GB2224058A

GB2224058A - Description and applications of an easy disconnect, motion decoupling top joint for OTEC

Info

Publication number: GB2224058A
Application number: GB8821094A
Authority: GB
Inventors: Martin Giles Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-09-08
Filing date: 1988-09-08
Publication date: 1990-04-25
Also published as: GB8821094D0

Abstract

The new joint which is illustrated in fig. 5 is designed to stop any motions from the top platform being transmitted to the CWP. In very bad weather when there is a danger of damage to the platform and/or the CWP, the CWP can easily be lowered below the surface of the sea where it is no longer exposed to such severe environmental conditions. The new joint can be beneficially applied to a ring semi submersible top platform whose design is optimised for minimum wind and wave induced motions. A laser welded sandwich construction of the CWP is proposed with syntactic foam placed between the inner and outer rings of the sandwich construction. The foam would make the pipe positively buoyant and it would be held in place by tension moorings attached to the sea bed similar to those used in tension leg platforms. (TLPs). <IMAGE>

Description

DESCRIPTION AND APPLICATIONS OF AN EASY DISCONNECT, MOTION DECOUPLING TOP JOINT FOR OTEC.

This invention relates to a new design of joint between the cold water pipe (CWP) and the top platform for an Ocean Thermal Energy Conversion (OTEC) power plant.

OTEC is the general description for the process by which energy can be extracted from the temparature difference availible between the warm surface waters and cool sub surface waters in the world's tropical oceans. It is a subject that has received little publicity in the U.K.

but which has been seriously investigated in the U.S.A. . A good overview of the subject is given by Ford et al (1983) and an illustration of possible OTEC plants is given in Fig. 1 (from ref. 1).

The temparature difference is due to the presence of a stable thermocline which results in a temparature of approximately 25 C in the surface layers and approximately 4 C at a depth of around 1000 m.

Because the temparature difference is so small the process has a low thermal efficiency and in order to produce a large power output, vast quantities of warm and cold water must be processed. However, the cold sub surface water has to be pumped slowly in order to minimise frictional losses and also so as not to disturb the stability of the thermocline.

This means that in order to transport the volume of water required, a large diameter CWP is needed. Calculations show that a pilot plant of approximately 40 MW capacity would require a pipe of 10 m. internal diameter with a length of about 1000 m.

Because of the size of the pipe, the design of the joint between the top of the pipe and the surface platform presents problems. If a fixed joint is used, the pipe will be subjected to forces due to the roll, surge, sway, heave, pitch, and yaw of the top vessel.

In order to reduce these forces a number of different types of joint have been suggested. One idea is to use a flexible pipe which would absorb the platform motion. However, the design of a reliable flexible pipe of 10 m. internal diameter able to withstand a 30 year design life immersed in sea water is far beyond the present state of the art. Alternative ideas which permit a certain degree of movement between the pipe and the top platform are illustrated in figs. 2,3 and 4 (from refs. 5,4 and 5 respectively). However, it is correct to say that no joints based on these principles have ever been constructed to the size that would be required for an OTEC application. Thus the reliability and development costs of such joints cannot easily be predicted.

The new design of joint which is proposed is illustrated in fig.5.

and is described in the following section.

Referring to the drawing the joint comprises of a Pick Up Pipe (PUP) (3), a bowl (2) and the CWP (1). It is possible for the PUP to be of smaller diameter than the CWP since, because of its short length it is possible to pump water fast through it in order to achieve the required flow rate without being subject to to unacceptably high side wall frictional losses.

(4) represents the top platform which for ease of illustration is drawn as a conventional semi-submersible although the ring semi-submersible displayed in fig.6. is likely to be more appropriate due to its superior motion characteristics. (5) represents catenary moorings anchoring the semi-submersible to the sea bed. It is likely that the power cables transmitting the electricity to shore would be run alongside the mooring cables. (6) illustrates the tension moorings holding the positively buoyant CWP to the seabed. (7) represents the mechanism that would be used to lower the CWP below the sea surface as might be required in very bad weather.

This could be achieved by sub sea winches or by altering the buoyancy of the CWP. (8) shows slack mooring chains which might be required if the motion between the CWP and the top platform should get out of phase and there was a danger of collision between the CWP and the top platform.

New Concepts for the Design of the CWP based on the New Joint.

A solid CWP which is sufficiently strong to withstand the bending forces imposed on it by the combined influences of wind, waves and current is too thick to be manufactured and is too expensive due to the mass of material required. Thus a sandwich construction (illustrated in fig.?) is proposed which is far stronger weight for weight than the solid pipe which it replaces. The difficulty of using a sandwich construction for a structure such as the CWP is getting access to the internal joints for the welding equipment. However, laser through welding offers the opportunity of solving this difficulty since (see fig.8, from ref.6) direct access to the internal joints is no longer required.

Using a sandwich construction opens up the possibility of using the space between the inner and outer rings in order to provide internal buoyancy to support the pipe's own weight. This can be achieved by either inserting syntactic foam into the empty spaces or by using compressed air.

Syntactic foam has the advantage of not losing its buoyancy if one of the rings becomes damaged. Also it would increase the strength and stiffness of the pipe as well as helping to inhibit corrosion between the rings.

Compressed air would have the advantage of being able to easily adjust the buoyancy of the pipe if it was necessary to lower the pipe below the surface of the sea in bad weather.

Claims

CLAIMS. I) Since the top platform and CWP are not attached to each other, motion of the top platform is not transmitted to the CWP. This reduces the number and the magnitude of the forces which are imposed on the CWP. 2) In very bad weather when there is a danger of damage to the platform and/or the CWP, the CWP can be lowered below the surface of the sea where it is no longer exposed to such severe environmental conditions. Since the top platform and the pipe are not fixed together this is a simple operation which can be carried out in the middle of a storm. 3) Since no solid parts move against each other there will be very little wear of the components that make up the joint. 4) Because of the simplicity of the joint it will be cheap to manufacture with low development costs. REFERENCES.

1) Cohen, R. 'Energy from the Ocean ' from Marine Technology in the 1990's published 1982 by the Royal Society, London.

2) Ford, G., Nibblett, C., Walker, L., 'Ocean Thermal Energy Conversion,' IEE procs., part A, volume 130, number 2, Mar. 1983, p. 93-100.

3) Pompa, J.A., Lunz, D.F. (Editors), 'NOAA/DOE CWP Structural Analysis Package,' September 1979, ORI Inc., prepared for NOAA under contract NDBO 03-78-G03-0504, U.S. data bank ref. OTEC 1530-1.

4) Science Applications Inc., 'OTEC Modular Experiment Cold Water Pipe Design Study,' prepared for NOAA under contract MO-A01-78-00-4142, SAl Report No. SAI-057-79-1049LJ, Dec. 1979, U.S. data bank ref. OTEC 1900-8.

5) TRW Energy Systems Group, 'OTEC Cold Water Pipe Preliminary Design Project - Final Report,' 20th November 1979, prepared for NOAA/DOE under contract MO-A01-78-00-4141, l-.S. data bank ref., OTEC 2000-17, vol. 1.

6) Vice-Chancellor's Report, 'The University of Newcastle upon Tyne,' availible from the Registrar's Office.