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Tenvig Offshore A.S Develops Low-Cost Early Oil Production Ship Figure 1 — Converted tanker fitted with unique stabilization system and turret- mooring system provides stable support for drilling, oil processing and storage. An entirely new concept in off- shore oil production facility, called the BTMM Production and Stor- age Ship, has been designed and developed by Tenvig Offshore A.S (Tenvig) of Oslo, Norway. The fully developed design of the ship and its systems was performed by Tenvig in cooperation with A/S Kongsberg Vapenfabrikk (KV). The latter firm designed the riser and riser-handling systems and Aukra Bruk A.S designed and engineered special parts of the ship's systems. The project's most outstanding feature, in addition to its storage capability, is the possibility of permitting early on-stream pro- duction at a modest investment. The planning and conversion pe- riod (ready for tie-in) is esti- mated to be 20 to 30 months after the decision is made to proceed. Also, the completion time is de- pendent on the characteristics of the oilfield. Estimates prepared by the designers indicate that such a project has the potential to reduce the general production costs. The concept involves using a ship, either new or converted, up to the VLCC size as a floating pro- duction facility. The main advan- tages in using such a ship are: large storage capacity, large deck payload capacity, ample deck space for equipment, gas and wa- ter injection capacity, easy to maintain and dock if necessary, and low conversion/building costs. The converted ship, or new, will be fitted with the BTMM Ship Stabilizing System, giving it mo- tion characteristics comparable to a semisubmersible structure — a necessity for using ships as float- ing production facilities in the North Sea as well as under other conditions. This system and the newly developed BT Turret Pro- duction Platform, together with KV's multibore riser system, have made this project possible. The turret-table mooring allows the ship to always choose the opti- mum heading against the prevail- ing wave trains. In designing the ship and its systems, North Sea weather con- ditions were used as a reference. Tests by the designers and calcu- lations have indicated high regu- larity and working time. Whether an existing ship is converted or a new ship is espe- cially designed for this purpose will have a minor effect on the overall technical concept. Convert- ing a modern tanker does not cre- ate classification problems, and when converted it will be able to remain on location during the life- time of the field without docking, provided proper protection, in- spection and maintenance are ar- ranged. Preliminary Quality As- surances have been given by clas- sification societies. Should dock- ing be necessary, however, then it is a relatively easy job to dis- connect the ship, pull the riser and proceed to a shipyard. It is considered that the BTMM Production and Storage Ship's main application will be on me- dium and marginal fields as well as for deep-water fields. The fea- sibility study and projects exe- cuted have been based on a 100,- 000-dwt tanker—a size which can accommodate a peak production of up to 100,000 barrels of oil per day and will provide a storage capacity of about 500,000 barrels. The design may easily be adapted to a larger tanker if increased capacities are desirable. For this purpose a larger turret produc- tion platform for three risers has been considered. The proposed production sys- tems onboard will be of well- known designs, and will be pro- vided to the client's specifications. All the high-pressure parts of the processing system are located in- side the turret production plat- form. Additional low-pressure processing facilities and storage are on the main deck and in the cargo tanks. The pipeline for transferring oil is connected ei- ther to a loading buoy or to a pipeline where this is possible. Off-loading also may take place from a boom at the stern of the ship. Various safety aspects have been considered in the design. Some of these considerations were: the distance between the living quarters and the hazardous area; location of living quarters in order to reduce heat transfer- ence, and a water-sprayed blast and fire wall. An anchor quick- release system combined with the emergency-generator-driven bow- thrusters enable the ship to move away from a danger zone. Stabilizing System The BTMM Ship Stabilizing System is the key to this project. The submerged adjustable stabil- izing pontoons, Figure 1, fore and aft have the capability of tying up huge oscillating water masses at a level in the sea where the energy is low. Besides, there is a time lag between the wave forces acting on the hull and on the pontoons, resulting in consid- erable dampening of the ship's motions and accelerations. This Tenvig system has been thoroughly model tested and high- ly sophisticated computer pro- grams imperative for the complex calculations and design of the pon- toons and their exact positioning were developed in cooperation with Franlab Marine (the French Petroleum Institute). Aukra Bruk A.S participated in the detailed design of the vertical movement and locking system. Tenvig is presently preparing a full-scale test of the system. The test is expected to be carried out in the North Sea with a 500 to 600-foot vessel. Production Platform The BT Turret Production Plat- form is octagonal in shape and is placed on top of a central turret table mooring. It comprises two or more decks on which all high- pressure processes are arranged, i.e., first stage separation as well as gas and water injections. Con- sequently, all transfers between the stationary turret and the ro- tating ship are arranged via flex- ible hoses under low pressure. In addition to the production facilities, the turret also carries a constant tension mooring sys- tem for eight piled anchors, the riser tensioners and the riser han- dling crane and equipment. A moon pool in the center of the turret is arranged to house the riser. The diameter of the moon (continued on page 40) custom builL.at low cost Before you order your next barge, get our estimate. We believe we can save you money. Quality workmanship and prompt delivery assured. For details, phone or write: IIAlfDCrfnPDAPE • MM Hi IIP UIbpAIII SHIPBUILDING & MFG. CO., INC. J^^im H U (I millllpH HAVRE DE GRACE. MD. • PHONE: 301 WE 9-2552 A subsidiary of M. P. HOWLETT, INC. - Est. 1875 • Nearly a century of'know how' in floating cranes and barges • 410 32nd St., Union City, N.J. Phone: 201-166-1566 May 1, 1980 ZIDELL 53
