Abstract

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.

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