DOT: Improving cycle time for production in deepwater
Deep Offshore Technology Conference to focus on cutting costs, increasing profits
The Deep Offshore Technology conference scheduled for Nov. 8-10 in Vitória, Brazil continues the tradition of this event by offering attendees a wide selection of enabling technology for deepwater operations. The conference, entering its 17th year, attracts attendees from the energy capitols of the world, including South America, Europe, the Middle East, Russia, Australia, Asia, and the US. Drawing on its wide base of experience, the technical conference addresses the complex and changing role of deepwater exploration and production in an evolving energy market.
The scope of this year’s conference includes more than 30 technical sessions, from lessons learned in field development to innovative tieback techniques, from field architecture and economics to new riser technology, and from project execution and management to advanced materials. The papers summarized below are a sample of the technology mix that will be available at this year’s conference.
New generation well design methods addresses deepwater HPHT design challenges
M. L. Payne
BP America Inc.
R. A. Miller
Viking Engineering L.C.
P. V. (Suri) Suryanarayana
Blade Energy Partners
Studies conducted for planning HPHT deepwater wells have identified a number of critical well and tubular design challenges involving stringent dimensional constraints (wellhead, reservoir evaluation, completion equipment, etc.) along with severe pressure, temperature, and tensile induced loads. The primary constraints include the current limitation of the subsea wellhead bore diameter of 18-3/4-in., the number and types of casings/liners required to isolate differing pressure regimes, and the required minimum dimensions for the production hole size, liner and tubing.
Traditional casing design methods are not adequate to enable viable well designs given this system of constraints, and thus a fit-for-purpose design basis is proposed in this paper. The ability and reasoning for advancing this basic design approach can be illustrated by considering the risk levels associated with potential failures of the involved casings.
Engineers have traditionally been trained and conditioned to attempt to design the well with a consistent and substantial design margin on all casings and liners. In reality, risk varies greatly depending on which casing or liner is being considered, and a more realistic risk profile for a deepwater well should define the risk levels based on the type of tubular involved.
In these discussions, assumptions are made that any failure in a particular tubular will occur at the worst location for such a failure. In cases concerning the containment of internal pressures, the worst location is accepted to be the top of the involved tubular. In such a failure of the surface casing, there is no redundant pressure containment capability of significance outside the surface casing. Thus, one must assume that such a failure would result in a release of wellbore fluids to the environment, including hydrocarbons if applicable to that hole section.
If a surface casing or other full string has failed, recovery would most likely involve running at least a patch and potentially another full string back to cover the point of failure. An alternate method might be to try to cut and pull a section of the casing and then re-run a tieback using an overshot patch, emergency tensioning sub, or other method. Attempted recovery in this manner would be both costly and inherently risky. Recovery from a liner failure is not a trivial matter; however, it could more likely be made by sidetracking, starting either with a section milling operation or by cutting a window.
Discussing potential failure modes, consequences and costs may be unappealing to engineers who are trained to design to avoid any potential failure. Nevertheless, the reality is that even a “robust” well design that appears to have large design margins against “all possible” loads still has a finite chance of failure. Such failure possibilities can stem from design limitations, installation problems, quality issues, unexpected loadings, and so on. Frank discussions about potential failure modes, consequences of failures, costs, and options for recovery are all key means of identifying better and more efficient means of well designs. In the case of deepwater HPHT wells, such discussions and identification of alternative design approaches are imperative.
Given the variation in risk levels depending on the type of string, one can consider what additional design options are possible. Results from studying this problem have identified a number of possibilities for increasing the number of casing points in deepwater well designs. One important area of study is the use of new, non-standard sizes.
To quantify the potential of non-conventional tubulars in a general sense, a special purpose well design algorithm was developed. This algorthim assumed that the required shoe depths for a given well were known. The design of each string must be performed in terms of defining the size, weight and grade of each string (whether liner or casing) to be run.
The goal of the calculation is to determine the size, weight and grade of the strings such that their performance properties exceed all loads during the service life. The constraints to the size selection typically include the design factors desired for classic burst and collapse pressure profiles, the diameter of the first string (dictated by the subsea wellhead), and constraints on the diameter to thickness ratio that can be manufactured
Given a set of assumptions on the size and type of tubing and joint connections that can be utilized, the step-wise approach to a well design becomes:
1. Select the OD of the first string subject to its constraints
2. Select the wall thickness to satisfy the pressure loads on this string, based on the appropriate design approach and rating criteria.
3. Identify the maximum OD of the next string, given the drift of the previous string, and the desired clearance in running the next string.
4. Repeat steps 2 - 3 for all strings.
The process of deepwater well design becomes one of allocating the available space given the primary constraints of the subsea wellhead system and the required completion. By “allocating” the radial space (e.g. the steel) from the deeper and less consequential drilling liner to the more critical full string, the designer is optimizing the integrity and reliability of the design from an overall standpoint. Though the lighter weight liner may not appear to be a robust design, it remains quite fit for purpose and it enables a more robust full casing string which is the most critical driver of overall well reliability relative to well control loading.
This paper goes on to illustrate a typical deepwater case using the algorithm, as well as the engineering and procurement challenges that present themselves when attempting to use the standard accepted casing sizes. The paper concludes with a detailed analysis of the factors that must be considered to successfully implement a custom Oil Country Tubular Goods (OCTG) program, and demonstrates that if conceived and planned correctly, a well can be designed in a fit for purpose and cost effective manner.•