Umbilical mandrel protects controls when surface BOP closes

New tool deals with landing string leak

Alain Chassagne
Elf
Dr. Tom Leeson
Expro

When the landing string leaks into the marine riser, BOP closure at the diverter can sever control lines. Here, a newly designed umbilical mandrel protects the controls. [21,676 bytes]

The ever-increasing requirement to provide equipment suitable for deep water oper ations continues to throw up challenges to traditional approaches in service pro vision. Extending the operating envelope of existing tool designs has limitations, and at some point it is necessary to take the alternative approach of beginning again with a conceptual design incorporating potentially radical new ideas.

To maximize the benefits of this approach, it is important to involve experienced and competent personnel at the conceptual design stage. To achieve this inclusive process, the industry must breach the barriers between client and contractor.

The challenge

Elf and Expro are developing a new tool, for deep-water operations, using a combined approach. Following initial HAZOPing of a proposed exploration testing program in deepwater, Elf identified a need to shut-in the surface diverter blowout preventer (BOP) below the rig floor in the event of a pressurized leak to the marine riser from the landing string.

In the event of a significant leak from the landing string to the marine riser, closing the diverter would protect personnel from the hazards of riser fluids and hydrocarbons being emitted at the rig floor. However, during testing operations, subsea control umbilicals strapped to the string would compromise the ability of the diverter to close on the piping and might be damaged in the process.

Deeper water provides more potential leak paths via the joints of the landing string. In addition, gas expansion from a deep leak may be greater, causing more riser fluids and hydrocarbon volumes to be expelled at the surface.

Tool development

Commissioning preliminary engineering to highlight the design issues and confirm the feasibility of a basic concept, Elf approached the well testing service sector to incorporate provision of a suitable tool within the testing services package. With Elf having already thought through the drivers and functional requirements, it was much easier for Expro to respond.

After evaluating the design concept developed by Elf, Expro returned with a revised tool design, configured to simplify the manufacturing process and minimize the rig-up time required as part of the test string installation process. The concept relies on carrying the production conduit through a load bearing mandrel, while protecting the umbilicals behind a series of interlocking housing plates, complete with locating pins to provide a means of attaching them to the main body of the tool.

The tool is designed to be run as part of the landing string, spaced out opposite the surface diverter to accommodate heave and tide. The unit permits up to three control umbilicals to be carried behind the protective housing plates. In the event of a shut-in, the diverter closes around the protective cover plates while the string remains connected subsea. The bulk of any riser returns are then diverted to the flare via the "poor-boy" knock-out system.

Technical development

Expro's detailed design identified a number of issues for further investigation:

How would the housing plates be fixed, once located?
How would the passage of riser fluid behind the housing plates be minimized?

A couple of test-jigs were designed to investigate and verify elements of the design avoiding expensive re-machining of the final tool.

It was also confirmed that the initial assumption regarding the tool diameter would not permit the tool to carry a sufficient bending load. This would have limited the operating envelope of the tool and required well testing operations to be postponed in even moderately poor weather. The limitation on the tool diameter had originally been set by the internal diameter of the surface diverter system.

A lateral solution was now required. Through contacts with rig equipment manufacturers, Expro was able to identify the possibility of a simple, inexpensive modification to the diverter element. This also permitted the mandrel design to be modified and the machining and manufacturing costs to be reduced.

It was only through such a flexible engineering approach, and a knowledge of rig operations, that the tool design was able to maintain the operations envelope and minimize the total costs. This was achieved by building maximum functionality into a simple design.

The first tool is currently in fabrication and due to begin operations in Angola shortly, providing the culmination of a successful collaboration between operator and service supplier.

The pooling of expertise has allowed lessons learned from previous projects to be incorporated into the design at an early stage. This partnering allowed the two to create an effective design the first time. Both companies hope this approach can be utilized again in the future.