Design and Installation of Submarine Pipelines and Risers against Severe Underscouring
Authors: Waghode Atul Ninu , Darekar Avinash Shankarrao , Samant Sachin 
Published: April 2012

A pipeline laid directly on the seafloor experiences boundary layer effects ofwaves and current-dominated flows close to the seabed in shallow andintermediate waters. Type of seabed soil plays a vital role in changingas-installed condition of pipeline throughout its lifespan. Hydrocarbon fieldslocated in the immediate vicinity of shores on the west coast of the Indianpeninsula experience some of the world's most severe seabed currents. Theseabed soil in these offshore regions is primarily fine loose sand with greatpotential for liquefaction. Due to a combination of strong underwater currentsand moving soil sediments, the seabed is considerably mobile and poses a greatchallenge to pipeline engineers in designing subsea facilities that canwithstand underwater scouring. The remedial measures for scour protection havea vital role in the feasibility of a project if the field is marginal. It alsoaffects the installation schedule to a greater extent considering a vey narrowweather window. This paper details the methodology used for optimum design andsafe installation of a subsea pipeline-riser system along with permanent scourprotection supports with minimal impact on installation costs and projectcompletion schedule.

Project Overview

Exploratory efforts carried out in the field have resulted in a gas discoveryin prospect A in the block. The field is comparatively marginal producing 2.4million standard cubic meters per day of gas containing more than 98% methanewith no CO2 and H2S. The field development involves installation of 2 unmannedwellhead platforms located in water depths of 25m and associated submarinepipelines. Gas from these platforms will be gathered on an existing wellheadplatform by 18" rigid pipelines. For hook-up of this pipeline majormodification works have also been planned at the existing platform which islocated at a distance of 30km from the coastal town of Suvali. The gathered gaswill be transferred to an onshore processing facility via an existing 24" submarine pipeline. The operator planned to develop the field on a fast trackwith the target of start of commercial production by the last quarter of2011.

Geographical Location

The hydrocarbon field is located in the Gulf of Khambhat, (also called the Gulfof Cambay) about 250 km North West from the city of Mumbai, India and around35-40 kms from the shoreline. As shown in Figure 1 Gulf of Khambhat istrumpet-shaped gulf of the Arabian Sea, indenting northward the coast ofGujarat state in western India, between the city of Mumbai and the Kathiawadregion of Gujarat. The gulf is 200km wide at its mouth between Diu and Daman, but rapidly narrows to 25km. It receives many rivers, including the Sabarmatiand Narmada (Ref. 1) in the same region. In the subtropical regions of theIndian subcontinent the direction of the wind is from South West during themonsoon season lasting from June to September. The shape of the Gulf ofKhambhat and its orientation contributes to a high tidal range in order of 12mand a high velocity of entering tides. At low tide, the bottom is left dry fora few kilometers away from the coast. Shoals and sandbanks are treacherous tonavigation, and all the gulf ports near the Gulf of Khambhat have suffered fromsilting caused by tides and flood torrents from the rivers. The field is wellknown for its harsh environment and very few installation contractorsparticipated in the bidding process anticipating difficulties to be facedduring installation.