Underwater Technology Vol 21 No 1

Summer 1995

A Personal View

Recognising the Challenge―Grasping the Opportunity

D Watson

Technical Papers

The Oil Spill Recovery Unit (OSRU) Concept

JHA Baker

Underwater Sound Perception and the Development of an Underwater Noise Weighting Scale

SJ Parvin and JR Nedwell

Abstract: The sensitivity of human hearing underwater for a 'wet' ear has been measured.  A total of 21 subjects underwent an underwater audiogram in an acoustic water tank at DRA Alverstoke.  A high quality underwater loudspeaker provided pure tone sound stimuli at the third octave band centre frequencies from 25 Hz to 16 kHz.  The results have enabled the underwater threshold of hearing to be defined.  The most sensitive hearing frequency was 800 Hz, and at a level of 41 dB SPL re.20mPa.  There is a general loss in hearing sensitivity underwater, which is most pronounced at frequencies above 2kHz and as a consequence, low frequency sound plays the greatest role in underwater sound perception by divers.  Consequently, a higher level of noise is permissible underwater than would be permissible in the air.  An example is presented of the use of a new underwater weighting scale, the dB(UW), as a means of assessing underwater noise hazard.

Scientific Diving Under Sea Ice in the Southern Ocean

C Robinson, HJ Hills, S Archer, RJG Leakey, PW Boyd and SJ Bury

Abstract: Scientific Diving techniques were employed during a 54 day oceanographic research cruise to the Bellingshausen Sea, Southern Ocean (65°S−72°S, 80°W−87°W), in order to position sampling and data collecting instrumentation beneath sea ice.  Eight Scientific Divers and a Field Diving Officer safely completed 112 individual dives (range 2−80 minutes, 2−28 m); 94 of these were roped dives through holes cut in 1 m thick sea ice.  Seawater temperature was −1.8°C, horizontal visibility 30 m+ and water depth 600m or more.  No problems were encountered with the diving equipment used.  Diving techniques enabled the collection of an important data set describing the dynamics of phytoplankton and zooplankton growth beneath sea ice.  Recommendations for future under-ice oceanic scientific diving include the use of dive tables with ascent rates of less than 15 m/min, the provision for therapeutic oxygen at the dive site, and adequate shelter for surface tenders.

The Raising of the Mary Rose: Archaeology and Salvage Combined

CTC Dobbs

Abstract: The Mary Rose was built between 1509 and 1511 and was a highly successful warship until she capsized and sank in the Solent in July 1545.  In 1982, after many years of painstaking search and investigation, the Mary Rose was recovered from the sea bed with unprecedented interest and support from the public.

In this paper, some of the problems associated with raising a fragile structure which also has great archaeological and historical importance are described, and aspects of the salvage programme are given from the perspective of one of the archaeological divers involved with the raising of the Mary Rose.  Finally, the importance of integrating the professions of both salvage and archaeology in such an operation is illustrated.

Meeting Reports

Man-Made Objects on the Seafloor: Discovery, Investigation and Recovery

R Pirie

New Technologies and Techniques in Underwater Science

M Breen

Underwater Technology Vol 21 No 2

Autumn 1995

A Personal View

A Green Sea?

D Lennard

Technical Papers

Technology Foresight: The Ivew from MTD

Sir Robert Hill

Advances in Mooring Line Damping

DT Brown, GJ Lyons and HM Lin

Abstract: This paper discusses recent fundamental and applied work investigating the damping forces induced on floating vessels and in particular catenary mooring lines.  A description of new tests being performed at University College London on large scale sections of mooring line to reveal damping contributions is also presented.

The contributions to damping from the mooring lines is high topical in that a number of floating product and storage units are currently being planned for hydrocarbon exploitation.  Indeed a number of schemes are already in operation at North Sea sites.  Design of the mooring system required to hold the vessel within a specified radius above the wellhead depends on an understanding of the imposed static and dynamic environmental loads.  The low frequency excitation caused by the random waves, and, to a certain extent, wind loading results in resonant motion responses in the horizontal plane leading to high mooring line forces.  Many of the loading mechanisms are well understood.  However, until recently the fluid induced forces acting on the moorings were assumed to have little influence on the vessel dynamic response.  Recent work has shown that the mooring system may under  certain circumstances provide up to 80% of the total damping available thus significantly reducing the peak line tensions.

