Subsea Deployment Systems Ltd.

SUBSEA INSTALLATION

WITHOUT

A HEAVY LIFT VESSEL

A STEP CHANGE IN SUBSEA INSTALLATION

OUTLINE METHOD

A typical installation will consist of the following steps:

Load-out Shallow Draught Surface Tow Ballasting and Trimming Deep Draught Surface Tow Submerged Tow Positioning Set-down Ballasting Float-off Load-out

The structure may be loaded-out into the Subsea Deployment Vehicle (SDV) by a variety of methods depending on the available equipment and draught. Load-out options include:-

Direct Lift Lift, Submerge and Float-over Float-over in a dry dock Submerge and Float-over using a floating dock or

submersible barge Submerged Docking

Shallow Draught Surface Tow

When there is limited water depth at the load-out location, the SDV and structure will be towed in Shallow Draught Surface Tow Mode until reaching a suitable location for flooding the hulls.

Shallow Draught Surface Tow

INTRODUCTION

The Subsea Deployment System (SDS) is a method of install-ing large subsea structures without a heavy lift vessel (HLV). It offers potential cost savings of 60% on multi-structure in-stallations and up to 80% on single structure installations compared to using an HLV. It can also install larger struc-tures in greater water depths in more environmentally hostile locations than is possible with current vessels.

The SDS is ideally suited to meet the growing demand for installing larger structures in deeper water.

The system is a potential step change in the installation of subsea structures.

ADVANTAGES

Low cost (OPEX and CAPEX) Low tech and largely failsafe Less weather critical (wave & ice) Potential to reduce the number of individual structures,

tie-ins and interfaces Potentially greater availability compared to HLVs Potential for phased installation Minimal dynamic loading

APPLICATIONS

Installation 100t to >1000t structures Water depth 100m to > 3000m Hostile environments Decommissioning Structures recovered without lifting to deck Minimal dynamic loading Large capacity >1000t structures Salvage Deployment vessel can be parked at wreck site for pro-

longed periods in all weather Structures recovered without lifting to deck Minimal dynamic loading

Patent: GB2464714

Ballasting and Trimming

When the water depth is suitable, the SDV will be ballasted down by flooding the hulls. The attitude of the SDV will be checked and adjusted as necessary.

Deep Draught Surface Tow

The SDV and structure will then be towed in Deep Draught Surface Tow Mode with only the castles and control chain towers breaking surface.

Submerged Tow

When the water depth is suitable, the tow vessel will pay out the tow wire and tow chain clump weight. The tow chain clump weight will cause the SDV to submerge.

The tow vessel can adjust its speed and the length of the tow wire to maintain the SDV at a suitable depth. The SDV may be lowered as the water depth increases by paying out on the tow wire. The position and inclination of the SDV will be monitored throughout by acoustic interrogation.

When approaching the field, the tow vessel will slow down and adjust the tow wire to keep the tow chain clump weight off the seabed until in a designated parking area.

The tow vessel will then pay out the tow wire until the clump weight rests on the seabed. The SDV and structure will now be “anchored” and float above seabed.

Ideally the length of tow wire between the clump weight and SDV will be marginally greater than the distance between the parking area and the final target location. This allows final set-down without the need to lift and re-position the tow chain clump weight.

Positioning

The SDV will be positioned by means of two control chains suspended from the installation vessel and lowered into the chain towers.

The height of the SDV will be adjusted by raising or lowering the control chains.

The position and orientation of the SDV will be adjusted by moving the installation vessel and / or the crane.

Deep Draught Surface Tow

Patent: GB2464714

Tow Transition

Lowering

Positioning of SDV

Tow Chain Clump Weight The tow chain clump weight is inserted into the tow wire to provide the necessary weight to submerge the SDV from the deep draught tow condition to the submerged tow condition. It also acts as an anchor for the SDV when parked on the seabed.

Ballast Chain Lockers Ballast chain lockers are placed at each corner above the hulls. They are used to trim the SDV to suit the weight and CoG of the structure prior to the tow.

They are also used to hold the ballast weight which replaces the structure after installation of the structure.

Stru

Solid Buoyancy Modules Solid buoyancy modules (syntactic foam) rated to the installation water depth are located above the hulls.

The buoyancy modules are located as high as is practical to maximise the separation between the CoG and CoB when the SDV is submerged. This ensures the stability of the SDV throughout all stages.

Structure - SDV Interface Beam The structure – SDV interface beams are used to support the structure between the hulls of the SDV. For a float-over load-out they will be fitted to the SDV but may be part of the lift for a lifted load-out.

Control Chains and Chain Towers The control chains are lowered into chain towers to gain control of the SDV during installation.

The weight of the chain supported by the SDV at the base of the chain towers is used to control the height of the SDV.

The length (weight) of chain suspended within the chain towers provides lateral and rotational control of the SDV.

Castles The castles are positioned above the majority of the solid buoyancy and will be the only part of the SDV apart from the chain towers that will protrude above the waterline in the deep draught condition. This facilitates fine tuning of the trim.

Longitudinal Pontoons The SDV consists of two longitudinal pontoons which have ballasting facilities. This enables the SDV c/w structure to operate at a shallow draught.

cture

Ballasting Platform / System The flooding and vent valves are manually operated from work stations next to the control chain towers by means of mechanical linkage where necessary.

The SDV will be ballasted by means of gravity alone and all valves will remain open during the submerged tow.

KEY FEATURES

Stability

The SDV is designed to be inherently stable throughout all stages of submergence.

