Description ofproposed paper: Burial ofsubmarine pipelines and cables is standard practice in offshore oil and gas regionswhere potentially damaging threats such as interaction with fishing gear anddragged anchors are high. The requirement to bury pipelines in Asia-Pacific hasnot had the same imperative as the North Sea for example but this region is nowseeing a growing demand for pipeline burial to increase mechanical protectionand stability. Performance benchmarks for most trenching systems are based onthe seabed soils typically found in the North Sea and consequently do notinclude the unusual carbonate rich seabed sediments that are prevalent in theAsia-Pacific region which are often cemented and pose a significant challenge to trenching. Application: Trenching isconsidered an activity with significant risk so it is particularly important toselect the correct tool for the field conditions and to establish realisticperformance criteria based on regional experience in the prevailing seabedsoils, including carbonate rich sediments. This paper addressesthe primary differences that exist in seabed sediments and trenching conditionsbetween Asia-Pacific and other regions and show how these dissimilaritiesimpact on the choice of burial equipmentand tool performance. Results, Observations, and Conclusions:A state of theart review of trenching methods and technologies is presented. Trenching casestudies from Australia and Asia are presented which highlight the challenges ofdesigning an appropriate protection and burial strategy for this region and providesindications of actual performance that can be expected across the range ofseabed conditions typically found in the Asia-Pacific region. This includesdata for trenching in harder cemented soils to help fill the gaps associatedwith performance predictions in carbonate sediments. Significance of Subject Matter: The significanceof this proposal paper can be summarised as follows:It provide a reference point onwhich to plan trenching work in Asia-Pacific region,It addresses the knowledge gapsassociated with the relative inexperience of trenching in Asia-Pacific regionPerformance benchmarking is extendedto include the carbonate rich and cemented sediments.

Production.Ccoynsht 1S?% OFFSHORE TECHNOLOGY CONFERENCE mug pap+, .as prepared for FwesematIwIat the offshcfe Techrwk-Jy C.xferenm held m Houston, Texas, 6.9 May 19S6 This paper was selected for presentatwn by tha OTC Program Commmee folbmng rewmv of mfornmlrx ccmtanmdm m ab%trwdsubmmed by the aultwr(s) Conlen!s of the PVC, as presemti, nave M M mwewed by tlw OIiWWrOTechnobgy Confereixe and are Wqect 10Ccrrecimnby the aullwr(s) The matanal, as pre$ented, dms rot neca$sarl~Me-d any posltwm01 the Offshore Techrmlogy Confeferw8 w IIS MIc%rs Pefrms!.mn to WY IS restrcted to an abstract of not more than S03 words Illustrates may nci ba copted The abstractshculd contain &onsp#cuaIsacknowledgmerdof where and by whom the pawr was presented Abstract A case history of two 8" flowlines which were trenched and backfilled in the Northern North Sea is presented. The majority of the trench was cut in sands and soft clay overlying the uneven surface of a hard clay, The trenching operation was successful with the pipe set at the required depth, but the post-backfill survey revealed long sections of both lines exposed at the surface. The problems were resolved by retrenching, dumping a screed of rock and then repeating the backfilling operation. Post-installation engineering work was performed to establish the likely cause of the pipe uplif?. A pipe flotation hypothesis was advanced and this paper concentrates on the work performed on this scenario. The conclusions reached were that during backfilling the mixing of the tine sand, silt, clay and shells produced a very high void ratio material that behaved as a slurry for long enough to allow the pipe to progressively uplift as the backfill plough passed, Soils to be trenched and backfilled should be investigated very carefully, not only for trenchability and in-service behaviour, but also for short term behaviour during backfilling and consolidation. -\ -\--',

A research study into water jetting of stiff cohesive soils has recently been performed by Perry Slingsby Systems Inc. The project is part of a cable and pipeline burial tool Research & Development project. The study program has set out to investigate the effect of water jetting on stiff to very stiff clay soils. To date, such soils have generally been beyond the capabilities of the industry's existing cable burial and maintenance ROV fleet. However, Perry Slingsby Systems have found that it is becoming an increasing requirement for operators to be able to extend the range of seafloor conditions in which burial can effectively take place. The intention is that the project will eventually lead to the delivery of a new range of high capability jet burial equipment. This paper describes the background and methodology of the testing program as carried out to date. It also details the selection of suitable soil test beds which were used to model stiff clay seafloor conditions. The test apparatus and procedures are also described and indications and discussion of the result trends obtained are given. Introduction Despite several well publicized claims to the contrary, there is a considerable amount of evidence that the majority of existing ROV cable jetting packages tend to prove ineffective at jetting in cohesive soils at shear strengths greater than about 50 kPa (classified as "firm" conditions). In fact Perry Slingsby's last major cohesive soil jetting research project in 1994 was geared towards establishing the burial capability of a range of equipment, known as the TXL250® ROV class, in soils up to this strength limit. These tests were described by Adamson and Kolle (1995). The intent of the new Soil Jetting Program is ultimately to develop an engineering model to predict soil trench formation under the action of an array of advancing water jets in stronger, "stiff" clay soils with strengths in the range of 50 to 125 kPa. Such a model is considered vital to designing an effective and power-efficient seafloor jet burial system as it allows the array of nozzles to be optimized and tuned for maximum possible performance over a range of conditions. Because there is only a relatively small amount of published research data appropriate to such soil conditions, further physical testing was considered to be an essential component in the validation of such a model. The Response to Market Requirements Without doubt, there is a general industry trend towards specifying post-lay cable burial via ROV water jetting systems at deeper burial depths and in stronger cohesive soils. In fact, there are increasing demands for future ROV type systems which can start to approach the burial performance of plow systems. It is this market pressure which has provided the incentive to develop a burial model that incorporates stiff clay jetting behavior based on reliable, accurate physical testing.

No abstract

Cables and pipelines are buried in deepwater to provide thermal insulation, on-bottom stability and physical protection. In the soft soils associated with deepwater regions cable ploughs and ROV based jet trenchers are the tools of choice.The Offshore Engineering Division of the Coflexip Stena Offshore (CSO) Group and Perry Slingsby Systems have contributed significantly to the development of deepwater cable and pipeline burial systems. Significant resources continue to be allocated to geotechnical research programmes to support product developments. This paper discusses the key technical issues with respect to deepwater cable and pipeline burial. The state-of-the-art of geotechnical engineering and research, and it's application to new burial product developments, is also presented.