in monograph [1] ways to solve the task of drill string bending strength as a flexible bar (model of drill string) gave only approximate estimates of strength properties of drill string, and did not describe projection of riser. Besides, specific features of offshore drilling limited by case of sea depth is more less than well depth, and the spudding-in of well did not mention.

Sarojan in [1] set a problem in general form, but solving of practically important tasks (straight and stability of drill string) was in simplified form. The task of riser construct strength in [1] did not solve. This approach significantly limits the practical value of theoretical results, because there is not the solving of this question in general form.

One of the main documents of oil and gas industry "Instruction of drill string calculation for oil and gas wells drilling" describes the check of static and fatigue strength of drill string section (rotary drilling on floating units) using Sarojans approximate formulas, limited by maximal sea depth only up to 150 meters. For sea depth more than 150 meters calculations of drill string stress, induced by horizontal movements of floating unit, rolling, sea flow and others do not exist.

Consequently, the problem of important parts of floating vessels projection (such as drill string and riser) use to be actual.
Now modem computer technologies of calculation and construction are introduced in the practice of offshore drilling units projection and the task of drill sting and riser construct strength can be solve in new calculation programming environment. In this case it is computer program Mathematica. The main target is to engender the computer methods of symbolic solutions searching of our tasks.

As a known, analytical (symbolic) solution in programming environments enable:
- to be standard solution with different numerical realizations;
- to observe the effect of main project parameters on final result, that impossible than numerical solutions are applied;
- to be compared with well-known solutions.

So, computer methods of project tasks solving allow to approach calculation task to construction task. Success of projection becomes independent from subjectivism of decision and depends on source data and art of designer.

In this report we represent the method of drill string and riser nonuniplanar bending modeling during offshore drilling operations (using computer program "Mathematica").

The main results include the fact, that modem computer program joined with traditional and well-known mathematical methods, applied by Russian oil and gas engineers-developed a new trend of computer algebra methods in tasks of calculations of extended-elastic, ponderable cons tractions, in this case, drill sting and riser.

This method is able to solve a number of new projection problems, such as:
• calculation and projection of riser for deep-water drilling;
• calculation and projection of drill string for spudding - in of new offshore well;
• calculation and projection of drill string and riser for emergency situation (this problem has never been studied).

Symbolic solutions for a number of projection tasks can be standards for constructing engineer to have opportunity to estimate numerical results, which other computer programs or traditional methods of calculation gives.

All computer calculations for projection of drill string and riser do not limited by sea depth. This fact means that there is new way to improve the construction solutions for development of deep-water oil and gas fields of Russian Arctic seas.

References
G.N. Коrn,, T.M. Korn. Mathematical handbook for scientists and engineers.// McGrow-Hill Company Inc. New York, Toronto. London, 1971, P.720
Srephen Wolfram. The Mathematica. Fourth Edition. Mathematica Version 4. Cambridge University Press. 1999. P.1470
Structural Steel Designer's Handbook./ Frederick S. Merrit, Editor. McGrow-Hill Book Company. New York, 1972. P.765

ARCTIC SHELF OIL AND GAS CONFERENCE 2004