Pieter Schelte, a Twin Tanker Vessel Concept for the Single Piece Removal of Large Topsides and Jackets
Authors: Schenkel Martijn , Spaan Jack , Varkevisser Leo 
Published: October 2008

In this paper a safe and cost effective approach to the removal of large oil and gas offshore platforms is presented. A dedicated vessel will be commissioned, named after the pioneer of offshore heavy lifting, Pieter Schelte Heerema (1908-1981). This vessel will be able to decommission platform topsides of up to 48,000 tonnes and jackets of up to 25,000 tonnes in a single lift operation. Delivery of the vessel to the market is set for the 2003 summer season. Detail engineering is in progress, and suitable tankers will be acquired shortly.

Introduction

In the last decades platform removal has attracted growing attention in the industry on account of the associated environmental and financial issues. Since the past few years solutions are proposed to counter the traditional approach of reversed installation. A number of concepts are under development aimed at reducing environmental and safety risks as well as reducing expenses by focussing at single piece removal of the larger topside facilities and jackets.

This paper presents the results of development work carried out on one of these concepts, "Pieter Schelte". Two 300,000 DWT tanker hulls provide the basis for this catamaran type vessel, which will have the following principle dimensions:Length 336 m 1100 ftWidth 118 m 387 ftDepth 29 m 94 ftSlot width 52 m 170 ft

The large size of the vessel provides ample stability and enables removal operations in relatively severe sea conditions. A hydraulic motion compensation system further improves vessel workability by reducing load impacts and accelerations during the removal operations.

When lifting a complete topside facility, the vessel lift system will attach to the legs of the topside Module Support Frame. Jackets can be removed in their entirety by lifting and tilting them onto the deck of the vessel.

Onboard the vessel, the platform components will be safe to survive the season's extreme weather conditions. Thus, the vessel will sail to shore and deliver these loads to suitable dismantling sites for scrapping. Alternatively, the topside can be delivered to an other platform location for re-use.

Distinct advantages of the vessel over existing heavy lift crane vessels are:minimum offshore preparatory work on topside facilities: single piece lifting; no cleaning and separation of process facilities; no re-instatement of lift points and rigging on each and every module;minimum subsea cutting operations: single piece jacket removal;maximum safety to personnel: very limited human activities offshore;maximum protection of the environment: handling of hazardous materials only within a confined onshore dismantling site;possible re-use of the entire topside.operations are reversible, thus making the unit also suited for platform installation.

Challenges Associated With Platform Removal

Platform Abandonment And Removal Legislation.

When an oil or gas field approaches the end of its economic life, the decline of production and ongoing maintenance costs no longer justify the continued operation of the production facilities. If prospective tie-ins from nearby discoveries are not economic, the owners have to consider how to abandon the field.