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Offshore construction and decommissioning vessel ('OCDV')

Objective

Objectives and problems to be solved: The objective of the Project is to develop a concept for a new type of Offshore Construction and Decommissioning Vessel (OCDV) devoted to safe, environmentally friendly and cost-effective removal and (re-) installation of offshore platforms and subset facilities. The key target is to prove the feasibility of the concept by obtaining Classification Approval of the Main Hull Scantlings, and by developing researched and viable design solutions for the main components and systems of the vessel. The development of the OCDV is based on an existing concept idea, where the OCDV takes the form of a U-shaped semi-submersible vessel which is equipped with a tie-down system to reduce and control its swell and wave induced vertical motions. The overall target of the Project is to conclude the vessel's operational methodology and workability, its main dimensions and its key operational equipment. The uniqueness of the Project is based upon the concept of removing (or installing) the topsides as one entity, carrying them from below. The proposed method of decommissioning allows the possibility for refurbishment and reuse of the offshore installations. Further, the method will maximise the safety of decommissioning personnel whilst minimising the potential risk of accidents and environmental damage by moving the major part of the dismantling (and hook-up) work from offshore to onshore. Additionally, the method provides an opportunity for development of marginal fields by providing low-cost, reused topsides facilities. Description of work: The work will be initiated by preliminary analysis and design/layout activities of the vessel hull combined with developing an outline method statement describing how the offshore operations shall be performed. At the same time functional requirements for - and initial design of - the vessel's key equipment shall be established. Based upon this initial study, the hydrostatic and hydrodynamic analyses will provide critical parameters and criteria for the structural design of the vessel and its systems. In parallel, a model of the vessel will be tested in an environmental tank to establish vessel performance and behaviour in various simulated sea-states. The results of the analyses and model tests will be used to perform the finite element analysis for computation of structural forces in hull and equipment under all global design conditions. Based on these forces, the hull design will be concluded and the main scantling drawings will be prepared and presented to the classification society for review and approval. At the same time research and development of a gendering system for horizontal positioning of the OCDV, of the retractable topsides support beams, and of the innovative hydraulic/mechanical tie-down system (tendons), including the design of the suction anchors and the study of their geographical applicability, will be conducted. The work has been split into 15 thematic work packages, and five main milestones have been identified, addressing critical intermediate results. Expected results and Exploitation Plans: The outcome of the work will be a detailed concept design for a vessel capable of handling the topside weights of about 90% of existing North Sea platforms. The subsequent phase will cover the detailed engineering and construction of the vessel. In a third phase, a consortium will be established around the OCDV capable of delivering a complete decommissioning package.
The outcome from the OCDV project is a viable design of a new type of heavy lift vessel, where the load is carried from underneath by several support beams protruding in the "moon pool" of the U-shaped vessel. The vessel is able to transport and install/remove construction elements, such as offshore oil and gas platforms of 14.000 T weight in single lift operations. The capacity can be increased but we have found 14.000 T to e an optimal size economically. The load transfer of cargo to/from fixed offshore structures is performed by ballasting respectively de-ballasting the vessel. Two different type of hull shapes have been developed in the project, a column stabilized unit with 2 pontoons and 8 columns and a catamaran-shaped unit with two pontoons and a transverse box above waterline connecting the two pontoons to a catamaran-shaped unit. The column stabilized unit is less weather sensitive compared to the catamaran-shaped unit and is better suited for operations in the North Sea whilst the catamaran shaped unit is less expensive and is better suited economically for operations in Gulf of Mexico and other areas with calm waters.

Three systems, essential for a safe load transfer operation, have been developed in the project; a Tie Down System (TDS), a Horizontal Positioning System (HPS) and a Load Transfer System (LTS). During offshore load transfers (by ballasting/ de-ballasting) the OCDV will be moored vertically by the TDS, having the effect of almost eliminating the vertical movements (heave, roll and pitch) caused by wave loads. A sub system to the TDS has been developed to stop vessel vertical motions without introducing snap loads in the tethers. A suction anchor, being part of the TDS, has been developed able to withstand the vertical tie down forces of some 1600-1900 T per anchor, even in a seabed consisting of sand. The anchors have been outfitted with suction pumps running constantly in order to maintain the suction force. A filter has been developed in order to avoid the soil plug disappearing whilst running the pump continuously. A large scale model of the anchor has been fabricated and tested successfully in the offshore environment with seabed of fine to medium sand. The whole TDS has been tested successfully on a model in a test basin outfitted with wave generators. The HPS has been developed for keeping the vessel in a target position, within a certain operational envelop relative to the platform substructure allowing the vessel to follow the wave frequency motions but reducing the second order motion of the vessel and by that minimizing the horizontal amplitude by about 50%.

The HPS will also secure the cargo to the vessel after lift off, working as a temporary sea fastening. The system has been successfully tested on a model in a test basin. A LTS has been developed supporting the platform Topside from underneath in several lifting points and thus distributing the load evenly. The system allows for vessel wave frequency motions when performing load transfer operation without introducing large dynamic loads into the platform. Two different systems have been developed and computer simulations have been performed to verify the system is working. The aim with the OCDV project was to develop a heavy lift vessel for later commercialisation, However the systems developed to the vessel could also be used in other marine applications and can be commercialised as individual business products.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

MASTER MARINE AS
Address
Oksenoeystien 12
1366 Lysaker
Norway

Participants (3)

DET NORSKE VERITAS A/S
Norway
Address
Veritasveien 1
1322 Hovik
NORWEGIAN GEOTECHNICAL INSTITUTE
Norway
Address
Sognsveien 72
0806 Oslo
REXROTH HYDRAUDYNE BV
Netherlands
Address
Kruisbroekstraat 1
5280 AA Boxtel