After Final Investment Decision, construction is usually organised into engineering, procurement, fabrication, transport & installation (T&I), hook-up, pre-commissioning and commissioning. Engineering freezes the design and produces IFC (issued-for-construction) documents. Procurement secures long-lead equipment such as subsea trees, umbilicals and major topside packages. Fabrication yards then build jackets, decks, modules and subsea structures. T&I contractors move these offshore and install them with heavy lift, pipelay and construction vessels. Once installed, systems are hooked up, tested, and gradually energized and pressurised during pre-commissioning and commissioning to demonstrate readiness for hydrocarbons. International guidelines group these as development and production activities, offshore pipeline operations and associated support operations. Ref
EPCI or EPCIC contracts bundle Engineering, Procurement, Construction, Installation and sometimes Commissioning under a single main contractor. During engineering, the contractor refines the concept, develops detailed design and defines installation methods. Procurement covers sourcing, expediting and inspecting all tagged equipment and bulk materials. Construction and onshore fabrication build modules, jackets, decks and subsea structures in yards. The installation scope then transports these to the field and installs them using specialist marine spreads, including heavy-lift, pipelay and support vessels. In EPCIC, the contractor may also execute offshore commissioning of systems up to handover. This integrated model reduces interfaces for the operator but shifts significant execution and schedule risk onto the contractor. Ref
Detailed engineering converts the selected development concept into constructible designs and work packs. It includes structural analysis of jackets, topsides and subsea frames; pipeline sizing and wall-thickness calculations; routing for flowlines, umbilicals and cables; and selection of materials and corrosion protection systems. Piping and instrumentation diagrams, control narratives and 3D models are developed to resolve clashes and confirm access for construction and maintenance. For subsea and SURF, route engineering accounts for seabed conditions, metocean data and on-bottom stability. Installation engineering defines lift points, rigging, seafastening and pipelay parameters. The output is a complete engineering dossier supporting procurement, fabrication, installation and later operations. Ref
Fabrication yards execute most heavy construction work onshore to minimise offshore time. Structural shops cut, profile and weld steel into sub-assemblies, which are then built up into legs, braces, decks and module frames. Welds receive non-destructive testing; surfaces are blasted and coated for corrosion protection. Mechanical, electrical and instrumentation teams install equipment, cable trays, cabling, tubing and insulation. Integration teams mount large packages such as compressors, generators and processing skids. As fabrication progresses, pre-commissioning begins: power and control cables are tested, instruments are calibrated and loop-checked, and systems may be function-tested with temporary utilities. Finished jackets and modules are weighed, sometimes trial-fitted, then prepared for load-out to transport barges. Ref
Load-out transfers jackets, decks and modules from the quay onto a barge or heavy transport vessel. Depending on weight and geometry, this may use skid beams, rocker arms, strand jacks or self-propelled modular transporters. Once on the vessel, the structure is sea-fastened with welds, braces and grillage designed to withstand transit forces. Naval architects analyse motions, stability and global strength for the transport route and seasons. During the voyage, marine procedures cover towing arrangements, weather limitations and contingency ports. At the field, sea-fastenings are cut, rigging is connected, and the structure is prepared for lift, launch or float-off, depending on the chosen installation method. Ref
Jacket installation typically starts with the heavy-lift vessel positioning alongside the transport barge. The crane hooks up the jacket using pre-designed slings and trunnions, lifts it free and either upends it in the water or installs it vertically, depending on configuration. The jacket is set on pre-levelled mudmats or piles and aligned using survey systems. Piles are then driven through pile sleeves and grouted to achieve the required foundation capacity. Follow-on works include installing boat landings, riser guards and conductor guides. Once the substructure is secure and surveyed, the topside is lifted or floated over and connected. This sequence is governed by detailed installation procedures and marine spread capabilities. Ref
For fixed platforms, topsides may be installed as multiple lifts or as a single integrated deck, depending on weight and crane capacity. The heavy-lift vessel positions the deck over the jacket, lowers it onto pre-set legs, and crews complete welding, bolting and levelling. For floaters such as FPSOs or semi-subs, construction may occur at a separate yard or shipyard, followed by tow-out and offshore mooring. Mooring lines and anchors are installed first, then the floater is connected and tensioned. Riser porches, umbilical pull-ins and export systems are subsequently hooked up. Integration continues with electrical, control and mechanical interfaces to subsea and export systems. Ref
Subsea production system installation starts with seabed preparation and deployment of templates or manifolds, usually from construction vessels with active heave-compensated cranes and ROV support. Wellheads are installed and wells drilled and completed. Subsea trees are then landed on the wellheads and locked in place. Manifolds, distribution units and other structures follow, aligned using acoustic and inertial positioning tools. Jumpers and flying leads connect wells to manifolds and manifolds to flowlines and umbilicals, typically using ROV-operated connection systems. Throughout, ROVs verify orientation, stab connections and leak-tight seals. After installation, hydraulic, electrical and communication tests confirm readiness before tie-in to the host facility. Ref
