Session: 01-02-01 Station Keeping - I

Submission Number: 156267

Mooring Spread Design Challenges for Stalk-On Riser Installations in Congested Offshore Fields 

Stalk-on method of riser installation is normally adopted where a flanged connection is not allowed at the pipeline to riser interface due to client or field specific requirements. Riser installation by stalk-on method requires an installation vessel with pre-installed davits which are utilized to lift the pipeline laid on seabed to the surface with pipeline tip effectively secured on to vessel deck. Prefabricated riser assembly is then lifted by vessel crane and the riser bottom end is aligned with pipeline end. After completion of offshore weld and field joint coatings, entire assembly of riser and pipeline is lowered to seabed with riser guided to riser clamps on the jacket during lowering process.

A majority of offshore field expansion works involves additional subsea structures and offshore platforms to be installed nearby existing offshore structures or production platforms complex. In many cases, risers to these newly installed offshore platforms are to be installed using stalk-on method due to requirements as mentioned above. Typically, a DP installation vessel is preferred for installation activities in congested fields with many subsea assets and offshore platforms in the vicinity of operation, for example, installation of a jacket. However, for stalk-on riser installation activities nearby existing offshore platforms, usage of DP vessels have limitations in terms of vessel size, thruster capacities, excursions envelopes, achievable operating seastates, tug interventions etc, due to additional loads imposed on DP system from davit lift operations. Furthermore, there are uncertainties related to the response of a DP vessel in the event of thruster failure during stalk-on installation activities. Therefore, installation vessel in moored configuration with a well-designed mooring spread is preferred. In addition, excursion envelopes for a moored vessel could be well predicted to ensure no collision or interference of vessel with existing nearby offshore and subsea assets during normal operating conditions and redundancy conditions. This paper aims to outline and describe the challenges faced and solutions adopted during the design and analysis of mooring spread of an installation vessel for one of the largest and critical stalk-on riser installation activity carried in Persian Gulf nearby an existing offshore production platforms complex, characterized by high external loadings on installation vessel from pipeline lifting operations.

Presenting Author: Toms Jacob MCDERMOTT

Presenting Author Biography: Mr. Toms Jacob is a Naval Architect and Chartered Engineer with more than 16 years of progressive and interdisciplinary experience in marine and offshore engineering sector. He started his career as plan approval engineer at Ship & Offshore Structures & Statutes Department at American Bureau of Shipping (ABS), Singapore. He had further worked with other classification societies, marine and offshore consultancy firms and offshore EPCI companies broadening his experience in the field of structural and statutory certifications of marine vessels and transportation and installation engineering of offshore and subsea structures. He is currently working as Senior Naval Architect in Offshore Installation Engineering Department at Mcdermott Dubai, UAE.

Toms holds a Bachelors degree in Naval Architecture & Shipbuilding from Cochin University of Science & Technology, India. He completed his Masters degree in the field of Subsea Engineering from University of Strathclyde, Glasgow, United Kingdom. His scholastic achievements include 2nd Rank in B.Tech Naval Architecture & Shipbuilding, 1st Rank in MSc Subsea Engineering and he is the winner of BP Scholarship for young talents in Subsea Engineering for academic year 2011-2012 at University of Strathclyde, Glasgow, United Kingdom.

Toms is a Chartered Engineer with the United Kingdom Engineering Council, member of The Royal Institution of Naval Architects, member of Society of Naval Architects and Marine Engineers and also a member of Society of Underwater Technology.