Abstract

Approximately 50% of all installations in the UK offshore sector are small steel jacket structures that are normally unmanned. These platforms often have severe limitations on the permissible lift weight of temporary well servicing equipment. During a study into the extent of this problem, it was observed that a significant proportion of gas production is reliant on wells producing via these normally unmanned installations, and that production from this source has declined dramatically in recent years. When compared to manned installations where (intervention is easier and therefore assumed more frequent), production decline is less severe.

An operator identified a production enhancement opportunity on an unmanned installation in the North Sea UK Sector. The well needed extensive intervention in order to be brought back to production using coiled tubing (CT). Due to the restricted crane capacity (less than 7.5 ton) on the platform, a conventional CT package could not be lifted on board. Through a collaborative project, an existing CT equipment package was modified to be lifted within this weight limit.

A boat spooling technique was used to install the CT string onboard the platform without using a crane as it was much heavier than the crane limit. Throughout the equipment modification project, the principles of efficient rig up and optimized fast, safe working where applied. The installation of this equipment on the platform was conducted quickly and easily, validating this approach and improving on previous experiences significantly.

This paper demonstrates how conventional equipment can be adapted to meet the challenges faced with platforms considered inaccessible to perform CT interventions.

Introduction

The application of Coiled Tubing (CT) interventions within the offshore environment has matured significantly over several decades. However, the use of CT to perform intervention services on small steel offshore platforms is a relatively uncommon practice. These installations create a nique challenge for access to perform intervention operations as many are classified as Normally Unmanned Installations (NUI).

Despite wells being constructed with a standalone completion design philosophy (similar to subsea wells), often intervention is required in many wells, particularly as North Sea infrastructure ages and matures. The need for CT operations is being realised more frequently by operators of wells on these types of installation in the North Sea.

These NUIs usually have severe limitations on size of deck area to work with, limited provision for personnel accommodation, and low crane lift capacity. To compound the problem, many of these platforms are getting older, and their cranes are being de-rated. Crane capacity is a function of boom angle. In bad weather conditions it is often necessary to extend the boom to avoid the risk of the supply vessel hitting the platform. Bad weather also affects the maximum safe lift possible from an offshore platform. Crane replacement, however, is difficult and very expensive.

As a consequence of these access restrictions, cost effective CT intervention is rarely performed on United Kingdom Continental Shelf NUIs. In the past, the only effective method of accessing wells on NUIs has been to use a drilling jack up rig or workover jack up rig. The availability of workover rigs suitable for this work has been extremely limited and the cost of using a drilling jack-up has become prohibitively expensive in the past five years.

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