Subsea pipeline construction begins with route selection, geophysical and geotechnical surveys, and on-bottom stability design. Pipes are manufactured and coated (typically anti-corrosion plus concrete weight), then transported to a pipelay vessel. On board, joints are welded, inspected and coated before entering the stinger and being laid onto the seabed by S-lay, J-lay or reel-lay methods. Shore approaches and tie-ins use specialised techniques such as horizontal directional drilling or hyperbaric welding. After lay, the line may be trenched, rock-dumped or otherwise protected. Risers connect seabed lines to surface facilities and can be rigid or flexible, using configurations like catenary or tower systems. Ref
Hook-up is the phase where installed structures and systems are physically and functionally connected offshore. After topsides or modules arrive on the jacket or floater, multi-discipline teams connect piping, cabling and tubing across module interfaces and to subsea, export and utility systems. Activities include spoolpiece installation, hot bolting, termination of electrical and instrumentation cables, control system integration, and completion of welds that could not be done onshore. Temporary power, construction lighting and scaffolding support the work. Hook-up often overlaps with mechanical completion and pre-commissioning, and it is a critical period for managing construction changes and punch-lists. Ref
Commissioning builds on pre-commissioning by energising systems in sequence and verifying that they operate together as intended. It typically follows a structured plan with subsystems grouped into commissioning packages. Electrical systems are energised, control systems are loaded with final logic, and unit operations (compressors, pumps, treatment packages) are started using inert or clean media. Integrated testing follows, sometimes including hot commissioning with hydrocarbons at reduced rates. Safety and emergency systems are thoroughly tested, including ESD logic and blowdown. Performance tests compare measured capacities, pressures and temperatures against design values. Only after successful commissioning and acceptance tests will the operator approve first oil or gas. Ref
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Offshore construction relies on a mixed fleet rather than a single “all-rounder.” Typical categories include heavy construction and heavy-lift vessels, pipelay vessels, diving support vessels, subsea construction / ROV support vessels, cable- and umbilical-lay vessels, rock installation and fallpipe vessels, survey vessels, anchor handling tug supply vessels, platform supply vessels, heavy transport vessels and accommodation barges or floatels. Each type is optimised for particular tasks and often equipped with dynamic positioning to hold station safely near structures. Contractors mix and match this fleet according to water depth, metocean conditions and scope, sometimes using multi-purpose construction support vessels that combine crane, ROV and lay capabilities in one hull. Ref
Heavy construction vessels and crane barges provide the lifting capacity to install jackets, topsides, large subsea structures and heavy pipeline components. They feature large offshore cranes, extensive deck space and powerful winch systems, often combined with pipelay or flex-lay equipment. Some modern heavy construction support vessels also carry DP2 or DP3 systems, enabling precise positioning without anchors while handling large loads. These ships support jacket setting, pile handling, topside lifts, heavy subsea module installation and sometimes decommissioning. Their crane capacities range from a few hundred tonnes on “heavy CSVs” up to several thousand tonnes on flagship units, making them central assets in most field development campaigns. Ref
Pipelay vessels are essentially floating factories that weld, inspect and install subsea pipelines. S-lay vessels weld joints along a horizontal firing line before the pipe curves over a stinger into an S-shaped profile toward the seabed. J-lay vessels orient the pipe nearly vertical, reducing bending and making them suitable for deep water. Reel-lay vessels spool pre-fabricated pipe onto a large reel onshore, then unspool and straighten it offshore for rapid installation. All configurations rely on tensioners, stingers and abandonment and recovery systems to control pipe curvature and tension. Choice of lay method depends on water depth, pipe size and coating. Ref
Diving support vessels provide the platform, equipment and life-support for commercial diving operations, especially saturation diving in deeper water. They combine DP capability, a moonpool, dive control rooms, handling systems and a saturation system with living chambers and a diving bell. From these vessels, divers perform tasks such as spoolpiece installation, clamp fitting, metrology, guide frame installation and subsea tie-ins where diver intervention is still preferred. DSVs must hold station accurately close to platforms or subsea assets, maintain safe launch and recovery envelopes and accommodate extensive auxiliary equipment. For shallower or less complex scopes, smaller air-diving spreads may be mobilised on multipurpose vessels or construction barges. Ref
Subsea construction vessels are specialist ships designed to execute complex underwater works using ROVs and deck equipment rather than divers. They typically feature DP2 or DP3 systems, large work-class ROVs, subsea cranes, winches and deployment frames for structures and tooling. These vessels handle manifold and template installation, jumper and flying-lead connection, valve operations, subsea cutting, dredging and decommissioning tasks. Their ability to work in deep water and harsh environments makes them essential in modern developments, where diver access is limited. Operators see them as flexible platforms that can switch between installation, inspection, maintenance and repair campaigns using modular, containerised tooling. Ref
