Operating Experiences From Second Generation Riserless Light Well Intervention (RLWI) in the North Sea
Authors: Sten-Halvorsen Vidar , Neumann Bjarne , Skeels Brian 
Published: July 2010

With the growing number of ageing subsea wells in the North Sea needing some sort of intervention, the demand for efficient subsea light well intervention service continues to grow. Such service has been in operation in the Norwegian sector for the past six years and is currently experiencing exponential growth with the addition of several more intervention systems spurred by continued long term commitments.

This paper presents a review of performance gains from second generation riserless light well intervention (RLWI) equipment currently working in the North Sea, focusing on the performance gains in-water umbilical management, safer deck handling, efficiency from remote in-situ fluid delivery, and hydrate control. Improved well control " barrier?? equipment now accommodates 1/3 longer length wireline tools, electric-powered control system architecture using ROV-type control cable (umbilical), autonomous grease injection capability, subsea monitored chemical injection and fluid exchange/ flushing capabilities have reduced fluid wastage and contingency tankage requirements; all introduced from recommendations and lessons learned from first generation RLWI systems

Introduction

Riserless Light Well Intervention (RLWI) has now established itself as a field-proven method for wireline intervention in subsea wells. Regular operations since 2006 from monohulls have shown great results on the production and recovery side and, now with two more systems coming on stream last season, a statistically significant number of well interventions have been performed. It is therefore now possible to establish reliable cost and time estimates for the potential well interventions being planned. This paper discusses the experiences gained from these operations.

Review of functionality upgrades Mark II

After several years of operation with the first system, market demand for more systems triggered the development of what became the Mark II system. Overall principles from the first generation were maintained for well barriers and overall system philosophy. However, to improve operational efficiencies, the control system architecture was updated, mitigating the possibilities of time lost due to handling of the large diameter IWOCS electro-hydraulic umbilical. The approach taken being to remove on-deck based surface power generation equipment and replaces the maze of hydraulic hoses and fittings associated with the cumbersome IWOCS umbilical with a ROV type fiber optic-electrical control cable. The subsea IWOCS controls powered by the cable features the same distributed control architecture found on newer work class ROVs, taking advantage of IP routing and control.

As it was expected that deepwater markets also would look for RLWI services as wells mature, the Mark II system was also prepared for easy adaptation for deepwater operations to water depths of 3000 m (10 000 ft) at a later stage. A thorough discussion of the Mark II system and its upgraded features has been presented in earlier OTC papers [3, 4].