Resolving the Right Support-Vessel Strategy for Offshore Coiled-Tubing Operations

Abstract

Whilst the coiled tubing technology continues to emerge as a versatile solution in the industry, it is often perceived as an expensive option by many operators. Offshore coiled tubing operations are usually more challenging than land CT operations due to involvement of several additional issues like requirement of Support Vessel, tighter equipment lay-out, higher lifting capacity requirements on site, sensitivity to weather/sea conditions etc. which may also have significant cost implications. With the expanding drilling limits in respect of operating depths, well trajectories, down hole pressures, hole sizes and other operating environment, the resulting wellbores are becoming increasingly challenging to down-hole service operations. The inability of the conventional ways of down-hole intervention like slick-line and e-lines to service such wellbores is opening up newer dimensions for coiled tubing applications. To cater to these increasingly challenging expectations, the coiled tubing size is gradually expanding and the length is extending, both demanding higher lifting capacities and bigger space on location. There are various ways offshore coiled tubing operations can be managed, in some cases, even without a support vessel. However more often a support vessel is required for running smooth operation. There are various types of support vessels available in market from tiny little workboats to highly sophisticated service vessels. The economics of any support vessel strategy are often mistaken; a low cost vessel may not necessarily be an optimized solution in terms of the overall economics of operations. In continuous operations, it is also common to see that sometimes the rate of coiled tubing servicing over takes the rate of generation of work-load, thereby creating the situation of resource idling. The present work demonstrates an analytical approach in formulating the right strategy on support vessel for offshore coiled tubing operations. It compares and evaluates various alternative modus operandi of running offshore coiled tubing operations and work its way up to identify the most appropriate strategy in terms of overall economics and safety considerations. The work also evaluates the sensitivity of this strategy on several technical, operational, organizational and environmental parameters, which commonly surround the actual working environment. Background The present work describes a process of resolving the right support vessel strategy for offshore coiled tubing operations based on analytical modeling and our relevant past experience in south East Asia region. A brief out-line of the operating environment on which most data used in this work are based, is described below.Water depth ranging from 30 feet to 100 feet.Limited lifting capacity availability on offshore structures, usually 2 to 5 tons.Typical offshore structures composing of well platforms which have moderate sized work decks and usually accommodate about ten conductors and well jackets which are small tripod structures usually accommodate a single or a few conductors in some cases. Due to extensive presence of brown fields in the region, there has been a steady rise in the requirement of coiled tubing works over recent years. Common application includes but not limited to wellbore clean-outs, matrix acidizing and fracturing applications, water control treatments, CT conveyed through tubing perforations, horizontal well treatments, logging, manipulation of down-hole flow control devices, CT fishing applications etc. Since the lifting capacity available at offshore locations is usually very limited, a self erecting crane package is commonly utilized to undertake heavy lifts. These cranes are usually modular type cranes which consist of individual low capacity (LC) and high capacity (HC) crane modules. The modules can be rigged up on offshore locations in series without the need for any external lifting mechanism on site.