Rock-installation and fallpipe vessels place graded rock accurately on the seabed to stabilise and protect offshore infrastructure. Using a vertical or inclined fallpipe, sometimes guided by an ROV at the tip, they can cover pipelines and cables, level the seabed and install scour protection around platforms or foundations. Dynamic positioning allows precise control of the vessel’s track while rock is pumped or gravity-fed down the pipe. This fleet plays a key role in pipeline and cable protection, seabed preparation before jacket installation and ballasting of platform legs. As water depths increase, DP fallpipe vessels enable rock placement down to several hundred metres and beyond, with sophisticated monitoring and survey systems. Ref
AHTS vessels are the workhorses for mooring and towing tasks. During construction, they install, recover and pre-tension anchors and mooring lines for rigs, barges and sometimes floating production units. They tow units between locations, assist with positioning during jacket installation and support barge manoeuvring in tight fields. Their powerful winches, stern rollers and open working decks allow safe handling of heavy chain, wire and synthetic moorings. AHTS vessels also provide supply and standby functions, delivering deck cargo, towing equipment and emergency support. While many construction campaigns now rely heavily on DP units, AHTS remain vital wherever anchored spreads or rig moves are involved. Ref
Platform supply vessels provide the logistics backbone for offshore construction. They shuttle equipment, tubulars, consumables, chemicals, fuel, food and personnel between shore bases and the offshore fleet. Their design features large open decks for project cargo, tanks for liquid products and often DP capability for safe alongside operations. During construction, PSVs support pipelay vessels, DSVs, heavy-lift units and platforms, keeping them stocked with weld consumables, grout, spare parts and ROV tooling. Efficient PSV scheduling and back-loading of waste and unused material are crucial for cost control and HSE performance, reducing the number of vessel trips and port calls required. Ref
Accommodation barges, ships and floatels provide living quarters when local platforms or FPSOs cannot house the peak workforce. These units offer cabins, offices, workshops and recreational facilities, and they connect to working platforms via gangways or crew boats. During intense hook-up and commissioning phases, they allow hundreds of personnel to stay close to the worksite, reducing helicopter or boat transfers and increasing productive hours. Some accommodation units are self-propelled DP vessels; others are moored barges supported by tugs and AHTS. Their presence influences planning for emergency response, POB (personnel on board) management and logistics, making them a central tool for managing labour-intensive construction periods. Ref
Foundation installation combines heavy mechanical tools and dedicated grouting systems. Hydraulic hammers mounted on cranes or frames drive piles through jacket legs or sleeves into the seabed, with internal lifting tools and upending frames handling long, heavy piles safely. For many jackets and templates, grout is then injected into annuli between piles and sleeves using high-capacity mixing, pumping and monitoring spreads. Specialist contractors supply grout mixing units, silos, pumps, hoses, injection clamps and field laboratories to control quality and document performance. These tools ensure structural load transfer and long-term integrity of platform foundations under cyclic wave and operational loading. Ref
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The construction phase concentrates a lot of risk in a relatively short time: intensive marine operations, complex lifting, subsea installation, simultaneous activities, and a big temporary workforce. Key challenges include safeguarding people during heavy lifts and vessel operations, protecting the environment from discharges, noise and seabed disturbance, and ensuring the integrity of new pipelines, structures and subsea equipment. Logistical complexity, crowded work sites and shifting scopes increase the potential for incidents if planning and coordination are weak. International EHS guidelines emphasise structured risk assessments, strong contractor management, robust marine procedures and emergency preparedness as core controls throughout development and pipeline operations, not just during drilling or production. Ref
SIMOPS occur when construction, commissioning and sometimes live production activities take place at the same location and time. That can mean hot work near hydrocarbons, lifting over live equipment, or construction crews sharing space with operating staff. The main challenge is that each activity might be individually safe, but their interactions create new hazards and escalation paths. Effective management relies on structured SIMOPS risk assessments, clear definition of “primary” and “secondary” operations, robust permit-to-work and isolation systems, and a central coordinator with authority to stop work. Good practice guidance stresses SIMOPS matrices, interface meetings and clear emergency plans to prevent conflicts and ensure everyone understands what else is happening around them. Ref
Waves, wind, currents and visibility directly influence whether marine operations can be performed safely. Heavy lifts, pipelay, DSV work and gangway transfers all have defined environmental operating limits; exceeding them increases risks of collision, line breaks, loss of position or dropped loads. As a result, “weather windows” become a major scheduling constraint, especially in harsh environments. Guidelines for offshore marine operations emphasise robust metocean data, conservative planning assumptions, and dynamic risk assessment as conditions change. Projects also need contingency plans and flexible sequencing to avoid pressure to work outside limits. Poor weather risk management can quickly translate into cost growth, as expensive vessels wait on weather or operations are aborted mid-task. Ref