Development and Applications of a Novel Underwater Laser Illumination System

S Tetlow and RL Allwood

Abstract: Severely limited underwater viewing remains a problem in the operation of remotely operated vehicles.  The recent availability of compact, diode-pumped green lasers has led to a renewed interest in the synchronous volume scanning technique as a means of overcoming some of the viewing problems.  However, one of the limitations with synchronous scanning is the limited depth of field achieved because of the optical arrangement used.  In an attempt to overcome this problem, an underwater laser illumination system has been developed at Cranfield University.  Stripes of laser light are used to illuminate a target and images from a conventional camera are processed to produce a computer generated composite image.  The system has been evaluated in an underwater laboratory and at various open-water sites.  Such a computer-integrated approach to underwater viewing has applications in areas such as macro- and micro-navigation and these are discussed.

An Assessment of Tidal Streams as Energy Sources in Orkney and Shetland

IG Bryden, CR Bullen, MS Baine and O Paish

Abstract: This paper summarises the findings of a major study into the feasibility of using rapid tidal currents to generate electricity in Orkney and Shetland.  The theory of tidal generation is briefly summarised and economic, social and environmental factors outlined.

Sea Manipulation of Heavy Loads Using Fresh Water: A Concept Study

JE Strangroom

Abstract: Practical trials using fresh water, which is about 2% less dense than sea water, as a buoyancy medium in sea water are described.  A modified hot-air balloon was attached underwater to a load of 4.7 tons and filled with fresh water pumped down from the surface.  Very delicate control of the lifting force could be achieved; the prolonged, steady force overcame bottom suction and the inertia and drag of the balloon largely restricted the acceleration and rates of ascent and descent.  Features of the method relevant to practical applications are also discussed.  Balloons capable of 1000 tons life can be built and operated at any depth using readily available pumps and hoses, and a tanker for the fresh water will be considerably cheaper to hire than a lifting barge.  Station-keeping by the surface vessel will be simplified, since it will not need to be vertically above the load.  Although currents could cause significant problems, it appears that in certain applications, particularly those involving very heavy loads at great depths, this method will have significant advantages over conventional techniques.

Meeting Reports

Subsea Raw-Water Injection

RL Allwood

The Potential of Robotic Systems in the Seas and Offshore (matching research effort to the offshore industry and science needs)

P Stevenson

Abandonment of North Sea Platforms

PW Penney

Book Reviews

EXPLORATIONS: My Quest for Adventure and Discovery Under the Sea

Reviewed by CP Summerhayes

Handbook for ROV Pilot/Technicians

Reviewed by D Norman

Underwater Technology Vol 21 No 3

Winter 1995−96

A Personal View

Towards a Technological Society

D Kirkley

Technical Papers

Deepwater Welding and Intervention Technology

JH Nixon and IM Richardson

Abstract: Within the next few years, the offshore oil industry will be seeking to exploit hydrocarbon reserves at depths inaccessible to saturation divers, and at which the welding processes currently in offshore use will not operate.  Alternative processes exist for the water depth range 500 to 1000 metres, and these have been shown to be viable in laboratory trials.  Further work is required to bring them to full operational status, and to integrate them with the wide range of other equipment required to carry out a complete underwater fabrication procedure without diver intervention.  Although alternative fabrication techniques exist, it is generally agreed that if hyperbaric welding can be shown to be reliable, and to produce acceptable joints, it will continue to be used by the offshore industry.

At present, no facilities exist for hyperbaric welding research at depths significantly greater than 1000 metres.  Cranfield is currently commissioning a 250 bar research facility, which can be used for undertaking studies into the performance and properties of arc welding at pressures equivalent to a water depth of 2.5km.

Testing Communications Systems on Diving Installations

V Flook

Abstract: The need for clear communications between diver and topside is obvious; safe practices depend on good communications, as do efficient operations and effective response to an emergency.  The requirements of communications systems have only recently been formalised.  The rationale behind the design of communications tests are presented and some of the problems which arise when these tests are applied to the specific problems of helium speech are discussed.

Tests made on commercial diving installations show that all parts of the system are important.  No matter how good the unscrambler, good performance may sometimes only be achieved by suing it in conjunction with a particular microphone.  Results of tests will be presented to demonstrate this and to demonstrate that the commonly held belief that speech is more intelligible without the unscrambler may sometimes be true.  The acoustic characteristics of the space from which the sound is being transmitted influences the intelligibility and two identical systems, used in different chambers, give very different performances

These results suggest that investing large sums of money to design new unscramblers may be wasteful and much small investments, to produce integrated systems to suit the particular space in which each will be used, will give much greater improvements in intelligibility.

