A proven subsea arrangement is the use of pipelines and umbilicals connecting subsea wells to the moored host facility (semi-submersible, FPSO, etc.). This subsea development scenario may require the following installation tasks:

  • Lifting and installation of subsea Christmas trees, manifolds, PLET/ PLEMs, UTA, SDU, etc.;
  • Laying of umbilicals and pipelines;
  • Subsea tie-ins.

The following factors need to be taken into account for selecting vessels for subsea development:

  • Installation task: What is to be installed?
  • Environmental conditions: How deep is the water in the installation site?
  • Vessel features: What requirements should the vessel meet?
  • Costs: What is the estimated budget?

The requirements of vessel features are divided into two categories in this chapter: basic requirements and functional requirements. The basic requirements describe what is needed to ensure vessel basic performance and safety. The functional requirements describe the critical actions needed to perform installation activities.

Basic Requirements for Vessels and Barges

Avessel’s performance and structure are basic requirements to be considered when selecting vessels and barges.

Vessel Performance

Vessel performance issues for and installation vessel include the following:

  • Buoyancy: The vessel should be buoyant at a certain loaded condition. There may be four buoyancy conditions for the vessel: upright position, trim, heel, and a combination of those conditions.
  • Stability: Stability means that the vessel can get back to its initial balanced position once the forces or moments applied on it have disappeared. Stability can be divided into “initial stability at small angle of inclination” and “stability at a large angle of inclination.” Initial stability has alinear relationship between inclination angle and uprighting moment.
  • Insubmersibility: Insubmersibility means that the vessel can remain buoyant and stable once one space or multiple spaces of the vessel are flooded. Sufficient stability and reserve buoyancy are ensured for all floating vessels in all stages of marine operations.
  • Sea-keeping: Sea-keeping means that the vessel can remain safe while navigating or operating at sea, even though it may be exposed to the severe forces and moments created by wind, waves, and current.
  • Maneuverability, speed, and resistance: Maneuverability of the vessel refers to the capability of the vessel to keep a constant navigation direction or change direction only according to the pilot’s desire. Speed and resistance refers to the speed capability of the vessel at the rated power of the main engine.

Vessel Strength

Vessel structures can experience three types of failures: strength failures, stability failures, and fatigue failures. A strength failure normally means that the stress of the vessel structure is larger than its Specified Minimum Yield Strength (SMYS). A stability failure means that the compressive stress of the structure is larger than the critical stability stress (e.g., Euler stress) and results in large displacements. Fatigue failure refers to the cracking or fracturing of the vessel’s structure due to continuous stress circulations. The strength of the vessel normally includes longitudinal strength and local strength. The strength of the vessel should not be exceeded during all vessel activities such as load-out, lifting, and transportation.

Functional Requirements

Generally speaking, a vessel is just a stable platform that sustains the specific equipment performing the required activities. The specific equipment is mobilized on board before carrying out the installation activities, and demobilized to the shore base once the required activities have been completed. The mobilization and demobilization consumes time and money. Consequently, specialized vessels with specific equipment for a certain type of job, such as pipe-laying vessels or diving support vessels, are often used. The main differences between the function requirements of a subsea hardware installation vessel and that of a pipe-laying/umbilicallaying vessel are the different requirements for specific equipment.

Vessel for Subsea Hardware Installation

Generally, subsea hardware can be installed by a discretional vessel with sufficient winch rope length and crane capacity with or without a heave compensator. The vessel may be a drill vessel, pipe-laying vessel, umbilicallaying vessel, or offshore support vessel. The critical vessel features for subsea installation normally include the following:

  • Deck space for the project equipment;
  • Deck load capacity;
  • Crane capacity and coverage;
  • Vessel sea behaviors (RAOs);
  • ROV requirements, such as two work-class ROVs for subsea structure installation, one work-class ROV, and one survey ROV for pipeline/ umbilical laying, monitoring, etc.
  • Accommodation capacity;
  • Transit speed (high transit speed has become important due to long transit distances as field developments are placed in deeper and deeper water);
  • Positioning requirements. Lifting and installation of subsea structures must be performed in a timely and safe manner, taking into account all site limitations:
  • Weather conditions;
  • Seabed soil conditions and visibility;
  • Other site constraints (mooring lines, other subsea structures, etc.). The deck handling system and the lifting devices on the vessel must be designed to control and prevent the pendulum movements of the subsea structures as they leave the deck storage area, pass through the splash zone, and land safely on the seabed. This dedicated installation equipment (cranes, winches, etc.) must be designed, built, and operated to suit the conditions in which they are to perform. More specifically, they also must take into account the structure’s installation criteria and constraints, as specified by the designers/manufacturers and national codes/certifying authority rules (DNV, API, ABS, etc.).

Vessel for Pipe and Umbilical Laying

Compared with the subsea hardware installation vessel, the selection of a pipe-laying vessel should include the following additional requirements for some specific equipment:

  • Capacity of tensioner: This is required based on the water depth and pipeline unit weight and buoyancy.
  • Abandon and recovery winch: This is required at the end of the pipe-laying process and if emergency conditions arise.
  • Davits capacity: This is required once davit activity is necessary for the offshore connection of pipeline or umbilicals.
  • Product storage capacity: This is required if the pipe joints or umbilical reels cannot be transferred from the storage barge to the laying vessel due to bad weather.


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