During shallow draught tow the hull spacing will provide good stability with the metacentre well above the CoG.

When the SDV is fully submerged the CoB is well above the CoG.

Dynamic Response

The control chain is not directly connected to the SDV and only rests within the chain tower, therefore the surface vessel and SDV can move independently.

Rapid vertical movement of the surface vessel only raises and lowers the control chain and not the SDV. Consequently there will be minimal variation in the down-line tension.

The mass of the SDV and structure is large compared to the weight of control chain and consequently the motion of the chains due to surface vessel motions causes minimal dynamic response of the SDV.

Installation Control

Lengths of chain suspended from the surface vessel are lowered into the chain towers to perform two main functions.

Primarily the weight of the chain supported at the base of the chain towers is used to make the SDV neutrally buoyant.

Secondly the length (weight) of chain suspended within the chain towers provides lateral control of the SDV.

The position and orientation of the SDV can be adjusted by moving the surface vessel.

SDV to Structure Interface

The interface connection between the subsea deployment vessel will vary depending on the structure to be deployed and the load-out method. It may be in the form of conventional slings, hydraulically operated pins or by means of a propriety connector such as Ballgrab.

In most cases this will involve the use of project specific interface beams which are attached to the structure at the design lift points and to standard connection points on the SDV. The interface beams may also serve as lifting beams if required by the load-out method.

Motion Relationship between SDV and Surface Vessel

Set-down

Once the DSV is in the correct position and orientation the structure will be landed by lowering the control chains. The control chains will then be fully lowered into the chain towers and temporarily disconnected. The control chain connector will be located on the top of the chain tower ready for reconnection after ballasting.

The weight of the control chains will contribute to the initial on-bottom stability, i.e. prior to ballasting.

Ballasting

Ballast weight will be added to the ballast chain lockers by the surface vessel crane to balance the weight of the structure. The SDV will then be slightly negatively buoyant and will rest on the structure. The SDV will now be disconnected from the structure.

Float-off

The installation vessel will now re-connect to the two control chains and raise them until the SDV is neutrally buoyant. Further raising of the control chains will lift the SDV clear of the structure.

The SDV will then be manoeuvred in a near reverse of the installation procedure until clear of any subsea assets. The control chains will then be removed completely from the towers allowing the SDV to float above the seabed while still anchored by the tow chain clump weight.

Patent: GB2464714

Control Chain Tower

WEATHER SENSITIVITY

Lift-out, Load-out, Float-out and Float-over

The load-out will be a weather sensitive operation; however, it will be performed in relatively protected locations. Consequently there are unlikely to be significant schedule delays.

Surface Tow

The surface tow can be done in relatively rough seas, structure sensitivity permitting. In most cases the transition to submerged tow will occur before reaching an exposed location.

Submerged Tow

The submerged tow of the SDV and structure is largely unaffected by the surface weather conditions. The limiting criteria are therefore more likely to be associated with the operational weather limitation of the tow vessel.

The towing operations can be suspended at any time by parking the SDV on the seabed and anchoring it with the tow chain clump weight.

Installation

During installation the SDV motions will be minimal in significant sea states; however, working conditions and ROV operating limits are likely to be the limiting criteria, typically Hs 3.5m. This compares with a typical limiting sea state of Hs 2.0m for a conventional lifted installation. In certain areas this may significantly increase the weather window and associated schedule benefits as demonstrated by the following chart.

Patent: GB2464714

SDS Inherent Safety

The potential for catastrophic failure is minimised by having a ‘close to neutral’ submerged weight of the SDV and structure.

Tow Rigging Failure

If there is rigging failure between the tow vessel and the tow chain clump weight, the chain clump weight will sink slowly to the seabed and act as an anchor for the SDV. If the water depth is greater than the length of rigging between the SDV and the tow chain clump weight, the SDV will be pulled under and will float at a known height above the seabed. Otherwise the SDV will float on the surface but will still be anchored by the weight of the chain clump weight.

If there is rigging failure between the SDV and the tow chain clump weight the SDV will float to the surface.

Loss of Buoyancy

During tow the SDV and structure will be marginally buoyant and will therefore be able to withstand some loss of buoyancy.

It is likely that any loss of buoyancy will be gradual. The trim of the SDV will be monitored and will give early warning of any loss of buoyancy. Consequently the SDV may be brought to a safe location for rectification. This could be simply by removing ballast chain.

Adverse Weather

Although adverse weather is unlikely to affect the SDV and structure, there may be occasions where the weather is unsuitable for the tow vessel. In such cases the tow chain clump weight will be lowered to the seabed leaving the SDV safely anchored.

Surface Vessel and SDV (Orcaflex)

Subsea Deployment Systems Ltd. Westhill, Scotland, UK

For further information please visit www.sdsltd-uk.com

© 2013 Subsea Deployment Systems Ltd.

U-shape SDV

The U-shape is a more compact design and typically has greater carrying capacity than the square shape SDV. However, this configuration requires a greater water depth since the structure is located below the SDV.

Square SDV

The square SDV can accommodate the full foot-print of the structure between the hulls and cross / interface beams. This results in a shallower draught than for the U-shape and would typically be used where there is a limited water depth at the load-out location or along the tow route. However, this design has less carrying capacity due to the relative heavy cross / interface beams.

SDS-PR-001-V2

SDV DESIGNS

The two main SDV configurations are square (centre pages) and U-shaped (front page) which facilitate different load-out methods. Both designs are constructed from modular units and the number of units can be varied to suit the required capacity.