Topsides Lessons Learned from Subsea Tie-Back Projects
Authors: Livingston Richard , Tong David , Wensel Eric , Whitworth Michael 
Published: October 2008

A project engineer responsible for a topsides upgrade on a subsea tie-back project, often faces challenges that may not be apparent based on his or her experiences on typical new build facility type projects. This paper is intended to help that engineer. It is a compilation of lessons learned from several project engineers having relevant experiences on subsea tieback projects.

Introduction

The paper is comprised of lessons learned as presented by four project engineers, each of whom have worked on several subsea tie-back projects. It focuses on topsides facilities issues on projects where there is a subsea tie-back to an existing facility. The paper attempts to avoid technical issues in the Subsea Team's scope and issues not specific to subsea tie-back projects. The objective is to better prepare a project engineer who is experienced with topsides new-build type facilities for some of the project challenges associated with subsea tie-backs.

In this paper, major issues are segmented into various categories including flowlines, chemical injection, flow line pigging, slug control, flare, relief and blowdown systems, metering, management of change, and subsea interfaces.

Flowlines

When the flowline comes on to the host platform, it becomes the Topside Design Team's scope. There are several design decisions that must be considered including the location of the flowline shutdown valves (SDV), the choice of flowline material and the selection of the various flowline instrumentation.

Location of Shutdown Valves.

The location of the flowline shutdown valves should be related to safety on the platform. The concern is if there is a failure (loss of containment) in the flowline / riser upstream of the shutdown valve, there would be no way of stopping the leak. As a worst case scenario the entire platform could be lost. Typically, there are three options to mitigate this risk:Install a subsea shutdown valve. This option is very expensive. Also, it may not be possible to install the shutdown valve very close to the platform, which would increase the volume of gas between the shutdown valve and the platform.Install the topsides shutdown valve where it would minimize risk of failure in the riser / flowline upstream of the valve. For instance, the shutdown valve could be located below the lowest equipment deck on a fixed jacket type platform where this was considered an issue. Some valves are not easily accessible and may now be located in the platform air gap or wave zone.Increase the integrity of the riser / flowline upstream of the shutdown valve. This can be achieved by providing any of the following:Added wall thicknessInsulationCorrosion protective coatingExplosion protection around flowline and SDVCorrosion monitoringLeak monitoringOrient flowlines to minimize impact of jet fireMinimize connections upstream of SDVAdd a second SDV

Flowline Materials.

Design pressures of 10-15,000 PSI in large flowline diameters have become more common over the last few years. These high pressures are especially common in subsea tie-backs.