GB2149916A - Buoyant seismic streamer array - Google Patents

Abstract

A seismic streamer array comprises nodes containing a filling gel separating hydrophone sections. The fill gel is a mixture of a synthetic hydrocarbon polymer e.g. polyisobutene having a high viscosity and a quantity of particulate material having a relative density less than 0.5 e.g. glass microspheres. The proportions of the mixture are chosen to provide the required buoyancy. The polymer may require to be heated in order to fill the nodes, but attains a high viscosity at the normal working temperature of the array. <IMAGE>

Description

SPECIFICATION A towed acoustic array structure This invention relates to a towed acoustic array structure such as is used for marine geological surveys.

A typical towed acoustic array is shown in the drawing. It comprises a number of node sections. The front of the array is connected via a forward isolator section to the ship's towing rope, whilst the rear of the array is connected via a rear isolator section to a trailing or tail rope.

One problem to be overcome is that of buoyancy of the array. In use the array is required to tow at a predetermined depth below the surface of the sea. The hydrophone sections may be contained in a large diameter plastics hose, which has a number of small hydrophones disposed along its length and a few electrical cables: The rest of the space within the hose is unoccupied. To achieve the required buoyancy it is common to fill such hose sections with a liquid of lower density than seawater, e.g. insulating mineral oil. The node sections, on the other hand, comprise a considerable amount of electronic hardware also enclosed in a tube or hose. Achieving the correct degree of buoyancy in the nodes usually requires that they be filled with material having a lesser density than that used for filling the hydrophone sections.

According to the present invention there is provided a towed acoustic array structure including alternating hydrophone sections and node sections, wherein the node sections each comprise a tube or hose enclosing electronic circuitry and electrical cabling, the remaining space within the node being filled with a gel consisting of a synthetic hydrocarbon polymer having a high viscosity and a relative density of less than 1.00 loaded with a quantity of evenly distributed particulate material having a relative density of less than 0.5, typically 0.3 to 0.4.

An example of a filling gel for a node is a mixture of a synthetic hydrocarbon polymer manufactured by polymerisation of olefins consisting essentialy isobutene, such as is sold under the trademark HYVIS by BP Chemicals Ltd. and glass microspheres such as are sold under the trademark FILLITE. A particular gel was made as follows. 340ml of polyisobutene was thoroughly mixed with 1 70ml of glass microspheres. The relative density of the resulting mixture was approximately 0.7. This compares with a relative density of insulating mineral oil of typically 8.00. Because of their high viscosity at ambient temperatures polyisobutene grades such as HYVIS 30 or HYVIS 1 50 require to be heaed during the mixing and filling processes.

Once a node has been filled with a heated gel and allowed to cool down to the normal array working temperature the viscosity of the gel is such that the glass microspheres will not migrate and the gel mixture remains stable over long periods of time, e.g. months.

Whilst the primary purpose of the gel is to provide a predetermined buoyancy to the node it also has thue useful property of being hydrophobic and thus contributes to the satisfactory electrical performance of the node equipment when immersed in the sea.

1. A towed acoustic array structure including alternating hydrophone sections and node sections, wherein the node sections each comprise a tube or hose encloding electronic circuitry and electrical cabling, the remaining space within the node being filled with a gel consisting of a synthetic hydrocarbon polymer having a high viscosity and a relative density of less than 1.00 loaded with a quantity of evenly distributed particulate material having a relative density of less than 0.5.

2. A towed array structure according to claim 1 wherein the high viscosity hydrocarbon polymer is a polymerised olefin consisting essentially of isobutene.

3. A towed array structure according to claim 1 or 2 wherein the particulate filler material is glass microspheres.

4. A towed acoustic array structure having nodes filled with a gel consisting of a synthetic hydrocarbon polyisobutene having a viscosity of 98.9"C in excess of 60OcSt and a relative density of less than 1.00 loaded with a quantity of evenly distributed glass microspheres having a relative density of less than 0.5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION A towed acoustic array structure This invention relates to a towed acoustic array structure such as is used for marine geological surveys. A typical towed acoustic array is shown in the drawing. It comprises a number of node sections. The front of the array is connected via a forward isolator section to the ship's towing rope, whilst the rear of the array is connected via a rear isolator section to a trailing or tail rope. One problem to be overcome is that of buoyancy of the array. In use the array is required to tow at a predetermined depth below the surface of the sea. The hydrophone sections may be contained in a large diameter plastics hose, which has a number of small hydrophones disposed along its length and a few electrical cables: The rest of the space within the hose is unoccupied. To achieve the required buoyancy it is common to fill such hose sections with a liquid of lower density than seawater, e.g. insulating mineral oil. The node sections, on the other hand, comprise a considerable amount of electronic hardware also enclosed in a tube or hose. Achieving the correct degree of buoyancy in the nodes usually requires that they be filled with material having a lesser density than that used for filling the hydrophone sections. According to the present invention there is provided a towed acoustic array structure including alternating hydrophone sections and node sections, wherein the node sections each comprise a tube or hose enclosing electronic circuitry and electrical cabling, the remaining space within the node being filled with a gel consisting of a synthetic hydrocarbon polymer having a high viscosity and a relative density of less than 1.00 loaded with a quantity of evenly distributed particulate material having a relative density of less than 0.5, typically 0.3 to 0.4. An example of a filling gel for a node is a mixture of a synthetic hydrocarbon polymer manufactured by polymerisation of olefins consisting essentialy isobutene, such as is sold under the trademark HYVIS by BP Chemicals Ltd. and glass microspheres such as are sold under the trademark FILLITE. A particular gel was made as follows. 340ml of polyisobutene was thoroughly mixed with 1 70ml of glass microspheres. The relative density of the resulting mixture was approximately 0.7. This compares with a relative density of insulating mineral oil of typically 8.00. Because of their high viscosity at ambient temperatures polyisobutene grades such as HYVIS 30 or HYVIS 1 50 require to be heaed during the mixing and filling processes. Once a node has been filled with a heated gel and allowed to cool down to the normal array working temperature the viscosity of the gel is such that the glass microspheres will not migrate and the gel mixture remains stable over long periods of time, e.g. months. Whilst the primary purpose of the gel is to provide a predetermined buoyancy to the node it also has thue useful property of being hydrophobic and thus contributes to the satisfactory electrical performance of the node equipment when immersed in the sea. CLAIMS

1. A towed acoustic array structure including alternating hydrophone sections and node sections, wherein the node sections each comprise a tube or hose encloding electronic circuitry and electrical cabling, the remaining space within the node being filled with a gel consisting of a synthetic hydrocarbon polymer having a high viscosity and a relative density of less than 1.00 loaded with a quantity of evenly distributed particulate material having a relative density of less than 0.5.

2. A towed array structure according to claim 1 wherein the high viscosity hydrocarbon polymer is a polymerised olefin consisting essentially of isobutene.

3. A towed array structure according to claim 1 or 2 wherein the particulate filler material is glass microspheres.

4. A towed acoustic array structure having nodes filled with a gel consisting of a synthetic hydrocarbon polyisobutene having a viscosity of 98.9"C in excess of 60OcSt and a relative density of less than 1.00 loaded with a quantity of evenly distributed glass microspheres having a relative density of less than 0.5.