Deepwater Tieback-Cementing-Design Challenges and Engineering Approach
Authors: Wilson Adam 
Published: March 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 178884, “Deepwater Tieback-Cementing-Design Challenges and Engineering Approach—Gulf of Mexico Case Study,” by Anouar Elhancha and Mohammed Dooply, SPE, Schlumberger, and Brian Koons, SPE, and Muhammad Saleh, Chevron, prepared for the 2016 IADC/SPE Drilling Conference and Exhibition, Fort Worth, Texas, USA, 1–3 March. The paper has not been peer reviewed.

The conventional design configuration of nested liners does not always allow for meeting the Bureau of Ocean Energy Management, Regulation, and Enforcement’s worst-case-discharge (WCD) criteria, and the long-string configuration presents operational limitations. Tieback casings and liners, on the other hand, enable reaching the well objective while meeting the WCD criteria. However, tieback casings and liners present several operational challenges during the well-construction phase. This paper focuses on cementing-design challenges and discusses the engineering techniques used to approach them.

Well-Construction Design Changes

To meet WCD criteria, existing well-construction designs for the deepwater Gulf of Mexico must be modified significantly. Deepwater wells must survive an uncontrolled blowout at the mudline, and the well must survive shutting in the blow-out, leaving the well fully evacuated to oil from the wellhead to the flowing zone.

These two WCD scenarios impose severe casing-collapse-pressure and axial-compression conditions. The severe casing-collapse pressure comes from a reduction of casing internal pressure because of an uncontrolled hydrocarbon-fluid flow to the wellhead and possible external-pressure increase because of annulus pressure buildup (APB) of trapped annulus fluid. The severe casing axial compression load comes from casing thermal compression load because of casing-temperature elevation from heat transfer of the uncontrolled hydrocarbon-fluid flow to the wellhead and from a casing deballooning effect of the severe-collapse-pressure condition under WCD.

Several casing-design options are analyzed for advantages and disadvantages with the consideration of meeting WCD criteria. The nested-liners option is susceptible to both collapse and axial failure along the liner body and at the hanger during WCD. Running long strings to depths greater than 15,000 ft is mostly limited by static hookload capacity of the current fleet of deepwater drilling rigs. Tieback strings are normally included in the casing design, with the objective of repairing damaged, worn, and corroded existing liner or casing and providing additional protection against troublesome intervals. Tieback liners, which are also called “scab” liners, extend existing liners farther uphole and are hung inside a previous casing, whereas tieback casings are run to the mudline and seated at the wellhead.

Several factors must be considered in tieback-string design. These include annular clearance, burst and collapse rating, hold-down capacity, and equipment performance against pressure and temperature conditions.