The Dynamic Response of Thermoplastic Hoses

PS McCarthy and PH Knight

Abstract:  A programme of experimental work has been carried out under the 'Umbilicals―the Future' joint industry umbilical research project, in which the behaviour of thermoplastic hoses in response to rapid pressure changes has been examined in detail.  The hose characteristics which have been investigated include steady state volumetric expansion, ageing, visco-elasticity and dynamic volumetric expansion.  Simulation results are presented which show the importance of correctly accounting for these effects which designing subsea well control systems, particularly for systems where there is no subsea hydraulic accumulation.

Meeting Reports

Recent Developments in Subsea Pigging and Isolation Techniques

RL Allwood

MARINFLEX '95 The Second European Conference on Flexible Pipes, Umbilicals and Marine Cables

DT Brown

SUBTECH '95 ― Addressing the Subsea Challenge

D Liddle

Second MAST Days and EUROMAR Market

B Woodward

Book Reviews

High Strength Steels in Offshore Engineering

Reviewed by WD Loth

Design and Safety Assessment for floating Installations

A Incecik

Underwater Technology Vol 21 No 4

Spring 1996

A Personal View

Underwater Robotics: Realising the Potential

Simon Corfield

Technical Briefing

The Southampton Oceanography Centre and the Legacy of the Challenger Expedition

CP Summerhayes and N Hamilton

Technical Papers

The Atlantic Meridional Transect: An oceanographic research programme to investigate physical, chemical, biological and optical variables of the Atlantic Ocean

DB Robins and J Aiken

Abstract: Oceanographers from the Plymouth Marine Laboratory, together with colleagues from the Southampton Oceanography Centre, University of Plymouth, University of Oviedo (Spain), National Aeronautics and Space Administration ( NASA, USA) and Brookhaven National Laboratory (USA) have started a programme of research which consists of a series of transects of the Atlantic Ocean between the United Kingdom and Falkland Islands.

The primary aim of the research is to measure physical, chemical, biological and optical variables in the upper 200m of the water column along the 12000 km transect in order to characterise the Atlantic Ocean over broad spatial scales.  This will help further the understanding of the role of the world's oceans in carbon cycles and provide valuable information for the calibration and validation of satellite remote sensing (i.e. for ocean colour and surface temperature of the oceans.  The use of 'ships of opportunity' is essential to carry out broad scale research, and the development and use of new technology is an important part of this process.  This report reviews the key issues and the relevance of such research and also highlights the new generation of technology that is required to work at these broad spatial scales.  The data presented here are generated at sea in 'real' or 'near real' time and they define the areas of greatest plankton abundance and productivity, as well as regions of hydrographic contrast.  In turn this will allow a more focused and targeted strategy for intensive sampling and analyses.  The role of the oceans in interacting with atmospheric carbon dioxide (one of the so-called greenhouse gases) is also discussed.

Measuring Abundance and Size Distribution of Zooplankton Using the Optical Plankton Counter in Underway Mode

CP Gallienne, DB Robins and DA Pilgrim

Abstract: The Optical Plankton Counter (OPC) is an optical instrument capable of large scale, rapid and continuous counting and sizing of zooplankton.  A towed and a laboratory version of the instrument is available.  A novel application of the laboratory model, in underway mode using a ship's uncontaminated sea-water supply, is the subject of this paper.  The work was carried out aboard the RSS James Clark Ross, the British Antarctic Survey (BAS) research and supply vessel, on her annual voyage south from the UK to the Falkland Islands during September and October 1995.  The instrument and its application are described, together with a review of some of the operational issues arising from such an application of the instrument.  Samples of the data are presented, to illustrate the scale of sampling possible with the system compared with traditional methods.  Some validation data from laboratory microscopic taxonomic analysis are also presented.

The Effective Use of Titanium in Subsea Applications

DK Peacock

Abstract: The effective and successful use of titanium in subsea applications requires the recognition, understanding and correct application of the combination of useful and unique physical, mechanical and corrosion resistant properties of titanium and its alloys.  These properties are reviewed and guidance is given on design, and fabrication using available products and cost-effective manufacturing methods.

Meeting Reports

Researching for Industry

RL Allwood

Umbilicals―The Future

PH Knight

Book Reviews

Pollution from Offshore Installations

Reviewed by G Plant

The Protection of the Underwater Cultural Heritage

Reviewed by J Blake