Construction brings multiple environmental stressors: noise from piling and vessels, seabed disturbance from anchors, piles and rock placement, and discharges from hydrostatic testing and vessel operations. Hydrotesting of pipelines and equipment uses large volumes of water that may contain corrosion inhibitors or biocides; international guidelines require treatment and controlled discharge to meet marine standards such as MARPOL. Ballast, bilge and drainage water must also be managed carefully, especially in sensitive or nearshore environments. Environmental and social checklists for offshore oil and gas stress early impact assessment, careful route and site selection, monitoring programmes and clear discharge standards as essential to minimise impacts during the development phase. Ref
Subsea pipelines face threats from installation-induced damage, unstable seabeds, free spans, third-party interference and dropped objects. Recommended practices such as DNV GL’s pipeline risk assessment standards identify hazards from anchors, trawl gear and ship collisions, alongside internal corrosion or hydrogen-induced cracking. During construction, inadequate on-bottom stability, poor backfilling or incorrect routing can create future integrity problems. Hydrotesting verifies strength and leak tightness but does not eliminate all threats. Risk-based approaches combine design codes, route engineering, protective measures (trenching, rock dump, mattresses), and inspection and repair strategies. Post-lay surveys and as-built documentation are critical milestones to confirm that installed lines meet design assumptions and risk criteria. Ref
Hook-up and commissioning bring a dense mix of construction, testing and energisation activities in a confined space. Challenges include managing a large multi-discipline workforce, tight schedules, changing work fronts and frequent SIMOPS with live utilities or neighbouring facilities. Technically, late design changes, incomplete vendor data, or fabrication deviations can create rework and punch lists offshore, where fixes are costlier and riskier. Commissioning teams must energise electrical and control systems, introduce fluids and test safety functions without compromising barriers. Good practice guidelines from offshore sectors highlight the need for clear leadership, integrated planning between construction and commissioning, robust permit-to-work and SIMOPS controls, and dedicated marine coordination and emergency preparedness during this phase. Ref
At peak construction, hundreds of people may be offshore across platforms, floaters, barges and vessels. Challenges include ensuring enough beds, coordinating helicopter and boat transfers, tracking personnel locations in real time, and maintaining emergency evacuation capacity. Marine coordination must juggle supply vessels, construction spreads and standby boats within limited sea room, often in busy shipping areas. H&S guidance stresses the importance of a dedicated marine and logistics coordination function, integrated POB tracking and clear lines of communication with external emergency services. Poor logistics planning can create fatigue, bottlenecks and “make-do” decisions that increase safety risk, particularly during intensive hook-up and commissioning campaigns. Ref
Before hydrocarbons can flow, operators must demonstrate to regulators and sometimes independent verifiers that construction and commissioning meet design, safety and environmental requirements. For offshore developments in many jurisdictions, this includes approvals of development plans, pipeline and facility design, and environmental assessments, followed by verification of construction, testing and as-built documentation. Agencies such as BSEE regulate design, construction and operation of facilities and pipelines on the outer continental shelf, issuing detailed rules and conducting inspections. Compliance with recognised standards (for structures, pipelines, marine systems) and EHS guidelines is part of the evidence package. Only after regulators are satisfied with integrity, safety management systems and emergency preparedness will start-up be authorised. Ref
Construction introduces or modifies major accident hazards such as loss of stability, structural failure, fire and explosion, and large releases from hydrotests or temporary systems. Risk management frameworks derived from offshore oil and gas practice stress identifying major accident hazards, defining prevention and mitigation barriers, and testing emergency response arrangements. During construction and commissioning, emergency plans must account for multi-vessel operations, temporary accommodation, SIMOPS and evolving layouts. Regular drills, clear command structures, and coordination with coast guards and rescue services are essential. International guidelines for offshore development highlight emergency preparedness and major accident risk as central EHS issues across all project phases, not just during steady-state production. Ref
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Industry: Offshore Oil & Gas | Wind Energy | Ship building | Offshore Logistics | Jobs & Roles |
Production Process: Exploration | Construction | Production | Decommissioning | Transport | Refining | Walk-to-Work |
Offshore Installations: FPSO | FLNG | Platforms | SOVs | CTVs | Sub-sea infrastructure | Tankers |
Safety: Access Control | POB | Workplace Safety | Workplace Health | Emergency | Training | Mustering | Regulations | Risk Assessment | Safety Assistance Technology |
Activity: Oil | Gas | Wind | Deep Sea Mining |
Areas: North Sea | Middle East | South Atlantic | Indian Ocean | Pacific Ocean |